Unravelling the oxygen influence in cubic bixbyite In$_2$O$_3$ on Raman active phonon modes by isotope studies

TL;DR

This study combines isotope substitution and Raman spectroscopy with theoretical calculations to analyze lattice dynamics and oxygen's role in cubic bixbyite In$_2$O$_3$, revealing how oxygen isotope composition affects phonon modes.

Contribution

It provides a detailed atomistic understanding of phonon modes in In$_2$O$_3$ using isotope studies and density functional theory, linking isotope effects to lattice vibrations.

Findings

Mode frequencies shift with oxygen isotope mass, with larger shifts in higher energy modes.

Experimental Raman data agree well with theoretical predictions.

Oxygen vacancies may cause detectable shifts in oxygen-dominated vibrational modes.

Abstract

In this study, we performed comprehensive investigations on the Raman active phonon modes in cubic bixbyite In$_2$O$_3$, 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 $^{16}$O and $^{18}$O isotope ratios, including their pure states. An increasing red-shift of the mode frequencies with increasing $^{18}$O 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 $^{16}$O isotopes and a MBE grown thin film as the $^{18}$O 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 In$_2$O$_3$ by means of Raman spectroscopy. For example, we speculate, that the presence of oxygen vacancies in In$_2$O$_3$ 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.