Synthesis, Phase Engineering and Optoelectronic Applications of Binary Arsenic Sulphide Thin Films from Orpiment to Dimorphite

Binary arsenic sulphide contains three thermodynamically stable compounds, orpiment (As2S3), realgar (As4S4) and dimorphite (As4S3), spanning a covalent layer network at the sulphur-rich end and discrete molecular cages at the arsenic-rich end. Thin-film research has concentrated almost entirely on As2S3, and no review has yet covered the binary system as a whole. This review brings together the phase diagram, the crystal structures of every stable phase and its polymorphs, the deposition methods, the structural and optical characterisation, the electrical and optoelectronic behaviour, and the device record. Deposition is compared across thermal evaporation, radio-frequency magnetron sputtering, pulsed laser deposition, plasma-enhanced chemical vapour deposition and solution routes, with attention to the two stoichiometry problems that govern film composition, incongruent evaporation of As2S3 and preferential sulphur sputtering from As-S targets. Phase engineering is treated as a distinct theme, since the arsenic-rich compositions crystallise as molecular cages whose polymorph ratio is set by growth rate, substrate temperature and annealing schedule. The review ends with the measurements that would bring the arsenic-rich compounds to the level of characterisation already available for As2S3, refractive index dispersion and non-linear coefficients, thin-film Raman assignments for each polymorph, and annealing schedules that identify which crystalline phase forms.