Growth and characterisation of earth-abundant semiconductor nanostructures for solar energy harvesting

Zinc phosphide (Zn3P2) is a compound semiconductor based on earth-abundant elements with functional properties ideal for solar cell applications. Cheap, abundant, and renewable energy sources are increasingly imperative due to the imminent threat posed by climate change. So far, zinc phosphide has not been exploited due to limitations in the fabrication of a high-quality material as a consequence of challenges in growth, controllable doping, and heterostructure formation. The maximum conversion efficiency (~6%) was obtained 40 years ago. One route with potential to circumvent or minimise the impact of the limiting factors is the growth of zinc phosphide in the form of nanostructures. By growing the material into nanoscale objects one opens up new elastic strain relaxation mechanisms, minimise the interface area, and loosen the lattice-matching constraints for high-quality epitaxial growth. This thesis focuses on the growth and characterisation with electron microscopy of (i) zinc phosphide nanowires grown through a vapour-liquid-solid approach, and (ii) nanopyramids and thin films grown through selective area epitaxy and lateral epitaxial overgrowth. The first part of this thesis will introduce the reader to the motivation behind the research with regards to renewable energy and a review of zinc phosphide (Chapter 1), the growth of nanostructures (Chapter 2) with focus on molecular beam epitaxy and nan-owires, and then the use of electron microscopy for characterisation of these structures (Chapter 3). The second part of this thesis will focus on the scientific results from the investigations into the growth of zinc phosphide nanostructures. First, the epitaxial growth of zinc phosphide nanowires using an indium-catalysed vapour-liquid-solid approach was investigated (Chapter 4). The growth parameters and mechanism were explored, as well as their impact on the functional properties of the material. Four different nanowire morphologies were achieved, and a followup study focused on the structure and formation of the zigzag ones (Chapter 5). The structure of the zigzag nanowires was found to be analogous to the twin super-lattices found in e.g. III-V nanowires, with one exception. Instead of the standard twin, the zinc phosphide nanowires were found to contain a heterotwin based on a ~monolayer thick inclusion of indium at the mirror plane. It was shown that they did not influ-ence the optoelectronic properties. A more general model for the superlattice formation was also developed, taking into account the non-polar nature of zinc phosphide. The last study explored zinc phosphide growth by selective area epitaxy (Chapter 6). The growth was limited to nanoscale holes using a nano-patterned oxide mask, where it proceeds through a vapour-solid mechanism. Zinc phosphide then forms nanopyramids enclosed by (101) facets, its most energetically stable configuration. If allowed to grow for long enough, the pyramids would coalesce into a thin film, via so-called lateral epitaxial overgrowth. This approach constitutes a step forward in the quest of high-quality, reproducible, and tunable growth of zinc phosphide, and offers a new pathway for its use as an earth-abundant photovoltaic material. Finally, the work is summarised at the end alongside an outlook of future research building on the findings presented in this thesis (Chapter 7). The supplementary information to the different chapters can be found in appendices at the end of the thesis.