Bandgap and Electronic Structure Determination of Oxygen

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Bandgap and Electronic Structure Determination of OxygenContaining Ammonothermal InN: Experiment and Theory M. Ruhul Amin, Tristan de Boer, Peter Becker, Jan Hertrampf, Rainer Niewa, and Alexander Moewes J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12369 • Publication Date (Web): 07 Mar 2019 Downloaded from http://pubs.acs.org on March 13, 2019

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The Journal of Physical Chemistry

Bandgap and Electronic Structure Determination of Oxygen - Containing Ammonothermal InN: Experiment and Theory M. Ruhul Amin,† T. de Boer,† Peter Becker,‡ Jan Hertrampf,‡ Rainer Niewa,‡ and Alexander Moewes∗,† †Department of Physics and Engineering Physics, University of Saskatchewan, 116 Science Place, Saskatoon, Saskatchewan , S7N 5E2, Canada ‡Institute of Inorganic Chemistry, University of Stuttgart, Pfaffenwaldring, 55, 70569 Stuttgart, Germany E-mail: [email protected] Phone: +1 (306) 966-6431. Fax: +1 (306) 966 6400

Abstract The electronic structure and band gap of InN synthesized by the ammonothermal method are studied by synchrotron-based soft X-ray absorption spectroscopy (XAS), emission spectroscopy (XES), X-ray excited optical luminescence (XEOL) spectroscopy, and density functional theory (DFT). The measured N K-edge XAS and XES spectra and the XEOL spectra are used to estimate the band gap of InN and it is found to be 1.7 ± 0.2 eV for both independent measurements, which is close to the initially reported values in the range of 1.89 eV - 2.10 eV for polycrystalline InN and about twice of the value recently obtained for single crystalline thin films between 0.70 eV and 1.0 eV. The possible origin of the measured increased band gap is discussed in terms

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of the presence of oxygen impurities and other impurity phases. Oxygen K-edge XES and XAS measurements are performed and reveal the presence of oxygen impurities. To gain insight in the structure of InN in the presence of oxygen impurities, we perform DFT calculations for hypothetical Wurtzite-type InO0.5 N0.5 and InO0.0625 N0.9375 and the known c-In2 O3 and find that the measured O K-edge spectra of the samples agree well with InO0.0625 N0.9375 . The XEOL measurements also confirm the presence of oxygen impurities, which are caused by substituting nitrogen atoms with oxygen atoms, and the impurity phase of In2 O3 in the samples.

INTRODUCTION Group III nitrides are attractive and widely studied semiconductor materials due to their potential applications. 1–8 Among them, InN is the least studied material but it has attracted intense interest due to its distinct physical and optical properties. Because of the relatively small band gap of InN, it has been used for technological applications in opto-electronic devices, particularly in high efficiency solar cells, 1–5 and in green and blue light emitting diodes and lasers. 6,7 Recently, an active debate has arisen about the optical band gap of InN, when a few groups reported that the band gap of InN is between 0.70 eV and 1.0 eV, 8–16 which is about half of the initially reported value in the range of 1.89 eV - 2.10 eV. 11,17–25 It was reported that high-quality InN single crystalline thin flims grown by molecular-beam epitaxy 8–12 or metalorganic chemical vapour deposition 14–16 yield the smaller band gap. On the other hand, larger band gaps have been reported for polycrystalline InN films grown by dc discharge, 17–21 reactive cathodic sputtering 22 or RF-sputtering. 23–25 Therefore, the synthesis process and impurities play a major role affecting the experimental band gap. In this paper, we have studied InN samples, which were synthesized by the ammonothermal method 26 for the first time. Possible explanations for a smaller band gap of InN include defects, non-stoichiometry and non-uniformity films 15 and Mie-resonance. 16 On the other hand, a strong Moss-Burstein effect due to the residual large carrier concentrations, 11,13,27 oxygen 2


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inclusion, 11,21,24,25,27 and quantum confinement due to spontaneous formation of needle-like nanocrystals 10 have been proposed as possible reasons for an increased band gap of InN.

