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

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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.