Carrier Dynamics in GaN films and InGaN/GaN Quantum Wells

基于InGaN/GaN量子威尔斯阱的发光二极管（LED）是新一代高效光源的基础，可减少全球照明所消耗的能源，从而对气候变化产生积极影响。然而，这些器件目前具有两个主要缺点：i）它们仅在光谱的蓝色部分中发射时才实现高效率，从而需要它们与黄光发射磷光体组合使用，这引入了固有损耗，以便产生白色光;以及ii）当它们在高电流下工作时它们的效率也降低，这两个缺点都与量子威尔斯中载流子的动力学以及它们嵌入其中的GaN膜有关。特别地，发光效率将由缺陷密度和阱两端的电场强度对载流子动力学的影响来确定。本计画将探讨当InGaN/GaN量子威尔斯在光谱的绿色部分发光以及在高载子密度下操作时，缺陷密度与电场强度中哪一个是限制InGaN/GaN量子阱效率的最重要因素（对应于高电流）。为此，博士生将研究一些特殊生长的样品，由我们在剑桥大学材料与冶金系的合作者提供，具有一系列缺陷密度和电场。依赖于温度和激发的光致发光光谱将被用作评估辐射和非辐射过程的相对重要性的方法（两者之间的平衡决定了发光效率）以及这些过程如何受到缺陷密度和电场的影响。这些研究也将以足够的分辨率进行，以确定这些影响在样本中的空间变化。与我们在剑桥的合作者进行的量子威尔斯的微观结构表征相比，这些信息特别有用。该项目属于EPSRC的“光电器件和电路”研究领域，由于其对照明能耗的影响，也与“能源应用材料”领域相关。

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.