Metal organic vapour phase epitaxy of AlN, GaN, InN and their alloys: A key chemical technology for advanced device applications

Abstract

This article reviews metal organic vapour phase epitaxy (MOVPE) processes developed for the group 13 nitrides AlN, GaN, InN and their alloys. The binaries are direct-gap semiconductors with respective bandgaps of 6.1eV for AlN, 3.4eV for GaN, and ∼0.6eV for InN, and adopt the hexagonal wurtzite crystal structure. The nitrides form continuous solid solutions, and are capable of being both n- and p-doped, thus making possible the growth of advanced heterostructure devices exemplified by GaN-based visible light-emitting diodes. Interest in nitride MOVPE from the late 1980s motivated significant work on single-source precursors, and also thermally labile nitrogen sources for use in two-source processes. The best developed of the former are azido compounds, which can have properties well tailored for low-temperature film deposition. However, the nitride MOVPE processes that have come to dominate device manufacturing since the mid-1990s depend on the reaction between ammonia and metal alkyl sources, and deposit GaN at temperatures usually above 1000°C. Most current nitride growth is performed heteroepitaxially on sapphire (0001) substrates, for which appropriate multistep growth initiation processes have been optimised. Current designs of nitride MOVPE reactor are engineered to avoid premature contact between the group 13 sources and ammonia, and feature in situ monitoring by optical means. The mechanisms of the growth chemistry are now understood to the extent that they are handled explicitly in multi-scale computational simulations of full processes. Particular recent advances in mechanistic understanding concern the role of nanoparticles that form in the gas phase, and which represent an important precursor loss channel. Methodologies for controlling the composition and properties of layers of GaN itself, and of ternary alloy layers with moderate (<25mole%) contents of InN or AlN, are well established. However, greater challenges are posed by growth of layers InN, AlN, and of alloys close to these two binaries in composition. New variants of MOVPE continue to be explored as a consequence, and include processes with pulsed alternating precursor introduction to enhance lateral migration of adatoms on the surface of the growing film. A further important new emphasis in recent years is the controlled growth of nanowire and nanorod arrays, which already include core-shell heterostructures of considerable sophistication.