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2013 Master Thesis
A novel Ohmic contact for AlGaN/GaN using
silicide electrodes
Supervisor
Professor Hiroshi Iwai
Department of Electronic and Applied Physics
Interdisciplinary Graduate School of Science and Engineering
Tokyo Institute of Technology
11M36260
Kana Tsuneishi
2
Contents
Chapter 1 Introduction
1.1Power device and requirement...........................................................................................5
1.2Feature of GaN and challenge............................................................................................7
1.3Common Ohmic contact process for AlGaN/GaN...............................................................8
1.4Purpose of this study ....................................................................................................... 11
Chapter 2 Fabrication and Characterization
2.1 Fabrication process ........................................................................................................15
2.2 Experimental principle...................................................................................................16
2.2.1 SPM cleaning and HF treatment ..................................................................................16
2.2.2 SiO2 passivation film by plasma-TEOS.........................................................................16
2.2.3 RF magnetron sputtering .............................................................................................16
2.2.4 PMA in N2 ambient......................................................................................................18
2.2.5 Dry etching by RIE......................................................................................................18
2.2.6 Wet etching with BHF..................................................................................................18
2.2.7 Transmission electron microscopy ................................................................................18
2.2.8 X-ray photoelectron spectroscopy ................................................................................20
2.2.9 Atomic force microscope ..............................................................................................21
2.3 TLM evaluation .............................................................................................................21
2.4 Electrical characteristics ................................................................................................23
2.4.1 J-V characteristics .......................................................................................................23
2.4.1.1 Thermionic emission .................................................................................................23
2.4.1.2 Image force inducted barrier lowering ......................................................................24
2.4.1.3 Field emission and Thermionic Field emission ...........................................................25
2.4.1.4 Thin surface barrier model .......................................................................................28
3
Chapter 3 NiSi2/Ni3P contact electrodes
3.1 Introduction ...................................................................................................................34
3.2 XPS analysis ..................................................................................................................35
3.3 TEM analysis .................................................................................................................37
3.4 Current-Voltage characteristics ......................................................................................37
3.5 Conclusion .....................................................................................................................40
Chapter 4 TiSi2 contact electrodes
4.1 Introduction ...................................................................................................................42
4.2 XPS analysis ..................................................................................................................42
4.3 AFM analysis .................................................................................................................43
4.4 Current-Voltage characteristics ......................................................................................44
4.5 Inducting C-V characteristics by I-V charactristics .........................................................46
4.6 Conclusion .....................................................................................................................47
Chapter 5 Conclusion
5 Conclusions ......................................................................................................................49
Acknowledgments................................................................................................................50
4
Chapter 1 Introduction
1.1 Power device and requirement
1.2 Feature of GaN HEMT and challenge
1.3 Common contact process for AlGaN/GaN
1.4 Purpose of this study
References
5
1.1Power device and requirement
Power device is used to power conversion which is shown to convert voltage, current,
frequency, coherent, number of phase and wave shape. Figure 1.1 shows relation of
frequency and switching capacity where power device is applied to electric home
appliance (microwave, electric fan and laundry machine) and industrial field (railroad
and robot weld) a wide range of fields. At current, power device is dominant Silicon (Si)
material with conversion efficiency of number of about 94% [1.1]. The reasons are
satisfactorily formed insulating film and high technology of crystal growth and
processing. However, conversion efficiency of power is a need to improve in order to
achieve further lower loss. Incidentally, the loss is determined by conduction loss and
switching loss. Silicon carbide (SiC) and gallium nitride (GaN) of new materials are
studied for using with power device that are expected to lower conduction loss from
character of material. Table 1.1 shows material parameter of Si, SiC and GaN [1.2].
Equation 1.1 shows the ideal specific on-resistance [1.3].
3
2
BDe
B
E
VR
(1.1)
Calculated SiC and GaN Ron related to conduction loss is lower loss than Si. Thus, they
can be expected to lower loss device.
6
103 104 105
100
101
102
103
104
Frequency (Hz)
Sw
itchi
ng C
apac
ity (
kVA
)
SiBipolar
IGBT
IPM/IGBT module
Si MOSFET
GTO
Thy
risto
r
HVDC
Railroad
UPS,DPS
EV/HEVRobot weld
Medical device
Air conditioner
Electric cooker
Switching Regulators
VTR, mobile phone
FPD driverMicrowave,Electric fan,laundry machine
Figure 1.1 Frequency and switching power device
Table 1.1 Material parameter of Si, SiC and GaN
Si 4H-SiC GaNEg (eV)Band-gap energy
1.1 3.2 3.4
e (cm2/Vs)mobility
1500 900 ~2000HEMT
specific inductive capacity
11.9 9.7 10
EBD (V/cm)breakdown field
3.0x105 2.5x106 3.0x106
(W/cmK)thermal conductivity
1.5 4.9 2.3
sat (cm/s)saturated velocity 1.0x107 2.2x107 2.7x107
Si 4H-SiC GaNEg (eV)Band-gap energy
1.1 3.2 3.4
e (cm2/Vs)mobility
1500 900 ~2000HEMT
specific inductive capacity
11.9 9.7 10
EBD (V/cm)breakdown field
3.0x105 2.5x106 3.0x106
(W/cmK)thermal conductivity
1.5 4.9 2.3
sat (cm/s)saturated velocity 1.0x107 2.2x107 2.7x107
7
1.2Feature of GaN and challenge
GaN has a larger band gap (3.4eV), larger saturation velocity (2.7×107cm/s), higher
thermal stability (2.3W/cmK), break down field (3.0×106V/ cm), and hetero field-effect
(HFET) can be high mobility (2100cm2/ Vs) in power devices as table 1.1.
