Optimising GaN light emitting structures on free-standing GaN substrates

氮化镓（GaN）是一种神奇的材料，可以发出耀眼的光。氮化镓发光二极管（led）大约在十年前首次出现，并已广泛应用于汽车、公共汽车和飞机的内部照明；交通信号灯、大型全彩显示屏和手机背光。GaN蓝光激光器将用于新一代DVD播放器，其音乐或图片的容量将是现有DVD的5倍以上。展望未来，氮化镓可能使高质量、高效率的白色照明成为可能，这将大大节省能源。另一个令人兴奋的发展可能是用于水净化的高效深紫外led，特别是在发展中国家。遗憾的是，我们目前还无法制造出上述提到的高效白光和深紫外led，因为还有一些关键的科学问题有待解决。为了成功地克服这些挑战，需要详细了解LED发光区域制造中涉及的复杂过程。它们由一种叫做InGaN的合金的薄层组成，它夹在更厚的GaN层之间，形成一种叫做量子阱的结构。这些量子阱比人的头发丝细5万倍。我们还必须了解限制光发射和优化LED中许多其他半导体层的导电性的过程。氮化镓材料和器件的一个主要问题是理解称为位错的缺陷的作用。GaN器件通常生长在蓝宝石或碳化硅上。由于氮化镓的原子间距与蓝宝石或碳化硅的原子间距不同，当氮化镓生长在这些材料上时，会形成大量的位错。最近，具有更少位错的独立体氮化镓的数量有限。因为它很难种植，所以很贵。然而，这种材料的两家生产商，三星康宁和Lumilog，已经免费为我们提供了一些这种材料，这样我们就可以优化GaN发光结构的生长：蓝色和绿色。这是一个做这件事的绝佳机会。据我们所知，世界上还没有人在GaN衬底上生长出绿色led。这项研究不仅将生产蓝色和绿色led，还将帮助我们了解位错在GaN led中的作用。

Gallium nitride (GaN) is an amazing material that can emit brilliant light. GaN light emitting diodes (LEDs) first became available about ten years ago, and are already used in a wide range of applications, including interior lighting in cars, buses and planes; traffic lights, large full-colour displays and backlighting in mobile phones. GaN blue lasers are about to be sold for next-generation DVD players, in which the DVDs will contain more than five times the amount of music or pictures as existing DVDs. Looking to the future, GaN may make possible high-quality, high efficiency white lighting which will produce major energy savings. Another exciting development could be high-efficiency deep ultra-violet LEDs for water purification, particularly in the developing world.Unfortunately, we are currently unable to make the high-efficiency white lighting and deep-UV LEDs referred to above because there are some key scientific problems that remain to be solved. To successfully surmount these challenges requires a detailed understanding of the complex processes involved in the fabrication of the light emitting regions of the LED. These consist of thin layers of an alloy called InGaN, which are sandwiched between thicker layers of GaN to make structures called quantum wells. These quantum wells are 50,000 times thinner than a human hair. We must also understand the processes that limit light emission and optimise the electrical conductivity of the many other semiconductor layers in an LED.A major problem with GaN materials and devices is understanding the role of defects called dislocations. GaN devices are normally grown on sapphire or SiC. Because GaN has different atomic spacings from the atoms in sapphire or SiC, a very high number of dislocations are formed when GaN is grown on these materials. Recently free-standing bulk GaN with far fewer dislocations has become available in limited quantities. Because it is very difficult to grow, it is expensive. However, two producers of this material, Samsung Corning and Lumilog, have offered us some of this material free-of-charge, so that we can optimise the growth of GaN light-emitting structures: both blue and green. This is a wonderful opportunity to do this. As far as we are aware, no one in the world has grown green LEDs on GaN substrates. This research will not only produce blue and green LEDs, it will also help us to understand the role of dislocations in GaN LEDs.