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Optical and Structural Characterization of Amorphous Carbon Films
By
Pratish Mahtani
A thesis submitted in conformity with the requirements for the degree of Master of Applied Science
Department of Electrical and Computer Engineering University of Toronto
© Copyright by Pratish Mahtani 2010
ii
Optical and Structural Characterization of Amorphous Carbon
Films
Pratish Mahtani
Master of Applied Science
Department of Electrical and Computer Engineering University of Toronto
2010
Abstract
A fundamental study of the correlations between ion energy, substrate temperature, and plasma
density with hydrogen content, percent sp2 bonding, optical gap, and refractive index of
hydrogenated amorphous carbon (a-C) films is presented. A strong dependency between the ion
energy used during deposition and the film’s microstructure is shown. Moreover, it is revealed
that the optical properties of the a-C films are controlled by the concentration and size of sp2
clusters in the film.
Through N2 mixing in the source gas, room-temperature nitrogen doped polymeric-like a-C films
were demonstrated for the first time. X-ray Photoelectron Spectroscopy revealed an increase in
the Fermi level of these films with increased nitrogen content.
A proof-of-concept a-C based transparent heat mirror (THM) was demonstrated. It was shown
that a-C acts as an oxygen-free protective barrier and anti-reflective coating for Ag films in the
THM, increasing the transmission in the visible region by 10-20%.
iii
Acknowledgments First and foremost, I would like to thank my supervisors, Professor Nazir Kherani and Professor
Stefan Zukotynski. The fundamental scientific approach and forward thinking that they
provided was a great base from which to conduct research. I am indebted to Professor Kherani
for his support, patience, guidance, and endless hours of analysis and discussion of my research.
I am grateful to Professor Zukotynksi for his careful and well thought out critique of my work.
This thesis would not have been possible without them.
A special thank you to Dr. Davit Yeghikyan and Dr. Tome Kosteski for mentoring me through
the early stages of my experiments and providing a structured, safe, and enjoyable work
environment in the lab. I would also like to thank them for supervising and assisting me in the
building of the RF PECVD system. Their experience in the field was a resource that I drew upon
countless times during my research.
Keith Leong has been a great colleague, tutor, and friend. I would like to express my deepest
gratitude for his extensive assistance in my research including providing training on the spectral
ellipsometer and FTIR spectrometer. I can only hope that his steadfast adherence to the
scientific approach has in-part rubbed off on me.
I would like to thank Paul O’Brien for his assistance with SEM, transmission/reflection, and
conductivity measurements. I would also like to thank Paul for his friendship and for being an
outlet for my hockey obsession.
Thank you to Dr. Adel Gougam for assisting me with four-point probe measurements and to Dr.
Honggang Liu for performing lifetime measurements. Also, I would like to express my gratitude
to Raymond Tsai for providing training on the DC Saddle Field system and introducing me to the
field of amorphous carbon. Thank you to Avikshit Mathur and Manish Goyal for providing
training on the E-beam, thermal evaporation, and sputtering systems. Thank you to Anton
Fischer for assisting me with the optical microscope. I would also like to thank Dr. Rana Sodhi
for performing XPS measurements.
Thank you to the NSERC Solar Buildings Research Network for their support of this research. A
special thank you to Professor Andreas Athienitis and Professor Stephen Harrison for their
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collaboration and development of this research project. I would also like to express my gratitude
to Professor Michael Collins for his assistance in transmission/reflection and emissivity
measurements. Thank you to Jarrett Carriere for assisting in developing the initial research plan
for this project.
Lastly, I would like to express my deepest appreciation to my friends and family for providing
support, encouragement, and when I needed it, a distraction from my research. Nothing is
possible without you.
v
Table of Contents Acknowledgments .......................................................................................................................... iii
Table of Contents ............................................................................................................................ v
List of Tables ............................................................................................................................... viii
List of Figures ................................................................................................................................ ix
List of Symbols and Acronyms ................................................................................................... xvii
1 Introduction ................................................................................................................................ 1
1.1 Properties of carbon ............................................................................................................ 1
1.2 Current state and trends in amorphous carbon research ..................................................... 5
1.3 Amorphous carbon as an optical coating ............................................................................ 6
1.4 Research overview .............................................................................................................. 7
2 Amorphous Carbon: An Overview ............................................................................................ 8
2.1 Deposition methods ............................................................................................................ 8
2.2 Plasma kinetics .................................................................................................................... 9
2.3 RF PECVD ........................................................................................................................ 11
2.4 Process variables in RF PECVD ....................................................................................... 16
2.5 Film growth processes ...................................................................................................... 20
3 Experimental Apparatus & Characterization Techniques ........................................................ 22
3.1 Deposition system ............................................................................................................. 22
3.2 RF PECVD system ........................................................................................................... 23
3.3 Sample preparation ........................................................................................................... 25
3.4 Sample set space ............................................................................................................... 27
3.5 UV-VIS-NIR Spectral Ellipsometry ................................................................................. 30
3.6 X-ray photoelectron and X-ray excited Auger electron spectroscopy .............................. 33
3.6.1 Overview ............................................................................................................... 33
vi
3.6.2 Elemental composition .......................................................................................... 34
3.6.3 Film Density .......................................................................................................... 36
3.6.4 Fermi level shifts ................................................................................................... 38
3.6.5 Carbon sp2 / sp3 bonding ratio .............................................................................. 41
3.7 Fourier Transform Infrared Spectroscopy ........................................................................ 45
3.8 Profilometry ...................................................................................................................... 50
4 Experimental Results ............................................................................................................... 51
4.1 Overview ........................................................................................................................... 51
4.2 Growth rate ....................................................................................................................... 52
4.3 Nitrogen content ................................................................................................................ 55
4.4 Hydrogen content .............................................................................................................. 56
4.5 Percent of sp3 Bonding ...................................................................................................... 64
4.6 Film density ...................................................................................................................... 66
4.7 Impurity doping ................................................................................................................ 69
4.8 Optical properties .............................................................................................................. 71
5 Analysis .................................................................................................................................... 83
5.1 Overview ........................................................................................................................... 83
5.2 Effect of deposition conditions on growth, electronic- and micro-structure of films ....... 84
5.2.1 C-RT and A-RT sample sets ................................................................................. 84
5.2.2 A-20 sample set ..................................................................................................... 92
5.2.3 C-N and A-N Sample Sets .................................................................................... 94
5.3 Relationship between film microstructure and optical properties .................................. 100
6 Applications: Transparent Heat Mirror .................................................................................. 106
6.1 Overview ......................................................................................................................... 106
6.2 Background ..................................................................................................................... 107
6.3 Experimental ................................................................................................................... 111
vii
6.4 Results ............................................................................................................................. 112
7 Conclusions and Future Work ................................................................................................ 116
7.1 Conclusions ..................................................................................................................... 116
7.2 Future Work .................................................................................................................... 118
viii
List of Tables Table 1.1: Categories of amorphous carbon (a-C) films ................................................................. 3
Table 3.1: C1s and N1s core-level binding corrected for charging effect (CN sample set) ......... 40
Table 3.2: Vibrational modes of molecular groups commonly found in a-C films ...................... 46
Table 4.1: Summary of sample sets .............................................................................................. 51
Table 4.2: N1s shifts for C-N sample set ...................................................................................... 69
Table 4.3: N1s shifts for A-N sample set ...................................................................................... 70
Table 6.1: Experimental details for deposition of three-layer THM structure ........................... 111
ix
List of Figures Figure 1.1: sp3 and sp2 bonding hybridizations of carbon. Black dots represent atomic positions.
Taken from [1]. ............................................................................................................................... 1
Figure 1.2: Ternary phase diagram of the various forms of amorphous carbon films.
Abbreviations are defined in Table 1.1. .......................................................................................... 4
Figure 2.1: Typical RF-PECVD System. ...................................................................................... 12
Figure 2.2: Generation of plasma in RF-PECVD. Neutrals are represented by ‘N’, ions are
represented by ‘+’, and electrons are represented by ‘-‘. .............................................................. 13
Figure 2.3: Formation of sheath regions near anode and cathode in RF PECVD. Neutrals are
represented by ‘N’, ions are represented by ‘+’, and electrons are represented by ‘-‘. ................ 13
Figure 2.4: Typical potential profile for RF PECVD chamber with cathode of smaller area than
anode. ............................................................................................................................................ 15
Figure 2.5: (a) Example of PECVD chamber where asymmetric gas flow leads to asymmetric ion
and radical distribution and consequently non-uniform film growth; (b) Example of PECVD
chamber which employs showerhead gas inlet which leads to symmetric gas flow profile and
uniform film growth. ..................................................................................................................... 18
Figure 2.6: Film growth processes in a-C (taken from [28]). ....................................................... 21
Figure 3.1: Schematic of RF PECVD chamber used for research discussed in this thesis. .......... 24
Figure 3.2: Schematic of optical system used for fitting ellipsometry measurements. ................ 32
Figure 3.3: (a) XPS: measurement of electrons emitted from core-level due to x-ray absorption
(b) XAES: measurement of secondary electrons emitted from valence-level carrying excess
energy created from core-level hole, created in process shown in (a), being filled. .................... 34
Figure 3.4: Band diagram depicting relationship between binding energy (BE), work function of
the sample (Φsample), work function of the electron analyzer (Φanalyzer), Fermi energy of the
sample (Ef), measured kinetic energy of the electron (Ek), and photon energy (hν) for XPS. Note
x
that since the sample and analyzer are both grounded, their Fermi energies are aligned. Ek’
represents the kinetic energy of the electron when it is emitted from the sample, and Ek
represents the kinetic energy of the electron measured on the electron analyzer. ........................ 35
Figure 3.5: Example XPS measurement used to determine elemental composition. The
measurement was taken on a sample in the A-20 sample set and shows two distinct peaks at two
binding energies: (i) 526eV corresponding to the binding energy of 1s electrons in oxygen and
labeled O1s in the figure, (ii) 284eV corresponding to the binding energy of 1s electrons in
carbon and labeled C1s in the figure. ............................................................................................ 36
Figure 3.6: Dependence of penetration depth, d, of x-ray incident at angle of θ on a film with x-
ray absorption length of L. ............................................................................................................ 37
Figure 3.7: Example of angle-resolved XPS measurements providing qualitative information of
film density. Note the curves are just a guide to the eye. ............................................................ 38
Figure 3.8: (a) “sandwich” method of measuring conductivity transversely through a thin-film (b)
shunting and unknown length issues that can occur with soft PLC:H film with pores, pinholes
and scratches. ................................................................................................................................ 39
Figure 3.9: Auger emission in sp3-hybridized carbon atom. ........................................................ 42
Figure 3.10: Two potential Auger processes in sp2-hybridized carbon atom that produce emission
of electrons with unique kinetic energies. ..................................................................................... 42
Figure 3.11: (a) XAES measurement (b) derivative spectra of XAES measurements with D-
parameter indicated on figure. ...................................................................................................... 43
Figure 3.12: Normalized transmission spectrum from FTIR measurement. ................................ 47
Figure 3.13: Transmission spectrum with background removed. ................................................. 48
Figure 3.14: Absorption coefficient spectrum with vibrational modes indicated. ........................ 49
Figure 4.1: Change in growth rate with temperature for A-20 sample set. .................................. 52
Figure 4.2: Change in growth rate with RF power for C-RT and A-RT sample sets. .................. 53
xi
Figure 4.3: Change in growth rate with N2 partial pressure for C-N and A-N sample sets. Note
the C-N sample set and A-N sample set had identical deposition parameters other than the
placement of the substrate and RF power used. For the C-N sample set, substrates were placed
on the cathode and an RF power of 5W was used while for the C-N sample set, substrates were
placed on the anode and an RF power of 20W was used. ............................................................. 54
Figure 4.4: Change in nitrogen content (at. %) with N2 partial pressure for films in A-N and C-N
sample sets. The curves serve as guides to the eye. ..................................................................... 55
Figure 4.5: Absorption of C-H modes for a-C films in the C-RT sample set. .............................. 56
Figure 4.6: Change in hydrogen concentration in a-C films in the C-RT sample set deposited at
different RF powers. The curve is a guide for the eye. ................................................................ 57
Figure 4.7: Percent of CHx bonding in the CH2 sp2 mode for C-RT sample set. ........................ 58
Figure 4.8: Absorption of C-H modes for a-C films in the A-RT sample set. ............................. 59
Figure 4.9: Change in hydrogen concentration in a-C films in the A-RT sample set deposited at
different RF powers. ..................................................................................................................... 60
Figure 4.10: Percent of CHx bonding in the CH2 sp2 mode for A-RT sample set. ....................... 60
Figure 4.11: Absorption of C-H modes for a-C films in the A-20 sample set. ............................. 61
Figure 4.12: Change in hydrogen concentration in a-C films in the A-20 sample set deposited at
different substrate temperatures. ................................................................................................... 62
Figure 4.13: Percent of CHx bonding in the CH2 sp2 mode for A-20 sample set. ........................ 63
Figure 4.14: Change in the percent of sp3-bonded carbon atoms for a-C films deposited at
different RF Powers in the C-RT and A-RT sample sets. The curves are guides to the eye. ...... 65
Figure 4.15: Change in the percent of sp3-bonded carbon atoms for a-C films deposited at
different substrate temperatures in the A-20 sample set. .............................................................. 65
xii
Figure 4.16: Difference in the oxygen concentration (O.C.) in the bulk and in the near-surface
(O.C.surface-O.C.bulk) for samples in the A-20 sample set. Measurements made by AR-XPS with
measurement at 70o representing the near-surface and measurement at 30o representing the bulk.
....................................................................................................................................................... 66
Figure 4.17: Difference in the oxygen concentration (O.C.) in the bulk and in the near-surface
(O.C.surface-O.C.bulk) for samples in the C-RT sample set. Measurements made by AR-XPS with
measurement at 70o representing the near-surface and measurement at 30o representing the bulk.
....................................................................................................................................................... 67
Figure 4.18: Difference in the oxygen concentration (O.C.) in the bulk and in the near-surface
(O.C.surface-O.C.bulk) for samples in the A-RT sample set. Measurements made by AR-XPS with
measurement at 70o representing the near-surface and measurement at 30o representing the bulk.
....................................................................................................................................................... 68
Figure 4.19: Refractive index (n) of a-C films in the A-20 sample set. These are intrinsic films
that were deposited on the anode at an RF power of 20W at several different substrate
temperatures. For clarity, an error bar is only shown for the first data point on the sample
deposited at 200oC. ....................................................................................................................... 71
Figure 4.20: Absorption coefficient (α) of a-C films in the A-20 sample set. These are intrinsic
films that were deposited on the anode at an RF power of 20W at several different substrate
temperatures. For clarity, an error bar is only shown for the first data point on the sample
deposited at 200oC. ....................................................................................................................... 72
Figure 4.21: Refractive index (n) of a-C films in the C-RT sample set. These are intrinsic films
that were deposited on the cathode at several different RF powers. For this sample set there was
no intentional heating of the substrate. For clarity, an error bar is only shown for the first data
point on the sample deposited at 60W. ......................................................................................... 73
Figure 4.22: Absorption coefficient (α) of a-C films in the C-RT sample set. These are intrinsic
films that were deposited on the cathode at several different RF powers. For this sample set
there was no intentional heating of the substrate. For clarity, an error bar is only shown for the
first data point on the sample deposited at 60W. .......................................................................... 74
xiii
Figure 4.23: Refractive index (n) of a-C films in the A-RT sample set. These are intrinsic films
that were deposited on the anode at several different RF powers. For this sample set there was
no intentional heating of the substrate. For clarity, an error bar is only shown for the first data
point on the sample deposited at 80W. ......................................................................................... 75
Figure 4.24: Absorption coefficient(α) of a-C films in the A-RT sample set. These are intrinsic
films that were deposited on the anode at several different RF powers, For this sample set there
was no intentional heating of the substrate. For clarity, an error bar is only shown for the first
data point on the sample deposited at 80W. ................................................................................. 76
Figure 4.25: Refractive index (n) of a-C films in the C-N sample set. Each film has a different
level of nitrogen content (at. %) due to the difference in the N2 partial pressure that was used in
the source gas. For this sample set the substrate was held on the cathode, the RF power was set
to 5W, and there was no intentional heating of the substrate. For clarity, an error bar is only
shown for the last data point on the sample with a nitrogen content of 8.97%. ........................... 77
Figure 4.26: Absorption coefficient (α) of a-C films in the C-N sample set. Each film has a
different level of nitrogen content (at. %) based on the N2 partial pressure that was used in the
source gas. For this sample set the substrate was held on the cathode, the RF power was set to
5W, and there was no intentional heating of the substrate. For clarity, an error bar is only shown
for the first data point on the sample with a nitrogen content of 8.97%. ...................................... 78
Figure 4.27: Refractive index (n) of a-C films in the A-N sample set. Each film has a different
level of nitrogen content (at. %) based on the N2 partial pressure that was used in the source gas.
For this sample set the substrate was held on the anode, the RF power was set to 20W, and there
was no intentional heating of the substrate. For clarity, an error bar is only shown for the first
data point on the sample with a nitrogen content of 17.04%. ....................................................... 79
Figure 4.28: Absorption coefficient (α) of a-C films in the A-N sample set. Each film has a
different level of nitrogen content (at. %) based on the N2 partial pressure that was used in the
source gas. For this sample set the substrate was held on the anode, the RF power was set to
20W, and there was no intentional heating of the substrate. For clarity, an error bar is only
shown for the first data point on the sample with a nitrogen content of 17.04%. ........................ 80
Figure 4.29: Change in the E04 gap with substrate temperature for A-20 sample set. .................. 81
xiv
Figure 4.30: Change in the E04 gap with RF power for C-RT and A-RT sample sets. The curves
are a guide to the eye. ................................................................................................................... 81
Figure 4.31: Change in the E04 gap with nitrogen content for C-N sample set. The curve is a
guide to the eye. ............................................................................................................................ 82
Figure 4.32: Change in the E04 gap with nitrogen content for A-N sample set. The curve is a
guide to the eye. ............................................................................................................................ 82
Figure 5.1: Change in plasma potential profile with increasing RF power for RF-PECVD
chamber with area of cathode smaller than area of anode. ........................................................... 86
Figure 5.2: Difference in relationship between growth rate and RF power for A-RT and C-RT
sample sets. The curves serve as guides to the eye. ..................................................................... 87
Figure 5.3: Relationship between hydrogen concentration and RF power for C-RT and A-RT
sample sets. The curve is a guide to the eye for the data points in the C-RT sample set. ........... 88
Figure 5.4: Relationship between %sp3 bonding and RF power for C-RT and A-RT sample sets.
The curves are a guide to the eye. Note that for the C-RT sample set, there appears to be two
distinct regions in the relationship; one at low power (<20W) in which only hydrogen
displacement is occurring and one at higher powers (>20W) in which both hydrogen
displacement and film penetration is occurring. For the A-RT sample set, due to the weaker
relationship between RF power and ion/radical energy, only the hydrogen displacement region is
apparent. ........................................................................................................................................ 91
Figure 5.5: Temperature dependent etching processes creating a net negative effect of substrate
temperature on growth rate of a-C films. ...................................................................................... 93
Figure 5.6: Relationship between growth rate and N2 partial pressure for A-N and C-N sample
sets. The curves are intended only as a guide to the eye. ............................................................. 95
Figure 5.7: Increasing N content (at. %) with increasing N2 Partial Pressure (%) for A-N and C-
N sample sets. ............................................................................................................................... 96
xv
Figure 5.8: Relationship between charge-corrected N1s shifts and nitrogen content for C-N and
A-N sample sets. The curves are a guide to the eye. Note that charge-corrected N1s shifts can
be taken as a qualitative measure of shifts in the Fermi level. An increase in the charge-corrected
N1s peak would represent an upward shift of the Fermi level toward the conduction band. ....... 97
Figure 5.9: Potential bonding configurations between nitrogen and carbon. Doping is only
possible in the configurations shown in (b), (e), and (h). Figure taken from [64]. ...................... 98
Figure 5.10: Simplified diagram of the density of states in a-C films. ....................................... 100
Figure 5.11: Photoemission spectra of a-C measured through Ultraviolet Photoelectron
Spectroscopy (UPS). The vertical axis represents the photoemission counts measured through
UPS and provides a qualitative assessment of the density of states in the valence band with the
Fermi level lying at 0eV. Note that the peak of the π-band lies closer to the Fermi level than the
peak of the σ-band. Taken from [28]. ........................................................................................ 101
Figure 5.12: Relationship between %sp2 bonding with E04 gap (black) and refractive index at
350nm (grey) for C-RT sample set. The curves are a guide for the eye. ................................... 102
Figure 5.14: Relationship between %sp2 bonding with E04 gap (black) and refractive index at
350nm (grey) for A-20 sample set. The curves are a guide for the eye. .................................... 103
Figure 5.15: Relationship between E04 gap and nitrogen content for C-N and A-N sample sets.
