Growth of high quality SnS van der Waals epitaxies on graphene buffer layer for photovoltaic applications

SnS van der Waals epitaxies (vdWEs) were grown by molecular beam epitaxy (MBE) on graphene buffer layer. Photo-absorption measurements indicate an indirect bandgap of ~1 eV and a direct bandgap of ~1.25 e V. Using this novel growth technique we observed significant lowering in the crystal size and the rocking curve FWHM compared to films deposited without the buffer layer. The absorption coefficient, α, is of the order of 10⁴cm^-1 with sharp cutoff in energy indicating low concentration of bandgap states. Hole mobility as high as 81 cm²V^-1s^-1 was observed for SnS films on graphene/GaAs(100) substrates. The layered structure and chemically saturated bonds for SnS and the graphene buffer layer stipulate that the interaction at the interface is dominated by vdW force. This significantly enhances the tolerance in the lattice mismatch leading the improvement in the crystallinity of the film.