Feature of GaN are made for hetero electron mobility transistor (HEMT) and can be
lateral power device structure. AlGaN/GaN HEMT is the ability to achieve
two-dimensional electron gases (2DEG) with sheet carrier concentration of 1013cm-2 or
higher close to the interface without intentionally doping. It has been shown previously
that piezoelectric effects can exert a substantial influence on charge density and
semiconductors grown in the (111) orientation. There is the problem that AlGaN is
stressed substrate direction which involve different thickness both center and edge.
Although, baffer layer can be glowed gradient technique, 8-inch epitaxial wafer is made
on Si (111) substrate. Being lager diameter substrate can be mass production once. In
conclusion it leads to reduction in cost.
Although, it is just case that must be forward a study for practical use form now on
challenging.
Another feature of GaN is lateral power device structure which can drift current lateral
direction. As a result, on resistance can be lower commonly vertical power device.
One of the issues of AlGaN/GaN HEMT is specific contact resistivity (con). Theoretical
on-resistance (Ron) limit exists for the lateral power device, which shows table1.1. The
specific on-resistance (RonA) is given by the sum of Rdrift, Rch, Rcon and cell length Lcell
as follow :
)2)((
)(
conchdriftconchdrift
cellconchdrifton
LLLRRR
LRRRAR
(1.2)
8
con
condrainsourcecon L
RRR2
Considering minimizing of the specific on-resistance, the optimized contact length Lcon
is given as follow:
chdrift
chdriftconcon RR
LLL
)(
In conclusion, the optimized contact length is proportional to the specific contact
resistivity of route, which needs to minimize somehow in this theory [1.2].
Drainsourse
Gate
Field plate
Lcon LchLdrift Lcon
Figure 1.2 Lateral power device structure
1.3Common Ohmic contact process for AlGaN/GaN
In this chapter, common Ohmic contact process of two examples is described. One the
common Ohmic contact process is Au/metal (Mo or Ni)/Al/Ti structure. Annealing
condition is a rapid thermal annealing system at optimum conditions of 850oC in N2
ambient for 30s, which is shown to figure 1.3 that transmission electron microscopy
(TEM). Excellent specific contact resistivity is 4.7×10-7cm2. High temperature
annealing result both intermixing of the metals and metal agglomeration. The
(1.3)
(1.4)
9
minimization of interfacial energy is believed to be the driving force for the Mo baling
up. And one type is the discrete TiN islands formed preferentially along threading
dislocations. The contact mechanism is shown figure 1.4 that two competing pathways
for electrons to be transported from the 2DEG to metal contacts, tunneling through
AlGaN layer and the direct conduction through TiN protrusions. This contact method
has problem that metal agglomeration and increasing threading dislocation which
concern large leak current causing [1.4,1.5]. Another the common Ohmic contact
process is inserted Si3N4 passivation layer in metal/AlGaN. Annealing condition is a
rapid thermal annealing at 800oC in N2 ambient for 90s, which is shown to figure 1.5
TEM. It seems that AlN as appointing allows. The formation of pseudo-morphic AlN at
the interface is key factor for the Ohmic contact behavior. As the contact is Ohmic, even
with an intact AlGaN has been created. The mechanism is arising 800 oC and more, Al
react with AlGaN to extract N and form N vacancy rich AlN layer. The deactivation of
Ti-AlGaN/GaN reaction has its origin in the introduction of Si3N4 passivation layer,
which acts as N source to Ti. Excellent specific contact resistivity is about 10-5order
cm2. AlN layer is sensitive annealing temperature, which need to be stable extraction
process of N from AlGaN layer [1.6].
Figure 1.3 TEM images that Au/Mo/Al/Ti after 850oC in N2 ambient for 30s
10
Annealing850oCN2 for 30s
Threading dislocates
AlGaN
GaN
Au/Mo/Al/Ti TiNAu/Mo/Al/Ti
Current
Figure 1.4 Conduction mechanism for Au/Mo/Al/Ti contacts
VN-rich AlN
AlGaN
GaN
Figure 1.5 TEM images that Au/Mo/Al/Ti/Si after 800oC in N2 ambient
N N
AlGaN
Au/Mo/Al/Ti
VN-rich AlNformation
GaN
Si or SiNxVN
VN VN
VN
anneal
Passivation layerCurrent
Figure 1.6 Conduction mechanism for Au/Mo/Al/Ti/Si contact
11
1.4Purpose of this study
In this thesis, novel methods are investigated by two approaches. This chapter, each
chapter is described about construction and contents of this thesis.
In chapter 1, power devices requirement, feature of GaN and challenging, commonly
Ohmic contact process are described as introduction.
In chapter 2, fabrication process and characterization methods are described, which
relate to chapter 3 and chapter 4 device fabrication process and result.
In chapter 3, inserted Ni3P in NiSi2 metal and AlGaN/GaN substrate is deliberation
result as contact electrodes. Inserted Ni3P is investigated by electronic characteristics
and physical analysis.
In chapter 4, TiSi2 metal approached using to contact electrodes which are investigated
by electronic characteristics and physical analysis.
Finally, in chapter 5 summarizes in this study.
Figure 1.7 shows the contents of this thesis, which is consisted of 5 parts.
