Relatively intense deep-green/yellow photoluminescence emission at ~600 nm is observed for InGaN/GaN multi quantum well (MQW) structures grown on bulk AlN substrates, demonstrating the potential to extend commercial III-Nitride LED technology to longer wavelengths. Optical spectroscopy has been performed on InGaN MQWs with an estimated In concentration of greater than 50% grown by metalorganic chemical vapor phase epitaxy at 750oC. Temperature- and power-dependence, time-resolved photoluminescence as well as spatially resolved cathodoluminescence measurements and transmission electron microscopy have been applied to understand and elucidate the nature of the mechanism responsible for radiative recombination at 600nm as well as higher energy emission band observed in the samples. A comparison between samples grown on bulk AlN and sapphire substrates indicate a lower degree of compositional and/or thickness fluctuation in the latter case. Our results indicate the presence of alloy compositional fluctuation in the active region despite the lower strain expected in the structure contrary to that of low In composition active regions deposited on bulk GaN substrates. Transient photoluminescence measurements signify a stretched exponential followed by a power decay to best fit the luminescence decay indicative of carrier hopping in the active region. Our results point to the fact that at such high In composition (>30%) InGaN compositional fluctuation is still a dominant effect despite lower strain at the substrate-epi interface.
It is of great technological importance to develop high quality III-Nitride layers and optoelectronic devices on Si substrates due to its low cost and wide availability as well as use of the highly matured Si microtechnology. Here we report on a novel scheme of substrate engineering to obtain high quality GaN layers on Si substrates. An ion implanted defective layer is formed in the substrate that partially isolates the III-Nitride layer from Si substrate and helps to reduce the strain in the film. The experimental results show substantial decrease in crack density, indicative of high interfacial tensile strain, with an average increase in the crack separation of 190 μm with crack free regions of 0.18 mm2 for a 2 μm thick GaN film. The optical quality and strain reduction in GaN film show strong dependence on the implantation conditions and the thickness of buffer layer. Moreover the GaN film grown on implanted AlN/Si substrate has better optical properties as compared to non implanted AlN/Si. In this paper we will show how the above mentioned scheme can resolve the issues related to cracks and dislocation density in the film that are detrimental to GaN based optoelectronic devices.
The growth of violet light emitting diodes (LEDs) was optimized using a statistical design of experiment (DOE) approach and several important interaction effects were found. The DOEs studied the effect of several variables on the well layer, the barrier layer, and the pAlGaN cladding layer. These variables included the gallium flow rate, the indium flow rate, the growth temperature, and the growth time for the well layer, the ammonia flow in the active region, the barrier growth time, and the Si doping of the barrier, as well as the growth time, growth temperature and Mg doping of the pAlGaN cladding layer. The LEDs were optimized based on combinations of several responses from photoluminescence and electroluminescence measurements. An overall process desirability was obtained, based on achieving the desired wavelength and maximizing the PL intensity and optical output power. Significant interactions between variables played a major role in the optimization of optical output power as well the emission wavelength. The understanding of these interactions led to the optimization of the LEDs both by improvements in the structure and improvements in the quality of the layers. Several of the interactions will be explained based on kinetic models of GaN growth by MOCVD.