Performance Enhancement of Light Emitting Diode Using Nanomaterials

Abstract

This thesis investigates light emitting GaN/InGaN LED and a layer of semiconductor nanocrystals for colour conversion. Unlike standard white LEDs, the device is configured to achieve high colour conversion efficiency via non-radiative energy transfer from the primary LED to the nanocrystals. newlineLED structures separation between quantum well and the surface and patterned standard bright LEDs are considered for the hybrid devices, which require close proximity of the nanocrystals to the quantum well. The development of the hybrid diode fabrication including process techniques for GaN LED and incorporation of the nanocrystals are presented with the emphasis on the differences with standard LED processing. newlineResults and analysis of optical and electrical characterization including photoluminescence (PL), micro-PL, time-resolved PL and electroluminescence (EL) together with current-voltage characteristics are presented to evaluate the device performance. A clear evidence of non-radiative energy transfer was seen in the carrier dynamics of both the LED and the nanocrystals when the quantum well nanocrystals separation was less than 10nm. Analysis of the results shows that in order to achieve sufficient for the white LED colour conversion, better surface passivation and nanocrystals with shorter excitation lifetimes and weaker Auger recombination and needed. newlineNano-scale optoelectronic devices have gained significant attention in recent years. Among these devices are semiconductor nanowires, whose diameters range from 100 to 200 nm. Semiconductor nanowires can be utilized in many different applications including light-emitting diodes and laser diodes. Higher surface to volume ratio makes nanowire-based structures potential candidates for the next generation of photo detectors, sensors, and solar cells. Core-shell light-emitting diodes based on selective-area growth of gallium nitride (GaN) nanowires provide a wide range of advantages. Among these advantages are access to non-polar m-plane sidewalls, higher active region area compared to conventional planar structures, and reduction of threading dislocation density. newline newline newlinevii| page newlineIn this work, metal organic chemical vapour deposition (MOCVD) was employed to grow GaN nanowires. GaN nanowires were grown selectively using a dielectric mask. The effect of mask geometry and growth conditions on the morphology and dimensions of the GaN nanowires were studied. It is shown that the pitch spacing between each nanowire has a significant effect on vii the geometry of the GaN nanowires. Growth of quantum wells around the GaN nanowires was also investigated. A feasible approach towards monolithically integrated multi-colour LEDs was presented, which is not possible using conventional planar LEDs. newline newline newline