Microstructure of gallium nitride thick films grown by CVD on conventional and LEO substrates

Large band gap semiconductors such as gallium nitride (GaN) are of interest for applications such as electroluminescent displays, laser printers, read-write laser sources, and sources for undersea optical communications. Crystal defects are the major limitation for GaN devices, particularly high densities of threading dislocations. When GaN is grown on conventional substrates, threading dislocation densities are on the order of 108–109 cm−2. Lateral Epitaxial Overgrowth (LEO), which uses an unconventional, engineered substrate and modified growth process, has been shown to reduce the density of threading dislocations in GaN films by several orders of magnitude.; This study compares the dislocation microstructure in two different LEO films to that in films grown on conventional substrates by HYPE and MOVPE. Microstructure and crystal deformation were studied using a variety of techniques, including diffraction contrast transmission electron microscopy, Burgers vector analysis of the dislocations, X-ray diffraction reciprocal space maps and backscattered electron Kikuchi pattern analysis.; The results of this study show that although the LEO process prevents a large fraction of the threading dislocations generated at the substrate/film interface from propagating into the overlayer, the price for this reduction is the formation of additional dislocation complexes which are not seen in conventionally grown films. In the isolated LEO bars, these are primarily longitudinal dislocations lying parallel to the bar axis and helical dislocations forming from threading dislocations. In the coalesced film, dislocation loops appear to emanate from the coalescence planes in both the (0001) and (11¯00) planes. The result is a total dislocation density on the order of 10 10 cm−2.; The insights gleaned from these experiments suggest that the interaction between the silica mask in the engineered substrate and the LEO film (be it a physical or chemical interaction) can greatly influence the microstructure of the film, and that the compressive stress imposed on the film during cool-down can be a source of the additional dislocation complexes observed in these samples.