Lighting the Future

下次你换一个钨丝灯泡的时候，简单想一下这个灯泡在它的生命周期中消耗了多少能量：超过2亿焦耳（大约相当于3吨煤所含的能量）！如果你考虑一下现在世界上有多少灯泡在使用，很明显，大量的能源被用来保持我们的家庭和办公室的照明。事实上，在英国，大约五分之一的用电量用于照明。因此，提高灯泡效率这样简单的事情可能会对英国的温室气体排放和化石燃料的使用产生巨大影响。一种新的半导体材料——氮化镓（GaN）——为照明问题提供了一个潜在的解决方案。氮化镓用于制造白光发光二极管（led）。这些固态光源已经比传统的钨丝灯泡效率高得多，并且有可能使效率提高十倍以上（比紧凑型荧光灯效率提高三倍）。为了实现这些巨大的效率改进，我们需要彻底了解我们制造LED的材料，以及它的结构，组成和性能如何影响LED的性能。我们还需要设计设备，尽可能充分利用我们对材料的了解。大量不同的因素影响led的效率，在这个项目中，来自剑桥、曼彻斯特、巴斯和斯特拉斯克莱德的科学家们正在汇集他们的专业知识，了解是什么限制了效率，并找到有利于我们所有人的解决方案，通过提供价格合理、高效的照明装置，这些装置将是持久的，并在家庭和办公室提供有吸引力的高质量的光。为此，我们将LED效率问题分解为若干相互关联的科学项目。氮化镓发光二极管是基于在硅或蓝宝石等其他材料上生长的薄层材料。电流通过LED的有源区域，从那里发出光。活性区由极薄的氮化镓和另一种半导体-氮化铟镓（InGaN）交替层组成。InGaN层只有10个原子层厚，被称为量子阱。在InGaN层中，正电荷载流子和负电荷载流子被捕获，并因此相互结合发出光。然而，GaN和InGaN晶体并不完美，它们结构上的缺陷会破坏发光过程，导致产生热而不是光，并降低LED效率。在我们项目的早期，我们计划通过关注材料缺陷、InGaN量子阱的详细小尺度结构和量子阱中产生的电场的项目来解决与光发射效率相关的基本问题。所有这些因素的影响将取决于注入LED的电量，一个主要问题是LED的效率在高注入电流下会下降。由于用于照明的高亮度led需要高电流，因此在固态照明充分发挥其潜力之前，我们也需要了解这个问题。我们开发的新材料和结构必须集成到工作的LED器件中，而这些器件的架构也是影响效率的关键因素。因此，我们的另一个研究项目将涉及设备设计。通过将我们开发的材料和设备的新想法结合在一起，我们的目标是生产高效的led，从而有利于我们的环境，价格便宜，并且与紧凑型荧光灯不同，它可以产生迷人的颜色，使家庭和办公室成为愉快和健康的地方。最后，我们希望我们的研究成果能够真正照亮您的未来！

Next time you change a tungsten filament light-bulb, give a brief thought to the amount of energy that bulb will have used up over its lifetime: more than 200 million Joules (about the same amount of energy as is contained in 3 tonnes of coal)! If you consider how many light bulbs must be in use in the world at this moment, it becomes clear that a huge amount of energy is spent keeping our homes and offices lit. In fact, about a fifth of electricity usage in the UK is for lighting. Hence, improvements in something as simple as light bulb efficiency could have an enormous impact on the UK's greenhouse gas emissions and use of fossil fuels.A new semiconductor material - gallium nitride (GaN) - provides a potential solution to the lighting problem. GaN is used to make white light-emitting diodes (LEDs). These solid state-light sources are already much more efficient than conventional tungsten filament light bulbs, and could potentially yield efficiency improvements of more than ten times (and be three times more efficient than compact fluorescent lamps). To achieve these vast improvements in efficiency, we need to thoroughly understand the material from which we make the LEDs, and how its structure, composition and properties influence LED performance. We also need to design devices which make the best possible use of everything we learn about the material. A large number of different factors influence the efficiency of LEDs, and in this programme, scientists from Cambridge, Manchester, Bath and Strathclyde are pooling their expertise to understand what limits the efficiency and find solutions which will benefit all of us, by providing sensibly-priced, highly-efficient lighting units which will be long-lasting and provide attractive high-quality light in homes and offices.To do this, we are breaking down the question of LED efficiency into a number of inter-linked scientific projects. GaN LEDs are based on thin layers of material grown on other materials such as silicon or sapphire. Electric current is passed into the active region of the LED, from which the light is emitted. The active region consists of very thin alternating layers of GaN and another semiconductor - indium gallium nitride (InGaN). The InGaN layers are only ten atomic layers thick and are called quantum wells. In the InGaN layers, positive and negative charge carriers become trapped and hence combine with one another giving out light. The GaN and InGaN crystals are not perfect, however, and defects in their structure can disrupt the light emission process, resulting in the production of heat rather than light and a reduction in LED efficiency. In the early years of our programme, we plan to tackle fundamental questions relating to light emission efficiency, by pursuing projects concentrating on the defects in the material, the detailed small-scale structure of the InGaN quantum wells and the electric fields which arise in those quantum wells. The effect of all of these factors will be dependent on the amount of electricity injected into the LED, and a major problem is that LED efficiency drops at high injection currents. Since high brightness LEDs for lighting require high electric currents, we will also need to understand this question before solid state lighting can reach its full potential.The new materials and structures we develop will have to be integrated into working LED devices, and the architecture of those devices is also a key factor affecting efficiency. Hence, another of our research projects will address device design. By bringing the new ideas we develop about materials and devices together we aim to produce LEDs that are highly efficient and thus beneficial to our environment, cheap to buy and, unlike compact fluorescent lamps, produce an attractive colour of light to make homes and offices pleasant and healthy places to be. In the end, we hope the products of our research really will be lighting your future!