Al Coordination and Ga Interstitial Stability in a β-(Al_0.2Ga_0.8)₂O₃ Thin Film

Alloying Al₂O₃ with Ga₂O₃ to form β-(AlGa_1-)₂O₃ opens the door to a large number of new possibilities for the fabrication of devices with tunable properties in many high-performance applications such as optoelectronics, sensing systems, and high-power electronics. Often, the properties of these devices are impacted by defects induced during the growth process. In this work, we uncover the crystal structure of a β-(Al_0.2Ga_0.8)₂O₃/β-Ga₂O₃ interface grown by molecular beam epitaxy. In particular, we determine Al coordination and the stability of Al and Ga interstitials and their effect on the electronic structure of the material by means of scanning transmission electron microscopy combined with density functional theory. Al atoms can substitutionally occupy both octahedral and tetrahedral sites. The atomic structure of the β-(Al_0.2Ga_0.8)₂O₃/β-Ga₂O₃ interface additionally shows Al and Ga interstitials located between neighboring tetrahedrally coordinated cation sites, whose stability will depend on the number of surrounding Al atoms. The presence of Al atoms near interstitials leads to structural distortions in the lattice and creates interstitial-divacancy complexes that will eventually form deep-level states below the conduction band (E_c) at E_c -1.25 eV, E_c -1.68 eV, E_c -1.78 eV, E_c -1.83 eV, and E_c -1.86 eV for a Ga interstitial surrounded by zero, one, two, three, and four Al atoms, respectively. These findings bring new insight toward the fabrication of tunable β-(AlGa_1-)₂O₃ heterostructure-based devices with controlled electronic properties.