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

Lead Research Organisation: University of Nottingham
Department Name: Sch of Physics & Astronomy

Abstract

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.

Planned Impact

Who will benefit from the proposed research?
1) Manufacturers and users of DUV devices for water purification.
2) Water engineering programmes in the UK and beyond.
3) Manufacturers of other DUV devices, especially for surface disinfection.
4) Manufacturers and users of optical devices operating in the ultraviolet.
5) Innovators in any field where semiconductors with band gaps from ~3.4eV to ~6.2eV are required.
6) Members of wider society, which increasingly requires high purity water with minimum carbon cost.

According to the World Health Organization, one-fifth of the world's population does not have access to clean drinking water. This is a major problem for practically all developing countries and therefore has stimulated active research in the water purification worldwide. Deep ultra-violet (UV) light (240nm-280nm) disinfection is one of the most promising methods for a water treatment. Deep ultra-violet (DUV) light of wavelength ~250nm attacks the DNA of micro-organisms and damages their genetic code and as a result stops their reproductive capability, making them harmless when consumed by humans. Unlike chemical water disinfection deep ultra-violet (UV) light disinfection 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 DUV light can be used for air and water purification and surface decontamination. Until recently the main DUV 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. The main aims of this project are the growth of free-standing wurzite AlGaN substrates by molecular beam epitaxy and MOVPE development of the first DUV AlGaN LEDs on AlGaN substrates. This is the first step towards developing commercially viable production of highly efficient DUV LEDs on AlGaN substrates for water purification and disinfection.

The PI and CoIs will communicate key results to industry and academics via refereed publications, the UKNC meetings/website, international conference presentations, Energy Technologies Research Institute, Nottingham, Cambridge and Bath websites and site visits to interested local companies. Project participants have a good track record in developing links with industry. The PI and CoIs and will give talks and presentations to existing and potential collaborators on request and as part of regular meetings with existing industrial collaborators in the nitride area and in the field of DUV devices and water purification. The expertise and contributions of all involved institutions are equally important to the success of the proposed project and all parties will be responsible for disseminating results to industry, academic colleagues and the general public. PI and CoIs will be involved in impact activities and in the dissemination of results. PhD students, post-docs and technical experts will be involved in design of web pages. Support and input will be sought from knowledge exchange experts, press offices, etc. in the Universities.

The Universities of Nottingham, Cambridge and Bath have an embedded Technology Transfer Offices (TTO). The TTOs include qualified intellectual property lawyers, experienced scientists and individuals with business-commercial experience. Where appropriate, intellectual property will be protected by filing and prosecution of patent applications. The Universities currently have many active spin out companies and seeks to ensure that technology is exploited to maximise the social and economic impact both locally and nationally to the benefit of the UK economy.

Publications

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Cuscó R (2015) Anharmonic phonon decay in cubic GaN in Physical Review B

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Novikov S (2015) Molecular beam epitaxy of free-standing wurtzite Al Ga1-N layers in Journal of Crystal Growth

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Novikov S (2016) Growth of free-standing wurtzite AlGaN by MBE using a highly efficient RF plasma source in Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena

 
Description Currently there is a high level of interest in the development of ultraviolet (UV) light sources for solid-state lighting, optical sensors, surface decontamination and water purification. It has long been established that UV light can be used for air and water purification and surface decontamination. Unlike chemical disinfection it will be fast; easy to use; will not require hazardous materials; has no danger of overdosing and does not produce toxic by-products. III-V semiconductor UV LEDs are now successfully manufactured using the AlxGa1-xN material system. The majority of UV 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 ~260 nm. However, there is a significant difference in the lattice parameters of GaN and AlN. Therefore AlxGa1-xN substrates would be preferable to those of either GaN or AlN for many ultraviolet device applications.

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 used the plasma-assisted molecular beam epitaxy (PA-MBE) technique for bulk crystal growth and have produced free-standing layers of zinc-blende GaN up to 100µm in thickness. We have shown that our newly developed PA-MBE process for the growth of bulk zinc-blende GaN layers can also be used to achieve free-standing wurtzite AlxGa1-xN wafers.
In this study wurtzite AlxGa1-xN layers with thicknesses up to 100µm and an AlN content, x, from 0 to 0.5 were successfully grown by PA-MBE on 2-inch and 3-inch sapphire and GaAs (111)B substrates. We have compared different RF nitrogen plasma sources for the growth of thick wurtzite AlxGa1-xN films including an HD25 source from Oxford Applied Research and a novel high efficiency source from Riber. We have investigated a wide range of the growth rates (V) from 0.2 to 3µm/h.

In X-ray diffraction (XRD) studies we observed a gradual improvement of the crystal quality of the wurtzite AlxGa1-xN layers with increasing thickness, which is typical for the standard bulk growth techniques.

We have grown AlxGa1-xN layers on GaAs substrates and subsequently removed the GaAs using a chemical etch in order to achieve free-standing AlxGa1-xN wafers. At a thickness of ~30µm, free-standing GaN wafers can easily be handled without cracking. Therefore, free-standing AlxGa1-xN wafers with thicknesses in the 30-100µm range may be used as substrates for further growth of AlxGa1-xN-based structures and devices. Our novel high efficiency RF plasma source allowed us to achieve such AlxGa1-xN thicknesses on 3-inch wafers in a single day's growth, which makes our bulk growth technique commercially viable.

