mena9510: advanced characterization methods fourier...
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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
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1D quantum harmonic oscillator
• Equally spaced vibrational levels
• Selection rule: ∆𝑛 = 1 transitions are allowed
=> spring constant (bond strength)
=> reduced mass
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1D quantum harmonic oscillator
At 290 K, , so most molecules are in the 𝑛 = 0
(ground) vibrational state.
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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
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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:
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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)
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• 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
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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
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• Atomic composition: isotope effect Ex: Li-OH complex in ZnO studied by Shi et. al.
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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
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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
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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
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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
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• Michelson interferometer (monochromatic wave)
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H vs. D treated b-Ga2O3
• Interferogram (multiple, discrete wavelengths)
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• Interferogram (continuous wavelength source)
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FFT
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𝑇 =𝐼
𝐼0= 𝑒−𝛼 𝜈 𝑑
(neglects surface reflections)
Linear absorption coefficient
𝐴 ≈ − log10 𝑇 = −𝛼 𝜈 𝑑 log 𝑒
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(neglects surface reflections)
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LVM! Broad absorption at low wavenumbers
due to free-carrier absorption
Bruker IFS 125HR FTIR spectrometer
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Bruker IFS 125HR FTIR spectrometer
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Interferometer compartment
• IR light sources
• Beamsplitter
• Fixed and moving mirrors
Bruker IFS 125HR FTIR spectrometer
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Sample compartment
• Cryostats (up to two)
• Other optical accessories (e.g., polarizer)
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Detector compartment
• IR detectors (4 internal, 2 external)
Bruker IFS 125HR FTIR spectrometer
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IR light sources
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Detectors
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Beamsplitters
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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.
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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.
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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
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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.)
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Experiments
Muon spin resonance (P.D.C. King, et al.)
Hall effect (T. Koida, et al.)
IR absorbance (W. Yin, et al.)
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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.
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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.
H shallow donors in In2O3
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.
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H shallow donors in In2O3
Uniaxial Stress – splitting pattern consistent with a defect with “quasi-
trigonal” symmetry.
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Combination of FTIR experiments and theory strongly suggest Hi is the
dominant shallow donor in In2O3.
Summary
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• 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…
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• 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
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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
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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).
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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
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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
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Oxygen dimers (O2) and trimers (O3) can also exist in as-grown crystals,
contribute to oxygen diffusion processes.
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Oxygen-vacancy (VO) defect (the Si-A center)
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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)
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Oxygen-vacancy defect complexes (VmOn)
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Additional oxygen-vacancy defect complexes formed by annealing irradiated Si.
Spectrum gets more complicated with additional impurities…
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Carbon (C) is also introduced during the growth of Si wafers from graphite in the
furnace.
Oxygen-vacancy defect complexes (VOn)
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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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