MENA9510: Advanced Characterization Methods Fourier ...folk.uio.no/vebjornb/MENA9510/lectures-2017/MENA9510_FTIR_2017.pdf · MENA9510: Advanced Characterization Methods Fourier Transform

MENA9510: Advanced Characterization Methods

Fourier Transform Infrared (FTIR)

spectroscopy (lecture)

Goals: To understand…

• Basic theory of vibrational spectroscopy.

• Key components of a FTIR spectrometer.

• Use of FTIR spectroscopy to characterize defects.

Philip Weiser, FTIR spectroscopy 09.10.2017 1

©2017 Philip M. Weiser

Vibrational spectroscopy

and

point defects in crystalline solids

09.10.2017 Philip Weiser, FTIR spectroscopy 2


1D quantum harmonic oscillator

• Equally spaced vibrational levels

• Selection rule: ∆𝑛 = 1 transitions are allowed

=> spring constant (bond strength)

=> reduced mass


1D quantum harmonic oscillator

At 290 K, , so most molecules are in the 𝑛 = 0

(ground) vibrational state.


1D quantum anharmonic oscillator

• ∆𝐸 increases with increasing n => modifies transition energies

• Selection rules are broken: ∆𝑛 ≥ 1=> permits overtones.

• Important effect for comparison of first-principles calcuations and

experiments

𝜒𝑒 is the anharmonicity constant


Electromagnetic spectrum

With use of wavenumber (reciprocal wavelength) unit, x-axis is proportional to energy.

h = Planck’s constant = 4.136x10-15 eV-s

c = speed of light = 2.998x1010 cm/s

Conversions:


Polyatomic molecules – «normal modes» of vibration

Vibrational spectroscopy

Water (H2O)

(http://www.chemtube3d.com/vibrationsH2O.htm)

Carbon dioxide (CO2)

(http://www.chemtube3d.com/vibrationsCO2.htm)


• Intrinsic absorption (valence to conduction band)

• Phonon absorption (vibrational modes of host lattice)

• Impurity/lattice defect absorption (electronic or

vibrational)

• Free carrer (intraband) absorption

Infrared (IR) absorption in semiconductors


localized vibrational mode (LVM)

Phonon absorption Impurity modifies host phonons

Eigenfrequencies are modified by neighbors, host lattice.

𝑚 = mass of impurity atom

𝑀 = mass of host atom

𝜒 = coupling between LVM and host ~2

Especially useful for identifying hydrogen-related defects.

𝜔2 = 𝑘1

𝑚+

1

𝜒𝑀

m

M


• Atomic composition: isotope effect Ex: Li-OH complex in ZnO studied by Shi et. al.


6Li-16O-H (3577.3 cm-1) 6Li-17O-H (3571.7 cm-1) 6Li-18O-H (3566.6 cm-1)

6Li-16O-D (2644.5 cm-1) 6Li-17O-D (2636.4 cm-1) 6Li-18O-D (2629.2 cm-1) 7Li-16O-D (2644.7 cm-1)

Note: LVMs at 6967.0 and 5191.2 cm-1 are the first overtones of the 3577.3 and 2644.5

cm-1 lines, respectively

• Absorption line strength: defect concentration


1000 1050 1100 1150 1200

0,0

1,0

2,0

3,0

peak amp = 3,21 cm-1

absorp

tion c

oeffic

ient (c

m-1

)

wave number (cm-1)

290 K

res = 1.0 cm-1

CZ-Si with FZ-Si spectral subtraction

peak center = 1107,4 cm-1

peak FWHM = 33 cm-1

Integrated abs. coef.

Peak amplitude

For Oi in Si,

• Bond orientation: polarization properties


D-treat H-treat Defect’s transition

moment has no

component along the

[1 0 2] direction

=> Limits possible defect

structures that need to

be investigated by theory

• Defect symmetry: uniaxial stress


Uniaxial stress is a

perturbation. It BREAKS

the orientational

degeneracy of the defect

within the crystal.

Splitting patterns of the

vibrational line under

different stress directions

give clues about the

defect symmetry.