In secondary ion mass spectrometry (SIMS) studies we observe uniform distributions of the Al, Ga and N content with depth in the AlxGa1-xN layers. The SIMS data also confirm that we were able to achieve a uniform distribution of Al content across the diameter of the wurtzite AlxGa1-xN layers.

Our results demonstrate that free-standing wurtzite AlxGa1-xN substrates can be achieved by PA-MBE using highly efficient RF plasma sources.
Exploitation Route Our results demonstrate that free-standing wurtzite AlxGa1-xN substrates can be achieved by PA-MBE using highly efficient RF plasma sources. Our results have demonstrated that MBE may be competitive with the other group III-nitrides bulk growth techniques in several important areas including production of free-standing zinc-blende (cubic) (Al)GaN and of free-standing wurtzite (hexagonal) AlGaN.
Sectors Agriculture, Food and Drink,Digital/Communication/Information Technologies (including Software),Electronics,Energy,Environment,Healthcare

 
Description According to the World Health Organization, one-fifth of the world's population does not have access to clean drinking water. This is a major problem for practically all developing countries and therefore has stimulated active research in the water purification worldwide. Deep ultra-violet (UV) light (240nm-280nm) disinfection is one of the most promising methods for a water treatment. Deep ultra-violet (DUV) light of wavelength ~250nm attacks the DNA of micro-organisms and damages their genetic code and as a result stops their reproductive capability, making them harmless when consumed by humans. Unlike chemical water disinfection deep ultra-violet (UV) light disinfection 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 DUV light can be used for air and water purification and surface decontamination. Until recently the main DUV 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. The main aims of this project are the growth of free-standing wurzite AlGaN substrates by molecular beam epitaxy and MOVPE development of the first DUV AlGaN LEDs on AlGaN substrates. This is the first step towards developing commercially viable production of highly efficient DUV LEDs on AlGaN substrates for water purification and disinfection. We have produced, for the first time, free-standing layers of zinc-blende GaN and AlGaN up to 100 µm in thickness and up to 3-inch in diameter. We have shown that our newly developed PA-MBE process for the growth of thick zincblende GaN layers can also be used to achieve free-standing wurtzite AlGaN wafers. We have demonstrated controlled doping and Al composition control for free-standing AlGaN layers. At a thickness of ~30 µm, free-standing GaN and AlGaN wafers can easily be handled without cracking. Therefore, free-standing GaN and AlGaN wafers with thicknesses in the 30-100 µm range may be used as substrates for further growth of GaN and AlGaN-based structures and devices. Using the new high efficiency plasma sources it is possible to grow such free-standing cubic and hexagonal GaN and AlGaN layers in a single day making PA-MBE a potentially viable commercial process. Our results have demonstrated that MBE may be competitive with the other group III-nitrides bulk growth techniques in several important areas including production of free-standing zinc-blende (cubic) (Al)GaN and of free-standing wurtzite (hexagonal) AlGaN. Prof S Novikov have publicised these results at several International conferences, including 3 Invited talks at the main Material conferences. We were invited to publish a review article on the subject in a high-impact journal - "Progress in Crystal Growth and Characterization of Materials". S V Novikov, A J Kent, C T Foxon "Molecular beam epitaxy as a growth technique for achieving free-standing zinc-blende GaN and wurtzite AlxGa1-xN" Progress in Crystal Growth and Characterization of Materials, 2017, V.63, N.2, P.P.25-39.
First Year Of Impact 2014
Sector Agriculture, Food and Drink,Digital/Communication/Information Technologies (including Software),Electronics,Energy,Environment
Impact Types Societal,Economic

 
Title Evolution of the m-plane Quantum Well Morphology and Composition within a GaN/InGaN Core-Shell Structure 
Description This dataset contains the results of scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM) Energy Dispersive X-ray (EDX) and Catodoluminescence (CL) measurements carried out on InGaN/GaN core-shell nanostructures. The samples are highly regular arrays of GaN etched cores onto which various InGaN layer thickness were grown using fixed metal organic vapour phase epitaxy (MOVPE) growth conditions. Three different growth time were used to grow InGaN layer with various thickness: 2min, 6min, and 18min, either with or without a GaN capping layer. SEM and AFM characterization techniques were used to assess the nanorod morphology and roughness of the lateral m-plane facets. TEM were used to investigate the structural properties and assess the InGaN thickness of the m-plane facets. EDX measurements were used to assess the InGaN layer composition of the m-plane facet. CL were used to assess the optical properties of each InGaN layer thickness. Correlation of SEM, AFM, TEM, EDX and CL allow to describe the and explain the growth mechanism of a thick InGaN shell grown on GaN NRs formed by combined top-down etching and regrowth. 
Type Of Material Database/Collection of data 
Year Produced 2016 
Provided To Others? Yes