InGaN/GaN Multiple Quantum Well Light-Emitting Diodes grown on Polar, Semi-polar and Non-Polar Orientations.

摘要

Cost effective solid-state lighting (SSL) is gaining much attention in recent years. As a result, there has been a great demand for high efficiency light emitting diodes (LEDs). InGaN/GaN multiple quantum well (MQW) based light-emitting diodes (LEDs) emitting in the blue/green region have emerged as promising candidates in realizing the next-generation SSL technology. InGaN/GaN quantum well structures for optoelectronic devices are conventionally grown on the c-plane (polar plane) which has a large polarization field. This large field within the quantum well structures results in a low rate of radiative recombination. This polarization issue is also partly responsible for the "green gap" or the poor external quantum efficiency observed for LEDs emitting in the green region of the visible spectrum and beyond. The alternative to this polarization issue is to grow on semi-polar orientations with a reduced field relative to the c-plane or on non-polar orientations which has zero polarization field. In this dissertation, alternative approaches to grow on semi-polar and nonpolar orientations are explored. The first of these approaches explores the possibility of growing on the facets of GaN nanowires that are oriented along desirable orientations from the perspective of polarization. A "proof of concept" LED structure, that has embedded voids, is overgrown on GaN nanowires. Three times improvement in the light-output power is observed for the LED overgrown on GaN nanowires relative to the conventional c-plane LED. The higher light-output power is attributed primarily to reduced piezo-electric fields and improved light extraction as a result of wave-guiding by the embedded voids. The second of these approaches explores the growth of MQW LEDs on semi-polar and non-polar bulk GaN substrates. A modified growth approach is used for incorporating higher amounts of indium to enable green-emitting MQWs. The challenges with these bulk GaN substrates and the effect of varying polarization fields on the different crystal orientations is discussed. Lastly, an approach to explore the possibility of an N-polar LED is demonstrated. The use of a polarity inverting layer for achieving p-GaN films on N-polar GaN is discussed. This technique is then incorporated to achieve a N-polar LED that has its MQWs grown on Npolar GaN, which is more advantageous for indium incorporation.