Note for both sample sets the refractive index remained constant at 1.6. For visual clarity the
refractive index curves were not included in the figure. The curves are a guide for the eye. ... 105
Figure 6.1: Normalized spectral emissive power of a blackbody radiator @ 5780K and blackbody
radiator @ 300K. Note that the solar spectrum can be approximated by a blackbody radiator @
5780K. ......................................................................................................................................... 107
Figure 6.2: Overview of multi-layer transparent heat mirror design. ......................................... 108
Figure 6.3: Infrared reflectance of state-of-the-art SnO2 (38nm)/NiCr (1nm)/Ag (9nm)/NiCr
(3nm)/Sn02 (38nm) structure. Taken from [67]. ........................................................................ 109
xvi
Figure 6.4: Transmittance of state-of-the-art SnO2 (38nm)/NiCr (1nm)/Ag (9nm)/NiCr
(3nm)/Sn02 (38nm) structure. Taken from [67]. ........................................................................ 110
Figure 6.5: Infrared reflection of a-C/Ag/a-C/glass THM optical system. Note that the spectra
for Ag/glass and glass are shown as references. ......................................................................... 112
Figure 6.6: Visible transmission of a-C/Ag/a-C/glass THM optical system. Note that the spectra
for Ag/glass and glass are shown as references. ......................................................................... 113
Figure 6.7: Visible transmission of a-C/Ag/a-C/glass THM using different Ag layer thicknesses.
The “Experimental” curve represents experimental measurements done on the a-C/Ag(20nm)/a-
C/glass THM that was fabricated. The “Model: 20nm Ag” curve represents the simulated
transmission for an a-C/Ag(20nm)/a-C/glass THM. The “Model: 10nm Ag” curve represents the
simulated transmission for an a-C/Ag(10nm)/a-C/glass THM. .................................................. 114
xvii
List of Symbols and Acronyms
A-20 Anode 20W sample set. In this sample set, substrates were placed on
the anode and the RF power was fixed at 20W. The varied parameter
was the substrate temperature.
a-C Amorphous Carbon (either hydrogenated or non-hydrogenated)
Ag Silver
A-N Anode Nitrogen-incorporation. In this sample set, substrates were
placed on the anode, no intentional heating was performed, and the
RF power was fixed at 20W. The varied parameter was the partial
pressure of n2 in the source gas.
A-RT Anode Room Temperature sample set. In this sample set, substrates
were placed on the anode and no intentional heating was performed.
The varied parameter was RF power.
at. % Atomic Percent
Au Gold
C1s Carbon core electron energy level
CH4 Methane gas
CHx Hydrocarbon Molecule
C-N Cathode Nitrogen-incorporation. In this sample set, substrates were
placed on the cathode, no intentional heating was performed, and the
RF power was fixed at 5W. The varied parameter was the partial
pressure of n2 in the source gas.
C-RT Cathode Room Temperature sample set. In this sample set,
substrates were placed on the cathode and no intentional heating was
xviii
performed. The varied parameter was RF power.
Cu Copper
DC Direct-Current
DCSF Dc Saddle Field
DLC:H Hydrogenated Diamond-Like a-C
E04 gap Photon energy at which the absorption coefficient reaches 104 cm-1
E-beam evaporation Electron Beam Evaporation
ECR Electron Cyclotron Resonance
Ef Fermi Energy
FTIR Fourier Transform Infrared Spectroscopy
Ge Germanium
GLC Graphitic-Like a-C
GLC:H Hydrogenated Graphitic-Like a-C
H Hydrogen
H2 Hydrogen gas
ICP Inductively-Coupled Plasma
k Extinction Coefficient
MW Microwave
n Refractive Index
N1s Nitrogen core electron energy level
xix
N2 Nitrogen gas
NiCr Nickel Chromium
O.C. Oxygen Concentration
O1s Oxygen core electron energy level
O2 Oxygen gas
PECVD Plasma Enhanced Chemical Vapor Deposition
PL Photoluminescent
PLC:H Hydrogenated Polymeric-Like a-C
PV Photovoltaic
RF Radio Frequency
RF-PECVD Radio Frequency Plasma Enhanced Chemical Vapor Deposition
sccm Standard Cubic Centimetres Per Minute
SnO2 Tin Oxide
TAC Tetrahedral Amorphous Carbon
TAC:H Hydrogenated Tetrahedral a-C
THM Transparent Heat Mirror
ULSI Ultra Large Scale Integrated Circuits
UV-VIS-NIR Ultraviolet Visible Near-Infrared
VLSI Very Large Scale Integrated Circuits
XAES X-Ray Excited Auger Electron Spectroscopy
xx
XPS X-Ray Photoelectron Spectroscopy
ZnS Zinc Sulphide
ZnSe Zinc Selenide
α Absorption Coefficient
λ Wavelength
%sp3 bonding Percent of sp3-hydridized carbon atoms
1
1 Introduction
1.1 Properties of carbon Carbon is one of the most widely studied elements. It is the basis of all life forms on our planet.
An entire branch of chemistry, organic chemistry, is dedicated to the study of hydrocarbon-based
compounds. Part of the fascination of carbon is the widely varying properties of its allotropes.
There are three main allotropes of carbon: diamond, graphite, and fullerene. Diamond is highly
transparent, an excellent electrical insulator, and the hardest and most thermally conductive of all
naturally occurring materials. In contrast, graphite is opaque, an excellent electrical conductor, a
thermal insulator, and one of the softest naturally occurring materials. Fullerenes represent a
relatively newly discovered set of carbon allotropes. As opposed to the semi-infinite network of
carbon atoms found in diamond and graphite, fullerenes exist in finite molecular forms such as
hollow spheres. Fullerenes are the focus for much of the work in nanotechnology and are the
basis for carbon nanotubes.
The versatility of carbon-based materials is a function of the different bonding hybridizations
that carbon atoms can form. The ground state electron configuration of carbon, 1s2 2s2 2p2, has
four valence (L-shell) electrons. Under appropriate conditions these valence electrons can form
hybridized orbitals such as sp3 or sp2 orbitals. This is illustrated in Figure 1.1. The formation of
sp1 orbitals are also possible, however, they do not play a significant role in the properties of a-C
films and thus will not be discussed further [1].
Figure 1.1: sp3 and sp2 bonding hybridizations of carbon. Black dots represent atomic
positions. Taken from [1].
2
In sp3 hybridization, four identical tetrahedrally oriented orbitals are formed with each orbital
being separated by an angle of 109.5o. Each of the four sp3 orbitals can form a single covalent
bond along the axis joining the nuclei of the two atoms involved in the bond. This type of bond
is known as a σ-bond and is the strongest of all the covalent bonds. Carbon bonded in the sp3
configuration forms an interlocking network of strong covalent bonds with each carbon atom
forming a σ-bond with its four nearest neighbours. Crystalline carbon bonded in this type of
configuration is known as diamond. It is this highly dense interlocking microstructure that leads
to the extraordinary hardness, transparency, and high electrical resistivity of diamond.
In sp2 hybridization, three identical co-planar trigonally directed sp2 orbitals are formed with
each orbital being separated by an angle of 120o. Each of the sp2 orbitals contains a single
valence electron with the remaining electron being held in the 2p orbital whose axis is
perpendicular to the plane containing the three sp2 orbitals. Each of the three sp2 orbitals can
form a single σ-bond with neighbouring atoms while the remaining 2p orbital can form a bond
with another 2p orbital of a neighbouring atom. The bond between the 2p orbitals is
perpendicular to the axis joining the nuclei of the two atoms involved in the bond. This type of
bond is known as a π-bond and is illustrated in Figure 1.1. The π-bond is weaker than the σ-
bond since there is a greater overlap of orbitals in a σ-bond than in a π-bond. As sp2 hybridized
carbon atoms come together, each carbon atom uses its sp2 orbitals to form single σ-bonds with
its three nearest neighbours. The remaining 2p orbital of each atom overlaps with the 2p orbitals
of each of its three nearest neighbours. This leads to a distributed π-bond which lies across the
entire plane of carbon atoms. Since all four valence electrons are used to bond with coplanar
atoms, adjacent planes are only held together by weak Van Der Waals forces. Crystalline carbon
bonded in this type of configuration is known as graphite. It is this microstructure of delocalized
π-bonding and weak inter-planar forces that lead to the high conductivity, opaqueness, and
softness of graphite.
Amorphous carbon (a-C) is a non-crystalline form of carbon that only has short range structural
order. The term a-C is also used to include hydrogen-containing (hydrogenated) forms of non-
crystalline carbon. While the two primary crystalline forms of carbon exhibit either 100% sp3
bonding (diamond) or 100% sp2 bonding (graphite), a-C films contain a mixture of sp3 and sp2
bonding. It is the ability to control the ratio of sp3 to sp2 bonds through growth conditions that
allows the development of a-C films that exhibit astonishingly different mechanical, electrical,
3
and optical properties. In addition to bonding hybridization, a second parameter, hydrogen
content, plays an important role in determining the film properties. Based on the percentage of
sp3 bonds and the percentage of hydrogen in the film, a-C films are separated into different
categories as shown in Table 1.1 and qualitatively illustrated in Figure 1.2 [1]. The naming of
these categories is based on historical publications and can be confusing at times. For instance,
some types of hydrogenated diamond-like carbon (DLC:H) are dark in appearance and thus the
optical properties are not at all like those of diamond. Moreover, it could be argued that the
mechanical properties of tetrahedral amorphous carbon (TAC) are more diamond-like than
DLC:H. Thus special care must be taken to distinguish between the properties associated with
the name of the category and the actual properties of a-C films in that category.
Table 1.1: Categories of amorphous carbon (a-C) films
Category Abbreviation sp3 (%) H (at.%)
Hardness (GPa)
Optical Gap (eV)
Density (g/cm3)
tetrahedral a-C TAC >65 <10 40-65 1.6-2.6 2.5-3.5
hydrogenated tetrahedral a-C
TAC:H >65 10-30
hydrogenated diamond-like a-C
DLC:H 30-65 20-40 20-40 0.8-4.0 1.8-3.0
hydrogenated polymeric-like a-C
PLC:H 60-80 40-65 soft 2.0-5.0 0.6-1.5
hydrogenated graphitic-like a-C
GLC:H <30 10-40 soft 0-0.6 1.2-2.0
graphitic-like a-C GLC <30 <10
4
Figure 1.2: Ternary phase diagram of the various forms of amorphous carbon films.
Abbreviations are defined in Table 1.1.
5
1.2 Current state and trends in amorphous carbon research In 1971, two researchers in the space science division of the Whittaker Corporation, Aisenberg
and Chabot, reported the room-temperature deposition of a-C films that exhibited many
diamond-like properties [2]. Ever since this time, researchers from across the world have been
working to understand this material and to find applications for it. Right from the beginning, the
most prevalent application explored for a-C was as a hard, scratch-resistant coating. Research
into this application reached a fever-pitch in 1989 when Liu and Cohen predicted the existence
of a β-C3N4 phase in nitrogen-incorporated a-C that was harder than diamond [3]. Although the
existence of the β-C3N4 phase has yet to be clearly achieved experimentally [4], the prevalent
research in the field of hard a-C coatings (DLC:H, TAC:H, TAC) has led to a maturity in the
understanding and fabrication of these films [5].
Today, the market for hard a-C coatings is rapidly growing and it currently represents close to a
billion dollar industry [6]. The most popular application of a-C is as a wear and corrosion
resistant coating for magnetic storage media (eg. computer hard drives) [5; 6]. Hard a-C
coatings (DLC:H, TAC:H, TAC) have also found applications as a low-friction, scratch-resistant
coating for polycarbonate sunglasses, razor blades, machining tools, and automotive components
[5; 6; 7]. In addition to their high hardness and low friction, these forms of a-C have also been
shown to be impermeable to liquids and to be chemically inert. This has led to exploration of a-
C coatings to protect biological technologies, such as artificial heart valves and joint implants,
against corrosion and diffusion [8; 9; 10; 11].
With the breadth of applications for hard forms of a-C, much of the research in the mechanical
properties of a-C has shifted from academia to industry [6]. Today, industrial institutions are
researching methods of optimizing hard a-C coatings to meet the needs of their specific
applications.
6
1.3 Amorphous carbon as an optical coating The superior mechanical properties of a-C have motivated most of the research in the field over
the past thirty years. While a-C films have excellent mechanical properties, the infrared
transparency, large optical gap, and the tunable refractive index (1.4-2.8) of these films also
present intriguing possibilities for optical applications. Also, while research into the mechanical
properties of a-C has been in-depth and has reached a level of maturity, studies into the optical
properties of a-C have been sparse and application specific.
Several researchers have explored the potential of using a-C as a replacement to SiO2 for the
interconnect dielectric used for Very Large Scale Integrated Circuits (VLSI) and Ultra Large
Scale Integrated Circuits (ULSI) [12; 13; 14; 15]. The high infrared transparency in a-C has also
led to some work in utilizing a-C as an anti-reflection coating for Ge and ZnSe infrared detectors
[16; 17; 18; 19; 20]. In addition, the large optical gap and tunable refractive index of a-C has led
researchers to explore the potential of a-C as an anti-reflection coating for Si solar cells [21; 22;
23]. There has also been some work in studying the photoluminescent (PL) properties of a-C
and looking into the possibility of using a-C as a replacement to ZnS in electroluminescent
devices [24; 25; 26].
Although these studies have examined the potential of a-C as an optical coating, they are
scattered and application-specific. There has been a lack of fundamental research in these
publications examining the relationship between the optical properties and the structural
properties and their correlation with the growth conditions of a-C. This type of fundamental
study is necessary in order to achieve the full potential of a-C as an optical coating.
7
1.4 Research overview Amorphous carbon films possess a combination of chemical inertness, high infrared
transmissivity, and tunable mechanical and optical properties. It is this unique combination of
properties coupled with the fact that these films can be deposited at room-temperature using
simple deposition techniques that make a-C an appealing alternative to other thin-film materials.
This is especially true in the solar industry where the exponential growth in demand and
competition in the industry has forced researchers to develop advanced cost-effective alternatives
to the standard technologies currently being used. The unique properties of a-C make it
appealing for a number of applications in the solar industry including: anti-reflection coatings for
Si photovoltaic (PV) cells, window layers for thin-film and heterojunction PV cells, surface
passivation layers, and low-emissive transparent heat mirrors for high efficiency windows.
Before a-C films can be utilized in any of these applications it is necessary to complete
fundamental research relating the optical properties of a-C to the structural and growth
conditions of these films.
In this thesis, experiments exploring the effect of growth conditions on the microstructural
properties and optical properties of a-C films are presented. The structure of the thesis is as
follows. In the following chapters (Chapters 2 and 3), a background on a-C film growth and
details regarding the experimental apparatus and characterization techniques used are discussed.
The results of these experiments are presented in Chapter 4, while the analysis of these results
and the defining relationships between the growth conditions, microstructural properties, and
optical properties of a-C films are discussed in Chapter 5. In Chapter 6, several applications for
a-C as an optical coating are suggested and one of these applications, a-C as a transparent heat
mirror coating, is experimentally demonstrated. Conclusions of this research are presented in the
final chapter.
8
2 Amorphous Carbon: An Overview
2.1 Deposition methods A variety of methods have been used to deposit a-C thin films. Typically, GLC and GLC:H are
deposited from sputtering a graphite target, while TLC and TLC:H are deposited using vacuum
arc deposition or pulsed-laser deposition, and PLC:H and DLC:H are deposited using either
sputtering or plasma-enhanced chemical vapor deposition (PECVD) [27; 28; 29; 30]. Since this
thesis investigates PLC:H and DLC:H films deposited using PECVD, only the PECVD method
is discussed further. In this section an overview of the important concepts relating to a-C films
deposited by PECVD is given. A complete review of plasmas and the PECVD technique is
beyond the scope of this thesis; interested readers are directed to the following sources for more
information [28; 31; 32].
One of the key features of PECVD is that a variety of substrates can be used because the
substrate temperature is kept low, typically below 250oC. Deposition can occur at a low
temperature because as indicated by the name, a plasma or glow discharge is used to generate the
reactive species necessary for film growth. A plasma is a volume of gas consisting of a high
density of charge carriers (ions and electrons) [27]. The high density of charge carriers in
plasmas, give it a number of distinguishing properties from non-ionized gases, and thus plasmas
are often referred to as the fourth state of matter.
In PECVD, a plasma is generated by an external electric field being applied to a volume of low-
pressure gas, typically less than 1 Torr. Before the electric field is applied, the volume of gas has
a very low density of charge carriers, created by cosmic radiation. Once the field is applied, the
charge carriers that are present accelerate and collide with neutral molecules (neutrals) in the gas.
Through these collusions, energy is transferred from the energetic charge carriers to the neutrals
causing ionization of the neutrals thus generating more charge carriers. Energetic carriers also
collide with electrodes and walls of the chamber causing ejection of new carriers into the gas. As
charge density builds in the chamber, the rate of recombination of charge increases. The
recombination of charge carriers occurs as electrons and ions in the plasma recombine with each
other or are lost due to collisions with the walls of the chamber. In time, a steady-state is reached
between the generation of new charge carriers and the recombination of existing charge carriers.
9
2.2 Plasma kinetics The plasma produced in typical PECVD systems consists of a mixture of ions, electrons, and
neutrals, with neutrals representing the majority of the species. The energy of the applied field is
transferred primarily to the electrons since their lower mass allows them to rapidly respond to the
applied field. Thus, the species within the plasma consist of high energy electrons and low
energy ions and neutrals; this is known as a low-temperature plasma [27].
The energetic electrons in the plasma play an important role as their high energy and low mass
allow them to collide inelastically with stable neutral molecules in the plasma resulting in one of
the following processes:
(i) Ionization: The energy transferred in an inelastic collision is used to eject an electron from a
neutral molecule, producing a positively charged ion and an additional electron.
Example: CH4 + e- → CH3+ + H + 2e-
(ii) Dissociation: The energy transferred from the inelastic collision is used to breakdown a
stable neutral molecule into neutral highly-reactive molecular fragment(s) known as neutral
radical(s). Note that in the context of this thesis, the term neutral radical(s) will be shortened
simply to radical(s). Thus in this thesis, the term radical is used to refer to neutral radicals.
Example: CH4 + e- → CH3 + H + e-
(ii) Excitation: The energy transferred from the inelastic collision is used to promote electron(s)
in the neutral molecule to a higher energy state. When these electron(s) relax to their ground
state, a photon is emitted. This relaxation process is responsible for the visible glow commonly
seen in plasmas.
