A comparative study of structural, compositional, thermal and optical properties of non stoichiometric (Zn,Fe)S chalcogenide pellets and thin films

Non-stiochiometric ternary chalcogenides Zn_1−xFe_xS, were prepared in the bulk form by co-precipitation of ZnS and FeS by Na₂S from aqueous solution containing FeSO₄ and ZnSO₄ and sintering of pellets of the co-precipitate repeatedly at 1073K in vacuum sealed quartz ampoules. Concomitant with the bulk form; thin fims of (Zn,Fe)S were synthesized by pyrolytic spray deposition method on quartz substrates from aqueous precursor solution containing ZnCl₂, FeCl₂ and thiourea in varying concentration under optimized conditions of substrate temperature (653K) carrier gas flowrate (11 l min⁻¹) and solution flow rate (8-6 ml min⁻¹).The structure, chemical composition, optical and thermoelectrical properties of the (bulk) pellets and thin films are studied as a function of initial solution concentration.X-ray diffraction of the pellets and thin films indicated the presence of solid solutions Zn_1−xFe_xS (sphalerite), while surface morphology as determined by SEM revealed a granular structure. Electrical resistivity of pellets and thin films, measured using two probe method (for pellets) and four probe van der Pauw method (for thin films) indicated that they are semiconducting while resistivity studies could not be carried out for a few thin films due to their high resistance (>20 MΩ).The chemical composition of the resulting solids as analyzed by X-ray fluorescence and that of thin films as analyzed by energy dispersive X-ray, reflected the composition of the solutions from which precipitation (for pellets) and deposition (for thin films) was carried out, with Fe contents up to x=0.4.SEM micrograph of pellets and thin films reveal that later have smaller grain size.Thermoelectric studies revealed that both solids and thin films possess the ability of ‘n’ as well as ‘p’ type conductivity.The diffuse optical reflectance measurements of pellets and transmittance measurements for thin films; as a function of wavelength reveal the dependence of direct optical band-gap on Fe content.