Bandgap and Electronic Structure Determination of Oxygen-Containing Ammonothermal InN: Experiment and Theory

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 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 1.89-2.10 eV for polycrystalline InN and about twice the value recently obtained for single crystalline thin films between 0.70 and 1.0 eV. The possible origin of the measured increased band gap is discussed in terms 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 into the structure of InN in the presence of oxygen impurities, we perform DFT calculations for hypothetical Wurtzite-type InO0.5N0.5 and InO0.0625N0.9375 and the known c-In2O3 and find that the measured O K-edge spectra of the samples agree well with InO0.0625N0.9375. 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 In2O3 in the samples.