Chapter1Introduction
Chapter2Fabrication and Characterization
Chapter3NiSi2/Ni3P contact
electrodes
Chapter5Conclusion
Chapter4TiSi2 contact
electrodes
Figure 1.7 Contents of this thesis
12
References
[1.1] “Substrates for GaN Gased Devices: Performance Comparisons and Market
Assessment” (2006)
[1.2] W. Saito, I. Omura, T. Ogawa and H. Ohashi, “Theoretical limit estimation of
lateral wide band- gap semiconductor power-switching device”, Solid-Stage Electronics
48 (2004) p. 1555-1562
[1.3] B. JAYANT BALIGA, “Power Semiconductor Device Figure of Merit for
High-Frequency Applications”, IEEE ELECTRON DEVICE LETTERS, VOL10,
NO.10 (1989)
[1.4] Liang Wang and Fitih M. Mohammed, “Differences in the reaction kinetics and
contact formation mechanisms of annealed Ti/Al/Mo/Au Ohmic contacts on n-GaN”,
JOURNAL OF PHYSICS, 101(2007)p.013702
[1.5] Liang Wang, Fitih M. Mohammed and Ilesanmi Adesidaa, “Dislocation -inducted
nonuniform interfacial reactions of Ti/Al/Mo/Au ohmic contacts on AlGaN/GaN
heterostructure”, Applied Physics Letter 87 (2005) p. 141915
[1.6] B. Van Daele and G. Van Tendeloo, “Mechanism for Ohmic contact formation on
Si3N4 passivated AlGaN/GaN high-electron-mobility transistors”, Applied Physics
Letter, 89 (2006) p. 201908
13
Chapter 2 Fabrication and Characterization
2.1 Fabrication process
2.2 Experimental principle
2.2.1 SPM cleaning and HF treatment
2.2.2 SiO2 passivation film by plasma-TEOS
2.2.3 RF magnetron sputtering
2.2.4 PMA in N2 ambient
2.2.5 Dry etching by RIE
2.2.6 Wet etching with HCl and BHF
2.2.7 Transmission electron microscopy
2.2.8 X-ray photoelectron spectroscopy
2.2.9 Atomic force microscope
2.3 TLM evaluation
14
2.4 Electrical characteristics
2.4.1 J-V characteristics
2.4.1.1 Thermionic emission
2.4.1.2 Image force inducted barrier lowering
2.4.1.3 Field emission
2.4.1.4 Thermionic-field emission
2.4.1.5 Thin surface barrier model
References
15
2.1 Fabrication process
Figure 2.1 shows contact device fabrication process. Wafers used in this study consist
of 26-nm-thick undoped Al0.25Ga0.75N on 1.3-�m-thick GaN, AlN/GaN layer stack,
epitaxially grown on Si (111) substrate. Hard mask is made by Plasma-TEOS, and then
Device isolation by RIE with Cl2. Oxide passivation is made by Plasma-TEOS. Contact
opening by buffered HF. Metal deposited by Supatter. Ti was formed 50nm-thick as
reference. TiSi2 was formed by cyclic deposition of 16-sets of Ti and Si layers,
20-nm-thick TiSi2 was formed. Similarly, Ni3P deposited 0.63-nm-thick before TiSi2. In
addition, NiSi2 was formed by cyclic deposition of 16-sets of Ni and Si layers,
20-nm-thick NiSi2 was formed. Similarly, Ni3P deposited 0.63-nm-thick before NiSi2.
Every TiN cap was formed 50nm-thick. Annealing was conducted in N2 ambient.
Current-voltage characteristics were measured through TLM patterns, where the current
pass through the two contacts.
Figure 2.1Contact device process and structure
i-Al0.25Ga0.75N (26nm)/i-GaN(1.3m)/buffer(AlN/GaN layer stack)/Si(111)
Chemical cleaning (SPM,HF)
Annealing in N2
Device isolation (RIE with Cl2)
Oxide passivation (plasma-TEOS)
Contact opening (Buffered HF)
Metal patterning (RIE with Cl2)
Metal deposition (Sputtering)・TiN(15nm)/Ti(60nm) [reference]・TiN(45nm)/TiSi2(20nm)・Ti(60nm) [reference]・NiSi2(10nm)・NiSi2(10nm)/Ni3P(0.1nm)
Current-voltage measurements
AlGaN
Buffer layer
GaN
Si sub.
d
SiO2
Electrode metal material・TiN(15nm)/Ti(60nm) [reference]・TiN(45nm)/TiSi2(20nm)・Ti(60nm) [reference]・NiSi2(10nm)・NiSi2(10nm)/Ni3P(0.1nm)
16
2.2 Experimental principle
In this section, in details of fabrication process describing in chapter 2.1 is explained
about some principle.
2.2.1 SPM cleaning and HF treatment
In the purpose of SPM cleaning and HF treatment are cleaning substrate surface. SPM
cleaning is one of the effective cleaning methods to degrease organic substance. HF
treatment is used to degrease oxidation products. Al0.25Ga0.75N/GaN substrate surface is
coated by organic resist film in order to protect from particles. The sample is degrease
using 1% HF treatment and Liquid solution which is made from H2SO4 and H2O2
(H2SO4: H2O2= 4:1). In this study, SPM cleaning is used on heating at 180oC for
Al0.25Ga0.75N/GaN substrate surface.
2.2.2 SiO2 passivation film by plasma-TEOS
Plasma chemical vapor deposition (CVD) is used Tetraethyl orthosilicate (TEOS) films
formation. Feature of CVD is formed using a rapid chemical reaction of TEOS at under
200oC temperature.
2.2.3 RF magnetron sputtering
Radio frequency (RF) magnetron sputtering is used to deposit metal (Ti, TiN, TiSi2,
NiSi2, Ni3P). Sputtering is one of the vacuum processes used to deposit thin films on
17
substrates. A high voltage across a low pressure gas is applied to become “plasma”
which is consisted of electrons and Ar gas ions in a high-energy state. After, the
accelerated plasma ions strike the “target” which is sputtering metal seed (Si, Ti, Ni,
Ni3P) as shown in figure 2.2. In this study, Ti, TiN, TiSi2, NiSi2and Ni3P that contact
electrode was deposited by sputtering process.
shutter
W
substrate
PlasmaAr+Ar+
W
WWW
Ti
Ti
TiTi
Ti
Figure 2.2 Illustration of sputtering
18
2.2.4 PMA in N2 ambient
Post metallization annealing (PMA) is needed to recover arising from processing
defects. In this study, different annealing conditions annealing temperature and time are
examined.