The ions and radicals produced in the plasma are predominantly responsible for film growth in
the PECVD process. Various types of ions and radicals can be produced depending on the type
of source gas used to produce the plasma. In the work reported in this thesis, methane (CH4) was
used as the source gas. The following lists the main ionization and dissociation processes found
in methane plasmas (note radicals are indicated by: `*`) [33; 34]:
10
CH4 + e- → CH3* + H* + e-
CH4 + e- → CH3+ + H* + 2e-
CH4 + e- → CH2* + H2
+ e-
CH4 + e- → CH* + H2 + H* + e-
CH4 + e- → CH4+ + 2e-
H2 + e- → 2H* + e-
H2 + e- → 2H+ + 3e-
While there are a number of ionic and radical species in methane plasmas, the three predominant
species are CH3+, CH3
*, and H* [35]. The specific role of these species in film growth is
discussed in Section 2.5.
11
2.3 RF PECVD PECVD systems are classified based on the source used to generate the electric field. The
primary types of PECVD sources are direct-current (DC), radio-frequency (RF), and microwave
(MW). There also exist a number of advanced configurations such as DC saddle-field (DCSF),
electron cyclotron resonance (ECR), and inductively-coupled plasma (ICP). DC configurations
(standard DC or DC saddle-field) are typically not used for a-C deposition since in these
configurations the source is coupled to the plasma through an external DC current. During the
deposition, highly resistive a-C film builds up on the DC electrodes and it becomes increasingly
difficult to maintain a stable plasma [27]. The work reported in this thesis uses the RF PECVD
configuration, which is the most commonly used configuration for a-C film deposition [28].
Rather than uses an external current, the RF PECVD configuration uses an alternating RF field to
couple power from the source to the plasma [27].
The typical configuration for an RF PECVD chamber is shown in Figure 2.1. The main features
are the power source, the impedance matching network, the two circular electrodes, pumping
outlet, and source gas inlet. The RF source, which operates at a frequency of 13.56MHz based
on regulatory requirements, is capacitively coupled to the chamber in order to suppress any DC
current. An impedance matching network is connected in series with the RF source in order to
prevent reflection of the RF power from the chamber. The two electrodes are typically of
different radii, with the smaller electrode typically being the cathode (powered) and the larger
electrode typically being the anode (grounded). The substrate can be placed on either the anode
or cathode but for hard a-C films, the substrate is typically placed on the cathode. The reasons
for this asymmetry in electrode area and the choice of substrate placement are explained later in
this section.
12
Figure 2.1: Typical RF-PECVD System.
The generation of a plasma in an RF PECVD chamber is depicted in Figure 2.2. The RF source
creates a field oscillating at a frequency of 13.56MHz. While the electrons in the gas volume are
able to oscillate at this frequency, the ions and neutrals1
1 Note the term neutral(s) is referring to both the stable neutral molecules introduced by the source gas and neutral radicals.
with their lower mobility, a factor of 102
lower than electrons, are unable to react to this oscillating field [27]. Thus the ions and neutrals
in the gas remain relatively stationary while the electrons oscillate between the electrodes. As
the electrons oscillate between the electrodes they collide inelastically with the neutrals and
induce ionization generating new ions and electrons. The generated ions remain stationary while
the generated electrons add to the oscillating flux of electrons and induce more ionization of
neutrals. This process continues until a steady-state is reached between the generation and
recombination of charge carriers.
13
Figure 2.2: Generation of plasma in RF-PECVD. Neutrals are represented by ‘N’, ions are
represented by ‘+’, and electrons are represented by ‘-‘.
As shown in Figure 2.3, in addition to the central plasma (glow) region, there also exist dark
space-charge regions near the electrodes known as the sheath regions. Due to the higher
mobility of electrons compared to ions, during the positive phase of electric field, the electron
current towards a given electrode is significantly higher than the ion current to the same
electrode during the negative phase of the electric field. This leaves a positive space charge
region near the electrodes known as the sheath regions. Since the system is capacitively-
coupled, the electron and ion currents must be balanced, thus a static electric field forms in the
sheath regions such that the electrodes develop a negative DC potential with respect to the
plasma. This negative potential is known as the self-bias potential.
Figure 2.3: Formation of sheath regions near anode and cathode in RF PECVD. Neutrals
are represented by ‘N’, ions are represented by ‘+’, and electrons are represented by ‘-‘.
14
Due to the capacitor-like nature of the sheath regions, the applied RF voltage is divided between
the sheaths according to their inverse capacitance and can be described by the following equation
[28]:
𝑉𝑉1
𝑉𝑉2= �
𝐴𝐴2
𝐴𝐴1�
2
(1)
where V1 is the self-bias potential of the electrode with area A1 and V2 is the self-bias potential
of the electrode with area A2. Thus if the electrodes are asymmetric in area, the electrode with
the smaller area will acquire the dominant self-bias potential. This is important when
considering the energy of ions and neutrals impinging on the electrode. Although, the sheath
region forces the net electron and ion fluxes towards the electrodes to become equal, the energy
that they have when they impinge on the electrodes is not. Due to formation of the field in the
sheath region, ions that enter the sheath regions are accelerated by the field and impinge on the
electrode with a high energy, as opposed to electrons which are retarded by the field. Ions that
accelerate through the sheath region collide with neutrals and thus both energetic ions and
neutrals impinge on the electrodes. As discussed in Section 2.5, for hard forms of a-C, high
energy ions and neutrals are desired. Thus the RF PECVD chamber is typically designed such
that the substrate lies on the smaller electrode. According to Equation 1, the smaller electrode
will acquire the dominant self-bias and thus the energy of the ions and neutrals impinging on the
smaller electrode will be greater than that of the larger electrode. An example of a typical
potential profile found in RF PECVD systems is shown in Figure 2.4.
15
Figure 2.4: Typical potential profile for RF PECVD chamber with cathode of smaller area
than anode.
16
2.4 Process variables in RF PECVD There are several process variables in RF PECVD that affect film growth. Understanding the
role of each of these process variables is essential in obtaining stable plasma conditions that will
deposit reproducible, high-quality films. The following provides a qualitative description for the
key process variables in a standard RF PECVD chamber:
(i) RF power: As described in the Section 2.3, the peak RF potential is divided between the two
sheath regions with the sheath region surrounding the electrode with the smaller area developing
the larger potential drop. Thus the greater the applied RF power, the larger the potential drop in
the sheath region. Since ions are accelerated through the sheath region towards the substrate, the
energy of ions impinging on the substrate increases with increasing RF power. In addition to
affecting the energy of ions, the applied RF power also affects the density and types of species
present in the plasma. As described in Section 2.2, gaseous state neutral molecules introduced
into the PECVD chamber can be ionized or dissociated into several different types of ions and
radicals. In each case, the energy produced through inelastic collisions with electrons acts as the
catalyst for the production of these species. Since the energy of electrons in the plasma is
coupled with the intensity of the applied field, the likelihood of production of a particular species
is dependent on the applied RF power.
(ii) Area of electrodes: As described in Section 2.3 and Equation 1, the self-bias developed by
each electrode is dependent on their respective areas, with the electrode with the smaller area
developing the larger self bias. Therefore, the potential profile of the plasma is dependent on the
ratio of areas of the two electrodes. If both electrodes are chosen with equal areas, then a
symmetric potential profile will develop. Typically, RF PECVD chambers are designed such
that one electrode is smaller than the other so that an asymmetric profile, like that shown in
Figure 2.4, develops. With an asymmetric profile, high energy ions will impinge on the smaller
electrode even at low RF powers. The ratio between the areas of the electrodes should be chosen
to create the potential profile that is suited for the type of film being deposited.
(iii) Pressure: Since many of the processes involved in the PECVD process are based on
collisions such as collisions of electrons with neutrals, the source gas pressure that is maintained
in the PECVD chamber plays an important role in determining the characteristics of the plasma.
Generally, lower pressures lead to higher energy ions and electrons due to a lower collision
17
frequency at lower pressures. If the pressure is driven below a threshold, the collision frequency
between electrons and neutrals reaches a point where there is not enough generation of charge
carriers to maintain a stable plasma. If the pressure is driven too high, due to the high frequency
of collisions, electrons are not be able acquire enough kinetic energy to ionize the neutrals in the
chamber and thus the generation of charge carriers will again be too low to maintain a stable
plasma. Thus for a given source gas, power, and flow rate, a range of pressures exist for which a
stable plasma can be generated.
In addition to the requirements of striking a stable RF PECVD plasma, pressure has a strong
effect in determining the value and distribution of ion energies impinging on the substrate. With
lower pressures, the initial flux of electrons towards the electrodes is higher. This necessitates a
larger developed DC self-bias on the electrodes to equalize the ion and electron fluxes. With a
higher potential drop in the sheath region, the energy of ions impinging onto the substrate will be
higher. Moreover, with a lower pressure, the number of collisions that these ions undergo in
their path through the sheath region to the electrode will be lower which will lead to a larger
mean energy and a sharper linewidth in the distribution of ion energies [27].
(iv) Gas flow rate: The rate and profile of how the source gas flows in and out of the PECVD
chamber also affects the film growth. There are two key events in the deposition process: (1)
breakdown of the source gas into ions and radicals, and (2) transport of the ions and radicals
towards the substrate. If the gas flow rate is too high, not enough time is given for this
deposition process to take place and the film growth rate will be low. In the extreme case,
growth will not occur at all and a plasma may not strike. If the gas flow rate is too low, the
plasma will reach an exhausted state where there will be a diminished concentration of neutral
molecules in the plasma leading to a reduced deposition rate. The optimal gas flow rate is
dependent on a number of factors including the geometry of the chamber, the other deposition
parameters being used, and the importance of growth rate in the deposition process.
The profile of how gas flows in and out of the PECVD chamber also affects film growth. If
profile of gas flow in the chamber is asymmetric then the production of ions and radicals in the
plasma will also be asymmetric which will result in non-uniform film growth. This is illustrated
in Figure 2.5. The source gas inlet ports and the pumping outlet ports in the PECVD chamber
should be designed to provide a symmetric gas flow profile to ensure uniform film growth.
18
Figure 2.5: (a) Example of PECVD chamber where asymmetric gas flow leads to
asymmetric ion and radical distribution and consequently non-uniform film growth; (b)
Example of PECVD chamber which employs showerhead gas inlet which leads to
symmetric gas flow profile and uniform film growth.
19
(v) Substrate Temperature: The temperature of the substrate plays an important role as it
provides thermal energy to solid-phase molecules in the growing film. Several temperature
dependent processes can occur during film growth and need to be chosen based on type of film
being grown and the type of substrate being used. Some of the processes include crystallization
of the film, surface migration, thermal desorption of solid-phase molecules in the film, thermal
expansion, increase/decrease of film growth rate, and increase/decrease in film stress.
20
2.5 Film growth processes The deposition process in RF PECVD can be broken down into three steps: (1) the production of
a plasma through inelastic collisions between energetic electrons and neutral molecules from the
source gas, (2) the formation of sheath regions near the electrode surfaces, and (3) the interaction
of energetic ions and radicals with the growing film. While the first two steps have already been
discussed in some detail, the third step is the focus of this section. Much of the work in
modeling the growth and microstructure in a-C films has been done by Robertson [28; 36; 37;
38; 39]. Robertson describes the microstructure of a-C as follows. The skeleton of the film is a
continuous network of sp3-bonded carbon atoms. The sp2-bonded carbon atoms form small
localized clusters that lie within this network. The quantity and size of these clusters can vary
depending on the energy of ions/radicals impinging on the growing film, the substrate
temperature, and the relative hydrogen concentration. The surface of an a-C film is fully
passivated with hydrogen, leaving it chemically inert [28]. Since the surface of the film is
chemically inert, film growth can occur only through abstraction of hydrogen from the surface or
subsurface bonding (subplantation) by energetic ions/radicals. The following is a list of the
primary film growth processes in a-C; an illustration of these processes is given in Figure 2.6.
(i) ion/radical subplantation: In order to penetrate through an interstitial site on the surface of a
growing a-C film, an ion or radical must have enough energy to overcome the repulsive inter-
atomic potential when passing through the films surface. This threshold energy is known as the
penetration threshold, Ep, and is approximately 32eV for the surface of a typical a-C film [28].
High energy ions/radicals that can overcome this threshold can penetrate several nanometers into
the film and bond with subsurface carbon atoms [38]. This subplantation process increases the
density and hardness of the a-C film. It is for this reason that high energy ions/radicals are useful
for the deposition of hard a-C films such as DLC:H, TAC:H, and TAC.
The energy of ions/radicals also affects the percentage of sp3 hybridized bonding in the film. For
each type of a-C film, there exists an optimal ion/radical energy for which maximum sp3 bonding
can be achieved. If the ion/radical energy is increased beyond this maximum, the excess energy
not used during the surface penetration is released as thermal energy. This thermal energy is
used to relax sp3 bonded carbon atoms to the lower energy sp2 configuration, and thus the
proportion of sp3 bonding in the film decreases.
21
(ii) hydrogen abstraction from surface: The surface of a growing a-C film is almost
completely passivated by hydrogen. However, surface hydrogen atoms can be abstracted leaving
highly reactive dangling bonds at the surface. There are two ways hydrogen can be abstracted
from the surface: (1) the surface hydrogen dissociates from its current bond to form a bond with
a passing hydrogen or hydrocarbon radical, (2) a high energy ion or radical collides with a
surface hydrogen atom and removes it from its current bond.
(iii) radical bonding to surface dangling bond: If a surface dangling bond is present, an
incoming hydrogen or hydrocarbon radical can form a bond with the dangling bond and
contribute to the film growth.
(iv) subsurface hydrogen abstraction: With their small size, hydrogen ions and radicals can
penetrate deep into the film’s bulk and abstract a hydrogen atom that is bonded in the bulk of the
film. The formed H2 molecule diffuses to the surface and desorbs from the film [28].
(v) ion or radical bonding to subsurface dangling bond: An energetic ion or radical can form
a bond with a subsurface dangling bond and contribute to the film growth. If the dangling bond
is deep within the film’s bulk, hydrogen radicals are the most likely to be able to penetrate to the
required depth to re-passivate the dangling bond.
Figure 2.6: Film growth processes in a-C (taken from [28]).
22
3 Experimental Apparatus & Characterization Techniques
3.1 Deposition system The primary part of the research plan for the work discussed in this thesis, was to relate the
growth conditions (temperature, plasma density, ion/radical energy, and nitrogen content) of a-C
films with their structural properties (film density, sp3/sp2 ratio, and hydrogen content) and
optical properties (refractive index and optical gap). In order to obtain meaningful results,
precise control over the growth conditions was necessary and thus the choice of the deposition
system used was vital.
In the initial stages of the research, a DC Saddle Field (DCSF) PECVD system was utilized for
deposition of the a-C samples. The DCSF system was designed and patented at the University of
Toronto and was the primary deposition tool available at the commencement of this research.
The primary advantage that the DCSF system has over the RF PECVD system is that it allows
independent control of ion energy and plasma density.
During the initial stages of the research, it became apparent that while the DCSF offers
advantages over the traditional RF PECVD system, it was not suitable for deposition of highly-
resistive a-C films. It was found that after the growth of more than 100nm of a-C film, the
DCSF system would become unstable. This loss of stability in the plasma was attributed to the
fact that the DCSF system uses an external DC current to couple the power source to the plasma.
As resistive a-C film built up on the electrodes of the DCSF system, it became increasingly
difficult to draw the external DC current and the plasma would become unstable. As stability
and reproducibility in film growth was particularly important for this research, it was determined
that it would be necessary to design and build an RF PECVD system to deposit the a-C films
needed for this research.
23
3.2 RF PECVD system The RF PECVD system that was designed and built for the research discussed in this thesis is
shown in Figure 3.1. As shown in the figure, a 13.56MHz power supply is connected to an
impedance matching network to prevent reflection of power from the chamber. The impedance
matching network consists of variable series and shunt capacitors that are connected to a
controller that automatically adjusts their impedance to obtain zero reflected power. The
electrodes for the system consist of a 12.5cm diameter stainless steel cathode and a 20cm
diameter stainless steel anode separated by 2.5cm. The asymmetry in electrode area was chosen
in order to create a potential profile that would lead to high energy ions impinging on the cathode
and low energy ions impinging on the anode. Since both the anode and cathode can act as the
substrate holder, this provides a wide range of ion energies that can be used for a-C film
deposition. The chamber shell, which is held at ground potential, is made of stainless steel and
has two optical viewports that allow the plasma and sheath regions to be viewed. The viewports
have stainless steel shutters that allow the chamber shell to maintain a constant potential profile.
It should be noted that even though the pumping arrangement of the system is asymmetric the
showerhead configuration of the cathode provided uniform deposition across the substrates. This
was confirmed through ellipsometry measurements which showed less than 1% variance in film
thickness across a 2cm x 4cm substrate.
24
Figure 3.1: Schematic of RF PECVD chamber used for research discussed in this thesis.
25
3.3 Sample preparation The substrates used for the a-C film depositions were double-side polished crystalline silicon (c-
Si) wafers with resistivity of 10-20 Ω-cm and thickness of 500µm.
Prior to deposition, contaminants such as dust and grease were removed from the surface of the
substrates using the following cleaning procedure:
• 15 minutes ultrasonic bath in acetone
• 5 minutes ultrasonic bath in de-ionized water
• 15 mutes ultrasonic bath in isopropyl alcohol
• 5 minutes ultrasonic bath in de-ionized water
• blow dried with desiccated air
• 1 minute dip in buffered hydrofluoric (HF) acid to remove native oxide
• rinse in de-ionized water
• blow dried with desiccated air
Following the cleaning, substrates were loaded into the PECVD chamber. Then the chamber
was sealed and pumped overnight with a mechanical and turbo-molecular pump to achieve a
base pressure of 10-6 mbar. The following day, the a-C film deposition was performed. The first
step in the deposition process was the introduction of the source gas. Once the source gas was
introduced into the chamber and the pressure stabilized, the substrate heater (if needed) was set
to the desired temperature. After the temperature had stabilized, the RF power was enabled, the
plasma ignited, and the a-C film deposition commenced.
Once the film deposition was complete, the remaining source gas was pumped out, then the
chamber was brought back up to atmospheric pressure, and the a-C samples were unloaded and
stored in an N2 atmosphere to prevent contamination of the film. During the deposition, the
electrodes and walls of the chamber are covered with a-C film. In order to maintain consistent
plasma conditions and prevent contamination for future depositions, this a-C film was removed
through reactive ion etching using an oxygen plasma struck in the RF PECVD chamber. The
reactive ion etching removed the a-C film by using oxygen ions and radicals to dissociate the
26
carbon-carbon (C-C) and carbon-hydrogen (C-H) bonds in the film to form stable CO2 and H2
gas phase molecules that could then be pumped out of the chamber. The reactive ion etch was
performed using the following parameters:
• source gas: O2
• flow rate: 20sccm
• chamber pressure: 40mTorr
• RF power: 25W
• duration: 45 minutes
27
3.4 Sample set space In order to examine the effect of growth conditions on the optical and structural properties of a-
C, five different sample sets were studied. For each sample set either RF power, temperature, or
the source gas was used as the independent varied parameter. The choice of RF power and
temperature as variable parameters was based on studies which showed that these parameters had
the strongest effect on the microstructure of a-C [40; 41; 42; 43]. The composition of the source
gas was also varied in order to allow for doping studies to be carried out. The details of each
sample set are explained below:
I. Cathode Room Temperature (C-RT): In this sample set, substrates were placed on the
cathode and no intentional heating was performed. The varied parameter was RF power.
Seven different RF powers were used in this sample set: 3W, 5W, 10W, 15W, 20W,
40W, and 60W. Due to the strong linkage between RF power and the energy of
ions/radicals impinging on the cathode, a wide range in ion/radical energies was
produced by varying the RF power. The aim of this sample set was to determine the
effect of ion/radical energy on the microstructural and optical properties of a-C films.
Specifically, the a-C film properties that were analyzed for this sample set were: growth
rate, film density, hydrogen content, percent of sp3-bonded carbon atoms, refractive
index, and absorption coefficient.
II. Anode Room Temperature (A-RT): In this sample set, substrates were placed on the
anode and no intentional heating was performed. The varied parameter was RF power.
Five different RF power were used in this sample set: 5W, 10W, 20W, 40W, and 80W.
