Growth of SnS van der Waals Epitaxies on Layered Substrates
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Growth of SnS van der Waals Epitaxies on Layered Substrates S.F. Wang, W.K. Fong, W. Wang, K.K. Leung and C. Surya Department of Electronic and Information Engineering and Photonic Research Centre, The Hong Kong Polytechnic University, Hong Kong, P.R. China. ABSTRACT In this paper we present systematic investigations on the growth of SnS van der Waals epitaxies (vdWEs) on different substrates, including crystalline and layered substrates, by molecular beam epitaxy (MBE). Experimental growth of SnS on conventional 3D substrates, such as GaAs, indicates strong interaction between the SnS layer and the substrate resulting in poor crystallinity in general. Substantial improvement in the film crystallinity can be obtained when the deposition is made on layered substrates, with saturated surface bonds, as observed in SnS films deposited on mica and crystalline substrates with a graphene buffer layer. Crystal size as large as one micron and rocking curve FWHM of 0.118° was observed despite the large lattice mismatches. This represents significant improvement over the reported value of ~3°. Several symmetric growth orientations are observed for films grown on mica substrates. The results indicate that weak vdW interactions between the saturated bonds of the substrate surface and the SnS unit layer which is an important factor for achieving high quality epitaxy layered materials. INTRODUCTION SnS is a promising photovoltaic material due to its close-to-ideal energy bandgap, of about 1.3 eV, and its high absorption coefficient at 1.5eV. In addition, the constituent elements are abundant, inexpensive and non-toxic, making it a promising candidate for large-scale deployment. Most importantly, SnS is a vdWE characterized by a 2-D layered structure with strong intra-layer covalent bonds and weak inter-layer interactions dominated by vdW forces. This class of materials is believed to possess high tolerances for mismatches in the lattice constant and the thermal expansion coefficients between the overlayer and substrate. This property may have important implications in a wide range of optoelectronic and electronic applications due to the unique potential of the growth of high quality films on low-cost substrates. Furthermore, a wide range of high quality heterojunctions consisting of different vdWEs, with large differences in lattice constants, may potentially be fabricated with very low interface states density [1]. In this paper, we investigate the growth of SnS on different substrates, including crystalline substrates and layered substrates. EXPERIMENT SnS thin films were deposited on different substrates including GaAs, graphene/GaAs, and mica sheets by molecular beam epitaxy (MBE) technique using 4N SnS as the evaporation source. The substrate temperature varied in the range of 200~300°C. The structural properties of
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the films were studied using high resolution XRD, and the top view and cross-sectional SEM were utilized to investigate the film morphology. RESULTS AND DISCUSSION MBE-grown SnS films on GaAs(100) and graph
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