Fundamental studies of zincblende nitride structures for optoelectronic applications

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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.