Due to the relatively weak relationship between RF power and the energy of ions/radicals
impinging on the anode, the ion/radical energy will increase only slightly while the
plasma density will show a strong increase with increasing RF power. The aim of this
sample set was to determine the effect of the density and types of species in the plasma
on the properties of a-C films. Specifically, the a-C film properties that were analyzed
for this sample set were: growth rate, film density, hydrogen content, percent of sp3-
bonded carbon atoms, refractive index, and absorption coefficient.
III. Anode 20W (A-20): In this sample set, substrates were placed on the anode and the RF
power was fixed at 20W. The varied parameter was the substrate temperature. Four
28
different substrate temperatures were used in this sample set: 35oC (no intentional
heating), 100oC, 150oC, and 200oC. The aim of this sample set was to determine the
effect of substrate temperature on the properties of a-C films. Specifically, the a-C film
properties that were analyzed for this sample set were: growth rate, film density,
hydrogen content, percent of sp3-bonded carbon atoms, refractive index, and absorption
coefficient.
IV. Cathode Nitrogen-incorporation (C-N): In this sample set, substrates were placed on
the cathode, no intentional heating was performed, and the RF power was fixed at 5W.
The varied parameter was the partial pressure of N2 in the source gas. Five different
partial pressures were explored: 0%, 5%, 10%, 25%, and 50%. The aim of this sample
set was to determine the potential of room-temperature doping in a-C films and the effect
of nitrogen content on the optical properties of the films. Specifically, the a-C film
properties that were analyzed for this sample set were: growth rate, nitrogen content,
Fermi level shifting, refractive index, and absorption coefficient.
V. Anode Nitrogen-incorporation (A-N): In this sample set, substrates were placed on the
anode, no intentional heating was performed, and the RF power was fixed at 20W. The
varied parameter was the partial pressure of N2 in the source gas. Five different partial
pressures were explored: 0%, 5%, 10%, 25%, and 50%. An RF power of 20W was
selected so that the ion/radical energies impinging on the substrate were similar to those
for the C-N sample set. This was done so that the only significant difference between
this sample set and the C-N sample set was the plasma density. The aim of this sample
set was to determine the effect of plasma density on the room-temperature doping of a-C
films. Specifically, the a-C film properties that were analyzed for this sample set were:
growth rate, nitrogen content, Fermi level shifting, refractive index, and absorption
coefficient.
The deposition parameters that remained fixed for all depositions were the flow rate and
pressure. The flow rate was set to 20sccm and the pressure was set to 60mTorr for all
depositions. These values were chosen based on an initial parameter scan that showed that the
use of these parameters led to a uniform and stable plasma profile.
29
In order to gauge the error associated with the deposition process (i.e. reproducibility of the
samples), one sample from each sample set was reproduced and the thickness and optical
properties of these samples were measured. It was found that the samples were highly
reproducible with variance in growth rate being less than 2% and variance in the optical
properties being less than 1%. The method of accounting for these errors is discussed in Section
3.5. Moreover, it should be noted that errors in the deposition parameters were accounted for in
the results. These errors, RF power (+/-3%), temperature (+/- 1oC), and partial pressure (+/-
0.7%), were assigned based on the specified tolerances of the respective controllers.
30
3.5 UV-VIS-NIR Spectral Ellipsometry UV-VIS-NIR spectral ellipsometry was used to determine the thickness and the complex index
of refraction of the a-C samples. The complex index of refraction, 𝑛𝑛�(𝐸𝐸), is defined as:
𝑛𝑛�(𝐸𝐸) = 𝑛𝑛(𝐸𝐸) + 𝑖𝑖𝑖𝑖(𝐸𝐸) (2)
where n(E) is the refractive index which describes the relative phase velocity of electromagnetic
radiation propagating through the material and k(E) is the extinction coefficient which describes
the absorption of electromagnetic radiation as it propagates through the material. The extinction
coefficient, k(E), is related to the absorption coefficient of the material, α(E), by:
𝛼𝛼(𝐸𝐸) = 4𝜋𝜋𝑖𝑖(𝐸𝐸)
𝜆𝜆
(3)
The method of ellipsometry is based on measuring the change in polarization state of light
reflected from an optical system (eg. air-film-substrate). The thickness and/or complex index of
refraction of the film can be found by performing a regression fitting of the measured data
against the theoretical change in polarization state of the optical system. This requires an initial
approximation of the thickness, n(E), and k(E) for each of the components of the optical system.
For the a-C samples, the optical system consists of three layers: air – a-C film – c-Si substrate.
The only unknowns lie in the a-C film. A 1st-order approximation for the thickness of the a-C
film was made by using values collected from profilometry measurements. Since the optical
properties of a-C vary greatly, a 1st-order approximation for n(E) and k(E) could not be made.
Thus in order to fit the measured data and obtain continuous values for n(E) and k(E), a
theoretical dispersion model for a-C was needed.
There are several common dispersion models that are used to describe semi-transparent films
such as the Cauchy, Sellmeier, Lorentz, Tauc-Lorentz, and Forouhi-Bloomer models. These
dispersion models can be differentiated on how they describe absorption in the material. For
amorphous semiconductors such as a-C, the description of absorption is complex due to the
existence of a band gap, tail states, and defect states. The Tauc-Lorentz and Forouhi-Bloomer
models were developed to describe the absorption characteristics of amorphous semiconductors.
Both models have been widely used, however, it has been shown that the parameters used in the
31
Forouhi-Bloomer model provide greater consistency with the actual electronic structure of a-C
films [44].
The Forouhi-Bloomer model describes the extinction coefficient for amorphous materials using
the following equation:
𝑖𝑖(𝐸𝐸) = 𝐴𝐴(𝐸𝐸 − 𝐸𝐸𝑔𝑔)2
𝐸𝐸2 − 𝐵𝐵𝐸𝐸 + 𝐶𝐶
(4)
where Eg, A, B, and C are positive non-zero constants characteristic of the medium. Using
Kramers-Kronig analysis, the real part of the refractive index can be found and is described by
the following equation:
𝑛𝑛(𝐸𝐸) = 𝑛𝑛(∞) + 𝐵𝐵𝑜𝑜𝐸𝐸 + 𝐶𝐶𝑜𝑜
𝐸𝐸2 − 𝐵𝐵𝐸𝐸 + 𝐶𝐶
(5)
where n(∞) is a positive non -zero constant representing the real part of the refractive index at
large photon energies, and Bo and Co are constants that depend on A, B, C, and Eg. Thus the
Forouhi-Bloomer dispersion model consists of five independent constants: A, B, C, Eg, and
n(∞).
For more information on the Forouhi-Bloomer model and the method of spectral ellipsometry,
interested readers are directed to the following sources [44; 45; 46].
Ellipsometric measurements were performed using the Sopra UV-VIS-NIR spectral ellipsometer.
The measurements were taken in the wavelength range between 275nm and 825nm at an angle of
incidence of 720. For fitting the measured data, a four layer optical system as shown in Figure
3.2 was used. The void+a-C diffusion layer was used to effectively describe the surface of the a-
C film. This layer provided greater accuracy to the physical representation of the optical system
as angle-resolved XPS measurements showed that the surface of the a-C layer was highly
porous.
32
Figure 3.2: Schematic of optical system used for fitting ellipsometry measurements.
Fitting was done using the Levenberg-Marquard method with a maximum of 50 iterations. The
parameters that were allowed to vary were the five constants of the Forouhi-Bloomer dispersion
model, the thickness of the a-C layer, the thickness of the void+a-C diffusion layer, and the
concentration of a-C in the void+a-C diffusion layer. All samples were fitted with an R-squared
convergence of approximately 0.99. Based on reproducibility experiments, an error of +/-1.8%
was estimated for thickness, while errors +/-0.2% and +/- 1.0% were estimated for the refractive
index and extinction coefficient, respectively.
33
3.6 X-ray photoelectron and X-ray excited Auger electron spectroscopy
3.6.1 Overview
X-ray photoelectron spectroscopy (XPS) and X-ray excited Auger electron spectroscopy (XAES)
were used to measure the composition, relative density, sp2/sp3 bonding ratio, and relative
position of the Fermi level for the a-C samples. A detailed review of XPS and XAES is beyond
the scope of this thesis. Interested readers are directed to the following sources for details on
XPS and XAES [47; 48; 49].
Both XPS and XAES are analytical methods that use an x-ray source to irradiate a sample under
ultra-high vacuum (UHV) in order to induce photoelectron emission in the sample. The emitted
electrons are collected by an electron detector and analyzer to measure the quantity and kinetic
energy of electrons emitted from the sample. As shown in Figure 3.3, XPS and XAES are
differentiated based on the photoelectron emission process being analyzed. In XPS, the energy
and quantity of core-level electrons emitted from the sample are measured, while in XAES the
energy and quantity of electrons emitted through the Auger process are analyzed. Core-level and
Auger electrons can be differentiated since the kinetic energy of electrons emitted from these two
processes lie in different energy ranges.
XPS was used to provide quantitative measurements of elemental composition, qualitative
measurements of film density, and qualitative measurements of Fermi level shifts for the a-C
samples doped with Nitrogen. XAES was used to provide quantitative measurements of the
sp2/sp3 bonding ratio in the a-C samples.
34
Figure 3.3: (a) XPS: measurement of electrons emitted from core-level due to x-ray
absorption (b) XAES: measurement of secondary electrons emitted from valence-level
carrying excess energy created from core-level hole, created in process shown in (a), being
filled.
XPS and XAES measurements were performed on a Thermo Scientific K-Alpha spectrometer.
Measurements were taken using a monochromatic source and the pass energy was set to 30eV.
In order to compensate for charging effects, a flood gun was used for the XAES measurements.
To determine the sp2/sp3 bonding ratio in the a-C samples, XAES measurements were also taken
on graphite and chemical vapour deposited (CVD) diamond references.
3.6.2 Elemental composition
Elemental composition can be found through XPS measurements by relating the kinetic energy
measured of an emitted core-level electron, Ek, to the core-level binding energy, BE, of a
particular element through the following equation:
𝐸𝐸𝑖𝑖 = hν − BE − Φanalyzer (6)
where hν is the x-ray photon energy and Φanalyzer is the work function of the electron analyzer.
This relationship is graphically illustrated in the band diagram shown in Figure 3.4.
As can be seen in the figure, the binding energy is defined as the energy difference between the
core-energy level and the Fermi level. Since this value is unique for each element, the elemental
composition of the sample can be found by comparing the counts of electrons occurring at
different binding energies. An example of elemental composition measurement is shown in
Figure 3.5. The energy scale was converted to binding energy using Equation 6 and each peak
35
was assigned to a specific element based on the best match between the peak’s energy and
known core-level binding energies. The counts were then corrected based on known
photoemission cross-sections of the selected elements and then the elemental composition was
found through the ratio between the areas under each peak. An error of 5% of the measured
value was accounted for these measurements based on the known tolerance of the system.
It should be noted that due to their low binding energies, electrons emitted from hydrogen and
helium cannot be detected in XPS systems, and thus composition found excludes contribution
from hydrogen. As discussed in the following section, the hydrogen content of the samples was
measured using Fourier Transform Infrared Spectroscopy.
Figure 3.4: Band diagram depicting relationship between binding energy (BE), work
function of the sample (Φsample), work function of the electron analyzer (Φanalyzer), Fermi
energy of the sample (Ef), measured kinetic energy of the electron (Ek), and photon energy
(hν) for XPS. Note that since the sample and analyzer are both grounded, their Fermi
energies are aligned. Ek’ represents the kinetic energy of the electron when it is emitted
from the sample, and Ek represents the kinetic energy of the electron measured on the
electron analyzer.
36
Figure 3.5: Example XPS measurement used to determine elemental composition. The
measurement was taken on a sample in the A-20 sample set and shows two distinct peaks at
two binding energies: (i) 526eV corresponding to the binding energy of 1s electrons in
oxygen and labeled O1s in the figure, (ii) 284eV corresponding to the binding energy of 1s
electrons in carbon and labeled C1s in the figure.
3.6.3 Film Density
Qualitative measurements on film density were obtained through XPS by taking measurements at
three different angles of incidence: 30o, 50o, and 70o. By varying the angle of incidence, the
depth that the x-ray beam penetrates the sample also varies with larger angles of incidence
leading to lower penetration depths. For example, if the angle of incidence of the x-ray is
denoted by θ, and the absorption length of x-ray photons in the film is denoted by L, then the
relationship between the penetration depth, d, with the angle of incidence can be approximated
by:
𝑑𝑑 = 𝐿𝐿 × cosθ (7)
37
This relationship is illustrated in Figure 3.6.
Figure 3.6: Dependence of penetration depth, d, of x-ray incident at angle of θ on a film
with x-ray absorption length of L.
By analyzing the change in film composition measured at different angles (depths), qualitative
information on the film’s density can be found. For example, a film with an oxygen
concentration that remains fairly constant with increasing depth can be concluded to be more
porous and less dense than a film that shows a strong decrease in oxygen concentration with
increasing depth. An example of this can be seen in the measurements shown in Figure 3.7. In
the figure, sample A shows a fairly constant oxygen profile while sample B shows a clear
decrease in oxygen concentration with decreasing angle (i.e. increasing depth).
38
Figure 3.7: Example of angle-resolved XPS measurements providing qualitative
information of film density. Note the curves are just a guide to the eye.
3.6.4 Fermi level shifts
In order to investigate the potential of nitrogen as a dopant in a-C, the dependence of the Fermi
level position with nitrogen content needed to be measured. For other semiconductors, this is
typically performed by measuring changes in the activation energy indicated in conductivity
versus temperature measurements. However, this method presents several challenges when
applied to the a-C samples studied in this thesis. Perhaps the greatest difficulty is presented by
the fact that PLC:H films, which were the type of a-C films investigated for doping in this thesis,
are highly resistive ~ 1015 Ω-cm [50]. With the equipment available, the only method of reliably
measuring the resistivity of this type of film is by performing the measurement transversely
through the “sandwich” method, illustrated in Figure 3.8. However as shown in the figure, due
to the soft, porous nature of PLC:H films, the “sandwich” method also poses several difficulties
6
6.5
7
7.5
8
8.5
9
9.5
10
10.5
11
30 35 40 45 50 55 60 65 70
Oxy
gen
Conc
entr
atio
n (a
t. %
)
Angle of Incidence (o)
Sample A
Sample B
39
as one needs to ensure that the metal contacts are not deposited over any pores, pinholes, or
scratches.
Figure 3.8: (a) “sandwich” method of measuring conductivity transversely through a thin-
film (b) shunting and unknown length issues that can occur with soft PLC:H film with
pores, pinholes and scratches.
In addition to issues associated with the high resistivity of the film, the fact that the PLC:H
samples were deposited at room-temperature also presented challenges. Since the samples were
deposited at room-temperature, any measurements on the film needed to be performed at or
below room temperature in order to prevent any annealing of the film. However, in order to find
the activation energy, conductivity versus temperature measurements are typically performed up
to at least 200oC to 300oC [51; 52].
XPS presents an optical alternative for measuring Fermi level shifts. As discussed in Section
3.6.2, binding energy is defined as the energy difference between a core-level electron and the
Fermi level. Thus shifts in the Fermi level can be found by measuring changes in the binding
energy of a material.
Several authors have shown XPS to be a reliable method for detecting Fermi level shifts in a-C
films [53; 54; 55]. In these studies, Fermi level shifts through XPS for DLC:H, TAC:H, and
TAC films were detected. To the best of our knowledge there is no study published on detecting
Fermi level shifts through XPS for PLC:H films.
When measuring Fermi level shifts through XPS, care must be taken not to confuse a shift in the
Fermi level with other effects that can cause shifts in the binding energy such as increased
resistivity or heteropolar bonding. Increased resistivity in the material can cause charging effects
that lead to a built-in retarding potential in the material which will decrease the kinetic energy of
40
electrons. This decrease in kinetic energy due to charging effects should not be falsely
associated with an increase in the binding energy of core-level electrons. In order to compensate
for charging effects, the C1s and N1s core-level binding energies were corrected using the
method outlined in [56]. In this method, the O1s binding energy on the surface of the a-C
samples is measured. Any shifts in the O1s peak in the samples is attributed to the charging
effect, and the C1s and N1s peaks are corrected by this shift. This method is argued to be valid
because the bonding environment of oxygen on the surface of a-C films does not change
significantly under different deposition conditions [56]. An example of C1s and N1s corrected
core-level energies is shown in Table 3.1.
Table 3.1: C1s and N1s core-level binding corrected for charging effect (CN sample set)
Nitrogen
(at. %)
C1s
+/-0.01
(eV)
N1s
+/-0.01
(eV)
O1s
+/-0.01
(eV)
ΔO1s
+/-0.02
(eV)
C1s corrected
+/-0.03 (eV)
N1s corrected
+/-0.03 (eV)
0 289.33 - 538.50 - 289.33 -
0.81 294.59 409.68 543.15 4.65 289.94 405.03
2.96 295.41 410.52 543.84 5.34 290.07 405.18
4.41 296.51 410.94 544.20 5.70 290.81 405.24
8.97 297.39 411.52 544.56 6.06 291.33 405.46
Heteropolar bonding, which is covalent bonding where the bonded electron(s) are not equally
shared by the atoms due to differences in electronegativity, can also cause a shift in binding
energy. Since nitrogen is more electronegative than carbon, the C1s binding energy can be
expected to increase due to an increase in nitrogen content [53; 55]. Therefore both the C1s and
N1s peaks need to be considered to determine if a shift in the Fermi level has taken place. Only
if both the charge-corrected C1s and N1s peaks have increased could one attribute the cause to a
shift in the Fermi level [55].
41
It should be noted that a quantitative value of the Fermi level shift cannot be obtained from XPS
measurements since the shift in the charge-corrected C1s peak is caused by both a shift in the
Fermi level and electronegativity effects. Thus as a means of comparing samples, the shift in the
charge-corrected N1s peak can be used as a qualitative measure of the shift in the Fermi level
[53]. Note an error of +/-0.01eV was accounted for in the measured C1s, N1s, and O1s peaks
based on the resolution of the measurement. This uncertainty also was accounted for when
assigning errors to the ΔO1s, C1s corrected, and N1s corrected values.
3.6.5 Carbon sp2 / sp3 bonding ratio
Using the Lascovich method [57; 58; 59; 60], XAES measurements were used to estimate the
ratio between sp2-hybridized and the sp3-hybridized carbon atoms in the film. The Lascovich
method is an empirical method which uses XAES measurements on carbon allotropes: diamond
(100% sp3 hybridized) and graphite (100% sp2 hybridized) to determine the percentage of sp2
hybridization in a-C films, which contain a mixture of sp3 and sp2 hybridization. The method
relies on the fact that the kinetic energy of an electron emitted from the Auger process is
sensitive to the energy levels present in the atom. For example, a sp3-hybridized carbon atom
has one core-level (1s) and one valence level (2sp3), and thus only one type of Auger emission
can take place; this is depicted in Figure 3.9. A sp2-hybridized carbon atom has one core-level
(1s) and two valence levels (2sp2 and 2p) and thus multiple Auger emission processes can take
place; this is depicted in Figure 3.10. Since sp2-hybridized carbon atoms have multiple valence
levels that can potentially be involved in Auger emission, one can expect the kinetic energy of
Auger electrons to have a broader profile in sp2-hybridized carbon atoms than in sp3-hybridized
carbon atoms. This can be extended from isolated atoms to condensed matter, as sp2-
hybridization leads to the presence of π-bands and thus a broadening of the valence band.
42
Figure 3.9: Auger emission in sp3-hybridized carbon atom.
Figure 3.10: Two potential Auger processes in sp2-hybridized carbon atom that produce
emission of electrons with unique kinetic energies.
A sample Auger measurement is illustrated in Figure 3.11. Note that the width of the dominant
peak, D, is more easily found by the derivative spectra, where the D-parameter in the derivative
spectra is represented by the distance between the maximum of the positive-going excursion and
the minimum of the negative-going excursion.
