Unravelling the oxygen influence in cubic bixbyite In2O3 on Raman active phonon modes by isotope studies

Abstract

In this study, we performed comprehensive investigations on the Raman active phonon modes in cubic bixbyite In₂O₃, an important oxide based, wide-bandgap semiconductor. Fundamental insights into the lattice dynamics are revealed, by determining the atomistic contribution to all modes and their frequencies by density functional perturbation theory calculations. Those simulations were performed for different compositions of ¹⁶O and ¹⁸O isotope ratios, including their pure states. An increasing red-shift of the mode frequencies with increasing ¹⁸O content for all modes, due to the increased atomic mass, is revealed. For the seven lowest energy modes, this relative shift is below 1%, whereas for the remaining 15 higher energetic modes a shift of about 5.5% was identified. All modes have energy contributions of both indium and oxygen lattice sites, except for one, which corresponds to a pure oxygen vibrational state. Applying Raman spectroscopy, those results could be verified experimentally with excellent agreement. Investigated samples consisted of a bulk single crystal with ¹⁶O isotopes and a MBE grown thin film as the ¹⁸O sample. Time-of-flight secondary ion mass spectrometry confirms the purity of the oxygen isotope in the sample. These isotopologue studies allow for a direct experimental access to fundamental material properties in cubic In₂O₃ by means of Raman spectroscopy. For example, we speculate, that the presence of oxygen vacancies in In₂O₃ would result in a shift of modes that are dominated by O-vibrations, e. g., E_g⁽⁴⁾ or A_g⁽⁴⁾, towards lower frequencies.