Phase transformations in gallium oxide layers

The three main crystalline modifications of Ga$_2$O$_3$, namely $\alpha$-phase, $\varepsilon$-phase, and $\beta$-phase were grown on sapphire substrates using the hydride vapour phase epitaxy (HVPE) method. The temperatures of the substrates and the values of the precursor fluxes required to obtain each phase exclusively were determined. It was observed that the metastable $\varepsilon$-phase easily transforms into a stable $\beta$-phase during annealing. However, the metastable $\alpha$-phase undergoes an intermediate amorphous phase during annealing, leading to flaking and collapse. This behavior arises from the excessively large increase in density ($\sim$10%) during the transformation from $\alpha$-phase to $\beta$-phase, which results in significant elastic stresses and an increase in the height of the phase transition barrier.