Study of Optical Properties of MOCVD-Grown Rutile GeO2 Films

TL;DR

This study investigates the optical properties of MOCVD-grown rutile GeO₂ thin films, revealing their emission characteristics, bandgap, and defect behavior, which are promising for deep-ultraviolet optoelectronic applications.

Contribution

It provides the first comprehensive optical characterization of crystalline r-GeO₂ films grown via MOCVD, linking domain size to emission and confirming a wide bandgap around 4.8-5.0 eV.

Findings

Broad visible cathodoluminescence peaks at 470 nm and 520 nm.

Bandgap estimated at approximately 4.75 eV from XPS.

Optical absorption edge near 250-260 nm indicating a wide bandgap.

Abstract

Rutile germanium dioxide (r-GeO$_2$) is a promising ultra-wide bandgap (UWBG) semiconductor, offering a high theoretical Baliga figure of merit, potential for p-type doping, and favorable thermal and electrical properties. In this work, we present a comprehensive optical investigation of crystalline r-GeO$_2$ thin films grown on r-TiO$_2$ (001) substrates via metal-organic chemical vapor deposition (MOCVD). Cathodoluminescence (CL) spectroscopy reveals broad visible emissions with distinct peaks near 470~nm and 520~nm. CL mapping indicates enhanced emission intensity in regions with larger crystalline domains, highlighting the correlation between domain size and optical quality. X-ray photoelectron spectroscopy (XPS) confirms the presence of Ge$^{4+}$ oxidation state and provides a bandgap estimation of $\sim$4.75~eV based on valence band and secondary electron cutoff analysis. UV--Vis transmittance measurements show a sharp absorption edge near 250--260~nm, corresponding to an optical bandgap in the range of 4.81--5.0~eV. These findings offer valuable insights into the defect-related emission behavior and band-edge characteristics of r-GeO$_2$, reinforcing its potential for future applications in power electronics and deep-ultraviolet optoelectronic devices.