Fourier Transform Infrared

(FTIR)

Spectroscopy and Spectrometers


• Michelson interferometer (monochromatic wave)


H vs. D treated b-Ga2O3

• Interferogram (multiple, discrete wavelengths)


• Interferogram (continuous wavelength source)


FFT



𝑇 =𝐼

𝐼0= 𝑒−𝛼 𝜈 𝑑

(neglects surface reflections)

Linear absorption coefficient

𝐴 ≈ − log10 𝑇 = −𝛼 𝜈 𝑑 log 𝑒


(neglects surface reflections)


LVM! Broad absorption at low wavenumbers

due to free-carrier absorption

Bruker IFS 125HR FTIR spectrometer




Interferometer compartment

• IR light sources

• Beamsplitter

• Fixed and moving mirrors



Sample compartment

• Cryostats (up to two)

• Other optical accessories (e.g., polarizer)


Detector compartment

• IR detectors (4 internal, 2 external)



IR light sources


Detectors


Beamsplitters


UiO Bruker IFS 125HR FTIR spectrometer

Source Beamsplitter Detector Spectral range (cm-1)

FIR Hg-arc Mylar 50 µm FIR DTGS 20-55

Hg-arc Mylar multilayer FIR DTGS 100-650

MIR Globar KBr DTGS 400-4800

Globar KBr MCT-broad 450-4800

Globar KBr MCT-mid 600-4800

Globar KBr InSb 1850-4800

NIR Tungsten CaF2 DTGS 4000-12000

Tungsten CaF2 InSb 4000-14000

Detectors cover similar spectral ranges but differ in region of

maximum sensitivity.


Signal-to-noise (S/N) ratio considerations

• Choose spectral region with best overlap of src/bms/dtc spectral

ranges.

• Spectral signal increases as the number of scans, n.

The spectral noise is n1/2.

=> signal-to-noise ratio (S/N) n1/2

• Spectral resolution

• Low (4 cm-1) vs. high (0.1 cm-1)

• Resolution ~(max. displacement of moving mirror) -1

• High resolution longer interferogram more noise

=> spectral resolution (S/N) -1

• Multiplex advantage - single scan measures entire spectral range in short

period of time.

• Throughput advantage – higher optical throughput (no slits) means more

light reaches the sample and detector.

• Precision Advantage – He-Ne laser used to control scanning mirror also

acts as an internal calibration standard.


FTIR Advantages (compared to dispersive instrument)

• Beer’s law – signal is proportional to defect concentration. Limits of detection

in the range of 1013 – 1015 cm-3 in a 1 cm thick sample.

• ‘Single-beam’ technique – reference and sample are not measured

simultaneously.

• Quantitative analysis requires calibration factor from another technique.

FTIR Limitations

Example of FTIR Spectroscopy to

Investigate Point Defects in Solids


Properties of H in In2O3

• Transparent conducting oxides (TCOs) have found widespread

applications as low-emissivity window coatings and transparent

electrodes.

• Unintentionally doped, as-grown crystals show strong n-type

conductivity, which is usually attributed to native defects.

• As-grown crystals contain hydrogen, which can act as a shallow

donor (e.g., in ZnO).

• What about In2O3? (In2O3 doped with Sn is most widely used

TCO.)


Experiments

Muon spin resonance (P.D.C. King, et al.)

Hall effect (T. Koida, et al.)

IR absorbance (W. Yin, et al.)


Theory

H should behave as a shallow donor in single crystal In2O3.

(S. Limpijumnong, et al. and W. Yin, et al.)

Most stable Hi+, quasi-3-fold symmetry

oxygen

indium

Metastable Hi+ positions

H shallow donors in In2O3

Thermal Stability

Rate of decay of free carrier

absorption is correlated with rate of

decay of 3306 cm-1 O-H line.


Isotope Effect

Exchange H for D.

Rate of decay of free carrier

absorption is correlated with rate of

decay of 2464 cm-1 O-D line.


Thinning Experiments

Hydrogenated In2O3 single crystal

thinned mechanically in small steps

Absorbance spectrum measured after

each step to determine int. abs. of 3306

cm-1 O-H line.

Rate of decay of free carrier absorption is

correlated with rate of decay of 3306 cm-1

line.



Uniaxial Stress – splitting pattern consistent with a defect with “quasi-

trigonal” symmetry.


Combination of FTIR experiments and theory strongly suggest Hi is the

dominant shallow donor in In2O3.

