Fourier Transform Infrared (FTIR) spectroscopy (lecture)

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1 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 M. Weiser Philip Weiser, FTIR spectroscopy 1

2 Vibrational spectroscopy and point defects in crystalline solids Philip Weiser, FTIR spectroscopy 2

3 1D quantum harmonic oscillator => spring constant (bond strength) => reduced mass Equally spaced vibrational levels Selection rule: n = 1 transitions are allowed Philip Weiser, FTIR spectroscopy 3

4 1D quantum harmonic oscillator At 290 K,, so most molecules are in the n = 0 (ground) vibrational state Philip Weiser, FTIR spectroscopy 4

5 1D quantum anharmonic oscillator χ e is the anharmonicity constant 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 Philip Weiser, FTIR spectroscopy 5

6 Electromagnetic spectrum With use of wavenumber (reciprocal wavelength) unit, x-axis is proportional to energy. Conversions: h = Planck s constant = 4.136x10-15 ev-s c = speed of light = 2.998x10 10 cm/s Philip Weiser, FTIR spectroscopy 6

7 Vibrational spectroscopy Polyatomic molecules «normal modes» of vibration Water (H 2 O) ( Carbon dioxide (CO 2 ) ( Philip Weiser, FTIR spectroscopy 7

8 Infrared (IR) absorption in semiconductors Intrinsic absorption (valence to conduction band) Phonon absorption (vibrational modes of host lattice) Impurity/lattice defect absorption (electronic or vibrational) Free carrer (intraband) absorption Philip Weiser, FTIR spectroscopy 8

9 Phonon absorption Impurity modifies host phonons localized vibrational mode (LVM) Philip Weiser, FTIR spectroscopy 9

10 Eigenfrequencies are modified by neighbors, host lattice. M ω 2 = k 1 m + 1 χm m m = mass of impurity atom M = mass of host atom χ = coupling between LVM and host ~2 Especially useful for identifying hydrogen-related defects Philip Weiser, FTIR spectroscopy 10

11 Atomic composition: isotope effect Ex: Li-OH complex in ZnO studied by Shi et. al. 6 Li- 16 O-H ( cm -1 ) 6 Li- 17 O-H ( cm -1 ) 6 Li- 18 O-H ( cm -1 ) 6 Li- 16 O-D ( cm -1 ) 6 Li- 17 O-D ( cm -1 ) 6 Li- 18 O-D ( cm -1 ) 7 Li- 16 O-D ( cm -1 ) Note: LVMs at and cm -1 are the first overtones of the and cm -1 lines, respectively Philip Weiser, FTIR spectroscopy 11

12 absorption coefficient (cm -1 ) Absorption line strength: defect concentration 3,0 CZ-Si with FZ-Si spectral subtraction 290 K res = 1.0 cm -1 peak amp = 3,21 cm -1 Integrated abs. coef. 2,0 peak FWHM = 33 cm -1 Peak amplitude 1,0 peak center = 1107,4 cm -1 For O i in Si, 0, wave number (cm -1 ) Philip Weiser, FTIR spectroscopy 12

13 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 Philip Weiser, FTIR spectroscopy 13

14 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 Philip Weiser, FTIR spectroscopy 14

15 Fourier Transform Infrared (FTIR) Spectroscopy and Spectrometers Philip Weiser, FTIR spectroscopy 15

16 Michelson interferometer (monochromatic wave) Philip Weiser, FTIR spectroscopy 16

17 Interferogram (multiple, discrete wavelengths) H vs. D treated b-ga 2 O Philip Weiser, FTIR spectroscopy 17

18 Interferogram (continuous wavelength source) FFT Philip Weiser, FTIR spectroscopy 18

19 Philip Weiser, FTIR spectroscopy 19

20 Linear absorption coefficient T = I = e α ν d I 0 (neglects surface reflections) Philip Weiser, FTIR spectroscopy 20

21 A log 10 T = α ν d log e (neglects surface reflections) Philip Weiser, FTIR spectroscopy 21

22 Broad absorption at low wavenumbers due to free-carrier absorption LVM! Philip Weiser, FTIR spectroscopy 22

23 Bruker IFS 125HR FTIR spectrometer Philip Weiser, FTIR spectroscopy 23

24 Bruker IFS 125HR FTIR spectrometer Interferometer compartment IR light sources Beamsplitter Fixed and moving mirrors Philip Weiser, FTIR spectroscopy 24

25 Bruker IFS 125HR FTIR spectrometer Sample compartment Cryostats (up to two) Other optical accessories (e.g., polarizer) Philip Weiser, FTIR spectroscopy 25

