Thermal stability and phase transformation of $\alpha$-, $\kappa (\epsilon )$-, and $\gamma$-Ga${}_2$O${}_3$ thin films to $\beta$-Ga${}_2$O${}_3$ under various ambient conditions

Abstract

Phase transitions in metastable $\alpha$-, $\kappa (\epsilon )$-, and $\gamma$-Ga${}_2$O${}_3$ films to thermodynamically stable $\beta$-Ga${}_2$O${}_3$ during annealing in air, N${}_2$, and vacuum have been systematically investigated via in-situ high-temperature X-ray diffraction and scanning electron microscopy. These respective polymorphs exhibited thermal stability to around 471-525${}^{\circ }$C, 773-825${}^{\circ }$C, and 490-575${}^{\circ }$C before transforming into $\beta$-Ga${}_2$O${}_3$, across all tested ambient conditions. Particular crystallographic orientation relationships were observed before and after the phase transitions, i.e., (0006) $\alpha$-Ga${}_2$O${}_3$ $\parallel$ $(\overline{4}02)$ $\beta$-Ga${}_2$O${}_3$, (004) $\kappa (\epsilon )$-Ga${}_2$O${}_3$ $\parallel$ (310) and $(\overline{4}02)$ $\beta$-Ga${}_2$O${}_3$, and (400) $\gamma$-Ga${}_2$O${}_3$ $\parallel$ (400) $\beta$-Ga${}_2$O${}_3$. The phase transition of $\alpha$-Ga${}_2$O${}_3$ to $\beta$-Ga${}_2$O${}_3$ resulted in catastrophic damage to the film and upheaval of the surface. The respective primary and possibly secondary causes of this damage are the +8.6% volume expansion and the dual displacive and reconstructive transformations that occur during this transition. The $\kappa (\epsilon )$- and $\gamma$-Ga${}_2$O${}_3$ films converted to $\beta$-Ga${}_2$O${}_3$ via singular reconstructive transformations with small changes in volume and unchanged surface microstructures.

15 pages, 6 figures, 1 table