Free-standing wurtzite AlGaN substrates for deep ultraviolet (DUV) devices.

紫外线（UV）光消毒是最有前途的水处理方法之一。与化学消毒不同，它将快速和易于使用，不需要有害物质，没有过量使用的危险，也不会产生有毒副产物。人们早已确定，紫外线可以用于空气和水的净化和表面净化。直到最近，这种应用的主要紫外线源是汞灯。然而，汞灯不方便携带，易碎，使用寿命有限，并且有处理问题。III族氮化物的最新发展使世界各地的研究人员考虑基于AlGaN的led作为一种可能的新替代DUV光源。如果能够开发出高效的设备，它们将易于使用，具有潜在的长寿命，不会易碎，并且可以使用电池操作，以便在偏远地区使用。改变活性AlGaN层的组成，将允许人们轻松地调整led的波长。这刺激了世界范围内对开发用于空气和水净化和表面净化的AlGaN基led的积极研究。这种DUV led在UV固态照明和药物检测方面也有潜在的应用。目前，第一个成功的半导体UV led是使用AlxGa1-xN材料系统制造的，覆盖了3.4eV （GaN）到6.2eV （AlN）的能量范围。大多数DUV led需要AlxGa1-xN层，其成分介于AlN和GaN之间的中间范围。例如，为了实现高效的水净化，这种AlxGa1-xN led需要在250-280nm波长范围内发射。然而，氮化镓和氮化铝的晶格参数有显著差异。阻碍DUV led进展的最严重的问题之一是缺乏合适的衬底来生长晶格匹配的AlGaN薄膜。不良匹配的结果是薄膜中的缺陷密度非常高，这可能会损害器件性能。目前大多数AlGaN LED器件都是在蓝宝石上生长的，很少在昂贵的GaN或AlN衬底上生长。衬底与活性AlGaN层之间的晶格不匹配导致了层的结构质量差、裂纹和当前DUV LED器件的不良形貌。因此，AlGaN DUV led的效率仍然很低。对于许多需要x~0.5的活性AlxGa1-xN层的紫外器件应用，具有适当Al含量的AlxGa1-xN衬底比蓝宝石，GaN或AlN衬底更可取。块状AlxGa1-xN衬底，其晶格常数和热膨胀性能与外延氮化物层相匹配，是制造高质量algan基DUV器件所必需的。分子束外延（MBE）通常被认为是一种生长极薄层的外延技术，其厚度由单层控制。然而，我们最近成功地使用了等离子体辅助分子束外延（PA-MBE）技术来生长大块晶体，并且我们生产了厚度达100微米的亚稳锌-闪锌矿（立方）GaN的独立层。我们已经通过生长直径达3英寸的独立锌混合氮化镓层证明了该工艺的可扩展性。该项目的主要目标是通过MBE生长独立的纤锌矿AlGaN衬底，全面分析其结构，光学和输运特性，以及在AlGaN衬底上开发第一个DUV AlGaN led的MOVPE。这是在AlGaN衬底上开发商业上可行的高效DUV led的第一步。通过MBE生长独立的纤锌矿藻类基质将在诺丁汉进行。MOVPE的生长和DUV LED外延层的测试将在剑桥进行。DUV led的制造和后续表征将在巴斯进行。

Ultra-violet (UV) light disinfection is one of the most promising methods for water treatment. Unlike chemical disinfection it will be fast and easy to use and will not require hazardous materials, has no danger of overdosing and does not produce toxic by-products. It has long been established that UV light can be used for air and water purification and surface decontamination. Until recently the main UV source for that application were mercury lamps. However, mercury lamps are not readily portable, are fragile, have a limited lifetime and have a disposal problem. The recent development of group III nitrides allows researchers world-wide to consider AlGaN based LEDs as a possible new alternative DUV light source. If efficient devices can be developed they will be easy to use, have potentially long life time, will not be fragile and will lend themselves to battery operation to allow their use in remote locations. Changing the composition of the active AlGaN layer, will allow one to tune easily the wavelength of the LEDs. This has stimulated active research world-wide to develop AlGaN based LEDs for air and water purification and surface decontamination. Such DUV LEDs will also have potential applications for UV solid state lighting and drug detection. The first successful semiconductor UV LEDs are now manufactured using the AlxGa1-xN material system, covering the energy range from 3.4eV (GaN) up to 6.2eV (AlN). The majority of DUV LEDs require AlxGa1-xN layers with compositions in the mid-range between AlN and GaN. For example, for efficient water purification such AlxGa1-xN LEDs need to emit in the wavelength range 250-280nm. However, there is a significant difference in the lattice parameters of GaN and AlN. One of the most severe problems hindering progress of DUV LEDs is the lack of suitable substrates on which lattice-matched AlGaN films can be grown. The consequence of a poor match is a very high defect density in the films which can impair device performance. Currently the majority of AlGaN LED devices are grown on sapphire and only rarely on expensive GaN or AlN substrates. The lattice mismatch between the substrate and the active AlGaN layer results in the poor structural quality of the layers, cracks and poor morphology of the current DUV LED devices. As a result the efficiency of the AlGaN DUV LEDs is still very low. AlxGa1-xN substrates with the proper Al content would be preferable to those of either sapphire, GaN or AlN for many ultraviolet device applications, which require active AlxGa1-xN layers with x~0.5. Bulk AlxGa1-xN substrates, which are matched in lattice constant and thermal expansion properties to epitaxial nitride layers are needed for fabrication of the highest-quality AlGaN-based DUV devices. Molecular beam epitaxy (MBE) is normally regarded as an epitaxial technique for the growth of very thin layers with monolayer control of their thickness. However, we have recently successfully used the plasma-assisted molecular beam epitaxy (PA-MBE) technique for bulk crystal growth and we produced free-standing layers of metastable zinc-blende (cubic) GaN up to 100 microns in thickness. We have demonstrated the scalability of the process by growing free-standing zinc-blende GaN layers up to 3-inches in diameter.The main aims of this project are the growth of free-standing wurzite AlGaN substrates by MBE, comprehensive analysis of their structural, optical and transport properties and MOVPE development of the first DUV AlGaN LEDs on AlGaN substrates. This is the first step towards developing commercially viable production of high efficient DUV LEDs on AlGaN substrates.Growth of free-standing wurzite AlGaN substrates by MBE will be carried out at Nottingham. MOVPE growth and testing of DUV LED epitaxial layers will be carried out at Cambridge. The fabrication and subsequent characterisation of DUV LEDs will be carried out at Bath.