Heteroepitaxy of perovskite (111) SrTiO₃ on wurtzite (0002) GaN using an artificial interface lattice design

Rational interface lattice design is the key issue for high quality integration of functional SrTiO₃ (STO) oxide on a GaN semiconductor. Based on the heteroepitaxy design principle for minimizing lattice mismatch at the interface, by inserting novel designed hexagonal Ti and spinel MgAl₂O₄ (MAO) buffer layers, epitaxial integration of the perovskite (111) STO onto wurtzite (0002) GaN was realized using pulsed laser deposition respectively. The epitaxial relationship was resolved to be (111)[110] STO//(0002)[1120] Ti//(0002)[1120] GaN and (111)[110] STO//(111)[110] MAO//(0002)[1120] GaN, respectively. Insertion of the ultrathin Ti and MAO buffer layers could reduce lattice mismatch at the STO/GaN interface from +13.3% to +6.5% and +3.3% respectively, thus inducing the epitaxial growth of STO. However, compared to STO with a three-dimensional growth mode on an MAO buffered GaN, STO on GaN with a designed Ti buffer layer displays a dense microstructure, flat surface and great interfacial state. By analyzing the out-of-plane atomic epitaxial matching model of STO/Ti/GaN, high quality integration of STO could originate from the release of the lattice mismatch strain by lattice distortion of the two unit-cell Ti buffer layer, and the low interfacial energy owing to the common Ti atom site and Ti-O at the STO/Ti interface.