Abstract
The physical properties of InGaN-based light emitting diodes (LEDs) and laser diodes (LDs) are investigated in this study. A strong focus of the thesis is to investigate the non-radiative recombination process which leads to the relative reduction of the efficiency with increasing current injection in an effect which is known as efficiency droop. The explanation of droop is often inconsistent and contradictory with different experiments or devices produced using different growth conditions. Whilst the literature suggests that Auger recombination, carrier leakage and a defect-related recombination are all separately the cause of droop, the physical cause of such loss mechanisms is often poorly explained. Results are presented in this thesis which show that there is a poor hole injection efficiency at low temperatures which is particularly problematic in devices which include electron blocking layers. The poor injection is expected to result in the escape of electrons that is exacerbated by an enhancement of the internal polarization fields. The reduction of the LED efficiency with increasing temperature where there are no hole injection issues is shown to be due to an increasing defect-related recombination rate. The temperature and pressure dependence of efficiency droop show that neither Auger recombination nor carrier leakage are required to explain droop. Evidence of carrier localization is presented by the "s-shape" dependence of the emission peak on temperature in an effect which is stronger for green LEDs (depth of 130meV) compared with blue LEDs (58meV). The weak pressure coefficients of the InGaN-based LEDs (green LED 1.20+/-0.06meV/bar at 5mA and blue LED 2.14+/-0.06meV/kbar at 5mA) are also partially expected to be due to carrier localization. Based on these findings, droop is expected to be caused by an increase in the defect-related recombination rate at high injection due to the increasing likelihood that carriers will occupy defect sites. A defect-related recombination model for droop is shown to be consistent with the temperature and pressure dependence of efficiency droop. Such processes are also shown to influence InGaN LEDs on silicon substrates and InGaN-based laser diodes. The findings of this thesis indicate that there is a strong influence of defect-related recombination, in addition to the internal polarization field strength, on the efficiency of InGaN-based emitters. Structural optimization of the device design and an in depth understanding of the types of defects involved are therefore required in order to achieve more efficient InGaN-based emitters.