2.2.5 Dry etching by RIE
Reactive ion etching (RIE) is used to both AlGaN etching and metal contact patterning.
Feature of RIE etching can be formed uniformly.
2.2.6 Wet etching with BHF
Buffer hydrogen fluoride (BHF) is used to SiO2 etching process. BHF etching can be
easily in a short time SiO2 layer.
2.2.7 Transmission electron microscopy
TEM is one of the electron microscopes. By irradiating electrons to the thin sample,
some electrons are scattered and others are transmitted. Because amount of transmitted
electrons depends on the structure or component of each portion, the specimen shape
and surface structure in addition to information of the internal material which is the
degree of cohesion, crystalline patterns, presence of lattice defect, and such as
orientation directions of the crystal can be known by observing the internal structure of
the sample. Typically a TEM consists of three stage of lens as shown figure 2.3. The
19
stages are condenser lenses are responsible for primary beam formation, whilst the
objective lenses focus the beam that comes through the sample itself. The projection
lenses are used to expand the beam onto the fluorescent or other imaging device, such as
film. In this study, NiSi2/AlGaN/GaN structure cross section surface image were
observed by TEM.
Figure 2.3 Illustration of TEM system
20
2.2.8 X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy (XPS) is one of the most effective method of
determining the elements, which composing the sample. XPS spectra are obtained by
irradiating a material with a beam of X-rays while simultaneously measuring the kinetic
energy (Ek) and number of electrons, which escape from the material being analyzed.
The relation of energies as follow:
(2.1)
Where h is energy of the x-ray, Ek is the kinetic energy of the emitted electron and Eb is
binding energy of emitted electron. Eb is determined by measuring Ek and his incident
X-ray energy, which is constant. Eb is observed to energy peak, which is determined by
composition of sample. In this study, the chemical composition of the sample was
measured by hard XPS at Spring-8 BL46XU as shown in figure 2.4. The mean free path
of excited electrons is about 10-nm thick. It is observed that NiSi2/AlGaN and
TiSi2/AlGaN interface reaction [2.1]
Figure 2.4 Illustration of XPS system
bk EEhv
21
2.2.9 Atomic force microscope
Atomic force microscope (AFM) is one of scanning tunneling microscope, which can
observed to surface state. AFM is detected to interatomic forces between prove and
sample. The probe is contacted to sample surface of weak force, which is controlled by
frequency, I or P gain to constant. After, it is observed to surface figure image. In this
study, it is observed AlGaN/GaN interface state.
2.3 TLM evaluation
Transmission line model (TLM) is used to specific contact resistivity (C) evaluation.
Figure 2.5 shows lateral contact resistance structure in top view (a) and side vies (b). As
shown in figure 2.5 (a), many squares are contact, wide length and contact gap are W, d
respectively. The contact gap is different distance, for example, d = 50, 100, 150, 200,
250, 300 m. In figure 2.5 (b), contact resistance and sheet resistance are Rc, Rsh .When
two-terminal contact resistance is measured total resistance (RT) from inter, by passing a
current I through the sample and measuring the voltage V across the two contacts as
follow:
(2.2)
Figure 2.6 shows total resistance-contact gap relation. When total resistance is zero (RT
(d = 0) =0), transfer length (LT) is obtained as follow:
(2.3)
shconT RW
dRR 2
2
dLT
22
The voltage is highest near the contact edge x=0 and drops nearly exponentially with
distance. ” 1 / e ” distance of voltage curve is defined as follow:
(2.4)
The transfer length can be thought of as that distance over which most of the current
transfer from the semiconductor into the metal or from the metal into the semiconductor.
The TLM model was later extended to two dimensions by the dual- level transmission
line model with the current allow to flow perpendicularly to the contact interface. A
comparison between the simple and the revise TLM shows a maxim contact resistance
deviation of 12% [2.2].
W
d
Rc
Rshn-type
p-type
Figure 2.5 Lateral contact resistance structure
(a) Top view
(b) Cross section
sh
CT R
L
23
d(m)
2Rc
RT(
)
2LT
Figure 2.6 Total resistance-contact gap relations
2.4 Electrical characteristics
In this section, it is described that considering owing metal-semiconductor contact.
Thermionic emission, field emission, thermionic field emission and deep donors related
to nitrogen vacancy are suggested to be the origin of surface donors producing thin
surface barrier model.
2.4.1 J-V characteristics 2.4.1.1 Thermionic emission
Figure 2.7 shows the flow of electron in an n-type Schottky diode that thermal
equilibrium (a) and applied forward bias (b) respectively. Bn is Schottky barrier height,
Ef is Fermi level, Ec is conduction band, bi is built in potential respectively. Showed
black dot of electron can transport upper barrier height which direction is shown arrows.
24
In figure 2.7 (a), there is thermal equilibrium, there is an equal and opposite flow of
electron. In figure 2.7 (b), caused Vapp is applied voltage, Fermi level Ef is moved upper
direction. Thermionic emission theory current density for Schottky diode as follow:
(2.5)
Where A* is effective Richardson constant, T is the absolute temperature, q is electronic
charge, Bn is Schottky barrier height, k is Bolzmann’s constant, Vapp is applied voltage
and n is ideality factor, which is related to the slope. Bn and n is obtained by fitting J-V
characteristics [2.3].
