Valence-Band Offset ofm-Plane GaN(11̄00) Films Grown on LiAlO₂(100) Substrates

Abstract

We present first-principles calculations on the heterojunction between a wurtzite GaN(1 100) film and a tetragonal LiAlO 2 (100) substrate. The relative barrier heights of different models of the GaNð1 100Þ k LiAlO 2 ð100Þ interface are examined as a function of the valence-band offset. The most favorable interface consists of fourfold-coordinated N and Ga, which has the lowest valence-band offset of 0.534 eV. This interface structure indicates that the nitrided LiAlO 2 (100) surface stabilizes the GaN/LiAlO 2 heteroepitaxial structures. Our findings explain a variety of experiments and may prove useful for improving the internal quantum efficiency of nonpolar GaN-based optoelectronic devices.A mong the alternative substrates for GaN epitaxy, the tetragonal lithium aluminum oxide (space group P4 1 2 1 2, -LiAlO 2 , LAO) was regarded as one of the most closely lattice-matched substrates. The epitaxial formation of a wurtzite m-plane GaN(1 " 100) film on a tetragonal LAO(100) substrate, i.e., GaNð1 "100Þ k LAOð100Þ, is nearly lattice matched and confirms that the lattice mismatches between wurtzite GaN and tetragonal LAO are 0.3% for GaN½0001 k LAO½010 and 1.7% for GaN½11 " 20 k LAO½001. 1-4) These small lattice mismatches suggest that LAO is a promising substrate material for the epitaxial growth of the hexagonal GaN thin film. In addition, the fabrication of m-plane GaN(1 " 100) film, i.e., the growth of GaN films oriented along nonpolar directions, has shown the promising potential for improving the internal quantum efficiency of GaN-based multiple-quantum-well (MQW) light-emitting diodes (LEDs) due to the absence of polarization-related electrostatic fields. 4) However, the nature of the atomic bonding between GaN epilayers and LAO templates has not been elucidated yet. The bonding configurations or electronic properties of heterojunctions can be characterized on the basis of the valence-or conductionband discontinuities. In this paper, we calculate the valenceband offset (VBO) of the optimized GaN(1 " 100)/LAO(100) heterostructure to elucidate the bonding configuration that enables strain relaxation and analyze the electronic charge density. The VBO using the standard bulk-plus-lineup procedure can be split into two terms as follows: 5,6)