43
Figure 3.11: (a) XAES measurement (b) derivative spectra of XAES measurements with D-
parameter indicated on figure.
44
Lascovich proposed that the percentage of sp2-hybridized carbon in a-C films can be found by a
linear extrapolation of the D-parameter measured in the a-C film with the D-parameters of
diamond and graphite [57]. Thus the percentage of sp2-hybridized carbon atoms can be found
through the following:
%𝑠𝑠𝑠𝑠2 = �𝐷𝐷𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 − 𝐷𝐷𝑑𝑑𝑖𝑖𝑠𝑠𝑠𝑠𝑜𝑜𝑛𝑛𝑑𝑑𝐷𝐷𝑔𝑔𝑔𝑔𝑠𝑠𝑠𝑠 ℎ𝑖𝑖𝑖𝑖𝑠𝑠 − 𝐷𝐷𝑑𝑑𝑖𝑖𝑠𝑠𝑠𝑠𝑜𝑜𝑛𝑛𝑑𝑑
� × 100% (8)
An error of +/-1% was accounted for the calculated value based on the step size used for this
measurement.
45
3.7 Fourier Transform Infrared Spectroscopy Fourier Transform Infra-Red (FTIR) spectroscopy was used to measure the hydrogen
concentration in the a-C samples. A complete review of FTIR is beyond the scope of this thesis.
Interested readers are directed to the following sources for details on the method of FTIR [61;
62; 63].
FTIR is an infrared (IR) spectroscopy method that uses a broadband source coupled with a
Michelson interferometer to perform IR transmission and reflection measurements. Using post-
acquisition processing with the Michelson interferometer allows the Fourier-transform of the
measured transmission or reflection intensity as a function of wavenumber to be found.
Therefore, even though a broadband source is used, measurements at particular wavenumbers
can be made.
FTIR measurements were made using a Perkin Elmer 2000 spectrometer. The spectral range
used was 400cm-1 to 5200cm-1 with a resolution of 1cm-1. Before a measurement was taken,
samples were left in the sample compartment for 10 minutes in order to allow desiccated air to
purge the compartment. This purge was found to be necessary as it prevented absorption from
water vapor and CO2 to effect measurement data. Moreover, in order to reduce background
noise in the acquired data, the average of 20 measurements was taken for each sample. All
measurements were taken on samples deposited on c-Si substrates. These substrates were chosen
for FTIR measurements due to the near-flat transmittance of c-Si in the spectral range of interest.
FTIR spectroscopy is commonly employed in measuring the molecular bonding in solids and
liquids. These measurements are based on observing the absorption arising from the vibrational
modes of different molecular groups. Since every molecule has its own natural vibrational mode
with the frequency generally lying in the IR region, by measuring the absorption in the IR, the
existence and concentration of particular molecular groups can be found. For example, Table
3.2 shows the vibrational modes of different molecular species commonly found in intrinsic and
nitrogen incorporated a-C films [39; 64]. By examining the IR spectra of an a-C sample,
information on the types of bonds and the quantity of particular molecular groups present in the
sample can be found.
46
Table 3.2: Vibrational modes of molecular groups commonly found in a-C films
Wavenumber (cm-1) Vibrational Mode Assignment
500-1000 sp2-bonded CH2 & CH3 bending
1300-1350 sp2 C-C, C-N, and C=N
1375 sp3-bonded CH2 & CH3 bending
1360-1380 C=N
1450 sp3-bonded CH2 and CH3 bending mode
1500-1510 C-N, C=N, and C=C
1550-1570 C=N
1600 C=N, and C=C
1620-1700 C=C (higher), and C=N(lower)
2000-2400 CN triple bond stretch (lower) ; CC triple bond stretch (higher)
2855 sp3 CH3
2920 CH2 and CH sp3
2975 CH2 sp2
3250 NH mode
3300 sp1 CH mode
3400 OH mode
47
Figure 3.12 shows an example of an FTIR transmission measurement on one of the a-C samples
normalized against the transmission of a bare c-Si substrate.
Figure 3.12: Normalized transmission spectrum from FTIR measurement.
In order to better indentify the absorption modes present in the a-C film, the background of the
spectra was removed using polynomial curve-fit algorithm provided in the TableCurve 2D
Automated Curve Fitting and Equation Discovery 5.0 software. As shown in Figure 3.13, once
the background was removed the location and size of the absorption peaks could easily be
identified.
48
Figure 3.13: Transmission spectrum with background removed.
The absorption spectrum of the a-C film was found using the following relation [65]:
𝛼𝛼 = −1𝑑𝑑
ln𝑇𝑇 (9)
where α is the absorption coefficient, d is the thickness of the film, and T is the normalized
transmission of the a-C sample with the background removed.
The resulting absorption spectrum of the a-C sample is shown Figure 3.14. Each absorption
peak was assigned to the nearest known vibrational mode for intrinsic a-C. As can be seen, the
dominant absorption peak lies between 2750cm-1 and 3050cm-1 and can be attributed to
hydrogen-carbon stretching modes.
49
Figure 3.14: Absorption coefficient spectrum with vibrational modes indicated.
A commonly used method of determining the hydrogen concentration in a-C films is to integrate
the area under the dominant CHx peak (2750cm-1 to 3050cm-1) in the absorption coefficient
spectrum and multiply this value by an empirically found oscillator strength [39; 66; 67; 68]:
𝐻𝐻.𝐶𝐶𝑜𝑜𝑛𝑛𝐶𝐶. = 𝐴𝐴�𝛼𝛼(𝜔𝜔)𝑤𝑤
3050
2750𝑑𝑑𝜔𝜔
(10)
where A is the constant 5X1020cm-2 which represents the empirically found absorption strength
correction factor. An error of 1% was estimated for the calculated hydrogen concentration.
This error was based on comparing the calculated hydrogen concentration for different fits of the
raw FTIR background spectra.
50
3.8 Profilometry Profilometry was used to obtain a 1st-order estimate of the thicknesses of the a-C films deposited.
This estimate was needed in order to perform spectral ellipsometry modelling.
Profilometry is a mechanical method of measuring the thickness of a thin-film material.
Designed similar to a record player, profilometers use a stylus connected to a tracking arm which
is free to move along two axes (vertical and horizontal) and is held down by a counter-balance
load. The thickness of a thin-film is measured by moving the stylus across a step between the
bare substrate and the film. The deflection of the tracking arm connected to the stylus is
converted to an electrical signal and displayed on screen.
As opposed to other methods of measuring thickness of thin-films such as scanning electron
microscopy (SEM) or transmissive electron microscopy (TEM), measurements made using
profilometry are simple, fast, and independent of the electrical and optical properties of the film
and substrate used. However, the advantage of simplicity and speed is countered by a loss in
accuracy. Since profilometry measures thickness by measuring deflection, a mask is required
during film growth to provide a step between the bare substrate and the film. Due to edge
effects, the growth rate of film near the mask will differ from the growth rate near the center of
the substrate and thus the thickness measured by profilometry near the mask will differ from the
thickness in the rest of the substrate. Moreover, since profilometry is a mechanical method,
measurements are dependent on the mechanical properties of the film and substrate (eg.
hardness, friction, surface roughness, etc.). Since profilometry was used only for a 1st-order
estimate of thickness, the loss in accuracy was more than justified by the simplicity and speed of
the method.
Profilometry measurements were made on a KLA-Tencor P16+ with the counter balance load set
to 0.5mg.
51
4 Experimental Results
4.1 Overview In this chapter, the results from measurements of growth rate, nitrogen content, hydrogen
content, percent of sp3-bonded carbon atoms, film density, and optical properties for the a-C
films studied in this thesis are presented. For convenience, a summary of the five sample sets
explored is provided in Table 4.1.
Table 4.1: Summary of sample sets
Sample
Set
Description
A-20 Anode 20W: These were samples deposited on the anode at an RF power of 20W.
The variable deposition parameter for this sample set was temperature which was
varied from ambient temperature (35oC) to 200oC.
C-RT Cathode Room Temperature: These were samples deposited on the cathode with no
applied heating. The variable deposition parameter for this sample set was RF
power which was varied from 3W to 60W.
A-RT Anode Room Temperature: These were samples deposited on the anode with no
applied heating. The variable deposition parameter for this sample set was RF
power which was varied from 5W to 80W.
C-N Cathode Nitrogen incorporated: These were samples deposited on the cathode with
no applied heating at an RF power of 5W. The variable deposition parameter for
this sample set was the partial pressure of N2 in the source gas which was varied
from 0% to 50%.
A-N Anode Nitrogen incorporated: These were sample deposited on the anode with no
applied heating at RF power of 20W. The variable deposition parameter for this
sample set was the partial pressure of N2 in the source gas which was varied from
0% to 50%.
52
4.2 Growth rate As explained in Section 3.5, the thickness of the a-C films were measured using spectral
ellipsometry with profilometry measurements acting as an initial estimation for the regression
fitting. The thickness of the a-C films ranged from 150nm to 1µm with most films having a
thickness between 300-600nm.
Growth rates were measured on all five sample sets. Figures 4.1 to 4.3 show the dependence of
growth rate on the varied deposition parameter for sample sets A-20, C-RT, A-RT, C-N, and A-
N, respectively.
As can be seen in Figure 4.1, the growth rate of a-C for the A-20 sample set shows a strong
dependence on the substrate temperature used for film deposition; with higher substrate
temperatures leading to lower growth rates. Figure 4.2 shows the dependence of growth rate on
the applied RF power for sample sets C-RT and A-RT. Both sample sets show a significant
increase in growth rate with increasing RF power. By comparing the figures, it is evident that
the growth rate in the C-RT sample set increases more rapidly with increasing RF power than the
A-RT sample set. This is the first indication that the placement of the substrate (at the anode or
cathode) plays a role in the film growth process.
Figure 4.1: Change in growth rate with temperature for A-20 sample set.
0
1
2
3
4
5
6
0 50 100 150 200 250
Gro
wth
Rat
e (n
m/m
in)
Temperature (oC)
53
Figure 4.2: Change in growth rate with RF power for C-RT and A-RT sample sets.
Figure 4.3 shows the dependence on growth rate on the percentage of N2 introduced in the source
gas. For the C-N sample set, the growth rate appears to be independent of the amount of N2
introduced when the nitrogen content is in the 0-25% range but as the nitrogen introduction
reaches 50% there is a small decrease in the film growth rate. The dependency of growth rate
with the percentage of N2 in the source gas appears to be quite different for the A-N sample set as
the growth rate increases monotonically as the nitrogen content is increased.
0
2
4
6
8
10
12
14
0 10 20 30 40 50 60 70 80 90
Gro
wth
Rat
e (n
m/m
in)
RF Power (W)
A-RT
C-RT
54
Figure 4.3: Change in growth rate with N2 partial pressure for C-N and A-N sample sets.
Note the C-N sample set and A-N sample set had identical deposition parameters other
than the placement of the substrate and RF power used. For the C-N sample set, substrates
were placed on the cathode and an RF power of 5W was used while for the C-N sample set,
substrates were placed on the anode and an RF power of 20W was used.
2
3
4
5
6
7
0 10 20 30 40 50 60
Gro
wth
Rat
e (n
m/m
in)
N2 in Mixing Bottle (% pressure)
A-N
C-N
55
4.3 Nitrogen content As described in Section 3.6, the nitrogen content in the a-C samples was measured using XPS.
Figure 4.4 shows the change in the nitrogen content (at. %) in a-C films deposited at different N2
partial pressures in the source gas. Only samples included in the A-N and C-N sample sets are
shown since only these sample sets included N2 in the source gas. The deposition parameters in
the A-N and C-N sample sets were identical other than the RF power and the placement of the
substrate. In the A-N sample set, an RF power of 20W was used and the substrate was held on
the anode, where as in the C-N sample set, an RF power of 5W was used as the substrate was
held on the cathode. As can be seen in Figure 4.4, for both sample sets, the nitrogen content in
the film increased as the N2 partial pressure was increased. One clear difference between the
sample sets is the rate at which the nitrogen content increased. As can be seen in Figure 4.4,
films in the A-N sample set showed a more rapid increase in the nitrogen content with increasing
N2 partial pressure than films in the C-N sample set.
Figure 4.4: Change in nitrogen content (at. %) with N2 partial pressure for films in A-N
and C-N sample sets. The curves serve as guides to the eye.
0
2
4
6
8
10
12
14
16
18
20
0% 10% 20% 30% 40% 50% 60%
N C
onte
nt (a
t. %
)
N2 Partial Pressure (%)
A-N Sample Set
C-N Sample Set
56
4.4 Hydrogen content Hydrogen content measurements were made on the three intrinsic a-C sample sets: C-RT, A-RT,
and A-20. As described in Section 3.7, the concentration of hydrogen in the amorphous carbon
samples was estimated by integrating the area under the overlapping CHx peaks (2750cm-1 to
3050cm-1) in the infrared absorption spectra.
The absorption spectra in the 2750cm-1 to 3050cm-1 region and the corresponding hydrogen
concentration for a-C films in the C-RT sample set are shown in Figure 4.5 and Figure 4.6,
respectively.
Figure 4.5: Absorption of C-H modes for a-C films in the C-RT sample set.
0
500
1000
1500
2000
2500
3000
3500
4000
2750 2800 2850 2900 2950 3000 3050 3100
Abs
orpt
ion
(cm
-1)
Wavenumber (cm-1)
3W
5W
10W
15W
20W
40W
60W
CH3 sp3
CH &CH2 sp3
CH2 sp2
57
Figure 4.6: Change in hydrogen concentration in a-C films in the C-RT sample set
deposited at different RF powers. The curve is a guide for the eye.
As can be seen in Figure 4.6, the hydrogen concentration monotonically decreases with
increasing RF power. In addition, by observing Figure 4.5, one can see that there are clear trends
in the ratios of the different CHx modes with increasing RF power. For example, as the RF
power is increased, the relative intensity of the CH2 sp2 mode decreases in comparison to the
CHx sp3 modes. This trend is quantified in Figure 4.7 which shows the percent of CHx bonding
in the CH2 sp2 mode for films deposited at different RF powers. As can be seen in the figure,
there is a rapid decline in the relative concentration of CH2 sp2 bonding in the a-C films as the
RF power is increased.
0
1
2
3
4
5
6
7
0 10 20 30 40 50 60 70
Hyd
roge
n Co
ncen
trat
ion
(1022
cm-3
)
RF Power (W)
58
Figure 4.7: Percent of CHx bonding in the CH2 sp2 mode for C-RT sample set.
The absorption spectra in the 2750cm-1 to 3050cm-1 region and the corresponding hydrogen
concentration for a-C films in the A-RT sample set are shown in Figure 4.8 and Figure 4.9,
respectively. As can be seen in Figure 4.9, there does not appear to be any clear trend between
the hydrogen concentration and the RF power used during deposition. The hydrogen
concentration changes only slightly as the RF power is decreased from 80W to 10W, however,
once the RF power is dropped to 5W there is a significant drop in the hydrogen concentration in
the film. While there does not appear to be any clear trend in hydrogen concentration with RF
power for this sample set, there is a trend in the ratio of the CH2 sp2 peak to the CHx sp3 peaks
with increasing RF power. Similar to the C-RT sample set, the relative intensity of the CH2 sp2
mode decreases as RF power is increased. This trend is depicted in Figure 4.10. However, by
comparing Figure 4.10 with Figure 4.7, it is clear that this trend is more prominent in the C-RT
sample set than in the A-RT sample set.
0
5
10
15
20
25
30
35
40
45
50
3 5 10 15 20 40 60
CH2
sp2
mod
e (%
)
RF Power (W)
59
Figure 4.8: Absorption of C-H modes for a-C films in the A-RT sample set.
0
500
1000
1500
2000
2500
3000
2750 2800 2850 2900 2950 3000 3050 3100
Abs
orpt
ion
(cm
-1)
Wavenumber (cm-1)
5W
10W
20W
40W
80W
CH2 sp2CH &CH2 sp3
CH3 sp3
60
Figure 4.9: Change in hydrogen concentration in a-C films in the A-RT sample set
deposited at different RF powers.
Figure 4.10: Percent of CHx bonding in the CH2 sp2 mode for A-RT sample set.
0
1
2
3
4
5
6
0 10 20 30 40 50 60 70 80 90
Hyd
roge
n Co
ncen
trat
ion
(1022
cm-3
)
RF Power (W)
30
32
34
36
38
40
42
44
46
48
50
5 10 20 40 80
CH2
sp2
Mod
e (%
)
RF Power (W)
61
The absorption spectra in the 2750cm-1 to 3050cm-1 region and the corresponding hydrogen
concentration for a-C films in the A-20 sample set are shown in Figure 4.11 and Figure 4.12,
respectively.
Figure 4.11: Absorption of C-H modes for a-C films in the A-20 sample set.
0
500
1000
1500
2000
2500
3000
3500
4000
2750 2800 2850 2900 2950 3000 3050 3100 3150
Abd
orpt
ion
(cm
-1)
Wavenumber (cm-1)
35C
100C
150C
200C
CH2 sp2CH &CH2 sp3
CH3 sp3
62
Figure 4.12: Change in hydrogen concentration in a-C films in the A-20 sample set
deposited at different substrate temperatures.
As can be seen in Figure 4.12, there appears to be a local maxima in the relationship between the
hydrogen concentration in the a-C films and the temperature of the substrate during deposition.
As the temperature is increased from ambient temperature, 35oC, where no intentional heating
was applied to the substrate, to 100oC, there is a significant increase in the hydrogen
concentration in the film. As the temperature is increased further to 150oC, the hydrogen
concentration remains relatively constant. However, once the temperature is increased to 200oC,
there is a significant decrease in the hydrogen concentration in the film.
The ratio of the CH2 sp2 mode to the CHx sp3 modes shows a different trend with increasing
substrate temperature. As can be seen in Figure 4.13, the relative concentration of CH2 sp2
bonding decreases as the temperature is increased from 35oC to 100oC and again as the
temperature is increased from 150oC to 200oC, however, between 100oC to 150oC, there does not
appear to be any change in the relative concentration of the CHx modes.
0
1
2
3
4
5
6
7
0 50 100 150 200 250
Hyd
roge
n Co
ncen
trat
ion
(1022
cm-3
)
Temperature (oC)
63
Figure 4.13: Percent of CHx bonding in the CH2 sp2 mode for A-20 sample set.
30
32
34
36
38
40
35 100 150 200
CH2
sp2
Mod
e (%
)
Temperature (oC)
64
4.5 Percent of sp3 Bonding As discussed in Section 3.6, the percent of sp3-bonded carbon atoms in the a-C samples was
estimated using XAES. Measurements were performed on the three intrinsic a-C sample sets: C-
RT, A-RT, and A-20. Due to overlapping modes introduced by nitrogen, the percent of sp3-
bonded carbon atoms could not be extracted from XAES measurements for the nitrogen-
incorporated sample sets: C-N and A-N.
Figure 4.14 shows the change in the percent of sp3-bonded carbon atoms (%sp3 bonding) for a-C
films in the C-RT and A-RT sample sets. For both these sample sets, the variable deposition
parameter was the applied RF power with all other parameters being held constant. These
samples sets only differ in the placement of the substrates, with substrates being held on the
cathode for the C-RT sample set and substrates held on the anode for the A-RT sample set. As
can be seen in the figure, both sample sets show decreasing trends in %sp3 bonding with
increasing RF power with the C-RT sample set showing a more rapid decrease than the A-RT
sample set. It is interesting to note that at low RF powers (below 20W), the C-RT and A-RT
sample sets show similar relationships between %sp3 bonding and RF power. However, as the
RF power is increased further, the %sp3 bonding in the C-RT sample set begins to decreases
rapidly while the A-RT sample set only shows a slight decrease.
