Phase-selective growth of $\hat{I}º$- vs ${\hat{I}}^2$-Ga${}_2$O${}_3$ and (In${}_x$Ga${}_{1-x}$)${}_2$O${}_3$ by In-mediated metal exchange catalysis in plasma-assisted molecular beam epitaxy

Abstract

Its piezo- and potentially ferroelectric properties make the metastable kappa polymorph of Ga${}_2$O${}_3$ an interesting material for multiple applications, while In-incorporation into any polymorphs of Ga${}_2$O${}_3$ allows to lower their bandgap. In this work, we provide a guideline to achieve single phase $\hat{I}º$-, ${\hat{I}}^2$-Ga${}_2$O${}_3$ as well as their (In${}_x$Ga${}_{1-x}$)${}_2$O${}_3$ alloys up to x = 0.14 and x = 0.17 respectively, using In-mediated metal exchange catalysis in plasma assisted molecular beam epitaxy (MEXCAT-MBE). The polymorph transition from $\hat{I}º$ to ${\hat{I}}^2$ is also addressed, highlighting the fundamental role played by the thermal stability of the $\hat{I}º$-Ga${}_2$O${}_3$. Additionally, we also demonstrate the possibility to grow ($\bar{2}$01) ${\hat{I}}^2$-Ga${}_2$O${}_3$ on top of $\hat{I}\pm$-Al${}_2$O${}_3$ (0001) at temperatures at least 100 °C above those achievable with conventional non-catalyzed MBE, opening the road for increased crystal quality in heteroepitaxy. The role of the substrate, as well as strain and structural defects in the growth of $\hat{I}º$-Ga${}_2$O${}_3$ is also investigated by growing simultaneously on three different materials: (i) $\hat{I}\pm$-Al${}_2$O${}_3$ (0001), (ii) 20 nm of ($\bar{2}$01) ${\hat{I}}^2$-Ga${}_2$O${}_3$ on $\hat{I}\pm$-Al${}_2$O${}_3$ (0001) and (iii) ($\bar{2}$01) ${\hat{I}}^2$-Ga${}_2$O${}_3$ single crystal.

Main text: 7 pages, 4 figures; Supplementary: 6 pages, 9 figures