26 Bruker IFS 125HR FTIR spectrometer Detector compartment IR detectors (4 internal, 2 external) Philip Weiser, FTIR spectroscopy 26

27 IR light sources Philip Weiser, FTIR spectroscopy 27

28 Detectors Philip Weiser, FTIR spectroscopy 28

29 Beamsplitters Philip Weiser, FTIR spectroscopy 29

30 UiO Bruker IFS 125HR FTIR spectrometer Source Beamsplitter Detector Spectral range (cm -1 ) FIR Hg-arc Mylar 50 µm FIR DTGS Hg-arc Mylar multilayer FIR DTGS MIR Globar KBr DTGS Globar KBr MCT-broad Globar KBr MCT-mid Globar KBr InSb NIR Tungsten CaF 2 DTGS Tungsten CaF 2 InSb Detectors cover similar spectral ranges but differ in region of maximum sensitivity Philip Weiser, FTIR spectroscopy 30

31 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 n 1/2. => signal-to-noise ratio (S/N) n 1/2 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) Philip Weiser, FTIR spectroscopy 31

32 FTIR Advantages (compared to dispersive instrument) 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 Limitations Beer s law signal is proportional to defect concentration. Limits of detection in the range of 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 Philip Weiser, FTIR spectroscopy 32

33 Example of FTIR Spectroscopy to Investigate Point Defects in Solids Philip Weiser, FTIR spectroscopy 33

34 Properties of H in In 2 O 3 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 In 2 O 3? (In 2 O 3 doped with Sn is most widely used TCO.) Philip Weiser, FTIR spectroscopy 34

35 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 In 2 O 3. (S. Limpijumnong, et al. and W. Yin, et al.) Most stable H i+, quasi-3-fold symmetry oxygen Metastable H i + positions indium Philip Weiser, FTIR spectroscopy 35

36 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. H shallow donors in In 2 O Philip Weiser, FTIR spectroscopy 36

37 Thinning Experiments Hydrogenated In 2 O 3 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. H shallow donors in In 2 O Philip Weiser, FTIR spectroscopy 37

38 Uniaxial Stress splitting pattern consistent with a defect with quasitrigonal symmetry. Combination of FTIR experiments and theory strongly suggest H i is the dominant shallow donor in In 2 O 3. H shallow donors in In 2 O Philip Weiser, FTIR spectroscopy 38

39 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) Philip Weiser, FTIR spectroscopy 39

40 In the lab View interior of the spectrometer. Measure transmission spectrum of a silicon wafer. Determine the concentration of interstitial-oxygen, [O i ], 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 Philip Weiser, FTIR spectroscopy 40

41 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) Philip Weiser, FTIR spectroscopy 41

42 References Hydrogen defects in In 2 O 3 T. Koida, H. Fujiwara, & M. Kondo. Hydrogen-doped In 2 O 3 as high-mobility transparent conductive oxide. Jap. J. Appl. Phys. 46, L (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, (2009). W. Yin, et al. Hydrogen centers and the conductivity of In2O3 single crystals. Phys. Rev. B 91, (2015). P. Weiser, et al. Symmetry and diffusivity of the interstitial hydrogen shallow-donor center in In 2 O 3, Appl. Phys. Lett. 109, (2016) Philip Weiser, FTIR spectroscopy 42

43 Interested in learning more about defects in semiconductors? Philip Weiser, FTIR spectroscopy 43

44 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 ~10 18 atoms/cm 3. 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 Philip Weiser, FTIR spectroscopy 44

45 Interstitial-oxygen (O i ) 28 Si- 16 O- 28 Si 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 (O 2 ) and trimers (O 3 ) can also exist in as-grown crystals, contribute to oxygen diffusion processes Philip Weiser, FTIR spectroscopy 45

46 Philip Weiser, FTIR spectroscopy 46

47 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 + O i VO Philip Weiser, FTIR spectroscopy 47

48 Oxygen-vacancy (VO) defect (the Si-A center) Philip Weiser, FTIR spectroscopy 48

49 Oxygen-vacancy defect complexes (V m O n ) Additional oxygen-vacancy defect complexes formed by annealing irradiated Si Philip Weiser, FTIR spectroscopy 49

50 Spectrum gets more complicated with additional impurities Carbon (C) is also introduced during the growth of Si wafers from graphite in the furnace Philip Weiser, FTIR spectroscopy 50

51 Oxygen-vacancy defect complexes (VO n ) Previous work at UiO has followed the evolution of oxygen-related defects following different irradiation treatments. (a) room temperature irradiation (b) hot irradiation (350 C) Trying to understand diffusion and dissociation mechanisms of these defects to control their impact Philip Weiser, FTIR spectroscopy 51

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