Effect of annealing on physical characteristics of the vacuum evaporated mixed phase SnxSy thin films

The mixed phase SnxSy films of the thickness of 200 nm were obtained by annealing in an indigenously designed furnace at 473 K, 523 K, 573 K and 623 K for 1 h at each temperature. Prior to annealing, the precursor 'Sn' and 'S' materials were mixed up in a ball milling setup and thermally evaporated at the vacuum pressure similar to 2 x 10(-6) mbar on corning glass substrate. The increase in the annealing temperature influenced the reaction kinetics of 'Sn' and 'S' materials and led to the formation of SnS phase at 473 K. The SnS phase existed from 473 K to 573 K and transformed to Sn2S3 at temperatures between 573 K and 623 K. Besides this, the SnS2 phase co-existed in all undertaken temperatures where intensity of its most prominent peak increased with increase in annealing temperature. The transmittance (%) values of the undertaken films increased with increase in the annealing temperature in the wavelengths varying from 400 nm (visible) to 1600 nm (NIR). The absorption coefficient (alpha) values decreased with increase in annealing temperature from 1 x 10(5) to 4 x 10(4) cm(-1) in the visible region and attained saturation values (similar to 2 x 10(4) cm(-1)) in the NIR region. The films possessed direct bandgap (E-g) with values increasing from 1.90 eV to 2.82 eV with increase in the annealing temperature from 473 K to 623 K; it also possessed indirect bandgaps with E-g values varying from 0.82 eV to 1.52 eV with increase in annealing temperature in the same temperature range. The 473 K annealed samples provided irregular shaped SnS grains indicated by bright crystallites which reached to their stable position with enhanced crystallinity at the annealing temperatures of 523 K and 573 K. The SnS grains, however, reduced to lesser number at 623 K due to growth of Sn2S3 crystallites at the expense of SnS. The 'S' rich SnS phase (SnS2) crystallites were seen represented by darker crystallites appeared at 523 K, 573 K and 623 K. The current-voltage (I-V) study of the FTO/Film/Ag structure confirmed the schottky diode formation between Ag and film interface. There are variations in the I-V behaviour of diodes which resulted in random changes in the values of 'ideality factor' and 'barrier heights'. The changes in diode parameters may be due to variation in the interface states, surface defects and barrier inhomogenities occurring due to compositional changes in SnxSy films.