Amorphous and crystalline GaNAs alloys for solar energy conversion devices

能源供应的低碳解决方案是我们社会最重要的目标之一。一个潜在的解决方案是使用太阳能。光电化学（PEC）分解水将太阳光直接转化为氢气是将太阳能转化为可储存燃料的最直接方法之一。由此产生的氢气是一种近乎理想的能量载体，有可能取代目前的能量运输方法，即电力和热量。PEC光电阳极（光电阴极）的材料选择对于高效制氢至关重要，需要耐腐蚀性以延长操作。用于光电阳极的半导体材料的带隙必须至少为2.0eV，但要小到足以吸收大部分太阳光。除了选择正确的带隙之外，导带和价带边缘必须跨越H +/H2和O2/H2O氧化还原电位，以便可以发生自发的水裂解。目前还没有任何材料能够完全满足这些要求。氮化镓（GaN）是该应用的优秀候选者，因为它具有约3.4eV的带隙、高机械硬度和高化学稳定性。由于GaN基固溶体中V族元素的强烈负弯曲，GaN的带隙可以被调节和减小。Berkeley的研究小组从理论上预测禁带宽度为2eV左右的GaNAs合金是理想的光电极材料，诺丁汉、Strathclyde和Berkeley的研究小组近年来联合研究了富氮端GaN1-xAsx合金的生长和性质。我们的合作得到了为期一年的EPSRC可行性研究资助（EP/G007160/1）的加强，这表明通过分子束外延（MBE）生长这种结构确实是可能的。我们已经成功地实现了GaN 1-xAsx合金在一个大的组成范围内生长的薄膜远低于正常的GaN生长温度。我们发现，具有高砷（As）含量（高于17%）的合金是非晶的，但尽管如此，GaNAs能隙随着As含量的增加而单调减小。光学吸收测量表明，随着As的增加，带隙从~3.4eV逐渐减小到~1.4eV。软X射线吸收光谱（XAS）和软X射线发射光谱（SXE）研究表明，随着非晶GaNAs合金中As组分的增加，导带下移，价带上移。我们的研究结果表明，非晶GaN 1-xAsx合金具有类似于随机晶体GaN 1-xAsx合金的短程有序。这些GaN1-xAsx合金涵盖了整个组成范围，不仅可用于PEC电池中的光阳极应用，用于制氢，而且还具有从紫外到红外操作的许多光学器件的技术潜力。GaNAs合金的非晶性质是特别有利的，因为可以使用低成本衬底，例如玻璃。具有短程有序性的非晶GaN_（1-x）Asx合金是一类潜在的新型太阳能半导体材料。然而，在实现这种器件之前，需要对GaNAs薄膜的生长和特性进行进一步的研究，特别是我们需要实现GaNAs材料的n型和p型掺杂。为了实现低成本器件，我们将尝试在低成本玻璃衬底上在相图的富N端生长GaN合金。GaN 1-xAsx层和结构将在诺丁汉通过低温MBE生长，并将在诺丁汉、斯特拉斯克莱德和伯克利进行仔细评估。作为拟议研究的结果，我们预计将开发一类新的III-V族材料用于太阳能转换器件，即非晶和晶体GaN 1-xAsx合金。我们希望首次展示基于GaN1-xAsx合金的光电化学水分解制氢装置。

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.