Band alignment at surfaces and heterointerfaces ofAl2O3,Ga2O3,In2O3, and related group-III oxide polymorphs: A first-principles study

Abstract

The band alignments at nonpolar surfaces and heterointerfaces of ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$, ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$, and ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ polymorphs, and three related group-III oxides, namely, ${\mathrm{Sc}}_{2}{\mathrm{O}}_{3}$, ${\mathrm{Y}}_{2}{\mathrm{O}}_{3}$, and ${\mathrm{La}}_{2}{\mathrm{O}}_{3}$, are investigated by using first-principles calculations. A non-self-consistent dielectric-dependent hybrid functional approach is adopted on top of semilocal density-functional calculations by using the Perdew-Burke-Ernzerhof functional tuned for solids (PBEsol) to accelerate the band alignment evaluation that involves surface and interface calculations. Among the five crystal structures considered, namely, corundum, \ensuremath{\beta}-gallia, \ensuremath{\kappa}-alumina, bixbyite (C-type rare earth), and A-type rare earth, the lowest energy phases are corundum, \ensuremath{\beta}-gallia, A-type rare earth, and bixbyite for ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$, ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$, ${\mathrm{La}}_{2}{\mathrm{O}}_{3}$, and the others, respectively, within PBEsol calculations. The ionization potential typically decreases in the order ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$, ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$, ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$, ${\mathrm{Sc}}_{2}{\mathrm{O}}_{3}$, ${\mathrm{Y}}_{2}{\mathrm{O}}_{3}$, and ${\mathrm{La}}_{2}{\mathrm{O}}_{3}$ within the same crystal structure and surface termination. This tendency is enhanced by the atomic relaxation-induced surface dipoles, where smaller cations tend to relax toward the bulk side compared to O ions, while larger cations tend to relax toward the vacuum side. The ionization potential and electron affinity differences at unrelaxed surfaces are good indicators of the interfacial valence- and conduction-band offsets, respectively, for ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$/${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ with a relatively small mismatch in the lattice parameters of the two phases.