Figure 4.15 shows the change in the percent of sp3-bonded carbon atoms (%sp3 bonding) for a-C
films in the A-20 sample set. For this sample set, all films were deposited at an RF power of
20W with the substrates held on the anode, however, each sample was deposited at a different
substrate temperature. As can be seen from the figure, the substrate temperature has a similar
effect as the RF power had in the C-RT and A-RT samples sets, as the %sp3 bonding shows a
generally decreasing trend with increasing substrate temperature. However, it is clear that for the
range in substrate temperatures explored, the %sp3 bonding is more weakly related to substrate
temperature than it is to RF power.
65
Figure 4.14: Change in the percent of sp3-bonded carbon atoms for a-C films deposited at
different RF Powers in the C-RT and A-RT sample sets. The curves are guides to the eye.
Figure 4.15: Change in the percent of sp3-bonded carbon atoms for a-C films deposited at
different substrate temperatures in the A-20 sample set.
0
10
20
30
40
50
60
70
80
90
0 10 20 30 40 50 60 70 80 90
%sp
3bo
ndin
g
RF Power (W)
C-RT
A-RT
57
58
59
60
61
62
63
64
65
66
67
0 50 100 150 200 250
%sp
3bo
ndin
g
Temperature (oC)
66
4.6 Film density In order to augment other structural characterization measurements and assist in the analysis,
qualitative film density measurements were performed on the three intrinsic sample sets: A-20,
C-RT, and A-RT. As explained in Section 3.6, the a-C film density can be qualitatively
measured by examining the change in oxygen concentration at increasing depth from the surface.
Samples that have a significantly lower oxygen concentration in the bulk of the sample as
compared to the surface can be deemed more dense than samples that have near constant oxygen
concentration from the surface to the bulk. Thus by measuring the change in oxygen
concentration in the film, a qualitative assessment of the a-C film density can be made.
For the A-20 sample set, the change in film density for different substrate temperatures can be
seen in Figure 4.16. As can be seen in the figure, the samples deposited at higher substrate
temperatures show a larger gradient in oxygen concentration and thus can be concluded to have a
relatively greater film density.
Figure 4.16: Difference in the oxygen concentration (O.C.) in the bulk and in the near-
surface (O.C.surface-O.C.bulk) for samples in the A-20 sample set. Measurements made by
AR-XPS with measurement at 70o representing the near-surface and measurement at 30o
representing the bulk.
-0.5 0 0.5 1 1.5 2 2.5 3
35
100
150
200
Change in Oxygen Concentration (at. %)
Tem
pera
ture
(o C
)
67
The change in film density for samples deposited at different RF powers in the C-RT sample set
can be seen in Figure 4.17. As can be seen in the figure, there is a significant change in the
oxygen gradient and thus film density as the RF power is increased from 3W to 10W. As the RF
power is increased from 10W to 20W the oxygen gradient appears to remain near constant (i.e.
changes are within the range of measurement error). As the RF power is increased beyond 20W,
the oxygen gradient and thus the film density once again increases significantly.
Figure 4.17: Difference in the oxygen concentration (O.C.) in the bulk and in the near-
surface (O.C.surface-O.C.bulk) for samples in the C-RT sample set. Measurements made by
AR-XPS with measurement at 70o representing the near-surface and measurement at 30o
representing the bulk.
The change in film density for samples deposited at different RF powers in the A-RT sample set
can be seen in Figure 4.18. As can be seen in the figure, as the RF power is increased there is a
significant increase in the oxygen gradient and thus density of the samples. It is also interesting
to observe that for this sample set, the samples deposited at 5W, 10W, and 20W, all show a
negative oxygen gradient meaning that there is more oxygen in the bulk of the film than near the
surface. This is an indication of the highly porous structure of these films.
0 1 2 3 4 5 6 7 8
3
5
10
15
20
40
60
Change in Oxygen Concentration (at. %)
RF P
ower
(W)
68
Figure 4.18: Difference in the oxygen concentration (O.C.) in the bulk and in the near-
surface (O.C.surface-O.C.bulk) for samples in the A-RT sample set. Measurements made by
AR-XPS with measurement at 70o representing the near-surface and measurement at 30o
representing the bulk.
-2 -1 0 1 2 3 4 5 6 7 8
5W
10W
20W
40W
80W
Change in Oxygen Concentration (at. %)
RF P
ower
(W)
69
4.7 Impurity doping In order to determine the effect of nitrogen content in a-C films, the relative position of the Fermi
level for films in the C-N and A-N sample sets were estimated through binding energy shifts
measured using XPS. This method was described in Section 3.6. For both the C-N and the A-N
sample sets, the percent of N2 in the source gas was varied from 0% to 50%. Both the C-N and
A-N sample sets produced PLC:H type a-C films with the deposition conditions of the sample
sets being identical other than the RF power and the placement of the substrate. In the A-N
sample set, an RF power of 20W was used and the substrate was held on the anode, where as in
the C-N sample set, an RF power of 5W was used as the substrate was held on the cathode. Due
to these differences, the nitrogen content (at. %) for these two sample sets differed.
As explained in Section 3.6, the change in the charge-corrected N1s binding energy provides a
qualitative measure of shifts in the Fermi level position with an increase in the N1s binding
energy representing a shift of the Fermi level toward the conduction band. The change in the
charge-corrected N1s binding energy for films with different levels of nitrogen content (at. %) in
the C-N and A-N sample sets are shown in Table 4.2 and Table 4.3, respectively. For both
sample sets it is clear that the Fermi level shifts towards the conduction band with increasing
nitrogen content (at. %). This indicates that nitrogen is acting as an n-type dopant for the a-C
films produced in these sample sets.
Table 4.2: N1s shifts for C-N sample set
N2 in source gas (%) Nitrogen (at. %) N1s corrected
+/-0.03 (eV)
ΔN1s
+/-0.03 (eV)
5 0.81 405.03 -
10 2.96 405.18 0.15
25 4.41 405.24 0.21
50 8.97 405.46 0.43
70
Table 4.3: N1s shifts for A-N sample set
N2 in source gas (%) Nitrogen (at. %) N1s corrected
+/-0.03 (eV)
ΔN1s
+/-0.03 (eV)
5 2.58 413.22 -
10 4.6 413.52 0.30
25 9.7 413.83 0.61
50 17 413.96 0.74
71
4.8 Optical properties In this section, the optical properties of the a-C samples, measured through spectral ellipsometry,
are presented. The results included in this section are the refractive index and absorption
coefficient from 300nm to 825nm as well as the calculated optical energy gap of the a-C samples
for all five sample sets: A-20, C-RT, A-RT, C-N, and A-N.
The refractive index and absorption coefficient spectra for the a-C films included in sample set
A-20 are shown in Figure 4.19 and Figure 4.20, respectively. For this sample set, all films were
deposited at an RF power of 20W with the substrates held on the anode, however, each sample
was deposited at a different substrate temperature. As can be seen from the figures, the films
deposited at 35oC, 100oC, and 150oC show similar refractive index and absorption coefficient
spectra (i.e. differences are within error bars), however, as the substrate temperature is increased
to 200oC, there is a noticeable shift in both the refractive index and absorption coefficient
spectra.
Figure 4.19: Refractive index (n) of a-C films in the A-20 sample set. These are intrinsic
films that were deposited on the anode at an RF power of 20W at several different
substrate temperatures. For clarity, an error bar is only shown for the first data point on
the sample deposited at 200oC.
1.52
1.54
1.56
1.58
1.6
1.62
1.64
1.66
1.68
1.7
1.72
250 350 450 550 650 750 850
n
λ (nm)
35 C
100C
150C
200C
1000C
1500C
2000C
350C
72
Figure 4.20: Absorption coefficient (α) of a-C films in the A-20 sample set. These are
intrinsic films that were deposited on the anode at an RF power of 20W at several different
substrate temperatures. For clarity, an error bar is only shown for the first data point on
the sample deposited at 200oC.
The refractive index and absorption coefficient spectra for the C-RT and A-RT sample sets are
presented in Figures 4.21 to 4.24. For both these sample sets, the variable deposition parameter
was the applied RF power with all other parameters being held constant. These samples sets
only differ in the placement of the substrates, with substrates being held on the cathode for the C-
RT sample set and substrates held on the anode for the A-RT sample set. As can be seen in
Figures 4.21 and 4.22, the applied RF power has a significant effect on the refractive index and
absorption coefficient for samples deposited on the cathode (C-RT sample set). As the RF power
is increased from 5W to 40W, both the refractive index and absorption coefficient spectra show
significant increases. However, as the RF power is increased from 40W to 60W, the increase is
0
0.5
1
1.5
2
2.5
3
3.5
250 350 450 550 650 750 850
α (1
04cm
-1)
λ (nm)
35C
100C
150C
200C
1000C
1500C
2000C
350C
73
less significant with the absorption coefficient spectra of these two samples being within the
range of the error bars.
Figure 4.21: Refractive index (n) of a-C films in the C-RT sample set. These are intrinsic
films that were deposited on the cathode at several different RF powers. For this sample
set there was no intentional heating of the substrate. For clarity, an error bar is only
shown for the first data point on the sample deposited at 60W.
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
250 350 450 550 650 750 850
n
λ (nm)
60W
40W
20W
15W
10W
5W
3W
74
Figure 4.22: Absorption coefficient (α) of a-C films in the C-RT sample set. These are
intrinsic films that were deposited on the cathode at several different RF powers. For this
sample set there was no intentional heating of the substrate. For clarity, an error bar is
only shown for the first data point on the sample deposited at 60W.
The trends for samples deposited on the anode (A-RT sample set) are not as apparent. As shown
in Figure 4.23, the only noticeable shift in refractive index occurs as the RF power is increased
from 40W to 80W, however, the magnitude of this shift is relatively small. The trend in the
absorption coefficient spectra, shown in Figure 4.24, is more apparent. Similar to the C-RT
sample set, the absorption coefficient increases with increasing RF power, however, once again
the magnitude of the shift is relatively small in comparison to the C-RT sample set. It is clear
from these results, that the refractive index and absorption coefficient of the samples deposited
on the anode (A-RT sample set) show a significantly weaker dependency on the RF power than
the samples deposited on the cathode (C-RT sample set). It should also be pointed out that the
refractive index and absorption coefficient spectra of the samples deposited on the anode appear
to be similar to that of the samples deposited at low RF powers (3W-5W) on the cathode.
0
2
4
6
8
10
12
14
250 350 450 550 650 750 850
α(1
04cm
-1)
λ (nm)
60W
40W
20W
15W
10W
5W
3W
75
Figure 4.23: Refractive index (n) of a-C films in the A-RT sample set. These are intrinsic
films that were deposited on the anode at several different RF powers. For this sample set
there was no intentional heating of the substrate. For clarity, an error bar is only shown
for the first data point on the sample deposited at 80W.
1.54
1.56
1.58
1.6
1.62
1.64
1.66
1.68
1.7
250 350 450 550 650 750 850
n
λ (nm)
80W
40W
20W
10W
5W
76
Figure 4.24: Absorption coefficient(α) of a-C films in the A-RT sample set. These are
intrinsic films that were deposited on the anode at several different RF powers; for this
sample set there was no intentional heating of the substrate. For clarity, an error bar is
only shown for the first data point on the sample deposited at 80W.
The refractive index and absorption coefficient spectra for the C-N and A-N sample sets are
shown in Figure 4.25 to 4.28, below. For both these sample sets, the variable deposition
parameter was the percent of N2 in the source gas, which was varied from 0% to 50%. The fixed
deposition parameters of these samples sets differed only in the RF power used and the
placement of the substrate. In the A-N sample set, an RF power of 20W was used and the
substrate was held on the anode, where as in the C-N sample set, an RF power of 5W was used
and the substrate was held on the cathode. Due to these differences, the nitrogen content (at. %)
for these two sample sets differed.
0
0.5
1
1.5
2
2.5
250 350 450 550 650 750 850
α(1
04cm
-1)
λ (nm)
80W
40W
20W
10W
5W
77
As can be seen in the figures, for both sample sets, the refractive index and absorption coefficient
spectra show a weak dependency with nitrogen content. For the C-N sample set, both the
refractive index and absorption coefficient spectra show only minor increases (i.e. shifts are
close to or within range of error bars) as the nitrogen content is increased from 0% to 4.41%;
however, when the nitrogen content is increased to 8.97%, a noticeable downward shift in the
refractive index and absorption coefficient spectra can be seen. The A-N sample set shows a
similar trend to the C-N sample set as the refractive index and absorption coefficient spectra
show only minor increases (i.e. shifts are close to or within range of error bars) as the nitrogen
content is increased from 0% to 4.6%. However, unlike the C-N sample set, as the nitrogen
content is increased beyond 8.97%, both the refractive index and absorption coefficient spectra
shift upwards rather than downwards.
Figure 4.25: Refractive index (n) of a-C films in the C-N sample set. Each film has a
different level of nitrogen content (at. %) due to the difference in the N2 partial pressure
that was used in the source gas. For this sample set the substrate was held on the cathode,
the RF power was set to 5W, and there was no intentional heating of the substrate. For
clarity, an error bar is only shown for the last data point on the sample with a nitrogen
content of 8.97%.
1.56
1.58
1.6
1.62
1.64
1.66
1.68
1.7
1.72
1.74
250 350 450 550 650 750 850
n
λ (nm)
8.97%4.41%2.96%5%0%
78
Figure 4.26: Absorption coefficient (α) of a-C films in the C-N sample set. Each film has a
different level of nitrogen content (at. %) based on the N2 partial pressure that was used in
the source gas. For this sample set the substrate was held on the cathode, the RF power
was set to 5W, and there was no intentional heating of the substrate. For clarity, an error
bar is only shown for the first data point on the sample with a nitrogen content of 8.97%.
0
0.5
1
1.5
2
2.5
3
3.5
4
250 350 450 550 650 750 850
α (
104
cm-1
)
λ (nm)
8.97%
4.41%
2.96%
5%
0%
79
Figure 4.27: Refractive index (n) of a-C films in the A-N sample set. Each film has a
different level of nitrogen content (at. %) based on the N2 partial pressure that was used in
the source gas. For this sample set the substrate was held on the anode, the RF power was
set to 20W, and there was no intentional heating of the substrate. For clarity, an error bar
is only shown for the first data point on the sample with a nitrogen content of 17.04%.
1.54
1.56
1.58
1.6
1.62
1.64
1.66
1.68
1.7
250 350 450 550 650 750 850
n
λ (nm)
17.04%
9.73%
4.60%
2.58%
0%
80
Figure 4.28: Absorption coefficient (α) of a-C films in the A-N sample set. Each film has a
different level of nitrogen content (at. %) based on the N2 partial pressure that was used in
the source gas. For this sample set the substrate was held on the anode, the RF power was
set to 20W, and there was no intentional heating of the substrate. For clarity, an error bar
is only shown for the first data point on the sample with a nitrogen content of 17.04%.
Typically for a-C films the optical energy gap is defined as the E04 energy, which is the photon
energy at which the absorption coefficient of the film equals 104 cm-1. The E04 energies for the a-
C films in the A-20, C-RT, A-RT, C-N, and A-N sample sets are shown below in Figure 4.29 to
Figure 4.32. Based on the 1.0% error attributed to the absorption coefficient, an uncertainty of
+/-0.2% was calculated for the E04 values. Since the E04 gap is linked directly with the
absorption coefficient of the film, the trends in the E04 gap for each sample set are the same as
the trends in absorption coefficient already discussed.
0
0.5
1
1.5
2
2.5
3
3.5
250 350 450 550 650 750 850
α (
104
cm-1
)
λ (nm)
17.04%
9.73%
4.60%
2.58%
0%
81
Figure 4.29: Change in the E04 gap with substrate temperature for A-20 sample set.
Figure 4.30: Change in the E04 gap with RF power for C-RT and A-RT sample sets. The
curves are a guide to the eye.
3
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
4
30 50 70 90 110 130 150 170 190
E 04ga
p (e
V)
Substrate Temperature (oC)
1.5
2
2.5
3
3.5
4
4.5
0 10 20 30 40 50 60 70 80 90
E 04G
ap (e
V)
RF Power (W)
A-RT
C-RT
82
Figure 4.31: Change in the E04 gap with nitrogen content for C-N sample set. The curve is
a guide to the eye.
Figure 4.32: Change in the E04 gap with nitrogen content for A-N sample set. The curve is
a guide to the eye.
3.2
3.3
3.4
3.5
3.6
0 2 4 6 8 10
E 04G
ap (e
V)
Nitrogen Incorporation (at. %)
3.3
3.4
3.5
3.6
3.7
3.8
3.9
4
0 5 10 15 20
Opt
ical
Gap
(eV
)
Nitrogen Incorporation (at. %)
83
5 Analysis
5.1 Overview In this chapter, the results presented in the previous chapter are analyzed. This analysis is broken
up into two sections. In Section 5.2, the effect of ion/radical energy, plasma density, substrate
temperature, and nitrogen incorporation on the growth rate, microstructural properties, and Fermi
level position are discussed. In Section 5.3, the relationship between the microstructure and
optical properties of a-C films is explored
84
5.2 Effect of deposition conditions on growth, electronic- and micro-structure of films
When reviewing the results presented in Chapter 4, it is apparent that many of the film properties
show similar trends as the primary deposition parameter is varied. Therefore, in order to analyze
the results and determine the relationships between different film properties, it is best to look at
all of the results within a sample set together and analyze how the deposition conditions affect
these film properties. Moreover, since some of the sample sets share the same variable
deposition parameter: (i) C-RT and A-RT, (ii) C-N and A-N, it will also be helpful to group
these sample sets together in the analysis.
5.2.1 C-RT and A-RT sample sets
The C-RT and A-RT sample sets shared the same variable deposition parameter, RF power, and
other than the placement of the substrate, they also shared identical fixed deposition parameters.
As explained in Section 2.3, one of the effects of increasing the RF power in an RF PECVD
system is that the types and density of ions and radicals generated in the plasma region will
change. This is intuitive since by increasing the RF power more energy is coupled to the free
electrons in plasma region and as these energetic electrons collide with neutral particles in
plasma region, new species and an overall greater density of ions and radicals are generated.
This increase in plasma density with increasing RF power will occur in both the C-RT and A-RT
sample sets since the placement of the substrate will not have an effect on this process.
However, by reviewing the results in the previous sections it is apparent that the placement of the
substrate has a significant effect on the film growth rate and microstructure. To appreciate how
the placement of the substrate (anode or cathode) can affect the growth rate and microstructure
of a-C films, one needs to review the potential profile of an RF PECVD chamber.
As explained in Section 2.3, the electrodes on a capacitively-coupled RF-PECVD system
develop negative DC biases with respect to the plasma potential. If the areas of the electrodes
are asymmetric, then the electrode with the smaller area develops the dominant self-bias
potential with the ratio of the self-biases being described by Equation 11:
VcathodeVanode
= � AanodeAcathode
�2
= 2.6 (11)
85
where Vcathode and Vanode are the magnitude of the self-biases on the cathode and anode with
respect to the plasma potential and Acathode and Aanode are the areas of the cathode and anode for
the RF PECVD system used for these experiments.
Since the area of the cathode was smaller than the area of the anode, the dominant self-bias fell
on the cathode and thus an asymmetric potential profile like that shown in Figure 5.1 would
develop in the chamber. As shown in the figure, as the RF power applied to the plasma is
increased, the magnitude of the self-biases on both electrodes would increase; however, in order
to maintain the ratio of the biases governed by Equation 11, the self-bias on the cathode would
have to increase by a factor of 2.6 greater than the increase in self-bias on the anode.
As explained in Section 2.3, the mean energy of ions and radicals impinging on an electrode is
primarily determined by the self-bias potential of the electrode. Since in the RF PECVD system
used here the dominant self-bias falls on the cathode, the energy of ions and radicals impinging
on the cathode is greater than on the anode. Moreover, as the RF power is increased the change
in self-bias and consequently ion/radical energy is significantly more pronounced on the cathode
than on the anode.
86
Figure 5.1: Change in plasma potential profile with increasing RF power for RF-PECVD
chamber with area of cathode smaller than area of anode.
