The Microstructure And Physical Properties Of GaN/SrTiO₃ Heterointerface

SrTiO3 is a typical transition metal oxide and is widely used as a substrate for the creation of high-Tc superconductors, large negative magnetoresistance or other functional materials. Recently, two-dimensional electron gas (2DEG) has also been observed at the surfaces of SrTiO3 (001), (110) and (111). The findings have sparked a new research of metallic oxide surfaces to improve this material’s technological importance. SrTiO3 surface can present high conductivity, blue-light emitting. Its lattice structure can form high-quality interfaces with other materials. GaN as the third generation semiconductor materials has directed band gap, higher saturated electron mobility, high breakdown electric field and high thermal conductivity. SrTiO3 can be demonstrated as a promising gate dielectric for GaN-based devices. The primary important GaN surface is the (0002) with hexagonal symmetry, and the (111) planes of cubic SrTiO3 provide the best atomic registry.Using first-principles calculations, we first calculate the surface energies, analyze the surface relaxations, structural distortions and electronic properties for the SrTiO3(111) terminations. Then, we study the adhesive strength and electronic properties of the GaN/SrTiO3 interfaces. The main results are listed as below:1. The structural and electronic properties of the SrTiO3(111) polar surfaces are studied using first-principles calculations. The structural distortions for Ti termination are larger than that for SrO3 termination. A negative charge density of -0.55e is concentrated at the surface of Ti termination, which may explain the recent finding of two-dimensional electron gas at the SrTiO3 (111) surface. A positive charge density of 0.64e is accumulated at SrO3 termination. The nonstoichiometric TiO and SrO2 terminations are also considered. Oxygen octahedrons tilt to the opposite direction in the adjacent cells along the stacking direction. TiO and SrO2 terminated slabs are both insulating with surface charge modified relatively small. The surface energies for all the SrTiO3 (001), (011) and (111) surfaces are calculated to compare the surface stability. The surface energy of Ti termination is lower than that of (111)-SrO3 termination and slightly larger than that of (011)-O2 termination.2. The Ga/SrO3 interface and N/Ti interface are energetically favorable among the atomic arrangements of the GaN/SrTiO3(111) interface, which illustrates that the the polarity of the initial surfaces would determine the stacking sequence. The Ga/SrO3(Ⅱ) interface is the most stable interface, because oxygen migrates from its initial position at the SrTiO3(111) surface and causes the shorter of Ga-O bonds. After oxygen vacancies are introduced in the Ga/SrO3(Ⅱ) interface, some new occupied states generate below the fermi level in comparison with DOS for perfect Ga/SrO3(Ⅱ) heterojunction, which may impact on the interfacial electrical conduction. The states are mainly contributed by the 4s4p states of Ga atoms above oxygen vacancies. The difference between the Ga atoms may be caused by the dangling bonds of the Ga atom near oxygen vacancy. We observe the occurrence of interfacial metallic states for the Ga/Ti interface. An electron density of 1.71 per surface unit is concentrated at the Ga/Ti interface. The states are mainly composed of N 2p, Ga 4s4p, Ti 3d, and O 2p states.