Crystal and Surface Structure of (AlxGa1–x)2O3

Abstract

Gallium oxide (β-Ga2O3) is a highly promising wide-band-gap semiconductor for high-power electronic applications due to its large calculated critical field strength and favorable material properties. In particular, alloying with aluminium offers a viable route for band-gap engineering, enabling a tunable increase of the band gap from β-Ga2O3 to Al2O3 while aiming to preserve high crystal quality and useful electron transport properties. Understanding both the bulk properties and surface structure of (AlxGa1–x )2O3 is therefore essential for future device processing and application.

In this work, β-Ga2O3 and Al-alloyed (AlxGa1–x )2O3 as-cleaved single crystals were investigated using X-ray scattering techniques, including X-ray diffraction (XRD), X-ray reflectivity (XRR), and surface X-ray diffraction (SXRD), complemented by atomic force microscopy (AFM) and low-energy electron diffraction (LEED). Single-crystal X-ray diffraction (SCXRD) was employed to determine the precise lattice parameters and to evaluate the bulk crystal quality. SXRD was used to investigate the surface structures of all the as-cleaved crystals. In addition, the actual aluminum composition, x, of the crystals was determined with an accuracy of approximately 1% by combining the average electron density obtained from XRR fitting with the lattice parameters derived from SCXRD.

The cleaved (100) surfaces of β-Ga2O3 and (AlxGa1–x )2O3 were found to cleave at the so called B-termination, corresponding to planes formed by edge-sharing oxygen octahedra and involving the breaking of the longest and weakest Ga–O bonds. SXRD refinements showed only minor deviations from the bulk atomic positions, with displacements of at most about 0.1 Å. AFM measurements revealed relatively large terraces separated by steps of approximately half the a-axis length, or about 0.6 nm, indicating that adjacent terraces possess the same atomic termination and are related by crystal symmetry. The β-Ga2O3 (100) surface was found to be nearly bulk-terminated and remained smooth up to 800 °C. For the Al-alloyed crystal, the SXRD data likewise indicated B-termination; however, the surface composition appeared to be Ga-rich, suggesting possible surface segregation after mild oxygen annealing. These results provide fundamental insight into the bulk ordering, alloy composition, and cleavage behavior of (AlxGa1–x )2O3 single crystals. Such knowledge is important for the controlled processing of these materials and for their future use in semiconductor devices where surface stability, structural perfection, and band-gap engineering are critical.