Metalorganic chemical vapor deposition of β-(AlxGa1−x)2O3 thin films on (001) β-Ga2O3 substrates

Abstract

Phase pure β-(AlxGa1−x)2O3 thin films are grown on (001) oriented β-Ga2O3 substrates via metalorganic chemical vapor deposition. By systematically tuning the precursor molar flow rates, the epitaxial growth of coherently strained β-(AlxGa1−x)2O3 films is demonstrated with up to 25% Al compositions as evaluated by high resolution x-ray diffraction. The asymmetrical reciprocal space mapping confirms the growth of coherent β-(AlxGa1−x)2O3 films (x < 25%) on (001) β-Ga2O3 substrates. However, the alloy inhomogeneity with local segregation of Al along the (2̄01) plane is observed from atomic resolution STEM imaging, resulting in wavy and inhomogeneous interfaces in the β-(AlxGa1−x)2O3/β-Ga2O3 superlattice structure. Room temperature Raman spectra of β-(AlxGa1−x)2O3 films show similar characteristics peaks as the (001) β-Ga2O3 substrate without obvious Raman shifts for films with different Al compositions. Atom probe tomography was used to investigate the atomic level structural chemistry with increasing Al content in the β-(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. Although the films exhibit alloy fluctuations, n-type doping demonstrates good electrical properties for films with various Al compositions. The determined valence and conduction band offsets at β-(AlxGa1−x)2O3/β-Ga2O3 heterojunctions using x-ray photoelectron spectroscopy reveal the formation of type-II (staggered) band alignment.