Metal Silicon(n-type)
qBn qbi
Ef Ef
Ec
Metal Silicon(n-type)
qBn qbiqVapp
Ef
Ef
Ec
qVapp
(a) Thermal equilibrium (b) Applied forward bias
Figure 2.7 Illustrate of the flow of electrons schematic energy-band diagrams
2.4.1.2 Image force inducted barrier lowering
Image-force barrier is interaction due to the polarization of the conducting electrodes
1exp* 2
nkT
qV
kT
qTAJ appBn
25
by the charge atoms of the sample. The image-force effect causes the energy barrier for
electron transport across metal-semiconductor interface to be lowered by
(2.6)
Where E is electric field and si is permittivity of Si. The actual energy barrier for
electron transport in Schottky barrier diode is (qBn -qimage) [2.3].
2.4.1.3 Field emission and Thermionic Field emission
Figure 2.8 shows referring to the energy band diagrams that forward bias (a) and
reverse bias. Thermionic emission (TE) over the barrier, field emission (FE) near the
Fermi level, and termionic-field emission (TFE) at an energy between TE and FE.
While FE is a pure tunneling process, TFE is tunneling of thermally excited carriers
depend on both temperature and doping level. A rough criterion can be set by comparing
the thrmal energy kT to E00 which is defined as
(2.7)
When kT >> E00, TE dominates and the original Schottky-barrier behavior prevails
without tunneling. When kT << E00, FE (or tunneling) dominates. When kT = E00, TFE is
siimage
qE
4
sim
NqhE
*200
26
the main mechanism which is a combination fo TE and FE. Under forward bias, the
current due to FE can be expressed as
(2.8)
Where
(2.9)
The much weaker temperature dependence here (absent in the exponential term)
compared to TE which is a characteristic of tunneling. The current due to TRE is given
by
(2.10)
(2.11)
This TFE peaks roughly at energy as follow:
00
00
00
expexp
)/cos(
**
E
qV
E
q
kT
q
kTEk
VqETAJ
FnBnn
FnBnTFE
)sin(
/exp**
11
00
kTckc
EVqTAJ FBn
FE
n
FBn V
Ec
4
log2
1
001
kT
EEE 00
000 coth
27
(2.12)
Where Em is measured from Ec of the neutral region. Under reverse bias, the tunneling
current can be much larger because a large voltage is possible. The currents due to FE
and TFE are given by as follow [2.4]:
(2.13)
(2.14)
where
(2.15)
)/(cosh 00
2 kTE
VqE FnBn
m
'expexp
)/(cosh**
00200
RBn
BnRTFE
qV
kT
q
kTEVqETAJ
RBn
Bn
Bn
RBnFE
VE
qV
k
EAJ
00
2/32
00
3
2exp**
)/tanh()/('
0000
00
kTEkTE
E
28
Metal Semiconductor
qBn
Ef
Ef
Ec
Metal
qBn
Ef
Ef
Ec
qVapp
Semiconductor
qVapp
Em
TE
TFE
FE
TE
TFE
FE
(a) applied reverse bias (b) applied forward bias
Figure 2.8 Illustrated energy-band diagrams electron conduction
2.4.1.4 Thin surface barrier model
A new thin surface barriers (TSB) model assuming the presence of extrinsic surface
donors was proposed, where the TFE /FE process through the TSB region determines
current transport. In metal-GaN contact,TFE /FE model cannot explain the measured
reverse I-V date quantitatively. Figure 2.9 shows TSB model (a) and the band diagram.
It is defined that TSB regions having a thickness D as shown figure 2.9, where a thin
surface is formed in each region by high density of unintentional surface donors. In each
TSB region, unintentional surface donors sharpen the Schottky it is assumed that
TFE/FE emission through TSB regions is the main mechanism of overall current
transport. In figure 2.9, the potential at the boundary x=D is defined as D.The potential
shape in TSB region is shape parabola, whose minimum potential is defined as 0. It is
defined V0 as the bias voltage at which 0=D holds, then V0 0 are given by the
following:
29
(2.16)
With
(2.17)
For V <V0
(2.18)
And for V >V0
(2.19)
The average forward saturation current density is given by the following equation:
(2.20)
The reverse saturation current density is given by one of the following two equations,
nS
DSB VD
qNV 2
00 2
D
Cn N
NkTV log
n
D
S
DS
D
S
DS
VV
DDVVqNN
NqN
2
20
0
00
21
2
nVV 0
kTnVqkTqV
kTEk
kTEEqTAJ
FnBn
BSF
/exp/exp
/cosh
/tanh*
00
00000
30
depending on whether the TFE process or the FE process is dominant. For TFE
2/1
00
00
/cosh)(
*
kTE
qVVq
k
EqTAJ B
nRSR
(2.21)
For FE
00
2/10
2/10
2/1000
2/300
//sin/
32exp*
Eqqktqqk
qEqETAJ
BBBB
BBSR
(2.22)
Here, is equation (2.20)-(2.22) denoted the ratio of the total TSB model are to the
total sample area. It can be shown that at high reverse voltages, the reverse ideality
factors satisfied the following relation: [2.5]
(2.23)
FD
DSR nN
Nn
1
1
31
Metal Semiconductor
qB
Ef
Ef
Ec
qVapp
Em =qm
qDq0
D Thin surfacebarrier (TSB)region
Figure 2.9 Illustrated band energy diagram of thin surface barrier (TSB)
model
32
References
[2.1] C. S Fadley “X-ray photoelectron spectroscopy: From origins to future directions”,
Nuclear Instruments and Methods in Physics Research A (2009) p.8-31
[2.2] D. K. Schroder: “Semiconductor Material and Device Characterization -Third
Edition-” Arizona State University Tempe, AZ (2006) p.138-140
[2.3] Y. Taur and T.H Ning “Fundamentals of MODEN VLSI DEVICES”, Cambridge
University press (1998) p.114
[2.4] S. M. SZE and KWOK K. KG: “PHYSICS OF SEMICONDUCTOR DEVICS
-Third Edition-” WILKY-INTERSCIENCE (2007) p.157-166
[2.5] Hideki Hasegawa and Susumu Oyama “Mechanism of anomalous current
transport in n-type GaN Schottky contacts”, American Vacuum Society (2002)
p.1647-1655
33
Chapter 3 NiSi2/Ni3P contact electrodes
3.1 Introduction
3.2 XPS analysis
3.3 TEM analysis
3.4 Current-Voltage characteristics
3.5 Conclusion
References
34
3.1 Introduction
In this chapter, nickel silicide (NiSi2) metal is described that propose to use
AlGaN/GaN contact electrode. In this study, it is aim that inserted Ni3P with NiSi2
moderate for Schottky barriers. Metal silicide is not ball up in high temperature
annealing [3.1]. Figure 3.1 shows TEM images that NiSi2 structure as-deposited (a) and
after 500 oC for 1min in N2 (b). As shown, it is expected to form uniform interface.
