Controlling vapor-liquid-solid growth of copper-seeded silicon microwire arrays for solar fuel generation
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The increasing population of the world is causing an increased demand for energy. The non-renewable fossil fuels are not a long-term option given the possibility of their extinction in near future and their contribution to global warming, for which renewable form of energy is an alternative. Solar energy incident on the surface of the earth is enough to meet the energy requirements of the planet but the devices that harvest this energy are not as efficient. A device was proposed which uses sunlight incident to split water molecule into hydrogen and oxygen. The hydrogen gas generated can be stored and used as a fuel to produce clean energy. This device uses arrays of doped silicon microwires that absorb the sunlight incident to generate carriers, that in turn are responsible for the water splitting reaction. The microwires are grown using a vapor-liquid-solid (VLS) mechanism and the properties of the microwires generated determine the efficiencies of the water splitting reactions. This work aims to understand the dependence of the microwire properties on the flow rates of the precursor gases and growth conditions used, to be able to grow p-n or p-i-n microwires suitable for efficient splitting of water. The flow rate of silicon precursor gas had direct dependence on the doping density and the number of sidewall facets of the microwires. It was observed that a change of silicon precursor flow rate during the growth resulted in electrically and mechanically defective microwires. The n-type nature of copper catalyst as a dopant in these microwires was established and the extent to which this impacts the accessible doping range in both p- and n-type microwires is well understood. This understanding was used to grow defect-free p+-intrinsic microwires and the abruptness of the transition region was studied.