The most prominent explanation for a larger band gap is oxygen inclusion. 11,21,24,25,27 Davydov et al. 11 showed that a sample with a band gap in the region of 1.8 - 2.1 eV contained up to 20% of oxygen. Recently, Yoshimoto et al. 24 reported that the optical band gap of polycrystalline InN increased from 1.55 eV to 2.27 eV with increasing oxygen contamination from 1% to 6%. More recently, it has been suggested that the larger band gap of InN upon incorporation of oxygen may be due to the formation of indium oxide nitride. Several authors 11,28–30 have studied indium oxide nitride at different oxygen concentrations and found that for higher oxygen concentrations, the thin film can be considered a solid solution between InN and In2 O3 , resulting in an increased band gap due to the large band gap (3.1 eV) of In2 O3 . 31–34 Still, there is not any clear experimental and theoretical evidence about the effects of oxygen in InN band gap but oxygen incorporation is commonly accepted and thus it needs further study.

To understand the band gap of InN and its origin, we have used synchrotron-based soft Xray emission (XES) and absorption spectroscopy (XAS), which are sensitive to the occupied and the unoccupied local partial density of states (pDOS), respectively, X-ray excited optical luminescence (XEOL) spectroscopy as well as density functional theory (DFT) calculations. XEOL spectra can be used for understanding the electronic structure, presence of defects, and luminescence properties of solids. 35,36 Previous studies have typically used the photoluminescence (PL) technique to determine the band gap of InN. 22,37,38 The use of the XEOL technique is new and it has been used here to estimate the band gap of InN and to identify the oxygen impurities and other impurity phases in InN. XAS and XES are used to probe directly the conduction band and valence band of the material, respectively. 31,39,40 The O K-edge XAS and XES measurements and XEOL measurements provide clear evidence

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for the presence of oxygen impurities and another impurity phase in InN. Meanwhile, to interpret the experimental band gap and spectra, we also have performed DFT calculations. The experimental results agree very well with the calculations. Finally, to understand the structure of InN in the presence of oxygen impurities, we have performed DFT calculations for the hypothetical Wurtzite-type InO0.5 N0.5 and InO0.0625 N0.9375 as well as the known cIn2 O3 .

EXPERIMENTAL SECTION Synthesis The ammonothermal synthesis process of the InN samples studied in this work is described in detail in Ref. 26 InN has been synthesized from InCl3 and KNH2 in supercritical ammonia at around 280 MPa at two temperatures leading to microcrystals with markedly different morphologies showing differing aspect ratios. In samples synthesized at 663 K furnace temperature mainly plate-shaped crystals with diameters up to 4 µm are present according to SEM, while at 773 K rod like crystals with lengths of up to 4 µm can be found. Figures 1(a) and 1(b) show typical SEM images of samples from ammonothermal synthesis. We also have used a high-purity (99.999%) c-In2 O3 sample which was obtained commercially from Sigma-Aldrich.

XAS, XES and XEOL Measurements XES and XAS spectra at the N K-edge and at the O K-edge were collected at the REIXS 41 beamline and the XEOL spectra were measured at VLS-PGM 42 beamline at the Canadian Light Source in Saskatoon, Saskatchewan, Canada. The absorption spectra were measured in the partial fluorescence yield (PFY) and the total electron yield (TEY) modes. TEY is very surface sensitive and is thus included to explore surface and PFY is used to probe the bulk. The resolving power, E/∆E, for the monochromator at REIXS is 8000, which corresponds 4


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to an energy resolution value, ∆E, of 0.06 eV at O-K-edge (500 eV) and of 0.05 eV at N K-edge (400 eV) XAS. All spectra were collected at room temperature with the incoming beam under 45o angle to the sample surface. The E/∆E for the spectrometer at the REIXS is 2000, which is equivalent to an energy resolution of 0.25 eV at 500 eV and 0.20 eV at 400 eV for XES. The XEOL data were collected using the Ocean Optics QE 65000 fast CCD spectrophotometer. 43 For all of the X-ray measurements, the powder samples were pressed into carbon tape, prior to transfer to the ultra-high vacuum chambers. The measured XAS and XES spectra were calibrated using hexagonal boron nitride (h-BN) for the N K-edge and bismuth germanium oxide (BGO) for the O K-edge. The XAS spectrum was calibrated using an initial h-BN (BGO) peak at 402.1 eV (532.7 eV) for N (O) K-edge, while the XES spectra were calibrated using elastic scattering features as this provides a common energy axis with the XAS measurements.

