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

在过去的10年中，氮化镓基发光二极管（LED）已被广泛用作各种光学显示器的有源发光元件，从交通信号灯到例如体育场馆中的大面积显示器。显示技术的这场革命之所以发生，是因为氮化镓LED不仅能够产生蓝光和绿色光，而且效率非常高，这两种属性在以前是其他类型的LED所不可能的。尽管在显示技术上有了这一革命性的飞跃，氮化镓LED仍然提供了进一步的机会，不仅可以开发更高效的显示器，可以用于电视，而且还可以开发非常高效的照明系统，即所谓的固态照明（SSL）。在大多数现代电视的核心是液晶显示单元，它能够显示当今的高清晰度节目。当电信号施加到液晶上时，液晶显示器通过透射或吸收光来工作。为了实现这一点，从晶体背面向观察者照射的光必须在特定方向上偏振，即构成光波的最大值和最小值位于特定方向。传统的光源，包括最新一代的LED，发射非偏振光，以使光适用于基于液晶的电视，这意味着光必须通过光偏振器，从而抑制大约50%的发射光。显然，这是一个效率低下的系统，如果一个有效的光源能够发出偏振光，电视显示器的总效率将大大提高。通过在新形式的模板（所谓的半极性和非极性晶体）上生长基于氮化物的LED，可以制造偏振光源，从而为我们提供显著节能的可能性。目前的基本科学问题，不仅管理如何以及光的偏振，而且光产生过程的效率还没有被理解。在这个项目中，我们将通过对材料和基础物理的全面研究来调查这些问题，这些材料和基础物理将使新一代液晶显示器的制造成为可能，这些显示器将用于低功耗电视。SSL被视为最有可能取代白炽灯泡和当前一代紧凑型荧光灯。仅从这一应用就可以判断出节能潜力的规模：“到2025年，SSL可以将全球照明用电量减少50%。仅在美国，这将减轻对133个新发电站（每个1000兆瓦）的需求，减少2.55亿公吨的二氧化碳排放，节省1150亿美元的电力成本。“（The Promise of Solid State Lighting for General Illumination，US Department of Energy，2000）。SSL系统的基础在于，可以通过使用蓝色发光LED和黄色发光磷光体的组合输出或者组合来自蓝色、绿色和红色发光LED的输出来产生白色光。到目前为止，实现的最高光生成效率为~ 70%，而SSL本身是一个非常高的数字，用于我们的办公室需要接近~ 90%的效率。到目前为止，这一进步被证明是不可能的，人们普遍认为这是由于内部电场引起的光发射速率的内在降低。通过在半极性或非极性模板上生长LED可以减少或消除这些场。使用这种方法的高效LED的前景仍然没有实现，主要是由于生长所需质量的晶体的困难。我们预计，通过使用新的和改进的晶体生长方法，这些问题可以克服允许SSL的承诺得以履行。

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.