Metalorganic Chemical Vapor Deposition of \b{eta}-(AlxGa1-x)2O3 thin films on (001) \b{eta}-Ga2O3 substrates

TL;DR

This study demonstrates the growth of high-quality ta-(AlxGa1-x)2O3 thin films on ta-Ga2O3 substrates via MOCVD, analyzing their structural, compositional, and electronic properties with a focus on alloy inhomogeneity and band alignment.

Contribution

It presents a systematic approach to grow and characterize coherently strained ta-(AlxGa1-x)2O3 films with up to 25% Al, revealing alloy inhomogeneity and band offsets.

Findings

Successful epitaxial growth of ta-(AlxGa1-x)2O3 films on ta-Ga2O3 substrates.

Observation of alloy inhomogeneity and local Al segregation.

Type-II band alignment at heterojunctions.

Abstract

Phase pure \b{eta}-(AlxGa1-x)2O3 thin films are grown on (001) oriented \b{eta}-Ga2O3 substrates via metalorganic chemical vapor deposition (MOCVD). By systematically tuning the precursor molar flow rates, the epitaxial growth of coherently strained \b{eta}-(AlxGa1-x)2O3 films are demonstrated with up to 25% Al compositions as evaluated by high resolution x-ray diffraction (XRD). The asymmetrical reciprocal space mapping confirms the growth of coherent \b{eta}-(AlxGa1-x)2O3 films (x < 25%) on (001) \b{eta}-Ga2O3 substrates. While the films show smooth surface morphologies, the alloy inhomogeneity with local segregation of Al along (-201) plane is observed from atomic resolution STEM imaging, resulting in wavy and inhomogeneous interfaces in \b{eta}-(AlxGa1-x)2O3/\b{eta}-Ga2O3 superlattice structure. Room temperature Raman spectra of \b{eta}-(AlxGa1-x)2O3 films show similar characteristics peaks as (001) \b{eta}-Ga2O3 substrate without obvious Raman shifts for films with different Al compositions. Atom probe tomography (APT) was used to investigate the atomic level structural chemistry with increasing Al content in the \b{eta}-(AlxGa1-x)2O3 films. A monotonous increase in chemical heterogeneity is observed from the in-plane Al/Ga distributions which was further confirmed via statistical frequency distribution analysis (FDA). Although the films exhibit alloy fluctuations, n-type doping with good electrical properties are demonstrated for films with various Al compositions. The determined valence and conduction band offsets at \b{eta}-(AlxGa1-x)2O3/\b{eta}-Ga2O3 heterojunctions using x-ray photoelectron spectroscopy (XPS) reveal the formation of type-II (staggered) band alignment.