Figure 1: (a) SEM image of platelets InN from ammonothermal synthesis at 663 K. (b) SEM image of rods InN from ammonothermal synthesis at 773 K.

DFT CALCULATION DETAILS Density functional theory (DFT) calculations using the available experimentally determined crystal structure of InN 44–47 based on the full-potential augmented plane-wave method with 5



scalar-relativistic corrections were performed using the WIEN2K software package. 48 For the exchange-correlation functional, the Perdew-Burke-Ernzerhof variant of the generalized gradient approximation (PBE-GGA) 49 was used. Calculations using the modified Becke Johnson (mBJ) exchange potential were conducted to obtain a more accurate estimate of the band gap. 50 The DFT calculations are used to calculate the band gap, the XAS and XES spectra and the pDOS. The spectra were calculated by multiplying the pDOS with the dipole transition matrix and the radial transition matrix. 51 The calculated spectra were broadened using the combination of a Lorentzian function to mimic the core-hole lifetime broadening, and a Gaussian function to mimic the instrumentation-related broadening. 52 Since the PBE-GGA underestimates the band gap of semiconductors, the calculated XAS and XES spectra were rigidly shifted by a constant amount to facilitate comparison with experiment. 52 In the experimental measurements, the XAS and XES spectra depend on the final state, but for the K-edge absorption, the final state has a core-hole in the nitrogen 1s energy level, 53,54 which tends to shift spectral weight to lower energies. To account for this effect, a 2 × 2 × 2 supercell was created and one core electron (1s) was removed from one of the nitrogen atoms in the supercell and one background lattice charge was added to the supercell to preserve charge neutrality.

RESULTS AND DISCUSSION N and O K-edge Absorption and Emission spectra The calculated and measured N Kα XES and 1s XAS spectra are shown in Fig. 2. At first, the experimental and calculated N K-edge XAS spectra of InN samples will be discussed, which are shown in Fig. 2(b). Black and red spectra shown in this panel correspond to the PFY of the rods and platelets of InN and dotted lines represent the TEY of them. The two absorption spectra for each sample, which are surface and bulk sensitive, respectively, match well, indicating that the surface is not differently contaminated and the same as the bulk. 6


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Figure 2: Measured and calculated N K-edge XES and XAS spectra. (a): Calculated and measured N K-edge XES spectra. Experimental and calculated (orange) N K-XES spectra of InN are compared with Wurtzite-type InO0.0625 N0.9375 (green) and InO0.5 N0.5 (blue). (b): Measured and calculated N K-edge XAS spectra. Experimental PFY (solid line) and TEY (dotted line) of InN are compared with core hole (solid line) and ground-state calculations (dash - dotted line) of InN, InO0.0625 N0.9375 and InO0.5 N0.5 . (c): Second derivatives of the XES spectra of N K-edge of InN, with peaks corresponding to valence band edges indicated by the arrows. (d): Second derivative of PFY (TEY) of XAS spectra of N K-edge of platelets InN in red solid line (dotted line) and rods InN in black solid line (dotted line) with peaks corresponding to conduction band edges indicated by the arrows.

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The measured XAS spectra of rods and platelets are identical, which is evidence that the electronic properties are not dependent on sample morphology, and are uniform throughout the sample. The measured overall spectral features are similar to those reported elsewhere for nanorods 20 and films. 12,20 The experimental XAS spectra of both InN rods and platelets are compared with the core hole (orange line) and ground state (dash - dotted orange line) calculations and it is found that both measured spectra agree better with the core hole calculation with all major features reproduced at approximately the correct energy position and at the correct peak height and therefore the effect of core hole needs to be taken into account. This agreement supports the structure and space group determined by X-ray diffraction. 44–47 This agreement also provides strong experimental support for the calculated band gap and electronic structure.

