Carrier Dynamics in GaN films and InGaN/GaN Quantum Wells

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Light-emitting diodes (LEDs) based on InGaN/GaN quantum wells are the basis of the new generation of highly efficient light sources, which are reducing the amount of energy consumed for lighting globally, and are thus having a positive impact on climate change. However, these devices currently have two major shortcoming: i) they only achieve high efficiency when emitting in the blue part of the spectrum necessitating their use in combination with a yellow-emitting phosphor, which introduces an inherent loss, in order to produce white light; and ii) their efficiency also reduces when they are operated at high currents, limiting their brightness.Both of these shortcomings are related to the dynamics of carriers in the quantum wells, and the GaN films in which they are embedded. In particular, the light emission efficiency will be determined by the effect on the carrier dynamics of both the defect density and the strength of the electric field across the well. This project will seek to determine which of defect density and electric field strength is the most important factor in limiting the efficiency of InGaN/GaN quantum wells when emitting in the green part of the spectrum and when operated at high carrier density (corresponding to high currents).For this purpose, the PhD student will study a number of specially grown samples, provided by our collaborators in the Department of Materials and Metallurgy at the University of Cambridge, with a range of defect densities and electric fields. Temperature- and excitation-dependent photoluminescence spectroscopy will be used as a method of assessing the relative importance of radiative and non-radiative processes (the balance between which determines the light emission efficiency) and how these are affected by defect density and electric field. These studies will also be conducted with sufficient resolution so as to determine how these effects vary spatially across the sample. This information in particular will be useful when compared to the characterization of the microscopic structure of the quantum wells carried out by our collaborators in Cambridge.This project lies in EPSRC's "Optoelectronic devices and circuits" research area and is also relevant to the "Materials for energy applications" area due to its impact on the energy consumption of lighting.