Initial nucleation of metastable γ-Ga2O3 during sub-millisecond thermal anneals of amorphous Ga2O3.

Beta-phase gallium oxide (β -Ga₂O₃) is a promising semiconductor for high frequency, high temperature, and high voltage applications. In addition to the β -phase, numerous other polymorphs exist and understanding the competition between phases is critical to control practical devices. The phase formation sequence of Ga₂O₃, starting from amorphous thin films, was determined using lateral-gradient laser spike annealing at peak temperatures of 500–1400 °C on 400 μs to 10 ms timescales, with transformations characterized by optical microscopy, x-ray diffraction, and transmission electron microscopy (TEM). The resulting phase processing map showed the γ -phase, a defect-spinel structure, first nucleating under all annealing times for temperatures from 650 to 800 °C. The cross-sectional TEM at the onset of the γ -phase formation showed nucleation near the film center with no evidence of heterogeneous nucleation at the interfaces. For temperatures above 850 °C, the thermodynamically stable β -phase was observed. For anneals of 1–4 ms and temperatures below 1200 °C, small randomly oriented grains were observed. Large grains were observed for anneals below 1 ms and above 1200 °C, with anneals above 4 ms and 1200 °C resulting in textured films. The formation of the γ -phase prior to β -phase, coupled with the observed grain structure, suggests that the γ -phase is kinetically preferred during thermal annealing of amorphous films, with β -phase subsequently forming by nucleation at higher temperatures. The low surface energy of the γ -phase implied by these results suggests an explanation for the widely observed γ -phase inclusions in β -phase Ga₂O₃ films grown by a variety of synthesis methods. [ABSTRACT FROM AUTHOR]