Kinetics studies on $\kappa$ to $\beta$-Ga$_2$O$_3$ phase transformations via in-situ high temperature X-ray diffraction

Abstract

The kinetics of the $\kappa$ to $\beta$-Ga$_2$O$_3$ phase transformation were investigated in five batches of nominally phase-pure $\kappa$-Ga2O3 thin films heteroepitaxially grown on c-plane sapphire, with film thickness ranging from 700 to 1100 nm, using in-situ high-temperature X-ray diffraction. Phase fractions were quantitatively extracted through modified Rietveld refinement that accounts for preferred orientation, and the transformation kinetics were analyzed using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. The applicability of the JMAK model to thin-film materials was evaluated and its lower and upper bounds for thin films and bulk materials were established. Based on this analysis, a method specifically suited for thin-film kinetic studies was developed and yielded reproducible and robust results across all five sample batches. The results indicate that the $\kappa$ to $\beta$ phase transformation in ~700-1100 nm films is best described as an interface-controlled, site-saturated nucleation with thickness-limited or effectively two-dimensional growth.