Growth Rate:
As presented in Section 4.2, the growth rate for both the A-RT and C-RT sample sets increased
with increasing RF power. Figure 5.2 shows the relationship between growth rate and RF power
was similar for both sample sets; however, the growth rate in the C-RT sample set was
consistently 20-30% more than the A-RT sample set.
The fact that both sample sets showed very similar growth rates even at high RF powers suggests
that plasma density is playing a primary role in controlling the growth rate. This is intuitive
since an increase in plasma density and the introduction of new reactive species in the plasma
87
would cause an increase in surface reactions on the growing film leading to an increase in growth
rate.
Figure 5.2: Difference in relationship between growth rate and RF power for A-RT and C-
RT sample sets. The curves serve as guides to the eye.
The fact that the growth rate in the C-RT sample set was consistently 20-30% more than the A-
RT sample set can be explained through the higher ion/radical energies impinging on the
cathode. As explained in Section 2.5, low energy ions & radicals do not have enough energy to
penetrate the surface of a growing a-C film and thus can only bond to the film if there is an
unpassivated dangling bond on the surface of the film. In contrast, higher energy ions and
radicals do not need to rely on surface dangling bonds since they can penetrate the surface and
bond to a carbon cluster within the bulk of the film. This increase in bonding possibilities for
high energy ions and radicals can explain the enhancement in growth rate seen in the C-RT
sample set in comparison to the A-RT sample set.
Microstructure:
As presented in Section 4.4, for the C-RT sample set there was a strong correlation between the
hydrogen concentration in the film and the RF power used during deposition while for the A-RT
0
2
4
6
8
10
12
14
0 10 20 30 40 50 60 70 80 90
Gro
wth
Rat
e (n
m/m
in)
RF Power (W)
A-RT
C-RT
88
sample set the hydrogen concentration seemed invariant with the RF power used. This
comparison is depicted in Figure 5.3. These results indicate that the ion/radical energy rather
than the plasma species/density is primarily responsible for determining the hydrogen
concentration in the film. By reviewing the film growth processes presented in Section 2.5, this
relationship between high energy ions/radicals and hydrogen concentration can be understood.
Due to the relatively low displacement energy of hydrogen, 2.5eV, surface hydrogen atoms can
be displaced from their bond through collisions with high energy ions/radicals. Moreover, high
energy hydrogen ions & radicals impinging on the film can penetrate into the bulk of the film,
bond with another hydrogen atom, and then desorb from the film. Thus there is a tendency for
the hydrogen concentration in the film to decrease as the mean energy of ions/radicals in film
deposition is increased. This explains why the hydrogen concentration has a strong dependency
on RF power for the C-RT sample set but not for the A-RT sample set.
Figure 5.3: Relationship between hydrogen concentration and RF power for C-RT and A-
RT sample sets. The curve is a guide to the eye for the data points in the C-RT sample set.
To understand why there is a significant drop off in hydrogen concentration for the sample
deposited at 5W in the A-RT sample set, density measurements need to be taken into account.
Without taking into account film density, hydrogen concentration measurements can be
0
1
2
3
4
5
6
7
0 20 40 60 80 100
Hyd
roge
n Co
ncen
trat
ion
(1022
cm-3
)
RF Power (W)
C-RT
A-RT
89
misleading at times since a film with a relatively low hydrogen concentration can actually have a
relatively high percentage of hydrogen if the film has a low enough density. Qualitative film
density measurements presented in Section 4.6, showed that the film density increased with
increasing RF power for both the C-RT and A-RT sample sets. Since for the C-RT sample set,
the hydrogen concentration and film density showed opposite trends with increasing RF power
(i.e. hydrogen concentration decreased while film density increased), it is evident that the percent
of hydrogen (at. %) in the films also decreases with increasing RF power. This is not the case in
the A-RT sample set where other than the sample deposited at 5W, the hydrogen concentration
remained near-constant. Thus it is not directly apparent what the trend in hydrogen (at. %) is for
the A-RT sample set. However, insight into this trend can be extracted by examining the ratio of
the C-H peaks in the infrared absorption spectra presented in Section 4.4.
The relative area of the CH2 sp2 peak can be used as a qualitative measure of the percent of
hydrogen (at. %) in the films, with a larger relative CH2 sp2 peak being attributed to a higher
percentage of hydrogen. The rationale for this stems from the fact that hydrogen prefers to bond
to sp3 hybridized carbon atoms because it represents a lower energy state than a bond with a sp2
hybridized carbon atom. Thus C-H sp2 bonding would be unlikely to occur unless the available
sp3 carbon bonds are near or at saturation due to a high percent of hydrogen (at. %) in the film.
It should be pointed out that it is also possible for a high relative intensity of the CH2 sp2 peak to
occur if a high percentage of the carbon atoms in the film are sp2 hybridized; however, as
indicated by the results shown in Section 4.4 and Section 4.5, for both the C-RT and A-RT
sample sets the percent of sp2 bonded carbon atoms and the relative intensity of the CH2 sp2 peak
show opposite trends with increasing RF power.
As shown in Section 4.4, the relative area of the CH2 sp2 peak decreases with increasing RF
power with the rate of decline being significantly larger in the C-RT sample set. This is
consistent with the ion/radical energy argument made above. For both the C-RT and A-RT
sample sets, the ion/radical energy will increase as the RF power is increased. However, this
increase is significantly larger for ions/radicals impinging on the cathode. Thus it would be
expected that the percent of hydrogen (at. %) in the film would decrease for both sample sets
with increasing RF power, with the rate of decrease being significantly larger for the C-RT
sample set.
90
This relationship between ion/radical energy and hydrogen (at. %) in the film can help explain
the trends in percent of sp3-bonded carbon atoms (%sp3 bonding) presented in Section 4.5. For
both the A-RT and C-RT sample sets, the trends in %sp3 bonding followed the same trends as the
hydrogen content (at. %). As the RF power increased, the %sp3 bonding in both sample sets
decreased, with the rate of decline in %sp3 bonding being significantly greater for the C-RT
sample set than the A-RT sample set.
The coupling between the hydrogen content (at. %) and the %sp3 bonding in the film occurs due
to the fact that the H-C sp3 bond is a lower energy bond for hydrogen in a-C than the H-C sp2
bond. Thus a high hydrogen content (at. %) will promote sp3 bonding in a-C. This type of a-C
film is classified as polymeric-like hydrogenated amorphous carbon (PLC:H) because the high
hydrogen content (at.%) causes most of the available carbon sp3 bonds to be filled with
hydrogen, leading to the microstructure of the film being a low density network of H-C sp3
clusters.
As the mean ion/radical energy rises, the hydrogen content (at.%) begins to decline due to more
ions/radicals impinging on the film with energies greater than the displacement energy of
hydrogen. Due to the relationship between hydrogen content (at. %) and %sp3 bonding in the
film, this decline in hydrogen content (at. %) will lead to a decrease in %sp3 bonding in the film.
In addition to reducing the hydrogen content (at. %), as the mean ion/radical energy rises further,
some ions/radicals will have enough energy to overcome the penetration threshold of the film
(32eV). Any excess energy that these ions/radicals possess above the penetration threshold is
transferred to thermal energy to the film. This thermal energy can be used to relax C-C sp3
bonds to the more stable C-C sp2 configuration [28], leading to a further decrease in %sp3
bonding in the film.
This relationship between %sp3 bonding and ion/radical energy is demonstrated in the results
shown in Figure 5.4. As can be seen from the figure, for the C-RT sample set, the relationship
between %sp3 bonding and RF power appears to have two distinct regions. For low RF powers
(<20W), the C-RT sample set appears to be following a similar relationship to the A-RT sample
set, but at higher RF power (>20W), the decline in %sp3 bonding in the C-RT sample set begins
to decline more rapidly. These two regions can be attributed to the two processes explained
above. At low RF powers (i.e. low ion/radical energies), ions/radicals impinging on the film
91
have only have enough energy to displace hydrogen from the film and thus indirectly decrease
the %sp3 bonding in the film. At higher RF powers (i.e. high ion/radical energies), ions/radicals
impinging on the film continue to displace hydrogen from the film but in addition ions/radicals
are now able to overcome the surface penetration threshold of a-C whereby their excess energy
transforms C-C sp3 bonds to more stable C-C sp2 bonds The reason why the A-RT sample set
never appears to enter this second region is due to the fact that the ion/radical energies rise more
rapidly on the cathode than on the anode with increasing RF power. Therefore, even at high RF
power (>20W), the ions/radicals impinging on samples held on the anode do not have enough
energy to penetrate the film and transform C-C sp3 bonds to C-C sp2 bonds.
Figure 5.4: Relationship between %sp3 bonding and RF power for C-RT and A-RT sample
sets. The curves are a guide to the eye. Note that for the C-RT sample set, there appears to
be two distinct regions in the relationship; one at low power (<20W) in which only
hydrogen displacement is occurring and one at higher powers (>20W) in which both
hydrogen displacement and film penetration is occurring. For the A-RT sample set, due to
the weaker relationship between RF power and ion/radical energy, only the hydrogen
displacement region is apparent.
0
10
20
30
40
50
60
70
80
90
0 10 20 30 40 50 60 70 80 90
%sp
3bo
ndin
g
RF Power (W)
C-RT
A-RT
hydrogen film penetration& hydrogen displacement
92
It is important to emphasize here that the relationship between hydrogen content (at. %) and
%sp3 bonding in a-C is concurrent and dynamic in nature as both of these film properties are
directly and indirectly related to film growth conditions such as ion/radical energy.
5.2.2 A-20 sample set
Growth Rate:
As presented in Section 4.2, there was a strong dependence between the growth rate and the
substrate temperature for the A-20 sample set. It was observed that as the substrate temperature
was increased, the growth rate of the film dropped significantly. This phenomenon has been
observed by other researchers [69; 70; 71], and has been attributed to a temperature dependent
etching of the growing a-C film [28]. Hydrogen radicals (i.e. atomic hydrogen) are highly
reactive and can act as an etchant for a-C when it impinges on the film. It has been reported, that
this etch rate increases with increasing temperature [28]. Therefore, even though the processes
involved in the growth of a-C film are independent of temperature, the temperature dependent
etching of the film causes increasing substrate temperature to have a net-negative effect on the
growth rate of the film. This is illustrated in Figure 5.5 [28].
93
Figure 5.5: Temperature dependent etching processes creating a net negative effect of
substrate temperature on growth rate of a-C films.
Micostructure:
As presented in Section 4.4, no trend can be seen between the hydrogen concentration and the
substrate temperature for the A-20 sample set. Thus it is necessary to look beyond hydrogen
concentration and density measurements in order to gain insight into the relationship between
substrate temperature and the percent of hydrogen (at. %) in the film.
Following the rationale provided in Section 5.2.1 for the A-RT sample set, the percent of
hydrogen (at. %) in the film can be extracted by examining the ratio of the C-H peaks in the
infrared absorption spectra. In Section 4.4, it was shown that the relative area of the CH2 sp2
peak declined with increasing substrate temperature. This indicates that the percent of hydrogen
(at. %) in the film also declined with increasing temperature.
94
This relationship between hydrogen content (at. %) with substrate temperature can be understood
through the relatively low displacement energy of hydrogen in a-C, 2.5eV. Due to the low
displacement energy of hydrogen, even at low temperatures, some of the hydrogen atoms in the
a-C film will acquire enough thermal energy to dissociate from their carbon host, diffuse, and
desorb from the film. As the substrate temperature is increased, the proportion of hydrogen
atoms that acquire this threshold energy will increase and the consequently the hydrogen content
(at. %) in the film will decrease.
This relationship between hydrogen content (at.%) and substrate temperature is partially
responsible for the decreasing trend in the percent of sp3-bonded carbon atoms (%sp3 bonding) in
the film with increasing substrate temperature presented in Section 4.5. Due to the promotion of
sp3 bonding by hydrogen, as the hydrogen content (at. %) in the film decreases, there is a
tendency for the %sp3 bonding in the film to also decrease. However, in addition to this process,
as the substrate temperature is increased, more thermal energy is provided to the growing film
which can be used to relax sp3 bonded carbon atoms to the more stable sp2 bond. Thus it is
likely that a combination of these two processes is responsible for the decrease in %sp3 bonding
in the film with increasing substrate temperature.
5.2.3 C-N and A-N Sample Sets
The C-N and A-N sample sets represent the two sample sets with nitrogen incorporation. The
only differences in the deposition conditions between these two sample sets were the placement
of the substrate and the RF power used. For the C-N sample set, the substrate was placed on the
cathode and an RF power of 5W was used, while for the A-N sample set the substrate was placed
on the anode and an RF power of 20W was used. These RF powers were chosen such that
intrinsic films deposited under the respective conditions had similar optical and structural
properties. Due to the strong dependence of ion energy on the structural properties of the film
(eg. hydrogen content, %sp3 bonding, etc.), it can be assumed that the only significant difference
in the deposition conditions between these two sample sets lies in the plasma species/density.
Therefore, by analyzing the differences in the trends between these two sample sets, the
dependence of the type and density of species in the plasma on the nitrogen content and doping
efficiency can be found.
95
Growth Rate:
As presented in Section 4.2, the trend in growth rate with increasing N2 partial pressure was quite
different for the C-N and A-N sample sets. As can be seen in Figure 5.6, even with no N2 in the
source gas, the growth rate in the A-N sample set was significantly higher than the C-N sample
set. This is to be expected due to the higher RF power and thus higher plasma density used in
the A-N sample set.
As the N2 partial pressure was increased, the growth rate in the A-N sample set increased while
the growth rate in the C-N sample set stayed near-constant. This suggests that the higher RF
power used in the A-N sample set is playing a role in determining the effect of N2 partial
pressure on the growth rate of the film. This can be explained by the fact that at a higher RF
power, the types and density of nitrogen ions and radicals will increase. The reactive species
produced at this higher RF power can provide additional bonding possibilities for the growing a-
C film and thus increase the growth rate. Therefore, increasing the N2 partial pressure can
increase the growth rate of the film, however, a threshold RF power is needed in order to
generate the necessary reactive species that will allow this effect to be seen.
Figure 5.6: Relationship between growth rate and N2 partial pressure for A-N and C-N
sample sets. The curves are intended only as a guide to the eye.
2
3
4
5
6
7
0 10 20 30 40 50 60
Gro
wth
Rat
e (n
m/m
in)
N2 in Mixing Bottle (% pressure)
A-N
C-N
96
The small drop-off in growth rate seen in the C-N sample set as the N2 partial pressure is
increased to 50% can be attributed to a change in the microstructure that was necessitated by the
increase in nitrogen content (at. %) in the film. This change in microstructure was confirmed
from AR-XPS measurements which showed a significant decrease in film density as the N2
partial pressure was increased to 50% in this sample set. As discussed in Section 5.3, this change
in microstructure is consistent with the trends in optical properties seen in this sample set.
Microstructure & Electronic Structure:
As presented in Section 4.3, the nitrogen content for both the C-N and A-N sample sets increased
with increasing levels of N2 partial pressure in the source gas. The results are repeated in Figure
5.7 for convenience. As can be seen from the figure, the rate of increase for the A-N sample set
was significantly higher than the C-N sample set. This difference can be explained through the
plasma species/density. The A-N sample set used a higher RF power than the C-N sample. This
higher RF power would lead to an increase in the plasma density and the introduction of new
reactive molecular nitrogen species in the plasma. With an increase in the density and types of
nitrogen reactive species in the plasma, the likelihood of nitrogen species impinging and bonding
on the growing film will increase and thus the nitrogen content in the film will also increase.
Figure 5.7: Increasing N content (at. %) with increasing N2 Partial Pressure (%) for A-N
and C-N sample sets.
0
2
4
6
8
10
12
14
16
18
20
0% 10% 20% 30% 40% 50% 60%
N C
onte
nt (a
t. %
)
N2 Partial Pressure (%)
A-N Sample Set
C-N Sample Set
97
The change in electronic structure has showed a dependency on the plasma density/species.
From the results presented in Section 4.7, it is evident that for the same percent of N2 in the
source gas, the charge-corrected N1s shifts and thus the Fermi level shifts were larger in the A-N
sample set than the C-N sample set however; this is at least partially due to the different nitrogen
content levels in these films. In order to more effectively compare these two sample sets, the
relationship between charge-corrected N1s shifts with nitrogen content needs to be compared.
Figure 5.8 illustrates this relationship for the two sample sets. As can be seen from the figure,
the C-N sample set shows a near-linear relationship between nitrogen content and the charge-
corrected N1s shifts, while the A-N sample set shows a near-logarithmic relationship.
Figure 5.8: Relationship between charge-corrected N1s shifts and nitrogen content for C-N
and A-N sample sets. The curves are a guide to the eye. Note that charge-corrected N1s
shifts can be taken as a qualitative measure of shifts in the Fermi level. An increase in the
charge-corrected N1s peak would represent an upward shift of the Fermi level toward the
conduction band.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 5 10 15 20
Char
ge-C
orre
cted
N1s
Shi
ft (e
V)
Nitrogen Content (at. %)
C-N Sample Set
A-N Sample Set
98
As explained in Section 3.6.4, charge-corrected N1s shifts can be taken as a qualitative measure
of shifts in the Fermi level with an increase in the charge-corrected N1s peak representing a shift
in the Fermi level toward the conduction band. Thus the differences shown in Figure 5.8
represent differences in doping efficiencies between the two sample sets. This difference in
doping efficiency can be explained through the different nitrogen species present in the plasma
of the two sample sets. As shown in Figure 5.9, only particular carbon-nitrogen bonding
configurations are potential doping configurations. One of the factors which determines the
carbon-nitrogen bonding configuration in the film is the type of molecular species in the plasma.
Of course, the types and density of reactive molecular species in the plasma are dependent on the
RF power used during deposition. The higher doping efficiency in the A-N sample set indicates
that the higher RF power used in this sample set increased the density of molecular species
necessary for carbon-nitrogen bonding in potential doping configurations.
Figure 5.9: Potential bonding configurations between nitrogen and carbon. Doping is only
possible in the configurations shown in (b), (e), and (h). Figure taken from [72].
It should also be noted that that although the microstructure of the intrinsic films formed in the
A-N and C-N sample sets are similar they are not identical. Thus it is also possible that
microstructure of films in the A-N sample set are more susceptible to doping than films in the C-
99
N sample set. Without having identical microstructures, it is not possible to know for certain
how much of a role the plasma species/density is having on the doping efficiency of the film.
100
5.3 Relationship between film microstructure and optical properties
In the previous section, trends in growth rate, microstructure, and electronic properties were
analyzed in order to define relationships between these properties and the deposition conditions
of the film. In this section, the relationship between the microstructure and optical properties of
a-C films is explored. This relationship can be best explained through Robertson’s model of a-C
films [28; 38]. Robertson describes the microstructure of a-C as a continuous network of sp3
bonded carbon atoms with the sp2 bonded carbon atoms forming small localized clusters that lie
within this network. As discussed in the previous section, the proportion of the film composed
by these sp2 clusters can vary depending on film growth conditions.
The quantity and size of the sp2 clusters play an important role in determining the film’s optical
properties. Carbon bonded in the sp3 configuration makes four strong σ-bonds with its bonding
partners, while carbon bonded in the sp2 configuration makes three strong σ-bonds and one weak
π-bond with its bonding partners. The σ-bonds form occupied σ-states in the valence band and
empty σ*-states in the conduction band, separated by a large σ- σ* gap. Similarly, the π-bonds
from the sp2 bonding form occupied π-states in the valence band and empty π *-states in the
conduction band, but separated by a smaller π - π * gap [1; 28]. This is illustrated in the
simplified density of states shown in Figure 5.10. For comparison, an experimentally measured
density of states of the valence band of an a-C sample is shown in Figure 5.11 [28].
Figure 5.10: Simplified diagram of the density of states in a-C films.
