Materials Challenges in GaN-based Light Emitting Structures

氮化镓(GaN)是一种可以发出灿烂光线的神奇材料。GaN发光二极管(LED)在大约10年前首次问世，并已广泛应用于汽车、公交车和飞机的室内照明；交通灯、大型全彩显示器和手机的背光。GaN蓝色激光即将用于下一代DVD播放器，其中DVD包含的音乐或图片数量将是现有DVD的十倍。展望未来，GaN可能会使高质量、高效率的白色照明成为可能，这将产生重大的节能效果。另一个令人兴奋的发展可能是用于净水的高效率深紫外光LED，特别是在发展中国家。不幸的是，我们目前无法制造上述高效率白光和深紫外光LED，因为还有一些关键的科学问题有待解决。要成功克服这些挑战，需要详细了解LED发光区的制造过程中涉及的复杂工艺。它们由一种名为InGaN的合金薄层组成，夹在较厚的GaN层之间，形成称为量子阱的结构。这些量子阱比人的头发还细5万倍。我们还必须了解限制发光的工艺，并优化LED中许多其他半导体层的导电性。在过去五年我们在GaN方面取得了巨大成功并使我们处于国际竞争地位之后，我们组建了一个由来自不同大学和行业的领先研究人员组成的团队，以解决限制GaN基LED性能的关键因素。我们理解的一个关键限制是为什么GaN蓝光LED会发出灿烂的光，尽管它们充满了称为位错的缺陷，这些缺陷应该会抑制量子阱产生的光发射。这一问题引起了激烈的辩论，2005年，两个主要国际会议专门讨论了这一问题。我们的理论是，发光的InGaN量子阱在纳米级的横向尺度上具有原子尺度的厚度起伏，因此光发射主要集中在远离位错的微小的纳米级区域。然而，UV LED的这种局域化要弱得多，因此不幸的是，位错强烈地猝灭了这些器件中的光发射。我们研究的一个主要目的是了解导致光发射的电子载流子是如何局域化的，并远离本来会猝灭光发射的缺陷，然后优化这种局域化。这可以通过对量子阱的生长进行工程设计来实现。为了理解量子井，我们不仅要检查它们发出的光，还要使用显微镜，让我们能够看到远小于光波长的物体，以成像发光区域内原子尺度的详细变化。由GaN制成的量子结构也具有很强的内部电场，可以减少光发射。我们将使用专业的显微镜技术来测量这些场，并研究减少它们的方法。另一个重点是开发新的方法来降低晶体中称为位错的缺陷密度。此外，我们还将研究LED中量子阱周围的GaN材料的电学性质，以了解是什么缺陷阻止了导电并减少了它们的发生。我们的研究涉及晶体种植者、电子显微镜专家、光学和电学表征技术专家、理论和实验物理学家、化学家和材料科学家。只有这种集成的方法才能解决GaN技术中具有挑战性的问题。

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 up to ten 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. Following on our highly successful work on GaN of the last five years which has put us into an internationally competitive position, we have put together a team of leading researchers from different universities and industry to attack the critical factors that limit the performance of GaN-based LEDs.One key limitation to our understanding is the reason why GaN blue LEDs emit brilliant light even though they are full of defects called dislocations that should quench the light emission arising from the quantum wells. This is hotly debated and in 2005 two major international conferences had special sessions devoted to discussing this topic. Our theory is that the light-emitting InGaN quantum wells have atomic scale thickness fluctuations on a nanometre lateral scale, and thus the light emission is mainly localised in tiny nanometre-scale regions away from the dislocations. However, this localisation is much weaker for UV LEDs, and so unfortunately dislocations strongly quench the light emission in these devices.A major thrust of our research is to understand how the electrical carriers whose interaction is responsible for the light emission are localised, and kept away from defects which would otherwise quench the light emission, and then to optimise this localisation. This may be achieved by engineering the growth of the quantum wells. To understand the quantum wells we will not only examine the light they emit, but use microscopes that allow us to visualise objects far smaller than the wavelength of light to image detailed, atomic-scale variations within the light emitting regions. Quantum structures made from GaN also have strong internal electric fields which can reduce the light emission. We will use specialist microscopy techniques to measure these fields, and study ways of reducing them.Another focus is to develop new methods of reducing the density of defects in crystals called dislocations. Additionally, we will study the electrical properties of the GaN material which surrounds the quantum wells in an LED, in order to understand what defects prevent electrical conduction and reduce their occurrence. Our research involves crystal growers, electron microscopists, experts in optical and electrical characterisation techniques, theoretical and experimental physicists, chemists, and materials scientists. Only this type of integrated approach can solve the challenging problems in GaN-based technology.