As we will discuss later, sufficient oxygen impurities were present to detect O K-edge XES and XAS spectra. To account for this oxygen impurities, we have performed DFT calculations for hypothetical Wurtzite-type InO0.5 N0.5 and InO0.0625 N0.9375 and compared them with measured N K-edge XAS and XES spectra of InN in Fig. 2. For the calculations of the hypothetical Wurtzite-type InO0.5 N0.5 , half of the nitrogen was substituted by oxygen in the experimental crystal structure of InN, and for hypothetical Wurtzite-type InO0.0625 N0.9375 calculations, a 2x2x2 supercell of InN was created and one nitrogen atom was substituted by an oxygen atom in the supercell, and the geometry was optimized afterwards for each cases. For force minimization, the crystal structures were allowed to relax for both cases. Y. Hattori et al. 27 theoretically found that oxygen is energetically favorable to exist mainly as singly charged isolated defect in InN. The ground state (dash - dotted line) and the core hole calculated N K-edge XAS spectra for InO0.0625 N0.9375 and InO0.5 N0.5 are presented in Fig. 2(b). It is clear from Fig. 2(b) that there is very little difference between the calculated InO0.0625 N0.9375 (green) and the measurements, while the calculated InO0.5 N0.5 (blue) does not agree with the measurements, indicating that oxygen does not affect the N-sites on a

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large scale when a small amount of oxygen is present in InN.

The N Kα XES excited at 441.1 eV, well above the N K absorption edge, and calculations of InN are presented in Fig. 2(a). The measured XES spectra of both types of InN samples show two features which are consistent with our calculations (orange). The calculated pDOS show that In s and O p hybridization contribute to the peak at 390.3 eV and In p, d and O p contribute to the peak at 394.0 eV. The authors in 12,20 also reported the same features for bulk InN systems. The calculated XES spectrum of InO0.0625 N0.9375 agrees better with the calculations of pure InN and measurements than InO0.5 N0.5 . Since the oxygen impurity concentration is small and the impurities only weakly affect the nearest N atoms, the N K-edge spectra are insufficiently sensitive to distinguish between pure InN and InN with a small amount of oxygen impurities.

We are now turning to the discussion of the measured O K-edge spectra which is presented in Fig. 3. To examine the crystal structure of InN in the presence of oxygen impurities, the O K-edge XAS and XES spectra of InN have been measured and compared with experimental and calculated c-In2 O3 , as well as calculated hypothetical Wurtzite-type InO0.5 N0.5 and InO0.0625 N0.9375 , shown in Fig. 3. For the calculation of In2 O3 , the experimental crystal structure of c-In2 O3 was used as an input. 31 It is shown in Fig. 3(b) that the experimental O K-edge XAS spectra of rods and platelets of InN are almost identical and the strong XAS signals confirm that oxygen impurities are present in the InN samples. The calculated and experimental XAS spectra for the c-In2 O3 reference agree very well. It is also observed that the O K-edge XAS spectra of c-In2 O3 bear some similarity to those of the InN samples, but there are some additional features and general broadening of features. On the other hand, comparing the XAS experimental and calculated spectra, we see that the spectrum of InO0.0625 N0.9375 agrees better with InN than InO0.5 N0.5 . Fig. 3(a) describes the O K-edge XES spectra of calculated and measured c-In2 O3 , measured InN, calculated InO0.5 N0.5 , and

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Figure 3: Measured and calculated O K-edge XES and XAS spectra. (a): Calculated and measured O K-edge XES spectra. Experimental (cyan) and calculated (purple) XES spectra of c-In2 O3 are compared with experimental XES spectra of platelets (red) and rods (black) of InN as well as the calculated hypothetical Wurtzite-type InO0.0625 N0.9375 (green) and InO0.5 N0.5 (blue). (b): Calculated and measured O K-edge XAS spectra. Measured PFY, core hole and ground state (dash - dotted) calculations of O K-edge XAS spectra of c-In2 O3 are compared with measured O K-edge XAS of platelets and rods of InN as well as core hole and ground state calculations of hypothetical Wurtzite-type InO0.0625 N0.9375 and InO0.5 N0.5 .

