Carrier Dynamics in Novel GaN Epilayers, InGaN/GaN Quantum Wells and MicroLEDs

Abstract

The work presented in this thesis investigates the carrier dynamics in novel GaN epilayers, InGaN/GaN quantum wells (QWs) and microLEDs. These novel materials have the potential to overcome the limitations of typical wurtzite (wz) GaN-based light-emitting diodes (LEDs) and realise a greater range of applications for GaN. The effect of thermal annealing on the optical properties of Mg-doped zincblende (zb) GaN epilayers is probed using photoluminescence (PL) and time-correlated single-photon counting (TCSPC) to investigate the carrier-dopant interactions. The presence of Mg in the zb-GaN epilayers causes a broad blue band to dominate the PL spectrum. The intensity of this band increases by a factor of five upon annealing in a nitrogen atmosphere. The increase in the intensity of the blue band was attributed to the reduction of Mg-H complexes that passivate the Mg acceptors and limits p-type conduction. Annealing also changed the decay dynamics of the blue band due to the reduction in the concentration of one-dimensional structural defects or zero-dimensional point defects. The efficiency droop behaviour of zb-InGaN/GaN QWs is investigated as a function of well width, number of wells and polarisation of the emitted light. The presence of a superlinear region in the zb droop curve indicates that non-radiative defects play a significant role at low temperatures in the zb-phase compared to the wz-phase. The emission from the zb-QWs appears to originate from perpendicular quantum wires (QWires) that form within the QW, giving rise to an asymmetry in the emission intensity when the polarisation is resolved along the perpendicular directions. The droop onsets appear to coincide with an increase of near band edge (NBE) emission in the spectra, indicating that carrier escape from the QW is contributing to the efficiency droop observed. The reduction of both NBE emission and the effect of efficiency droop as QW width and number of QWs are increased support this attribution. In collaboration with Plessey Semiconductors Ltd, microLEDs of various sizes are investigated using luminescence mapping to determine the cause of the drop in efficiency as microLED diameter is reduced. There is no significant evidence of mechanisms such as etching-induced Shockley-Read Hall recombination, electric field effects and limited carrier injection present in the microLEDs. The combination of a lower fractional contact area for the small microLEDs and the uniform distribution of injected carriers led to the observed reduction in electroluminescence efficiency as the microLED diameter was reduced in the Plessey devices.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Darren Graham (Supervisor) & David Binks (Supervisor)