Molecular Beam Epitaxy of SnSe Thin Films for Future Optoelectronic Applications

Van der Waals (vdW) layered chalcogenides have strongly direction-dependent properties that make them interesting for certain photonic and optoelectronic applications. Orthorhombic tin selenide (α-SnSe) is a triaxial vdW material with strong optical anisotropy within layer planes, which has motivated studies of optical phase and domain switching. As with every vdW material, controlling the phase and orientation of crystal domains during growth is key to reliably making wafer-scale, high-quality thin films, free from twin boundaries. We demonstrate a fast and easy optical method - that applies to all triaxial vdW materials - to quantify domain orientation in single-phase α-SnSe thin films made by molecular beam epitaxy (MBE). We use our method to confirm a high density of twin boundaries in α-SnSe epitaxial films on MgO substrates, with square symmetry that results in degeneracy between α-SnSe 90° domain orientations. We then demonstrate that growing instead on a-plane sapphire, with rectangular lattice-matched symmetry that breaks the α-SnSe domain degeneracy, results in single-crystalline films with preferred orientation, with twin domains all-but-eliminated. We then perform experiments attempting ferroelastic α-SnSe domain switching and epitaxial stabilization of metastable cubic SnSe using other substrates. Our bottom-up SnSe film syntheses by MBE are enabling for future applications of this vdW material that is particularly difficult to process by top-down methods.