Fundamental studies of zincblende nitride structures for optoelectronic applications

在过去的15年里，基于GaN的发光二极管（led）在可用性和效率方面发生了一场革命，它主要发出蓝光，但也发出绿光。这些LED被广泛应用于显示器、局部单色照明，最重要的是所谓的固态照明（SSL），换句话说就是LED灯泡。在LED灯泡中，白光是由LED发出的蓝光与荧光粉发出的黄光混合而产生的。一些LED光被荧光粉转化为黄光，这一过程被称为“下转换”。LED灯泡的最大优点是比白炽灯或紧凑型荧光灯效率高得多。照明用电约占全球发电量的20%，被认为是家庭用电中最浪费的部分。基于GaN led的SSL有可能将照明使用的能源消耗减少4倍。下转换蓝光本质上是浪费能源的，如果荧光粉可以被发光其他颜色（绿色和红色）的LED取代，与蓝色LED性能相似，SSL的效率可以提高15 - 20%。目前，根据美国能源部（Bardsley N et al. 2015“美国能源部2015固态照明研发计划”），蓝色和绿色led的功率转换效率（PCE）分别为60%和22%，以提供足够的光来照亮一个房间。根据这份报告，2020年的个人消费支出目标是80%的蓝色和35%的绿色。为了实现上述PCE的改善，蓝色和绿色排放者的IQE必须显著提高。然而，尽管经过数十年的研究和开发，蓝色和绿色排放者的最佳IQE值已经趋于稳定，使用传统技术取得任何进一步重大改进的希望渺茫。在这个项目中，我们建议开发一种新形式的技术，可以导致led的IQE显著增加。目前，构成led主动发光区域的氮化镓基晶体是这样的，原子排列成六角形。其结果是，当一个电子被注入晶体以产生光时，光发射过程是缓慢的，留下足够的时间让电子通过其他不导致光发射的过程失去能量。这些“非辐射”过程限制了led的IQE。在这项工作中，我们提出以这样一种形式生产立方氮化镓晶体，使电子产生光的时间大大减少，从而提供了提高效率的led的可能性。实现这一目标需要克服若干挑战。首先，在晶体结构上没有太多错误的情况下制备立方氮化镓是非常困难的。要克服的主要问题是晶体在原子层堆叠的方式上有缺陷的自然倾向。我们打算深入研究晶体生长过程，使我们能够消除这个问题，并生产出具有更少缺陷的晶体。其次，我们必须能够控制GaN导电的能力，这样我们才能成功地制造led。为了生产高导电性材料，我们将研究限制电导率的工艺。第三，我们将确定发光过程的细节，以确定是否实现了更高效率的承诺。

Over the last fifteen years there has been a revolution in terms of the availability and efficiency of GaN based light emitting diodes (LEDs) that emit principally blue light but also green light. These LEDs have found widespread application in displays, local monochrome lighting and most importantly so-called Solid State Lighting (SSL), in other words LED light bulbs. In an LED bulb, the white light is produced by mixing the blue light from an LED with yellow light from a phosphor. Some of the LED light is transformed into yellow light by the phosphor in a process called "down-conversion". The biggest advantage of LED bulbs is that they are much more efficient than incandescent or compact fluorescent light bulbs. Lighting uses about 20% of global electricity production, and has been identified as the single most wasteful component of domestic electricity use. SSL based on GaN LEDs, has the potential to reduce the consumption of energy used by lighting by a factor of 4. Down-converting blue light intrinsically wastes energy, and if the phosphor could be replaced by LEDs emitting other colours (green and red) with similar performance as the blue LED, the efficiency of SSL could be improved by 15 - 20%. At the moment, according to the US Department of Energy (Bardsley N et al. 2015 "US Department of Energy 2015 Solid-State Lighting R&D Plan"), the Power Conversion Efficiencies (PCE) of blue and green LEDS when operated to give sufficient light to illuminate a room are 60% and 22% respectively. The target PCE figures for the year 2020 from this report are blue 80% and green 35%. To achieve the stated improvements in PCE means that the IQE of blue and green emitters has to be increased significantly. Yet despite decades of research and development the best IQE values for both blue and green emitters have plateaued with little promise of any further significant improvements being achieved using the conventional technology.In this program we propose to develop a new form of technology that could lead to LEDs with significant increases in IQE. At the moment the GaN based crystals that make up the active light emitting regions of LEDs are such that the atoms are arranged in a hexagonal pattern. This has the consequence that when an electron is injected into the crystal to generate light, the light emission process is slow, leaving plenty of time for the electron to lose energy by other processes that do not lead to light emission. These "non radiative" processes limit the IQE of LEDs. In this work we propose to produce cubic GaN crystals in such a form that the time for electrons to generate light is greatly reduced, thus offering up the possibility of LEDs with increased efficiency.There are several challenges to be overcome in achieving this goal. Firstly the fabrication of cubic GaN without too many mistakes in the crystal's structure is very difficult. The main problem to be overcome is a natural tendency for the crystals to have faults in the way layers of atoms stack. We intend to study in depth the crystal growth process enabling us to eliminate this problem and produce crystals with many fewer faults. Secondly we must be able to control the ability of the GaN to conduct electricity so that we can successfully fabricate LEDS. We will investigate the processes whereby the conductivity may be limited with the aim of producing high conductivity material. Thirdly we will determine the details of the light emission process to determine whether the promise of higher efficiency is fulfilled.