| Due to the unavailability of a practical, economical GaN homoepitaxial substrate, the optimization of structural properties of GaN thin films on heteroepitaxial substrates (e.g. sapphire) is vital to obtain high-performance GaN-based devices such as long-lifetime lasers and UV photodetectors. The introduction of the 'two-step' method for growth of GaN on sapphire was a key improvement in the development of GaN-based optoelectronic and electronic devices, since it lowered threading dislocation (TD) densities to the 10 8--109 cm-2 range. These levels would be unacceptable in GaAs- and InP-based devices, but were sufficiently low for rapid (Al,In)GaN device development to closely follow.; The two-step method, consisting of a low-temperature GaN 'nucleation layer' followed by high-temperature GaN growth, was studied in detail by our group in a step-wise manner. We gained insight into the mechanisms by which TDs generate and propagate in GAN thin films on sapphire. The early morphology and distribution of high-temperature GaN islands on the nucleation layer was found to be closely related to the eventual TD density at higher film thicknesses: sparsely nucleated, larger high-temperature GaN islands led to lower TD density.; Although various device structures had been successfully fabricated and tested on dislocated films, the development of lateral epitaxial overgrowth (LEO) of GaN led to unprecedented improvements in device performance, such as demonstrated by Nakamura et al. for long-lifetime blue lasers. This technique typically consists of masking a 'seed' GaN layer with a stripe-patterned mask such as SiO2, and regrowing such that material grows first from the stripe openings and then laterally over the mask. We found that these overgrown regions typically have 'local' TD densities of <10 5 cm-2. We first optimized GaN LEO on a morphological and structural basis, thereby developing a reproducible process for obtaining coalesced overgrown films with low TD densities. These were then used as the structural templates for device regrowth and fabrication in order to determine the effects of TDs on device performance. We successfully fabricated and tested p-n junctions, AlGaN/GaN field-effect transistors, solar-blind AlGaN/GaN UV photodetectors, InGaN/GaN multiple-quantum-well lasers, and AlGaN/GaN heterojunction bipolar transistors on both wings (overgrown regions) and windows (dislocated areas). In so doing, we found that although TD reduction is vital in photodetectors and lasers (for example), other devices such as field-effect transistors may tolerate TDs at typical moderate (∼108 cm-2 ) levels. |
