Se/Sn flux ratio effects on epitaxial SnSe thin films; crystallinity & domain rotation

We report on the effects of Se/Sn flux ratio to the growth and thermoelectric properties of SnSe thin films on MgO (100) substrate. All films were epitaxially grown at 300 °C with various Se/Sn flux ratios of 0.8, 0.9, 1, 3, 5, and 7. The crystal structure of the obtained SnSe thin films was orthorhombic (Pnma space group) with a-axis (h00) orientation. XRD Φ-scan studies showed an existence of orthorhombic domains, whose rotation can be modulated by the Se/Sn flux ratio. A structure consisting of 0° and 90° rotated domains was observed in all samples, while additional 45° rotated domains were simultaneously observed in the samples with higher Se/Sn ratio of 5 and 7. Especially, 135° rotated domains were observed in sample Se/Sn = 7. Interestingly, the best crystalline film was obtained at the Se/Sn = 1. We found that Seebeck coefficients were positive at room temperature, decreased with temperature and became negative for the sample Se/Sn = 1, indicating bipolar transport in the samples. A maximum PF value of 3.74 μWcm−1K−2 was obtained at 550 K for sample Se/Sn = 0.8.