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，它将快速且易于使用，不需要危险材料，没有过量使用的危险，也不会产生有毒副产品。紫外光可用于空气和水的净化以及表面去污，这一点早已得到证实。直到最近，这种应用的主要紫外线来源是汞灯。然而，汞灯不容易携带，易碎，寿命有限，而且存在处置问题。第三族氮化物的最新发展使世界各地的研究人员可以考虑将AlGaN基LED作为一种可能的新的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，不会脆弱，并将适合电池操作，以便在偏远地区使用。改变有源AlGaN层的组成，将允许人们轻松地调整LED的波长。这激发了全球范围内积极的研究，开发用于空气和水净化以及表面净化的AlGaN基LED。这种DUV LED还将在紫外固态照明和药物检测方面有潜在的应用。第一批成功的半导体紫外光LED现在使用AlxGa1-xN材料系统制造，覆盖的能量范围从3.4 eV(GaN)到6.2 eV(AlN)。大多数DUV LED要求AlxGa1-xN层的成分介于AlN和GaN之间。例如，为了高效的水净化，这样的AlxGa1-xN LED需要在250-280 nm的波长范围内发光。然而，GaN和AlN的晶格参数存在显著差异。阻碍DUV LED发展的最严重问题之一是缺乏合适的衬底来生长晶格匹配的AlGaN薄膜。匹配不佳的结果是薄膜中的缺陷密度非常高，这可能会影响器件性能。目前，大多数AlGaN LED器件生长在蓝宝石上，很少生长在昂贵的GaN或AlN衬底上。衬底和有源AlGaN层之间的晶格失配导致了当前DUV LED器件的结构质量差、裂纹和形貌差。因此，AlGaN DUV LED的效率仍然很低。在许多紫外光器件应用中，具有适当Al含量的AlxGa1-xN衬底比蓝宝石、GaN或AlN衬底更适合于需要x~0.5的有源AlxGa1-xN层的应用。为了制造高质量的AlGaN基DUV器件，需要在晶格常数和热膨胀特性上与外延氮化物层相匹配的块状AlxGa1-xN衬底。分子束外延(MBE)通常被认为是一种生长非常薄的薄膜的外延技术，其厚度是单层控制的。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，并制备了厚度高达100微米的亚稳闪锌矿(立方)GaN。该项目的主要目标是通过分子束外延生长纤锌矿结构的AlGaN衬底，对其结构、光学和输运特性进行综合分析，并在AlGaN衬底上开发出第一批DUV AlGaN LED。这是在AlGaN衬底上开发商业上可行的高效DUV LED生产的第一步。利用分子束外延技术生长独立的纤锌矿AlGaN衬底将在诺丁汉进行。DUV LED外延层的MOVPE生长和测试将在剑桥进行。DUV LED的制造和随后的特性将在Bath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.