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

Beta-phase gallium oxide ( beta-Ga2O3) is a promising semiconductor for high frequency, high temperature, and high voltage applications. In addition to the beta-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 & DEG;C on 400 mu s 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 gamma-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 gamma-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 beta-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 gamma-phase prior to beta-phase, coupled with the observed grain structure, suggests that the gamma-phase is kinetically preferred during thermal annealing of amorphous films, with beta-phase subsequently forming by nucleation at higher temperatures. The low surface energy of the gamma-phase implied by these results suggests an explanation for the widely observed gamma-phase inclusions in beta-phase Ga2O3 films grown by a variety of synthesis methods.

Automated summary

Beta-phase gallium oxide (β-Ga2O3) is a promising semiconductor for high frequency, high temperature, and high voltage applications. The formation sequence of different phases during thermal annealing of amorphous thin films was investigated, revealing that the gamma-phase nucleates first and then transforms into the thermodynamically stable beta-phase at higher temperatures. These findings are critical for controlling practical devices.