Thermal annealing of sequentially deposited SnS thin films

The influence of thermal treatment with the number of deposition cycles on the properties of SnS films on CdS and ZnS substrates was investigated. Annealing under an argon atmosphere made amorphous SnS films formed in one deposition cycle crystalline, but did not markedly change the crystallinity of SnS films formed after multiple deposition cycles. All annealed films were consistent with the orthorhombic phase of herzenbergite SnS, and no additional Sn-containing phases were identified. The CdS/SnS films maintained their initial stoichiometric composition of tin monosulphide after annealing. The films deposited on ZnS substrate films were rich in tin and poor in sulphur and their composition was unaffected by thermal annealing. Both the deposited and thermally annealed films possessed uniform pinhole-free surfaces. Only minor changes in the optical transmittance and reflectance spectra of the CdS/SnS films were observed after annealing, whereas the spectra of the ZnS/SnS films exhibited substantial changes after annealing. As the annealing temperature increased, the absorption edge of the ZnS/SnS films shifted to a longer wavelength. The optical bandgap of the CdS/SnS films was indirect and decreased from 1.28 eV for the three-deposition-cycle CdS/SnS film to 1.22 eV after annealing at 460 degrees C. The ZnS/SnS films showed a similar change: the bandgap of 1.39 eV for the unannealed films decreased to 1.23 eV after annealing. All deposited and annealed SnS films showed p-type conductivity and their photoconductivity increased with the increasing annealing temperature. Solar cells with reverse structures were fabricated; their performance decreased with the increasing annealing temperature of the SnS film.