Feasibility study of growth by MBE of As doped GaN layers for photoanode applications in hydrogen production by photoelectrochemical water splitting

能源供应的低碳解决方案可能是我们社会最重要的目标之一。潜在的解决办法包括使用水力能源、生物质能、太阳能、风能和地热能。目前，将能源从生产地输送到需要地的主要方法有两种-电力和热力。然而，在未来，新的方法可能会占主导地位。最有前途的载体之一是氢气（H2），它可以通过水裂解产生，并可以通过燃料电池轻松地转化为电能和热能。光电化学（PEC）电池在阳光照射下，具有将水裂解为氢气和氧气的能力。这种电池在受到太阳辐射的光电极表面产生电荷。光电阳极（光电阴极）材料的选择对于使用PEC方法高效制氢至关重要。用于光阳极的半导体材料需要适当的带隙。带隙必须在太阳光谱的理想范围内才能吸收光子。除了选择正确的带隙，导带和价带边缘需要对齐水裂解氧化还原电位。因此，理想的带隙约为2.0eV。第二个要求是光电阳极材料在水溶液中具有耐腐蚀性，可长期运行。在化合物半导体中，上述要求指向III族/氮化物。氮化镓（GaN）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。在过去的几年里，氢燃料电池是美国能源部（DOE）大规模计划的主题。参与这一方案的小组之一设在劳伦斯伯克利国家实验室。Walukiewicz教授在那里进行的理论计算表明，GaN 1-xAsx材料系统是最有前途的光阳极材料之一。然而，理论上预测和实验证实的Ga-N-As系统的一个大的兼容性差距。到目前为止，在GaN 1-xAsx层中报道的最高浓度为x~ 1%。在诺丁汉大学，我们的小组已经广泛研究了GaN基固溶体的分子束外延（MBE）生长超过十年。我们已经非常详细地研究了生长和性能的GaN 1-xAsx层的MBE制备，使用等离子体源的活性氮。由于我们在这一领域的专业知识，我们已经接触了W。Walukiewicz与GaN 1-xAsx材料的请求，用于光电阳极应用在PEC电池制氢。尽管我们花了很多精力研究这种材料系统的生长，但对光阳极材料的特殊要求与我们以前的应用有很大不同。我们需要研究显著不同的MBE生长条件，以满足PEC光阳极应用中所需的更高As含量的要求，并确定是否可以满足这一要求。因此，我们正在申请通过MBE生长具有高As含量（0.05%）的GaN 1-xAsx的短期可行性研究

The move towards low carbon solutions for our energy supply is probably one of the most important aims for our society. The potential solutions include the use of hydro energy, biomass energy, solar energy, wind energy and geothermal energy.Currently there are two main methods to transport energy from the primary source where it is produced to the place where it is needed - electricity and heat. However, in future new methods may become dominant. One of the most promising carriers is hydrogen (H2), which can be generated by water splitting and can be easily converted into electricity and heat by means of fuel cells.Photoelectrochemical (PEC) cells, illuminated by sunlight, have the ability to split water into hydrogen and oxygen. Such cells generate electronic charge at the surface of a photoelectrode subjected to solar radiation. The choice of material for the photoanode (photocathode) is crucial for efficient hydrogen production using the PEC method. Semiconductor materials used for photoanodes require the proper band gap. The band gap must be in the ideal range of the solar spectrum to absorb photons. In addition to choosing the correct band gap, the conduction and valence band edges need to be aligned to the water splitting redox potentials. Therefore, the ideal band gap is around ~2.0eV. The second requirement is for the photoanode material to be corrosion-resistant in water solutions for long periods of operation. In compound semiconductors the above requirements point towards group III/nitrides. Gallium nitride (GaN) 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. Hydrogen fuel cells are the subject of a massive Department of Energy (DOE) programme in the USA during the last few years. One of the groups involved in this programme is based at the Lawrence Berkeley National Laboratory. Theoretical calculations performed there by Prof. Walukiewicz suggest that the GaN1-xAsx material system is one of the most promising materials for the photoanodes. However, a large miscibility gap was theoretically predicted and experimentally confirmed for the Ga-N-As system. The highest concentrations reported so far in GaN1-xAsx layers is x~1%. At the University of Nottingham, our group has studied extensively growth by molecular beam epitaxy (MBE) of GaN-based solid solutions for more than a decade. We have studied in great detail the growth and properties of GaN1-xAsx layers prepared by MBE, using a plasma source for active nitrogen. As a result of our expertise in this area, we have been approached by Prof. W. Walukiewicz with a request for GaN1-xAsx material for photoanodes applications in PEC cells for hydrogen production. Even though we have spent a lot of effort studying the growth of this material system, the particular requirements for the photoanode material are significantly different from our previous applications. We need to investigate significantly different MBE growth conditions in order to satisfy the requirements for the higher As content needed in the PEC photoanode application and indeed to determine if this requirement can be met. Therefore, we are applying for a short feasibility study of the growth by MBE of GaN1-xAsx with a high As content (0.05