Fabrication and characterization of GaN-based light emitting diodes

Abstract

Detailed investigations of GaN-based light emitting diodes (LEDs) with InGaN/GaN multiple quantum wells (MQWs), grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrates, have been performed. Optimization of the growth parameters such as growth temperature, pressure and III-V ratio has been achieved. Two different growth modes including the step-flow and spiral growth mode for the MQWs region were used to investigate the impact of device's performance. It was found that the spiral growth mode may dominate under low growth rate conditions and substantial increases in the root mean square roughness of the films. The domination by spiral growth mode may result in large strain non-uniformity in the MQWs and it will significantly reduce the reliability of the devices. The structural, electrical, optical and thermal properties of the GaN films and LEDs were characterized using atomic force microscopy, transmission electron microscopy, high resolution x-ray diffraction technique, I-V measurement, low-frequency noise measurement, photoluminescence, electroluminescence and thermoreflectance measurement. To reduce the dislocation density of the GaN thin films, conventional epitaxial lateral overgrown (ELOG) and facet-controlled ELOG were applied to the growth of GaN thin films. For the fabrication of the LED devices, these two techniques may not be a suitable method to obtain highly reliable devices due to the localization of dislocations. Thus, it is important to develop novel growth processes for the enhancement of the devices properties. In this thesis, we present a novel LED structure in which the MQW layer is grown on top of a Nano-ELOG (NELOG) layer using a SiO₂growth mask with nanometer-scale windows. The optoelectronic properties and hot-electron integrity of the devices were characterized by electroluminescence, I-V, low-frequency noise and thermoreflectance measurements. Significant improvement in the device properties were observed compared to a standard device fabricated on a low temperature GaN buffer layer without utilizing the NELOG layer.