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InO0.0625 N0.9375 . The calculated and measured O K-edge XES spectra of c-In2 O3 agree well. It is found that both experimental spectra of rods and platelets of InN are identical. It is noted that the measured XES spectra of InN show a sharp peak at around 528.5 eV. As for the XAS, the calculated XES spectrum of InO0.5 N0.5 does not match well with these measurements but the calculated InO0.06 N0.94 does. However, the spectrum would agree better in height, width and positions of the peaks if the oxygen concentration is less than 6% but the required supercell of 2×2×3 instead of the current 2×2 ×2 would render such calculations computationally prohibitive. Those impurities are caused by substituting nitrogen atoms with oxygen atoms. Experimental determination of the oxygen content by the hot carrier gas extraction technique for these bulk samples resulted in 4 wt.%, 26 meaning a maximum of about 3 % oxygen substituting nitrogen in the InN phase.

Band Gap The N Kα XES and 1s XAS spectra of InN are used to estimate the band gap of InN samples. The band gap is defined as the energy difference between the top of the valence band and the bottom of the conduction band. To determine the band edges of our InN samples, the second derivatives of the XAS and XES spectra are taken. 31,39,40 In this method, the top of the valence band and bottom of the conduction band are taken to be the first peaks above the noise at the upper edge of the XES and at the lower edge of the XAS spectra, as indicated with arrows in Fig. 2(c) and Fig. 2(d), respectively. This method is less ambiguous than other methods like linear extrapolation yielding a more reproducible band gap. Note that, the XAS second-derivative peak is found at the same energy position (396.9 eV) in both the PFY mode (solid line) and TEY mode (dotted line) for both rods and platelets InN which is presented in Fig. 2(d). However, the authors in Ref. 12 found an 0.6 eV larger separation for TEY XAS and claimed that this separation is due to the Fermi level being pinned high above the conduction band minimum at the surface due to the intrinsic electron 11



accumulation.

As mentioned earlier in the calculations section, XAS probes the conduction band in the presence of core hole instead of in the ground state. Therefore, the band gap can only be determined correctly from XES and XAS onsets once the core hole shift has been accounted for. The core hole shift is calculated by the energy difference between the conduction band onset in the ground and core hole state calculations. For InN, a negligible core hole shift (0 eV) is observed so that the experimentally determined band gap corresponds to the XES and XAS onsets.

Table 1: Measured, calculated and previously reported band gaps for InN. The measured band gap is denoted by ∆expt. and the calculated band gap is denoted by ∆mBJ using mBJ functional. The experimental techniques and calculation methods for previously reported band gap determination are also included. All numbers are in eV. ∆expt.

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Other expt. ∆ 0.7 8,9,11,16 0.8 12,13,16 0.9 10 1.0 14,15 1.89 17,22,23 1.90 15,20,21,24 1.95 18 2.0-2.1 16,19,25

Determination method Optical absorption 8,9,13–16,18,19,23–25 Photoluminescence 8–11,13,14,21,22 XES and XAS 12,20 GGA -mBJ 27 (calc.) DFT - G0 W0 44 (calc.) DFT - LDA 45,46 (calc.)

The experimentally determined band gap of both rods and platelets of InN samples is found to be 1.7 ± 0.2 eV for both PFY and TEY modes, which is presented in Table 1 along with a summary of the band gap values established in the literature. Piper et al. 12 have used XAS and XES measurements to estimate the band gap for wurtzite InN (000¯1) thin films and found to be 0.8 eV and 1.4 eV for TEY and TEY modes, respectively, by the linear extrapolation method. The authors in Ref. 20 estimated the band gap for polycrys12


