Study of semi-polar and non-polar nitride based structures for opto-electronic device applications

Over the last 10 years gallium nitride based light emitting diodes (LEDs) have found widespread use as the active light emitting element of various optical displays, ranging from traffic lights to large area displays in, for example, sports stadia. This revolution in display technology has occurred because gallium nitride LEDs have not only the ability to generate blue and green light but also very efficiently, both of these attributes were not previously possible with other types of LED. Despite this revolutionary leap forward in display technology gallium nitride LEDs offer still further opportunities of developing not only even more efficient displays that can be used in televisions but also very efficient lighting systems, so called Solid State Lighting (SSL).
At the heart of most modern televisions is a liquid crystal display unit that is capable of displaying today's high definition programs. The liquid crystal display works by either transmitting or absorbing light when an electrical signal is applied to the crystal. For this to occur the light that is shone from the back of the crystal towards the viewer has to be polarised in a particular direction, i.e. the maxima and minima that make up the light wave light lie in a particular direction. Conventional light sources, including the latest generation of LED, emit unpolarised light so to make the light suitable for use in a liquid crystal based television means that the light has to be passed through a light polariser thus rejecting approximately 50% of the emitted light. Clearly this is an inefficient system and the overall efficiency of television displays would be greatly improved if an efficient light source could emit polarised light. By growing the nitride based LEDs on new forms of template, so-called semi-polar and non-polar crystals, it is possible to fabricate polarised light sources offering us the possibility of significant energy savings. At the moment the fundamental scientific questions that govern not only how well the light is polarised but also efficiency of the light generation process are not understood. In this program we will investigate these issues by making a comprehensive study of both the materials and underlying physics that will enable the fabrication of a new generation of liquid crystal based displays for inclusion in low power consumption televisions.
SSL is viewed as the most likely replacement for incandescent light bulbs and the current generation of compact fluorescent lamps. From this application alone the scale of the potential for energy saving can be judged by the following: "By 2025, SSL could reduce the global amount of electricity used for lighting by 50%. In the US alone this would alleviate the need for 133 new power stations (1000 MW each), eliminate 255 million metric tons of CO2 and save $115 billion of electricity costs." (The Promise of Solid State Lighting for General Illumination, US Department of Energy, 2000). The basis for SSL systems is that white light can produced by either using the combined output of a blue light emitting LED and a yellow light emitting phosphor or be combining the output from blue, green and red light emitting LEDs. The highest light generation efficiency achieved so far is ~70%, while in its self is a remarkably high figure for SSL to be employed in our offices requires efficiencies approaching ~90%. This step forward has so far proved impossible and it is widely believed that this is due to intrinsic reductions in the rate of light emission caused by internal electric fields. These fields can be reduced or eliminated by the growth of LEDs on semi-polar or non-polar templates. The promise of highly efficient LEDs using this methodology remains unfulfilled principally due to the difficulties of growing crystals of the required quality. We anticipate by using novel and improved methods of crystal growth that these problems can be overcome allowing the promise of SSL to be fulfilled.

We aim to develop group III- nitride based quantum well structures which will enable the development of both polarised and ultra high efficiency light emitting diodes (LEDs). Polarised LEDs can be used in a wide range of applications such as in low-power flat-panel displays, machine vision, free-space optical communication and microscopy. The production of ultra high-efficiency LEDs is an essential step before solid state lighting (SSL) that use white light LEDs can be widely deployed to replace the current lighting systems based on incandescent light bulbs and compact fluorescent lamps. The potential economic impact of improved LEDs for displays can be gauged by the fact that in 2008 ca. 50% of the approximately 200 million televisions sold worldwide used flat panel liquid crystal displays, for which efficient polarised LEDs would be an ideal source of back-lighting. Such devices could result in significant energy savings, as could SSL which a recent (2010) US Department of Energy report estimated that if high-efficiency low-cost LEDs become widespread in the US, over $20 billion per year electricity savings would be made.
The development of polarised LEDs and LEDs with high efficiencies for use in SSL will impact on the manufacturers of LEDs and those companies that incorporate these devices in systems. We are already in close contact with major international LED manufacturers through both formal and informal collaborative projects. The UK has a strong presence in the application of "commodity" LEDs in applications that include domestic and commercial lighting, signals and displays. Although applications are a step further on from fundamental LED design, an appreciation of the potential of new technology will allow companies in these fields to design "for the future". We have close links with the full range of UK industry involved with nitride-based LEDs. Intellectual property protection is extremely important to ensure that the economic outputs arising from our work can be exploited fully and will be transferred to companies through licensing of IP generated within the project. The project is also intended to produce knowledge of benefit to the broad community of group III-nitride researchers and related industries such as growth equipment manufacturers. The results will be publicly disseminated as widely as possible, with the exception of those relating to patentable inventions. Cambridge University Enterprise and University of Manchester Intellectual Property Limited will subsequently lead any patenting procedure should patentable inventions arise and indicate the point at which results can be released for external publication. Subsequently, additional funding applications may be made in collaboration with interested representatives from industry (e.g. to the TSB) to perform additional more applied research needed to support the development of promising structures into full commercial devices. This transfer of knowledge generated during the course of the three year program will directly boost the productivity and competiveness of this sector of UK industry. Along with these direct economic benefits the PhD students on the project will develop skills and knowledge that will satisfy the current skills shortage in the rapidly expanding displays industry.
The development of improved energy-efficient technology such as low energy displays and SSL will help to reduce the UK's total electricity usage, assisting the UK Government's efforts to reach the legally binding target of a 34% reduction in CO2 emissions by 2020. This will bring improved quality of life to the wider public through provision of lighter, more energy efficient thinner displays for televisions, laptops etc. The public would also benefit from improved scientific education and an increased awareness and understanding of energy-efficient technologies as a result of the publicity surrounding the project and our outreach activities.