Optical studies on non-stiochiometric (Zn, Fe)S chalcogenide bulk pellets prepared by coprecipitation

Non-stiochiometric ternary chalcogenides Zn1−xFexS (x = 0.005, 0.013, 0.05, 0.231, and 0.43) bulk powders were prepared by coprecipitation of ZnS and FeS by adding Na2S to aqueous solutions containing FeSO4 and ZnSO4. Pellets of resulting material were sintered at 1123 K in vacuum-sealed quartz ampoules maintained at 10−2 torr for a total of 165 h. Direct optical bandgaps were found to decrease with increase in Fe content viz. 2.1 eV < Eg < 3.3 eV for 0.43 > x > 0.005. A subsidiary peak attributed to photoionization transitions of Fe ions appeared at lower energy values (around 2.7 eV) whose intensity increased with increase in Fe content. Room temperature photoluminescence emission studies (PLE) of the pellets were obtained with λexc = 225 nm, revealed UV region peaks centered at energies above that of bandgap (Eg), and correspond to free exciton emission. While those of energies below that of Eg are attributed to defects due to vacancies and interstitials. The PLE peaks in visible region (viz. 491, 518, and 596 nm) are attributed to transitions involving impurity levels. Peak at 617 nm is expected to be due to transitions associated with Fe2+ levels localized at 2.1 eV below the (Zn, Fe)S conduction edge. The various bypassing processes leading to the deactivating behavior of Fe2+ as evident from decreasing luminescence peak intensity with increasing ‘x’, have been explained qualitatively.