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

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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.