Revealing epitaxial relationships at Ga${}_2$O${}_3$ interfaces with p-type oxides

Abstract

p-type oxide contact layers such as Cr${}_2$O${}_3$ and NiO are attracting increasing interest in pn-heterojunctions with n-type monoclinic $\beta$-Ga${}_2$O${}_3$ for high-power electronic devices and other extreme environment applications. However, scientific understanding of their epitaxial relationships remains incomplete. In this work we investigate the epitaxial relation of Cr${}_2$O${}_3$ and NiO layers to (001) and ($\bar{2}$01) out-of-plane oriented Ga${}_2$O${}_3$ substrates. Surprisingly, we find that, for the most commercially relevant (001)-orientation of the Ga${}_2$O${}_3$ substrate, the epitaxial relationships are Cr${}_2$O${}_3$ (0001) and NiO (111) $\parallel$ Ga${}_2$O${}_3$ (101), both at the non-intuitive $\chi$ = 22.5 ${}^{\circ }$ angle with respect to the substrate normal. We explain this unusual discovery by the interfacial atomistic bonding dominated by oxygen sublattice equivalence of these Cr${}_2$O${}_3$ and NiO polar surface orientations to the tilted Ga${}_2$O${}_3$ (101), rather than Ga${}_2$O${}_3$ (001) substrate surface planes. Furthermore, we assign the in-plane orientation for Cr${}_2$O${}_3$ on ($\bar{2}$01)-oriented Ga${}_2$O${}_3$ as: Cr${}_2$O${}_3$ $[12\bar{3}0]$ $\parallel$ Ga${}_2$O${}_3$ $[010]$ with two in-plane rotational domains. Interface modeling confirms the in-plane orientation for Cr${}_2$O${}_3$/$(\bar{2}01)$ Ga${}_2$O${}_3$ and shows that strained O-terminated Ga${}_2$O${}_3$ $(\bar{2}01)$ surfaces have the lowest interfacial energy with Cr${}_2$O${}_3$ (0001). Beyond establishing the specific epitaxial relationships for Cr${}_2$O${}_3$ and NiO on Ga${}_2$O${}_3$, this work provides a systematic methodology for the unambiguous structural characterization of heterointerfaces involving materials with markedly different crystal symmetries.