Formation mechanism of 2D SnS2 and SnS by chemical vapor deposition using SnCl4 and H2S

SnS2 and SnS are two-dimensional (2D) semiconductors with distinct properties, as they exhibit a different type of conduction. They are of interest for applications in nanoelectronics, optoelectronics and sensors. To enable these applications, the deposition of SnS2 and SnS layers with a well-controlled phase, crystallinity and thickness at the nanometer level is required on large-area substrates. Chemical vapor deposition (CVD) of SnS2 and SnS using SnCl4 and H2S has previously been reported to give micrometer level polycrystalline SnS2 and SnS layers, which were insulating due to the uncontrolled grain orientations. In this work, we investigate the formation mechanism and phase control of nanometer level 2D SnS2 and SnS by SnCl4/H2S CVD. Nanometer level and phase-pure 2D hexagonal SnS2 and orthorhombic SnS layers are obtained. The SnSx phase depends on both the temperature and the H2S/SnCl4 concentration ratio. Compared to the formation of the SnS2 phase, the formation of the SnS phase is favorable at higher temperature and, surprisingly, at a higher H2S/SnCl4 concentration ratio. This is explained by the catalytic decomposition of H2S by SnS2 with the formation of H-2, where the as such generated H-2 reduces SnS2 to SnS at temperatures equal to or higher than 350 degrees C. By adjusting the conditions of the CVD process, the product can be tuned to either n-type SnS2 or p-type SnS, as demonstrated by back-gated field effect transistors.