Microstructures of aluminum gallium nitride epitaxial layers

Stress relief mechanisms and microstructures of AlxGa 1-xN thin films were investigated by growing samples by MBE and MOCVD. For investigation of stress relief mechanisms, a series of eight GaN samples were grown using MOCVD with AlxGa1-xN interlayers ranging from xAl=0.14 to xAl=1. Each successive interlayer in a given sample was increased in thickness and followed by a GaN probe-layer. A multi-beam optical stress sensor (MOSS) was used to monitor the stress in the sample during the growth process and determine the onset of stress relaxation. The thicknesses determined for stress relief onset in the interlayers were compared with calculations of Griffith's Criterion for hexagonal thin films and found to closely follow the predicted thicknesses of surface crack formation. For investigation of microstructures in AlxGa1-xN thin films, several sets of samples were grown by MOCVD, with varying pressure, temperature, and composition, and by MBE with varying temperature. The samples were examined by transmission electron microscopy, including [101¯0] selected area electron diffraction (SAED) patterns and weak beam dark field images taken with g=(0002) and g=(1¯21¯0). The MOCVD samples with composition variation were examined with [112¯0] SAED patterns, and the MBE-grown samples were examined using z-contrast imaging. All the MOCVD samples showed signs of ordering, while none of the MBE-grown samples did. In addition, the ordering was shown to be forming as thin plates of ordered material on the (0001) planes, anisotropic within the plane. Some MBE-grown samples were shown to have strong composition modulations arranged in bands arranged parallel to the surface of the sample, due to a balance between strain energy in the samples and the interfacial energy occurring between regions of high and low xAl. The samples grown by MOCVD were shown to have signs of phase separation in addition to the ordering observed. These samples show enhanced ordering in the system when there is low surface mobility. Phase separation will destroy the more quickly forming ordering in the samples, but lower surface mobility will inhibit the formation of strong phase separation, preserving the ordered structure.