Figure 3.2 shows Sheet resistivity-annealing temperature, on AlGaN or GaN, stable
resistivity. Thus, high temperature annealing is expected to form uniform interface and
low resistivity.
In this chapter, NiSi2 20-nm thickness (Ni: 0.5-nm, Si: 1.13-nm, 16set stack structure)
electrode selected, and inserted Ni3P 0.63-nm and Ti (reference) 1-nm thickness.
Annealing temperature different conditions was examined to Current-voltage
measurement. In following Contact devices fabrication process is shown as chapter 2.1.
TNiSi2 = ~10nm10nm
(a) as deposited
Si(100)
10nm
(b) RTA: 500oC, 1min
NiSi2
Si(100)
Epoxy
Si substrate
8 sets of Si(1.9nm)/Ni(0.5nm)n-Si(100) sub., Nd=3x1015 cm-3
(a) NiSi2 structure and as deposited (b) 500oC for 1min in N2
Figure 3.1 TEM images at NiSi2 structure as-deposited
35
0 200 400 600 800Annealing temperature (oC)
600
400
800
0Sh
eet
resi
stiv
ity
(/s
q)
GaN
Si
200
AlGaNSiO2
Figure 3.2 Sheet resistivity-annealing temperature that AlGaN or GaN
3.2 XPS analysis
Analysis by XPS, it can be observed interface reaction. Figure 3.3 shows Ni (a) and Ga
(b) spectra that NiSi2 and inserted Ti between NiSi2 and AlGaN at 800oC for 1min in N2.
In figure 3.3 (a), NiSi2, NiSix phase or NiGa that binding energy peak was observed
about 854.45-eV and 853.65-eV respectively. In Ni2p 2/3 spectra, inserted Ti to NiSi2 is
possible that form Ni-rich silicide or NiGa. NiSi2 should be decomposed NiSix phase
formed. In figure 3.3 (b), binding energy peak Ga-N and Ni-Ga was observed about
1118.36-eV and 1116.8-eV respectively. In Ga2p 2/3 spectra, magnitude of NiGa is same
both NiSi2 and inserted Ti with NiSi2, which is about 1-nm thickness by calculation. It is
possible that NiGa is formed by high annealing temperature. As a result, Ti insert with
NiSi2 that can be reaction with ease than NiSi2only.
36
852853854855856
NiSi2/AlGaN
NiSi2/Ti/AlGaN
NiSi2
Ni2p3/2
Ni-rich silicideand/or NiGa
Binding energy (eV)
h=7938.85 eV, TOA=80o
RTA:800oCN2 for 1min
Inte
nsity
(a.
u.)
856 855 854 853 852
(a) Ni spectra
11151116111711181119112011211122
NiSi2/AlGaN
NiSi2/Ti/AlGaN
Ga2p3/2
GaN
NiGa
h=7938.85 eV, TOA=80o
RTA:800oCN2 for 1min
Inte
nsity
(a.
u.)
1122 1120 1118 1116Binding energy (eV)
(b) Ga spectra
Figure 3.3 Ni and Ga spectrum at 800oC for 1min in N2
37
3.3 TEM analysis
Analysis by TEM image, it can be observed device structure vertical image. Figure 3.4
shows NiSi2 and inserted Ti between NiSi2 and AlGaN at 800oC for 1min in N2. In
figure 3.4 (a), after annealing, NiSi2 layer is continued no agglomeration. Although
there is observed void at the pointing area. It is possible that the void is after fell out. In
figure 3.4 (b), it is increased surface roughness and is exsisted to low density void. It is
considered that Ti decomposed to NiSi2 layer. It is not observed TiN agglomeration
described in section 1.3, because it is possible to sppresse reaction of NiSi2 feature
effect. In conclusion, high temperature annealing NiSi2 is continued uniform layer but,
void is formed at AlGaN surface area. Thus, it is difficult for defining Schottky barrier
model.
(a) NiSi2/Ti/AlGaN interface (b) NiSi2/Ti/AlGaN interface
Figure3.4 TEM image at 800oC for 1min in N2
3.4 Current-Voltage characteristics
Figure 3.5 shows NiSi2 and inserted Ti or P between NiSi2 and AlGaN at 800oC for
1min in N2. Inserted Ti with NiSi2 is larger current at low voltage range (0~2.3 V).
Although, at high voltage range (2.3~5.0 V) Inserted Ni3P with NiSi2 observed. Figure
38
3.6 shows current-voltage at inserted Ni3P (a) or Ti (b). In inserted Ni3P (a), 800 oC
annealing large current. It is suggested that Ni3P effect obtained. .It is possible that Ni3P
doping Schottky modulation, but there is no evidence in details. In inserted Ti (b), over
800 oC, it is observed Ohmic characteristics. It is possible that Ni3P doping Schottky
modulation, but there is no evidence in details. Figure 3.7 shows changed measurement
temperature (-30~23oC) that inserted Ti or Ni3P between NiSi2 and AlGaN. Both
samples are decreased current lower temperature. And, inserted Ti is lager current at 1 V,
inserted Ni3P is lager current high voltage range (over 1V). It is possible that
-5 -4 -3 -2 -1 0 1 2 3 4 5
0
10
20
Voltage (V)
Cur
rent
(m
A)
30
-30
-20
-10
with Ti
with Ni3P
NiSi2
RTA:800oCN2 for 1min
Figure 3.5 NiSi2 and inserted Ti or Ni3P between NiSi2 and
AlGaN at 800oC for 1min in N2.
