Amorphous and crystalline GaNAs alloys for solar energy conversion devices

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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.