Fundamental studies of zincblende nitride structures for optoelectronic applications

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.

Our proposed research will lead to an understanding of the underlying science needed for the exploitation of zincblende (zb) GaN in light emitting diodes (LEDs), particularly those that emit in the very challenging green spectral region where efficiencies are currently low. Such green light emitting diodes have the potential to substantially improve the efficiency and functionality of solid state lighting (SSL). Under the terms of the 2015 Paris Agreement on climate change, the UK is committed to achieving net zero emissions of greenhouse gases by the end of the century. This requires a rate of reduction in fossil fuel usage even greater than that which is embodied in the 2008 Climate Change Act. Switching to LED blubs, which use less than a fifth of the energy of conventional bulbs, can assist significantly with this aim, since about 20% of the total UK electricity generation is used for lighting. However, the current generation of bulbs are expensive and the colour balance of the light they emit can make them unattractive to consumers.
The development of high quality zb-GaN emitters and hence in the longer term green and blue LEDs with improved efficiency will enable SSL based on red-green-blue (RGB) LED bulbs. (Red LEDs with high efficiency based on phosphide materials are already available). Through such a change, the cost of SSL can be reduced by eliminating expensive rare-earth doped phosphors. Also, the substrates we will use for zb-GaN are based on large area, low cost Silicon wafers and will hence bring the cost of high quality SSL down further. The colour of light emitted by RGB LED bulbs could be user tunable, so that the new bulbs will satisfy user preferences whilst realising large energy savings. This colour tunability can be used to improve health (for example mitigating seasonal affective disorder, jet-lag and some types of insomnia), also helping to increase alertness and performance in the workplace. We can also utilise energy efficient RGB lighting to perform additional functions, such as wireless data transfer via optical communication technologies ("LiFi"), mitigating the growing problem of insufficient bandwidth for WiFi traffic.
The UK lighting supply chain consists of around 1700 companies. For these companies to maintain a commercial lead, they must have access to novel materials and devices, for which an understanding of the underlying physics and materials science is vital. The commercial relevance of the materials studies we will perform here is underlined by the involvement of two UK companies, Anvil Semiconductors and Plessey Semiconductors who are well-placed to exploit the results of this project, creating revenue and employment. These and other UK companies will also benefit from the availability of post-doctoral researchers trained through this project. As well as transferring their expertise into industry, we will educate the broader public about our research and its potential impact. One way this will be achieved is using a social media strategy which we have developed with emphases on communicating with schools and young people, disseminating our research via the national media, and interaction with policy makers. We will also interact with the public face-to-face at events such as the Cheltenham Science Festival and Bluedot at Jodrell Bank, addressing their concerns about the energy crisis and how SSL can affect this major global challenge. We will communicate with policy-makers via engagement with parliamentary processes. These dialogues are particularly important in ensuring take-up of SSL as a key energy-saving technology, as well as providing the public with insight into the underlying science.