39
-5 -4 -3 -2 -1 0 1 2 3 4 5
0
0.1
0.2
Voltage (V)
Cu
rre
nt
(mA
)
0.3
-0.3
-0.2
-0.1900oC
800oC
with Ni3P
RTA:N2 for 1min
700oC
600oC
500oC
(a) inserted Ni3P
-5 -4 -3 -2 -1 0 1 2 3 4 5
0
0.1
0.2
Voltage (V)
Cur
rent
(m
A)
0.3
-0.3
-0.2
-0.1
900oC
800oC
with Ti
RTA:N2
for 1min
700oC
600oC
500oC
(b) inserted Ti
Figure 3.6 Depend on annealing temperature current-voltage
40
10-3
10-2
10-1
10-7
10-6
10-5
10-4
0 1 2 3 4 5
23oC
-30oC
23oC
-30oC
With Ni3P
With Ti
RTA:800oCN2 for 1min
Voltage (V)
Log
(A)
Figure 3.7 Changed measurement temperature
inserted Ti or Ni3P between NiSi2 and AlGaN
3.5 Conclusion
NiSi2 electrode and insert Ti or Ni3P electrode is examined for AlGaN/GaN. After high
temperature annealing, NiSi2 electrode should be decomposed NiSix phase formed by
XPS, which is continued uniform by TEM image. Although void is formed at AlGaN
surface area. Thus, it is difficult for defining Schottky barrier model. Current voltage is
observed lager current at high voltage range (2.3~5.0 V). It is possible that Ni3P doping
Schottky modulation, but there is no evidence in details.
References [3.1] S. P. Murarka,”SILICIDES FOR VLSI APPLICATION”, Bell Telephone
Laboratories, (1983)
41
Chapter 4 TiSi2 contact electrodes
4.1 Introduction
4.2 XPS analysis
4.3 AFM analysis
4.4 Current-Voltage characteristics
4.5 Inducting C-V characteristics by I-V charactristics
4.6 Conclusion
References
42
4.1 Introduction
In this chapter, titanium silicide (TiSi2) metal is described that propose to use
AlGaN/GaN contact electrode. In this study, it is aim that TiSi2/AlGaN interface is
uniform after high temperature annealing. The metal silicide can be easy to form by
semiconductor process, which can control to rerate composing formation (4.1). TiSi2
electrodes expect to decompose AlGaN by forming Si3N4, which approach by form
thermochemistry reaction as follow:
8.6kJ/molNSi11
2TiN
11
3Ga
4
3Al
4
1TiSi
11
3NGaAl 4320.750.25 (4.1)
TiSi2, TiN and Si3N4enthalpy of formation are 133.9 kJ/ mol, 265.5 kJ/ mol and 744.8
kJ/ mol respectively.
TiSi2 20-nm thickness (Ti: 0.5-nm, Si: 1.13-nm, 16set stack structure) electrode selected.
Annealing temperature different conditions is examined to Current-voltage
measurement. In following Contact devices fabrication process is shown as chapter 2.1.
4.2 XPS analysis
To learn details of TiSi2/AlGaN interface reaction that is used by XPS analysis. Figure
4.1 shows Ga and Al spectrum that TiSi2 and Ti (reference) at 900oC annealing
temperature for 1min in N2. In Ga2p2/3 spectrum, energy shift was observed. Similarly,
Al spectrum energy shifted. So, TiSi2 electrodes suppressed reaction role at
metal/AlGaN that should be formed uniform interface. As a result, Al and Ga atoms in
AlGaN layer remain intact with TiSi2 electrodes, which can extract of N to remain
uniform interface.
43
1115111611171118111911201121
Ti(10nm)
TiSi2(10nm)
Ti-Ga
Ga-N
900oC, 1min in N2
(h=7940eV, TOA=80o)
BE=0.16eV
Ga 2p3/2 spectra
Binding energy (eV)
Inte
nsi
ty (
a.u
.)
(a) Ga spectra
155915601561156215631564
Binding energy (eV)
Inte
nsity
(a.
u.)
Ti(20nm)
TiSi2(20nm)
Al-N
(h=7940eV, TOA=80o)Al spectra900oC, 1min in N2
(b) Al spectra
Figure 4.1 XPS at Ga and Al spectrum that TiSi2 and Ti (reference) at 900oC annealing
temperature for 1min in N2
4.3 AFM analysis
Figure4.2 shows AlGaN interface that as-AlGaN (a) and eliminated Ti (b) or TiSi2 (c).
44
Eliminated metal condition is SPM cleaning10min and 40% HF etching 10s.In figure
4.2 (a) shown, surface state is observed frat which RMS is 0.17-nm on 1000nmsq.
Although, it is observed that uniform surface and some black point that defect of AlGaN
substrate. In figure 4.2 (b) shown, after 750oC 20 min in N2 annealing Ti that eliminated
inter face. It is observed some small metal. It might be suggested that TiN metal
agglomeration. In figure 4.3 (c) shown, after 950oC 20 min in N2 annealing TiSi2 that
eliminated inter face. It is observed that nearly uniform and some spot grain metal. That
might means constant contact formation.