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talline nanorods and films of InN using the combination of XES and XAS measurements and found that it is 1.9 eV using the linear extrapolation method. It should be noted here that we have also examined our measured band gap using the linear extrapolation method. We found that our experimentally determined band gap is unchanged at 1.7 ± 0.2 eV, reflecting that the experimentally determined band gap is insensitive to the specific technique used to determine the onset of spectral weight. The experimental uncertainty originates from the monochromator resolution and the energy calibration. This determination of the band gap is in contrast with the calculated band gap using PBE-GGA which results in a metallic state for Wurtzite-type InN and a band gap of 0.89 eV using the mBJ exchange-correlation potential. 50 This underestimation of the band gap is typical of LDA and GGA exchange correlation functional and is due to the fundamental limitations of DFT. Specifically, the formal electronic band gap, ∆g = I - A, where I is the first ionization potential and A is the electron affinity may not match the Kohn - Sham derived band gap due to a derivative discontinuity in the exchange - correlation functional. 55 Our experimentally measured band gap agrees well with determinations made using other experiments , 17–25 which are shown in Table 1. All our measurements were performed at room temperature. Prior work 56 demonstrates that the band gap of InN varies weakly with temperature. Specifically, the variation of band gap for InN was found to be 0.023 eV between 4.2 K and 300 K. As such, the band gap measured experimentally is representative of the intrinsic electronic band gap.

X-ray Excited Optical Luminescence (XEOL) Spectra Recently, researchers have been using X-ray excited optical luminescence (XEOL) spectra to explore electronic structure, presence of defects, and luminescence properties of solids. 35,36 Here, we have performed XEOL measurements on InN for a variety of excitation energies to probe the depth dependence of luminescence. Figure 4 shows XEOL spectra of InN at three excitation energies at 90 eV (Fig. 4(a)), 165 eV (Fig. 4(b)), and at 200 eV (Fig. 4(c)). 13



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Figure 4: XEOL spectra of InN platelets, c-In2 O3 and the variation of photon attenuation length with excitation energy. Experimental XEOL spectra of InN at three different excitation energies are presented in (a), (b) and (c). The XEOL spectra exhibit features located at 274 nm, 288 nm, 305 nm, 328 nm, 351 nm, 425 nm, 667 nm, 921 nm and at 973 nm. To fit the two broad peaks located at 425 nm and at 667 nm, three Gaussians and the luminescence spectrum of c-In2 O3 (green) are used. The variation of photon attenuation length (logarithmic scale) with excitation energy for InN is presented in (d) with data from Ref. 57 The three stars labeled in this graph represent the three excitation energies in our measurements.

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XEOL spectra of rods (not shown) and platelets (in Fig. 4) of InN are identical. Therefore, the XEOL spectra are independent of sample morphology. The lowest excitation energy for InN was at 90 eV, which corresponds to 20 nm photon attenuation length and due to the limitation of excitation energy range of the VLS-PGM beamline, 42 the highest excitation was at 200 eV. However, the largest photon attenuation length in this energy range for InN is 270 nm when the excitation energy is 165 eV. The wide variation of excitation energy probes the depth from 20 nm to 270 nm of the sample. For InN, the variation in photon attenuation length with excitation energy is shown in Fig. 4(d). 57 The authors in 58 reported the cathodoluminescence spectra of fluorine-doped c-In2 O3 , which matches well with our spectra, but they were not able to measure the spectra lower than 330 nm and higher than 975 nm due to the instrumental limitations. They observed two broad peaks at 410 nm and 650 nm and claimed that the origin of the former peak corresponds the indirect conduction band to valence band transition of c-In2 O3 and the 650 nm peak originates from oxygen defects.

Figures 4(a) - 4(c) reveal the individual Gaussians and the c-In2 O3 luminescence spectrum (green) that fit the experimental broad luminescence spectral features located at 425 nm and at 667 nm best for all three cases. We also have used more Gaussians in high and low wavelength regions to fit the XEOL spectra which are not presented in Fig. 4. Note that, the luminescence feature of c-In2 O3 at around 667 nm is due to oxygen defects. 58–60 It is shown that the c-In2 O3 luminescence spectrum is present for all excitation energies and its intensity is almost identical for all excitation energies (for different attenuation lengths), indicating that c-In2 O3 might be present in the InN samples. According to the Gaussian fitting, a small g3 Gaussian feature is observed in the same position (at 651 nm) of luminescence spectrum of c-In2 O3 . The g3 is much narrower (FWHM: 100 nm) than the c-In2 O3 spectrum (FWHM: 223 nm) and can thus be uniquely distinguished despite occurring at the same energy. Interestingly, the peak of Gaussian g3 feature lines up exactly with the N K-

