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The effect of sub-oxide phases on the transparency of tin-doped
gallium oxide
K. Lim1,2, L. T. Schelhas2, S. C. Siah3, R. E. Brandt3, A. Zakutayev4, S. Lany4, B. Gorman5, C. J.
Sun6, D. Ginley4, T. Buonassisi3, M. F. Toney2
Supplementary Information
Experimental details
- Thin films deposition
Ga2O3:Sn thin films (100-400 nm thickness) were synthesized on 50x50 mm Eagle XG glass
substrates were using combinatorial Pulsed Laser Deposition (PLD) with mutually orthogonal
composition (2–10% Sn) and temperature gradients (250–520 °C). The 50 mm ceramic Ga2O3
and SnO2 targets were ablated using ultraviolet KrF excimer laser (248 nm wavelength) with 1–2
J/cm2 energy density and 10 Hz repetition rage, in 10-3 Torr of gas (wither pure Ar or 1 % O2
mix) in a vacuum chamber with 10-8 Torr base pressure.
- Thin film characterization
Spatially-resolved characterization of the resulting combinatorial film libraries was performed on
rectangular 4x11 grid of points. Chemical composition and film thickness was mapped using xray fluorescence (XRF, Fischer XDV, Si drift detector). Physical properties were characterized
using spatially-resolved collinear four-point probe measurements and optical
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transmittance/reflectance spectroscopy (both custom build). The results were analyzed using a
custom set of functions implemented in Igor PRO software.
- XAS measurement
Fluorescence mode is used for both Ga K-edge and Sn K-edge XAS measurements, and the size
of X-ray illumination area is approximately 200 x 200 μm2. The Ge detector is used to collect the
fluorescence data in both Ga K-edge and Sn K-edge. X-ray absorption of Ga and Sn foils are
measured to calibrate the energy shift in X-ray, and also used as reference data to determine the
oxidation state of each element. We use the Athena and Artemis programs to analyze XAS data.
- X-ray diffraction
The energy of the X-ray is 12.495 keV (0.992 Å), and Δ2theta is 0.0023. The Si(111)
monochromator is employed. We convert the angle of 2theta from the wavelength of synchrotron
to the wavelength of Cu K alpha for easier comparison.
- TEM
Specimen preparation was completed using standard FIB cross-sectional techniques using Ga+
ion energies between 30 and 2 kV in an FEI Co. Helios 600 dual column FIB / SEM. In-situ
manipulation was completed before final thinning and ion damage removal at low energies.
Specimens were further analyzed in an FEI Co. Talos 200FX using TEM, STEM, and electron
diffraction modes. TEM images were acquired in bright field and dark field modes and compared
with STEM images using bright field, annular dark field, and HAADF modes. EDS elemental
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maps were acquired in STEM mode using the instrument’s 4 SDD detectors at a count rate of
approximately 300,000 cps. EDS data was analyzed using Bruker Esprit software v4.0.
- XAS data processing
In the XAS signal, we can transform from energy space to wave vector space using the following
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relationship, k  2m( E  E0 ) /  . Where k is the electron wavenumber, m is the electron mass,
E0 is the K-edge absorption energy of Sn, and ħ is the Planck’s constant.1 The spectra are
weighted by k2 to compensate for the amplitude decay. The Fourier-transform is applied to the
k2-weighted spectra, from k = 1 to k = 12 Å-1 range. By the Fourier transformation, we can
transform the wave vector space to real space, which is desirable for EXAFS analysis.
- Interpretation of the Sn-K edge EXAFS
We employ EXAFS to determine the local structure near the Sn element to better understand its
role as a dopant. Figure S4(a) and (b) show the Fourier transform of the EXAFS from all 4
conditions in both GNO and GWO. The EXAFS transforms are all quite similar with a first
nearest neighbor at about 2 Å, but importantly, we find that there is no second nearest neighbor
peak. This means that Sn in the films is locally disordered or in a nanocrystalline environment.2
This result is consistent with the XRD that there is no signal from Sn metal, SnO, or SnO2 since
amorphous or nanocrystalline phases can be difficult to detect by XRD. For Sn2+ or Sn4+
substituting for the Ga3+ in the Ga2O3 lattice, the second peak was observed in previous study.3
To lend insight into the Sn environment, we determine the distance between the Sn and the first
nearest neighbor by fitting the spectra, using ATOMS and FEFF6 codes imbedded in Artemis
program.4 We show the fitting results of every point at GNO and GWO films in Table S1, and
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we see that the fitting results are almost same at every point. We find that the distance between
the Sn and the first nearest neighbor is about 2.03 Å. The distance between Sn and the first
nearest neighbor in SnO, SnO2, Sn2+ in amorphous Ga2O3, and Sn4+ in amorphous Ga2O3 are
2.04 Å, 2.05 Å, 2.03 Å and 2.12 Å, respectively.3,5 And the distance of Sn-Sn in Sn metal is
around 2.21 Å; therefore, we can conclude that the first nearest atom is oxygen.5,6 Thus, the Sn
K-edge XANES and EXAFS results suggest that most of the Sn exists in nanocrystalline or
amorphous SnO, SnO2, and substituting for Ga3+ in amorphous/nanocrystalline gallium oxide
phases.
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Figure S1. The XRD data of the GNO and GWO films. The spectra of (i) correspond to the point
of 520 °C, (ii) correspond to 400 °C, (iii) correspond to 320 °C, and (iv) correspond to 280 °C. At
low temperature, the Ga2O3 is amorphous or polycrystalline, and at high temperature (520 °C) the
Ga2O3 is preferentially oriented in the (-201) direction. The peak at ~39 degree corresponds to (402) peak of the beta crystalline Ga2O3.
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Figure S2. k2-weighted EXAFS spectra for (a) GNO and (b) GWO films. The x-axis is
proportional to the distance between the Sn and neighboring atoms, and the intensity in the yaxis is related to the number of atoms at that distance.
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N
ΔEo (eV)
ΔReff (Å)
520 °C
4.89(9)
7.22(46)
2.03(0)
0.011
400 °C
4.76(6)
7.33(57)
2.03(1)
0.016
320 °C
4.79(9)
7.20(46)
2.03(0)
0.016
280 °C
4.83(14)
7.44(66)
2.03(1)
Film
σ2 (Å-2)
R-factor
GNO
0.015
0.0070
520 °C
4.81(13)
7.47(69)
2.03(1)
0.019
400 °C
4.92(15)
6.94(77)
2.03(1)
0.021
320 °C
4.91(13)
7.69(63)
2.03(1)
0.020
280 °C
4.86(7)
7.19(64)
2.03(1)
0.019
GWO
Table S1. Sn K-edge EXAFS fitting parameters from the spectrum of 400 °C point at the GNO
film. The fitting range is 0.8 Å < R < 2.0 Å. In the table, N is the coordination number of atoms,
ΔE0 is an energy shift, ΔReff is the distance between the Sn atom and the nearest atom, and σ2 is
the Debye-Waller factor.
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Reference
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S. Calvin, XAFS for Everyone. (CRC Press, 2013), p.8.
S. C. Siah, S. W. Lee, Y. S. Lee, J. Heo, T. Shibata, C. U. Segre, R. G. Gordon, and T.
Buonassisi, Applied Physics Letters 104 (24), 242113 (2014).
S. C. Siah, R. E. Brandt, K. Lim, L. T. Schelhas, R. Jaramillo, M. D. Heinemann, D. Chua,
J. Wright, J. D. Perkins, C. U. Segre, R. G. Gordon, M. F. Toney, and T. Buonassisi,
Applied Physics Letters 107 (25), 252103 (2015).
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