Summary


• Vibrational spectroscopy – how/why it works

• Point defects in solids – localized vibrational modes and the

information they provide about the atomic compositions,

concentrations, orientations, and symmetries of defects

• FTIR spectrometers – Michelson interferometer, non-destructive

technique, high S/N ratios in a relatively short period of time,

accessible spectral ranges

• Defect charcterization – hydrogen-related defects in transparent

conducting oxides, oxygen-related defects in Si (to name a few)

In the lab…


• View interior of the spectrometer.

• Measure transmission spectrum of a silicon wafer.

• Determine the concentration of interstitial-oxygen, [Oi], in an as-grown

Si wafer measured at room temperature.

• Identify different oxygen-related LVMs observed in Si measured at low

temperature.

Group 1 (Thursday) – room temperature

Group 2 (Friday) – low temperature

=> Share data between both groups.

References


Vibrational spectroscopy and defects in semiconductors

Identification of Defects in Semiconductors (ed. M. Stavola), Vol. 51B in Semiconductors

and Semimetals (Academic Press, Boston, 1999).

R. S. Drago. Physical Methods for Chemists. 2nd ed. (Saunders College Publishing, 1992).

M. Fox. Optical Properties of Solids. 2nd ed. (Oxford University Press, 2010).

FTIR spectrometers/spectroscopy

P.R. Griffiths and J.A. De Haseth. Fourier Transform Infrared Spectrometry. 2nd ed. (John

Wiley and Sons, Inc, 2007).

D.C. Harris. Quantitative Chemical Analysis. 7th ed. (W. H. Freeman, 2006).

References


Hydrogen defects in In2O3 T. Koida, H. Fujiwara, & M. Kondo. Hydrogen-doped In2O3 as high-mobility transparent

conductive oxide. Jap. J. Appl. Phys. 46, L685-687 (2007).

B. B. Baker, et al. Motional characteristics of positively charged muonium defects in In2O3.

AIP Conf. Proc. 1583, 323 (2014).

S. Limpijumnong, et al. Hydrogen doping in indium oxide: An ab initio study. Phys. Rev. B

80, 193202 (2009).

W. Yin, et al. Hydrogen centers and the conductivity of In2O3 single crystals. Phys. Rev. B

91, 075208 (2015).

P. Weiser, et al. Symmetry and diffusivity of the interstitial hydrogen shallow-donor center in

In2O3, Appl. Phys. Lett. 109, 202105 (2016).


Interested in learning more about defects in semiconductors?

Case 2: Oxygen-related defects in Si

• Silicon is the dominant material in the semiconductor industry

(integrated circuits, photovoltaics, etc.)

• Majority of solar-grade Si is grown by the Czochralski (CZ)

method, contains oxygen impurities at concentrations of ~1018

atoms/cm3.

• Oxygen defects and preciptates play a tremendous role in Si!

– Immobilize dislocations => improve the mechanical strength

– Trap metallic impurities (gettering) => decrease device

failure

– Thermal double donors and light-induced degradation =>

lack of control over device conductivity


Interstitial-oxygen (Oi)

28Si-16O-28Si defect “molecule”

Multitude of LVMs at low temperature from

different normal modes and isotopic

combinations.

3 (antisymmetric stretch) = 1107 cm-1,

used to determine [O] in Si wafers at

room temperature


Oxygen dimers (O2) and trimers (O3) can also exist in as-grown crystals,

contribute to oxygen diffusion processes.


Oxygen-vacancy (VO) defect (the Si-A center)


Oxygen forms an Si-O-Si bridge in the

vacancy, weak Si-Si bond between

remaining two Si atoms.

Si vacancies (V) produced by irradiation

with high energy particles.

V + Oi VO

Oxygen-vacancy (VO) defect (the Si-A center)


Oxygen-vacancy defect complexes (VmOn)


Additional oxygen-vacancy defect complexes formed by annealing irradiated Si.

Spectrum gets more complicated with additional impurities…


Carbon (C) is also introduced during the growth of Si wafers from graphite in the

furnace.

Oxygen-vacancy defect complexes (VOn)


Previous work at UiO has followed the evolution of oxygen-related defects

following different irradiation treatments.

(a) – room temperature irradiation

(b) – hot irradiation (350C)

Trying to understand diffusion and dissociation mechanisms of these defects to

control their impact.

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