Pressure-temperature dependence of nanowire formation in the arsenic-sulfur system

Arsenic sulfide (As_xS_y) nanowires, nanodroplets, and micro-islands were synthesised on fused silica substrates using an evaporation-condensation process at reduced pressures (70 mTorr-70 Torr) in sealed ampoules. Microstructural control of the deposited thin film was achieved by controlling initial pressure, substrate temperature, and substrate surface treatment. Microstructures were characterised using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Surface chemistry and topography of the substrates were characterised using x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), respectively. Semi-quantitative image analysis and basic curve fitting were used to develop an empirical model to mathematically describe the variation of microstructure as a function of initial pressure and substrate temperature and map out the regions of different microstructures in P-T space. Nanowires of an amorphous, semiconducting material that is transparent in the visible-LWIR region such as As₂S₃, provide new opportunities for the development of novel nanophotonic and electronic devices and also provide an excellent opportunity to model (and control) microstructure development from nanometre to micron scales in a physical vapour deposition process.