Amorphous and crystalline GaNAs alloys for solar energy conversion devices

The move towards low carbon solutions for our energy supply is one of the most important aims for our society. A potential solution is the use of solar energy. The direct conversion of sunlight into hydrogen by photoelectrochemical (PEC) water-splitting is one of the most direct methods to transfer solar energy into a storable fuel. The hydrogen gas thus produced is a near ideal carrier of energy, with the potential to supersede the current methods of energy transportation, namely electricity and heat. The choice of material for the PEC photoanode (photocathode) is crucial for efficient hydrogen production, with a need for corrosion-resistance for prolonged operation. The band gap of semiconductor materials used for photoanodes must be at least 2.0eV, but small enough to absorb most sunlight. In addition to choosing the correct band gap, the conduction and valence band edges must straddle the H+/H2 and O2/H2O redox potentials so that spontaneous water splitting can occur. Currently there is no material that fully satisfies these requirements. Gallium nitride (GaN) is an excellent candidate for this application since it has a band gap ~3.4eV, high mechanical hardness and high chemical stability. The band gap of GaN can be adjusted and decreased due to strong negative bowing in the GaN-based solid solutions with group V elements. The group from Berkeley have theoretically predicted that GaNAs alloy with a band gap ~2eV could be the ideal photoelectrode material.The Nottingham, Strathclyde and Berkeley groups have jointly investigated the growth and properties of GaN1-xAsx alloys at the N-rich end of the phase diagram during recent years. Our collaboration was strengthened by a 1 year EPSRC feasibility grant (EP/G007160/1), which showed that it is indeed possible to grow such structures by molecular beam epitaxy (MBE). We have succeeded in achieving GaN1-xAsx alloys over a large composition range by growing the films much below the normal GaN growth temperatures. We discovered that alloys with a high Arsenic (As) content, above 17%, are amorphous, but despite this fact the GaNAs energy gap decreases monotonically with increasing As content. Optical absorption measurements reveal a continuous gradual decrease of band gap from ~3.4eV to ~1.4eV with increasing As. Soft x-ray absorption spectroscopy (XAS) and soft x-ray emission spectroscopy (SXE) studies have shown that the conduction band moves down and valence band moves up as the As composition increases in amorphous GaNAs alloys. Our results indicate that the amorphous GaN1-xAsx alloys have short-range ordering that resembles random crystalline GaN1-xAsx alloys. These GaN1-xAsx alloys cover the whole composition range and can be used not only for photoanode applications in PEC cells for hydrogen production, but also have technological potential for many optical devices operating from the ultraviolet to the infra-red. The amorphous nature of the GaNAs alloys is particularly advantageous since low cost substrates such as glass could be used. Amorphous GaN1-xAsx alloys with short-range ordering are potentially a new class of semiconductor materials for solar energy conversion. However, before such devices can be realised further research on the growth and properties of GaNAs films is required and in particular we need to achieve n- and p-doping of the GaNAs material. To achieve low cost devices we will attempt to grow GaNAs alloys at the N-rich end of the phase diagram on low cost glass substrates. The GaN1-xAsx layers and structures will be grown at Nottingham by low temperature MBE and will be carefully evaluated at Nottingham, Strathclyde and Berkley. As a result of the proposed research, we expect to develop a new class of III-V materials for solar energy conversion devices, namely amorphous and crystalline GaN1-xAsx alloys. We expect to demonstrate for the first time photoelectrochemical water-splitting devices for hydrogen production based on GaN1-xAsx alloys.

Who will benefit from the proposed research? 1) Manufacturers and users of water-splitting devices for hydrogen production. 2) Innovators in any field where semiconductors with band gaps from ~3.4 eV to ~1 eV are required. 3) Manufacturers of solar energy conversion devices, especially low cost solar cells. 4) Manufacturers and users of optical devices operating from the ultraviolet to the infra-red. 5) Low carbon energy programmes in the UK, USA and beyond. 6) Members of wider society, which increasingly requires energy with minimum carbon cost. How will they benefit from the proposed research? The direct conversion of sunlight into hydrogen by photoelectrochemical (PEC) water-splitting is one of the direct methods to transfer solar energy into a storable fuel. Research into PEC water-splitting based on GaN materials is entirely absent in the UK. Our project aims to develop low cost PEC based on GaNAs alloys and therefore to achieve a breakthrough in this field from within the UK nitride community. It will be of particular benefit to the UK, because the project will allow us to use the expertise of our US partners in this area. The GaNAs alloys cover the whole composition range and can be used not only for photoanode applications in PEC cells for hydrogen production, but also have technological potential for many optical devices operating from the ultraviolet to the infra-red, including low cost solar cell applications. The amorphous nature of the GaNAs alloys is particularly advantageous since low cost substrates such as glass could be used for solar cell fabrication. The Universities of Nottingham and Strathclyde are major centres for energy research. Applicants from Nottingham are the active members of the Energy Technologies Research Institute and therefore the results of the current studies will be immediately available to the all member of the Institute. The Strathclyde applicant is fully engaged in the Energy Theme of the Scottish Universities Physics Alliance and at the same time leads the Condensed Matter and Materials Theme. Our results will be disseminated to a number of semiconductor and device manufacturers within the UK and the USA. We will invite local companies to attend seminars and industry based events at the Nottingham and Strathclyde Universities. We will visit UK and US companies to discuss results and to give seminars where requested. Both Nottingham and Strathclyde are active participants in the UK Nitride Consortium, which includes a healthy number of industrial partners. PIs from UNOTT and UStrath 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 web pages design. Support and input will be sought from knowledge exchange experts, press offices, etc. in both Universities and SUPA. The PIs will communicate key results to industry and academics via refereed publications, the UK Nitride Consortium meetings/website, international conference presentations, Energy Technologies Research Institute, School of Physics websites and site visits to interested local companies. The University of Nottingham has an embedded Technology Transfer Office (TTO), which has responsibility for the assessment of intellectual property arising from research projects. Strathclyde has a strong record of exploitation and the Research office provide excellent support in this area. The UNOTT and UStrath have a good track record in developing links with industry. The licensing executives will identify potential licensees or potential commercial collaborators. IP rights will be distributed between Nottingham, Strathclyde, Berkeley and any third parties according to their level of involvement in the project.