101
Figure 5.11: Photoemission spectra of a-C measured through Ultraviolet Photoelectron
Spectroscopy (UPS). The vertical axis represents the photoemission counts measured
through UPS and provides a qualitative assessment of the density of states in the valence
band with the Fermi level lying at 0eV. Note that the peak of the π-band lies closer to the
Fermi level than the peak of the σ-band. Taken from [28].
Due to their smaller gap, the π - π * states create the dominant band gap in a-C. Therefore, the
quantity and size of the sp2 clusters in the a-C microstructure will determine the optical
properties of the film.
This relationship between the sp2 content and the optical properties of the film is illustrated in
Figure 5.12 for the C-RT sample set. As can be seen, both the E04 gap and refractive index show
a strong dependency with the sp2 content as the E04 optical gap monotonically decreases and
refractive index monotonically increases with increasing sp2 bonding in the film. It is also
interesting to note that the E04 gap and refractive index show an S-shape dependency with the sp2
content with the peak dependency occurring at around 50% sp2 bonding.
102
Figure 5.12: Relationship between %sp2 bonding with E04 gap (black) and refractive index
at 350nm (grey) for C-RT sample set. The curves are a guide for the eye.
The dependency between sp2 content and the optical properties of a-C films is also seen in the A-
RT and A-20 sample sets. Figure 5.13 and Figure 5.14 show the dependence of the E04 gap and
refractive index on the sp2 content for the A-RT and A-20 sample sets, respectively. As seen
from the figures, for both sample sets, the E04 gap decreases and the refractive index increases
with increasing sp2 content. It should be noted that the refractive indices displayed in Figures
5.12 to 5.14 are for a wavelength of 350nm. The choice of this wavelength was based on the fact
that for the films with E04 gaps above 3eV, the refractive index only showed appreciable changes
at wavelengths below 400nm. Therefore in order to show the dependence between sp2 content
and the refractive index for all the films, it was necessary to show the refractive index at a
wavelength below 400nm.
1.5
1.7
1.9
2.1
2.3
1.5
2
2.5
3
3.5
4
4.5
20 30 40 50 60 70 80 90 100
n @
350
nm
E 04G
ap (e
V)
%sp2 content
103
Figure 5.13: Relationship between %sp2 bonding with E04 gap (black) and refractive index
at 350nm (grey) for A-RT sample set. The curves are a guide for the eye.
Figure 5.14: Relationship between %sp2 bonding with E04 gap (black) and refractive index
at 350nm (grey) for A-20 sample set. The curves are a guide for the eye.
1.6
1.65
1.7
3.4
3.5
3.6
3.7
3.8
3.9
4
4.1
15 20 25 30 35 40 45 50
n @
350
nm
E 04G
ap (e
V)
%sp2 bonding
1.55
1.6
1.65
1.7
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
4
32 34 36 38 40 42 44
n @
350
nm
E 04G
ap (e
V)
%sp2 bonding
104
Due to difficulties associated with overlapping nitrogen and carbon modes in the XAES spectra,
%sp2 bonding measurements were not taken on the C-N and A-N sample sets. Thus the
relationship between %sp2 bonding with the optical properties of the film cannot be directly
verified for these sample sets. However, as illustrated in Figure 5.15, it is apparent that there is a
strong relationship between the E04 optical gap and the nitrogen content (at. %) of the films. For
both sample sets, the E04 optical gap decreases with increasing nitrogen content. This
relationship is expected since increasing the nitrogen content will lead to an increase in the
density of donor states that lie within the band gap and thus lower the E04 optical gap in the film.
It should also be noted that for the C-N sample set, when the nitrogen content is increased to 9%
the trend in declining E04 gap is broken and the E04 gap shows a slight increase. This change
suggests a change in microstructure of the film related to the increase in nitrogen content. This is
consistent with the growth rate measurements analyzed in Section 5.2.3, where the C-N sample
set showed a sudden decrease in growth rate when the nitrogen content was increased beyond
9%. The fact that no discontinuity in the trends in growth rate or E04 gap are seen for the A-N
sample set suggests the microstructure of intrinsic films in this sample set have a greater capacity
for nitrogen incorporation and thus do not require a change in microstructure as the nitrogen
content is increased.
105
Figure 5.15: Relationship between E04 gap and nitrogen content for C-N and A-N sample
sets. Note for both sample sets the refractive index remained constant at 1.6. For visual
clarity the refractive index curves were not included in the figure. The curves are a guide
for the eye.
3
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
4
0 5 10 15 20
E 04G
ap (e
V)
Nitrogen Incorporation (at. %)
C-N
A-N
106
6 Applications: Transparent Heat Mirror
6.1 Overview In the previous sections, it was shown that the a-C films show a wide range in optical properties
which can be precisely tuned through film growth conditions. In addition, it was shown that
these films can be moderately doped even at room-temperature deposition. These properties
coupled with the superior mechanical properties of a-C make these films appealing for a number
of optical applications including: anti-reflection coating for Si photovoltaic (PV) cells, window
layer for thin-film heterojunction PV cells, and low-absorption surface passivation layer for c-Si
wafers.
One of the more interesting applications of a-C which has yet to be explored in depth is the use
of a-C in a multi-layer transparent heat mirror (THM) coating. In the following section, an
overview of THM coatings is provided and the application of a-C films in these coatings is
discussed. In addition, results for initial THM designs are provided and an outlook for future
work is given.
107
6.2 Background A transparent heat mirror (THM) coating is an optically selective coating which is transparent to
solar radiation but reflective for mid-infrared radiation. These coatings are used in a number of
applications with the most common being high-efficiency, low-emissive windows for residential
and commercial buildings. These low-emissive windows have been shown to significantly
reduce heating loads in homes and buildings by suppressing radiative thermal losses [73; 74].
The key in understanding THM coatings is to note that the wavelength spectrum emitted by the
sun, which can be approximated by a blackbody radiator at T~5780K, is different from the
spectrum emitted by an object at room temperature, which can be approximated by a blackbody
radiator at T~300K. This is illustrated in Figure 6.1. Notice that the peak of the solar spectrum
is between 400nm-900nm, where as the peak of the spectrum of a blackbody radiator at 300K is
between 5μm-20μm. Therefore, an effective THM coating can be made by designing it such that
it has a high transmittance from 400nm-900nm and a high reflectance from 5μm-20μm.
Figure 6.1: Normalized spectral emissive power of a blackbody radiator @ 5780K and
blackbody radiator @ 300K. Note that the solar spectrum can be approximated by a
blackbody radiator @ 5780K.
108
One method of designing a THM is to sandwich a thin-layer of a transition metal (eg. Au, Ag,
Cu, etc.) between films with low absorption coefficients (low-k) and appropriate refractive
indices. This is depicted in Figure 6.2. The thin-metallic layer provides the required reflection
in the mid-infrared while the low-k layers act as anti-reflective layers thereby enhancing the
transmission in the solar region. Additional requirements of the low-k film are that it be non-
absorbing in the mid-infrared to allow reflection from the thin-metallic layer and also have a
high hardness so that it can prevent scratches and oxidation to the metallic film.
Figure 6.2: Overview of multi-layer transparent heat mirror design.
Since most wide band gap materials cannot satisfy all the specified requirements, typically
multiple layers of different materials are used. For example, the state-of-the-art THM coating
developed by Martin-Palma et al. uses a five-layer SnO2 (38nm)/NiCr (1nm)/Ag (9nm)/NiCr
(3nm)/Sn02 (38nm) structure [75]. In this structure, the Ag layer provides the reflection in the
mid-infrared while the NiCr layers act as an oxygen-free protective layer for the Ag and the Sn02
layers act as anti-reflective layers to maximize transmissivity in the solar region. The NiCr and
SnO2 layers also provide a small boost to mid-infrared reflection of this structure. The
transmission and reflection spectra for this structure are shown in Figure 6.3 and Figure 6.4.
109
Figure 6.3: Infrared reflectance of state-of-the-art SnO2 (38nm)/NiCr (1nm)/Ag (9nm)/NiCr
(3nm)/Sn02 (38nm) structure. Taken from [75].
110
Figure 6.4: Transmittance of state-of-the-art SnO2 (38nm)/NiCr (1nm)/Ag (9nm)/NiCr
(3nm)/Sn02 (38nm) structure. Taken from [75].
While this structure achieves the necessary properties for a THM, the use of five-layers made
using three different materials creates an expensive and complicated manufacturing process.
The motivation for exploring the application of a-C for THM coatings is based on the unique
combination of superior mechanical and tunable optical properties that these films possess.
With this combination, a-C films can act as both the anti-reflective and protective layers in a
THM structure. Moreover, it has been shown in other studies that a-C films are highly
transparent in the mid-infrared and thus suitable for THM coatings [30; 76]. It should be noted
that another study [76] has explored the use of a-C in a THM coating however, in this study the
a-C coating was deposited using sputtering. The a-C based THM studied in this thesis used
PECVD a-C which has the advantage of having tunable optical and structural properties. This
tunability is key as it allows a-C films with optimal optical properties to be used to create a high
quality THM coating.
111
6.3 Experimental The THM design that was explored in the present study was a three-layer a-C/Ag/a-C structure.
The THM structure was deposited onto fused silica glass substrates. The choice of Ag as the
metallic layer was based on transmission studies performed elsewhere which showed that Ag has
a lower absorption coefficient in the visible region than the other transition metals [77].
The thicknesses and deposition conditions of each layer are summarized in Table 6.1. The
thickness of the Ag layer was based on the minimum thickness needed to achieve a uniform
layer. The refractive index and thicknesses of the a-C layers were selected in order to achieve a
continuous admittance profile through an air/a-C/Ag/a-C/glass optical system. The details of the
admittance method for designing a THM are described elsewhere [78].
Table 6.1: Experimental details for deposition of three-layer THM structure
Layer Thickness (nm) Deposition System Deposition Conditions
a-C 51.6 RF PECVD RF Power: 10W
Substrate: Cathode
Temperature: Ambient
Pressure: 60mTorr
Flow: 20sccm
Ag 20 E-beam Growth Rate: 1.5A/s
a-C 55.1 RF PECVD RF Power: 10W
Substrate: Cathode
Temperature: Ambient
Pressure: 60mTorr
Flow: 20sccm
Transmission measurements in the visible region were performed using a UV-Visible
spectrometer while the reflectivity in the infrared was measured using Fourier Transform
Infrared spectroscopy.
112
6.4 Results Reflection measurements in the infrared and transmission measurements in the visible region are
shown in Figure 6.5 and Figure 6.6, respectively. As can be seen from the figures, the a-C/Ag/a-
C/glass structure had a reflection of about 95% in the infrared while the transmission in the
visible peaked at about 50%. While the reflection in the infrared is on par with the state-of-the-
art structure developed by Martin-Palmer et al., the transmission in the visible is significantly
lower.
Figure 6.5: Infrared reflection of a-C/Ag/a-C/glass THM optical system. Note that the
spectra for Ag/glass and glass are shown as references.
0
10
20
30
40
50
60
70
80
90
100
5 10 15 20 25
Refle
ctio
n (%
)
Wavelength (µm)
Ag/glass
a-C/Ag/a-C/glass
glass
113
Figure 6.6: Visible transmission of a-C/Ag/a-C/glass THM optical system. Note that the
spectra for Ag/glass and glass are shown as references.
By comparing Figure 6.6 with Figure 6.4, it is apparent that the dampened transmission in the
visible region can be attributed to our Ag layer. As can be seen from the figures, the
transmission of the Ag/glass reference that Martin-Palmer et al. deposited had a transmission that
was 20% to 30% higher than the transmission of our Ag/glass reference. This difference in the
transmission of the Ag layers can be explained by the different thicknesses that were used as
Martin Palmer et al. used a Ag layer with a thickness of 9nm while the thickness of our Ag layer
was 20nm. We were unable to reduce the thickness of our Ag layer since our attempts to do so
would lead to Ag layers that would consist of non-uniform islands which would not produce the
necessary reflection in the infrared.
The role of the thickness of the Ag layer in determining the visible transmission of the a-C/Ag/a-
C/glass THM was confirmed through optical simulations. As shown in Figure 6.7, by reducing
the Ag thickness, the visible transmission of the a-C/Ag/a-C/glass THM can be significantly
increased. In fact, by comparing Figure 6.7 with Figure 6.4, it can be seen that by reducing the
Ag thickness to 10nm, the visible transmission of the a-C/Ag/a-C/glass THM is actually about
5% higher than the state-of-the-art THM.
0
10
20
30
40
50
60
70
80
90
100
400 450 500 550 600 650 700
Tran
smis
sion
(%)
Wavelength (nm)
glass
a-C/Ag/a-C/glass
Ag/glass
114
This increase in visible transmission can be explained by the fact that the state-of-the-art THM
requires the Ag layer to be sandwiched by NiCr layers to provide protection against scratches
and oxidation. Since a-C is able serve the dual role of being both an anti-reflective and
protective layer, these highly absorbing NiCr layers are not required in the a-C based THM and
thus the a-C based THM is able to achieve a higher transmission in the visible region.
Figure 6.7: Visible transmission of a-C/Ag/a-C/glass THM using different Ag layer
thicknesses. The “Experimental” curve represents experimental measurements done on
the a-C/Ag(20nm)/a-C/glass THM that was fabricated. The “Model: 20nm Ag” curve
represents the simulated transmission for an a-C/Ag(20nm)/a-C/glass THM. The “Model:
10nm Ag” curve represents the simulated transmission for an a-C/Ag(10nm)/a-C/glass
THM.
The focus of future research on the a-C/Ag/a-C THM will be on developing a method for
depositing a thinner uniform layer of Ag. One potential method of achieving this goal is to use
115
magnetron sputtering to deposit the Ag layer as it has been reported in other studies that a
uniform layer of Ag of less than 10nm can be grown by using this method [67].
116
7 Conclusions and Future Work
7.1 Conclusions A fundamental study defining relationships between growth conditions, electronic,
microstructural, and optical properties of a-C films has been carried out. The key results of this
research are summarized below:
I. Role of ion/radical energy on film microstructure: It was found that both the hydrogen
content (at. %) and the percent of sp3-bonded carbon atoms decreased as the ion/radical
energy used during film deposition was increased. The decrease in hydrogen content
with increasing ion/radical energy was attributed to the low displacement energy of
hydrogen in a-C (2.5eV). The decline in the percent of sp3-bonded carbon atoms with
increasing ion/radical energy was observed to occur in two stages. At low ion/radical
energies, the percent of sp3-bonded carbon atoms had a moderate decline with increasing
ion/radical energy while at high ion/radical energies the decline became rapid. In the low
ion/radical energy region, the decline in the percent of sp3-bonded carbon atoms was
attributed to its dependence on hydrogen content. Since bonding with a sp3-hybridized
carbon atom represents a relatively low energy state for hydrogen in a-C, a co-
dependence between hydrogen content and the percent of sp3-bonded carbon atoms in a-
C develops, whereby a decline in hydrogen content leads to a decline in sp3 bonding. In
the high ion/radical energy region, the dependence between hydrogen content and the
percent of sp3-bonded carbon atoms continues, however, a second process also comes
into factor. As the ions/radical energy is increased beyond a threshold, ions/radicals are
able to overcome the surface penetration threshold of a-C (32eV) whereby their excess
energy transforms C-C sp3 bonds to more stable C-C sp2 bonds.
II. Role of substrate temperature on film microstructure: The substrate temperature was
found to have a weaker dependence than ion/radical energy on the film microstructure.
By increasing the substrate temperature, a decline in hydrogen content (at. %) and a
decline in the percent of sp3-bonded carbon atoms was found. This relationship between
substrate temperature and film microstructure was attributed to the thermal energy
provided to hydrogen atoms in the film by increased substrate temperature. As hydrogen
117
atoms develop enough thermal energy they dissociate from their carbon host, diffuse, and
desorb from the film. The coupling between hydrogen content (at. %) and sp3 bonding
leads to the decline in the percent of sp3-bonded carbon atoms in the film.
III. Relationship between optical properties and film microstructure: It was determined that
the optical properties of a-C films are primarily controlled through the carbon sp2/sp3
bonding ratio. This was attributed to the fact that the sp2-bonded carbon atoms introduce
π and π* states which lead to an effective decrease in the optical gap of the film. By
regulating the ion/radical energy and substrate temperature, the sp2/sp3 bonding ratio and
thus the optical properties of the a-C film can be precisely controlled over a wide range of
values. E04 optical gaps from 1.7eV to 4.0eV and refractive indices at 550nm from 1.6 to
2.2 were demonstrated.
IV. Doping of a-C: By introducing N2 in the source gas, it was shown that nitrogen acts as an
n-type dopant to hydrogenated polymeric-like a-C (PLC:H) films. This was the first time
nitrogen doping of PLC:H films has been demonstrated. By comparing Fermi level shifts
detected from XPS measurements, it was found that the plasma density was playing a
role in determining the doping efficiency of these films: films produced at a higher
plasma density showed a higher doping efficiency.
The precise control of the optical properties of a-C films demonstrated in this work led to several
applications of a-C films for solar collection devices being suggested. One of these potential
applications, a-C/Ag/a-C transparent heat mirrors (THM), was explored in depth. The a-C/Ag/a-
C THM is promising as it provides a simpler, more cost-effective alternative to the state-of-the-
art THM which is a five-layer SnO2/NiCr/Ag/NiCr/Sn02 structure. Moreover, it was
demonstrated that the a-C/Ag/a-C THM provided enhanced transmission in the visible region
when compared with the state-of-the-art five-layer structure. The primary advantage of the a-C
based THM structure is that owing to its excellent mechanical properties and tunable optical
properties, the a-C film can act as both the protective layer and anti-reflective layer for a THM
structure. Thus the a-C based THM can use fewer layers which allows for simpler fabrication
and improved transmission in the visible region.
118
7.2 Future Work The fundamental study completed in this thesis clearly defines relationships between the growth
conditions, electronic, microstructural, and optical properties of a-C films, opens up numerous
potential applications for a-C coatings. In particular, the demonstration of n-type doping and the
precise control of the optical properties of these films over a wide range of optical gaps and
refractive indices are particularly attractive for solar energy applications. The following is a
description of new research projects that have emerged based on the work completed in this
thesis:
I. Transparent Heat Mirror (THM) coatings: As described in this thesis, a-C THM
coatings are a promising alternative to the five-layer state-of-art THM coatings. The
main focus of future work will be to develop a method of depositing thinner uniform Ag
layers. By reducing the thickness of the Ag layer the transmission of the a-C/Ag/a-C
THM coating should reach 80% in the visible region. One potential method of reducing
the Ag layer thickness while maintaining a uniform surface includes depositing a thick
layer of Ag (eg. 20nm) and subsequently attempting to uniformly etch the layer down.
Another potential method includes exploring other deposition techniques for Ag
including sputtering and electrochemical deposition.
II. Anti-reflective coatings for hydrogenated amorphous silicon – crystalline silicon
heterojunction solar cells: The demonstration of the precise control of the optical
properties of a-C films over a wide range of refractive indices opens up the potential of
applying a-C films as an anti-reflective coating for heterojunction solar cells. State-of-
the-art anti-reflection coatings for high efficiency solar cells use a MgF2/ZnS bi-layer.
By controlling the refractive index of the a-C film during film growth an a-C bi-layer or
graded-layer can be grown. This a-C bi-layer would be a significantly simpler and more
cost-effective alternative to the MgF2/ZnS bi-layer as it can be grown in a single
processing step using the same deposition facilities used for hydrogenated amorphous
silicon films.
III. Amorphous carbon – crystalline silicon heterojunction solar cells: The demonstration of
n-type doping of hydrogenated polymeric-like amorphous carbon (PLC:H) opens up the
119
possibility for these films to be considered in a-C – crystalline silicon heterojunction
solar cells. The ability to tune the optical gap in a-C makes these films appealing as
active layers for solar cells. In order to investigate the potential of this solar cell
structure, the passivation properties of PLC:H on crystalline silicon need to be studied.
In addition, depositions at higher plasma densities should be investigated to determine if
the doping efficiency of these materials can be increased further.
120
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