Structural transformation of $\alpha$- and $\kappa$-Ga$_2$O$_3$ thin films on sapphire upon annealing in air

The results of a study of the effect of annealing on the structural transformation of thin films of $\alpha$- and $\kappa$-Ga$_2$O$_3$ obtained by chloride vapor-phase epitaxy on $c$-plane of a sapphire substrate are presented. It has been shown that upon annealing of an $\alpha$-Ga$_2$O$_3$ film during 60 min at a temperature in the region of the polymorphic transition of 550–575$^\circ$C, its homogeneity begins to be disrupted, and a partial polymorphic transition is observed with the formation of small $\beta$-Ga$_2$O$_3$ crystallites. As a result of high-temperature annealing of the $\kappa$-Ga$_2$O$_3$ film in air for 30 min at a temperature $T$=850$^\circ$C, the $\kappa\to\beta$ transition and recrystallization of the original film into a homogeneous $\beta$-Ga$_2$O$_3$ film were observed. It was found that the resulting film predominantly consists of mutually oriented domains. Using transmission electron microscopy, the orientation relationships between the film and the sapphire substrate were determined. The structure of boundaries between domains has been studied.