Wafer-Scale Growth of Ultra-Thin SnSe_x (x=1,2) by Low-Temperature MOCVD

Two-dimensional van der Waals (vdW) crystals are a recently emerging class of electronic materials, and among them, Mo-, W-based transition-metal dichalcogenides (TMDs) have been successfully synthesized in the form of wafer-scale thin films until now. On the other hand, the group IV metal chalcogenides including Sn-based compounds have been limited in their applications with small flakes or thick films despite their promising properties. The reason is that the absence of suitable precursors and low growth temperature made it difficult for the materials to diffuse on the substrate and grow into a large-area thin film. In this work, we demonstrate gas-phase synthesis ultra-thin SnSe<i>_x</i> (<i>x</i>=1 and 2) films with high crystallinity and wafer-scale homogeneity. It is enabled by low-temperature metal-organic chemical vapor deposition (MOCVD) in which relatively compact and non-toxic methyl ligand precursors are used. Also, we overcome the temperature mismatch between high precursor decomposition temperature (~650°C) and low growth temperature (~200°C) by designing the separate functional zones with independent thermal control. Through this preferable growth condition, SnSe₂ ultra-thin film (3~5 nm) can be ultimately synthesized in wafer-scale. Afterward, <i>n</i>-type SnSe₂ thin film can be polarity change to <i>p</i>-type SnSe by thermally induced phase transition in inert and stable condition. SnSe thin film can also keep high crystallinity and homogeneity across the wafer despite the crystal structure change. Both <i>n</i>-type SnSe₂ and <i>p</i>-type SnSe thin films can be applied as p-n junction channel array with type II band alignment. This study will serve as a stepping stone to accelerate the electronic and thermoelectric device applications of high quality SnSe<i>_x</i> (<i>x</i>=1,2) thin film in the wafer-scale.