Evolution of crystalline structure in SnS thin films prepared by chemical deposition

• Cubic or orthorhombic SnS thin films deposited from a chemical bath at 20 or 40 °C.
• SnS film with band gap 1.66 eV deposited at 20 °C followed by SnS film of 1.1 eV at 40 °C.
• The optical band gap of 1.66 eV for cubic and 1.1 eV for orthorhombic are relevant as photovoltaic absorber.

Crystal structure and morphology undergo significant evolution in thin films of tin(II) sulfide prepared by chemical deposition, over a narrow interval of bath temperature of 20–40 °C, but has not been recognized in previous studies. The chemical bath is constituted using tin(II) chloride, triethanolamine, ammonia(aq.) and thioacetamide. At bath temperature of 20 °C, the deposition rate of the film is 10 nm/h; and at 24 h, a film of thickness 260 nm is obtained. This film is compact and with a predominantly cubic (Cub-) crystalline structure. At 40 °C, the deposition rate is 25 nm/h, and a film of 600 nm in thickness is deposited in 24 h. However, this film has evolved into vertically stacked platelets of orthorhombic (OR-) crystalline structure. The transition from compact-to-platelet morphology as well as from Cub-to-OR-crystalline structure is observed near a deposition temperature, 35 °C. The Cub-SnS has a characteristic high optical band gap, 1.67 eV (direct gap; forbidden transitions) with an electrical conductivity, 10−7(Ω cm)−1; both properties being un-affected when films are heated at 300 °C in a nitrogen ambient. In OR-SnS, the band gap is 1.1 eV (indirect gap; allowed transitions). The electrical conductivity of such films is notably higher, 10−4 (Ω cm)−1, which increases further by an order of magnitude when the films have been heated at 300 °C in nitrogen.

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