4.4 Current-Voltage characteristics
Figure 4.3 shows Current-Voltage on annealing time dependence that Ti (a) and TiSi2
(b). In figure 4.3 (a), Ti is observed Ohmic characteristics at 750oC, after high
temperature was decreasing current. In figure 4.3 (b), TiSi2 is observed Ohmic
characteristics at 950oC, under 900oC is observed non-liner current. Figure 4.4 shows
Current at 2V –annealing time. TiSi2 is constant current value, but Ti is non-constant
current. It might be suggested that different conducting mechanism. Figure 4.5 shows
specific contact resistance (c) for Ti and TiSi2. Ti is shown Ohmic behavior at 5-20min,
TiSi2 is shown sometime. TiSi2 can be obtained constant Ohmic contact at 950oC.
Specific contact is detained by chapter 2.3 calculation.
45
-5 -4 -3 -2 -1 0 1 2 3 4 5
0
1
2
Voltage (V)
Curr
ent
(m
A)
3
-3
-2
-1
TiN/Ti
750oC
850oC
800oC
900oC
700oC
950oC
RTA:N2 for 1min
-5 -4 -3 -2 -1 0 1 2 3 4 5
0
1
2
Voltage (V)
Curr
ent
(m
A)
3
-3
-2
-1
TiN/Ti
750oC
850oC
800oC
900oC
700oC
950oC
RTA:N2 for 1min
Ti
(a) Ti electroces
-5 -4 -3 -2 -1 0 1 2 3 4 5
0
1
2
Voltage (V)
Cur
rent (m
A)
3
-3
-2
-1
TiN/TiSi2
950oC
750oC
800oC
900oC
700oC850oC
RTA:N2 for 1min
(b)TiSi2 electrodes
Figure 4.3 Current-Voltage on annealing time dependence
46
700 800 900 10000.0
0.4
0.8
1.2
1.6
Temperature (oC)
Cur
rent
@2V
(m
A) Ohmic
non-linear IV
OhmicTi
TiSi2
non-linear IV
Figure 4.4 Current at 2V –annealing time Ti and TiSi2
10-4
10-3
10-2
10-1
c(
cm2 )
10 100
Annealing time (min)2 5 20 501
TiSi2 950oC
750oC
Ohmic for Ti
Figure 4.5 Specific contact resistivity Ti and TiSi2
4.5 Inducting C-V characteristics by I-V charactristics
In this chapter, it is examined inducting C-V characteristics by I-V charactristics. TiSi2
is suggested that stable interface by XPS and TEM image. Figure 4.6 is shown
C-applied voltage that calaculated by chapter 2.4.
47
4.6 Conclusion
TiSi2 electrode is examined for AlGaN/GaN contact. It is obtained at 950oC Ohmic
characteristics that stable contact more annealing time. Thus, TiSi2 contact mechanism
is different commonly Ti conduction model. Figure4.6 shows TiSi2 conduction model.
TiSi2 is extracted N vacancy [4.1] model which can decrease contact resistance. High
temperature is need and stable contact can be obtained. Ti conduction model is shown at
chapter1.3.
TiSi2 TiSi2VN VN
VN
VN VNVN
VN
VN
VN
VN
VN
VN
VNVN
VNVN
Figure 4.6 TiSi2 conduction mechanism
References
[4.1] D. C. Look, G.C. Farlow, J.R. Drevinsky , D. F. Bliss and J. R. Sizelove: “ On the
nitrogen vacancy in GaN”, APPLIED PHYSICS LETTER, vol 83, No. 17 (2003)
48
Chapter 5 Conclusion
49
5 Conclusions
In this study, two way of contact metal are examined for AlGaN/GaN, which is
described summary as follows.
(1)NiSi2/Ni3P contact electrodes
Inserted Ni3P between NiSi2 and AlGaN electrode is examined for AlGaN/GaN.
After high temperature annealing, NiSi2 electrode should be decomposed NiSix phase
formed by XPS, which is continued uniform by TEM image. Although void is formed at
AlGaN surface area. Current voltage is observed lager current at high voltage range
(2.3~5.0 V). It is suggested that Ni3P doping Schottky modulation, but there is no
evidence in details. In conclusion, Inserted Ni3P with NiSi2 electrode is obtained stable
current, which means using for Schottky gate.
(2)TiSi2 contact electrodes
TiSi2 electrode is examined for AlGaN/GaN. By XPS Al and Ga atoms in AlGaN layer
remain intact with TiSi2 electrodes, which can extract of N to remain uniform interface.
In 950oC annealing temperature, Ohmic characteristic was obtained, which is stable
resistance for annealing time. TiSi2 is extracted N vacancy model which can decrease
contact resistance. High temperature is needed, but stable contact can be obtained. In
conclusion, TiSi2 is suggested that stable Ohmic contact, which may be lower specific
contact resistivity to be extracted more N vacancy.
In conclusion, TiSi2 can be expected to stable contact metal layer after high temperature
annealing. Especially, it is feature that extract of N to remain uniform interface.
50
Acknowledgments
I would like to express my gratitude to my supervisor Prof. Hiroshi Iwai for his
continuous encouragement and advices for my study. He also gave me many chances to
attend conferences. The experiences are precious for my present and future life.
I thank to Prof. Takeo Hattori, Prof. Kenji Natori, Prof. Nobuyuki Sugii, Prof. Akira
Nishiyama, Prof. Kazuo Tsutsui, Prof. Yoshinori Kataoka, Associate Prof. Parhat Ahmet,
and Associate Prof. Kuniyuki Kakushima for useful advice and great help whenever I
met difficult problem.
I thank to research colleagues of Iwai Lab. for supporting my study.
I would like to appreciate the support of secretaries, Ms. Nishizawa and Ms.
Matsumoto.
Finally, I would like to thank my parents Yasuhiko and Rika, my sister Asuka and
Chiaki for their endless support and encouragement.
Kana Tsuneishi
January, 2013