15



edge XES and XAS separation of 1.7 ± 0.2 eV for InN, therefore, it could be a band-to-band transition of InN. The authors in Refs. 22,37,38 observed band-to-band transition at 1.8 eV of InN in their PL measurements. The g1 and g2 Gaussian features at 422 nm and at 520 nm might be due to the oxygen defects for In2 O3 . This is because the origin of PL spectra of In2 O3 in the range from violet to green such as at 506 nm in Ref., 61 at 425 nm, 429 nm, 442 nm, and at 466 nm in Ref., 62 and 442 nm, 468 nm, and at 524 nm in Ref. 63 is due to oxygen impurities. Therefor oxygen impurities and In2 O3 impurity phase are present in InN. The authors in 11 found the band gap of 1.8 - 2.0 eV for InN and claimed the reason for an increased band gap is to formation of optically transparent solid solution of InN-In2 O3 at an oxygen concentration of about 20%. Several other authors 28–30 also suggested a solid solution between InN and In2 O3 and found increased band gap. Note that, the positions of all the Gaussians remain at the same position for different excitation energies. The luminescence features at 274 nm, 288 nm, 305 nm, 328 nm, and 351 nm are due to presence of N2 gas produced by irradiation-induced decomposition of InN as assigned previously . 64,65 There are some defects, which are leading to the features at 921 nm and at 970 nm.

CONCLUSIONS We have studied the electronic structure and the band gap of ammonothermal InN experimentally and theoretically using soft X-ray synchrotron spectroscopy measurements and DFT calculations, respectively. The measured and calculated band gaps of InN are found to be 1.7 ± 0.2 eV and 0.89 eV, respectively, which are in excellent agreement with prior experiments 17–20,22–25 and calculations, 44–46 respectively. The band gap of InN obtained from the XEOL spectra confirms the measured value from XAS and XES spectra. The origin of the measured band gap is discussed in terms of the presence of oxygen impurities and impurity phases of InN. The experimental O K-edge XAS and XES spectra of InN and DFT calcula-

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tions for hypothetical Wurtzite-type InO0.5 N0.5 and InO0.0625 N0.9375 and the known c-In2 O3 suggest that less than 6% oxygen impurities are present in our InN samples. Those impurities originate from substituting nitrogen atoms with oxygen atoms. This oxygen content is an excellent agreement with a previously experimentally determined value. 26 The XEOL spectra also confirm the presence of oxygen impurities and impurity phase of In2 O3 in InN.

AUTHOR INFORMATION Corresponding Author ?

E-mail: [email protected]

ORCID Alexander Moewes: 0000-0002-9218-2280 M. Ruhul Amin: 0000-0002-1289-4713 T. de Boer: 0000-0002-6898-5040 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The experiments for this paper were performed at the Canadian Light Source, which is funded by the Canada Foundation for Innovation, the Natural Science and Engineering Research Council of Canada (NSERC), the National Research Council Canada, the Canadian Institutes of Health Research, the Government of Saskatchewan, Western Economic Diversification Canada, and the University of Saskatchewan. M.R.A. is grateful to the Government of Saskatchewan for financial support of Saskatchewan Innovation and Opportunity Scholarship. We acknowledge support from NSERC and the Canada Research Chair program. The authors also acknowledge Compute Canada. The calculations presented in this paper were performed on the Cedar high-performance computing cluster, which is the part of the West17



Grid (www.westgrid.ca) and Compute Canada Calcul Canada (www.computecanada.ca). We also thank the German Research Foundation (DFG) for financial support within the project FOR 1600: Chemie und Technologie der Ammonothermal-Synthese von Nitriden.

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Bandgap and Electronic Structure Determination of Oxygen

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