Effect of annealing conditions on the optical properties and surface morphologies of (2¯01)-oriented β-Ga2O3 crystals

[Display omitted] •Suitable annealing conditions for the (2¯01) surface of the β-Ga2O3 crystals were studied.•Flat terraces were formed at 300 °C after annealing in air.•Faceting on the (2¯01) surface began to expose (100) and (001) surfaces at 600 °C.•The bulk properties of the crystals changed through oxygen intake at 900 °C.•Different surface orientations have significantly different thermal stability and reactivity. The bulk properties of β-Ga2O3 are sensitive to temperature and atmosphere; therefore, suitable annealing conditions are required. This study was performed to investigate the effects of annealing conditions on the (2¯01) surface of β-Ga2O3 crystal using atomic force microscopy, optical transmission/ luminescence spectroscopy, and X-ray photoelectron spectroscopy. Annealing was performed in air and in evacuated glass capsules at different temperatures. After annealing at 900 °C in air, the sample became a colorless insulator, and faceting was observed. On the other hand, sample decomposition was observed after annealing in the evacuated capsules. While, after annealing at 300 °C in air, step-terrace formation was observed. With increase of the annealing temperature, faceting was observed after annealing at 600 °C exposing the (100) and (001) surfaces. These results demonstrate the need for optimized annealing conditions for different surface orientations to obtain desirable β-Ga2O3 surfaces for heterointerfaces.