Measuring and Then Eliminating Twin Domains in SnSe Thin Films Using Fast Optical Metrology and Molecular Beam Epitaxy

Abstract

van der Waals (vdW) layered chalcogenides have strongly direction-dependent (i.e., anisotropic) properties that make them interesting for 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 orientation of crystal domains during growth is key to reliably making wafer-scale, high-quality thin films, free from twin boundaries. Here, we demonstrate a fast optical method to quantify domain orientation in SnSe thin films made by molecular beam epitaxy (MBE). The in-plane optical anisotropy results in white-light being reflected into distinct colors for certain optical polarization angles and the color depends on domain orientation. We use our method to confirm a high density of twin boundaries in SnSe epitaxial films on MgO substrates, with square symmetry that results in degeneracy between SnSe 90° domain orientations. We then demonstrate that growing on a-plane sapphire, with rectangular lattice-matched symmetry that breaks the SnSe domain degeneracy, results in single-crystalline films with one preferred orientation. Our SnSe bottom-up film synthesis by MBE enables future applications of this vdW material that is particularly difficult to process by top-down methods. Our optical metrology is fast and can apply to all triaxial vdW materials.

Recent Advances in 2D Material Theory, Synthesis, Properties, and Applications

“…(d) The SnSe 7 coordination polyhedron. Reproduced with permission from ref . Copyright 2022 American Chemical Society.…”

“…(e–h) Demonstrations of structure and phase control through epitaxy: (e) eliminating twin domains. Reproduced with permission from ref . Copyright 2022 American Chemical Society.…”

“…The ferroelastic distortion that creates the distinction between armchair and zigzag directions leads to a system of 90°twin boundaries, across which these directions (and their associated anisotropic properties) exchange. Mortelmans et al 134 showed that these 90°twins in SnSe can be eliminated by epitaxial growth on a substrate with a rectangular and lattice-matched inplane unit cell (Figure 8e), making possible large-area thin films with uniform in-plane anisotropy. Chang et al 135 demonstrated through epitaxial growth on 6H-SiC(0001) that monolayers of SnSe are indeed polar (Figure 8f), and exhibit in-plane ferroelectricity at room temperature.…”

Review of Nanolayered Post-transition Metal Monochalcogenides: Synthesis, Properties, and Applications

“…Chin et al 187 used individual elemental source materials to study the self-limiting stoichiometry in SnSe and revealed that within a limited range, changing the Se:Sn flux ratio does not influence the film stoichiometry, but only rotates the primary crystallographic orientation from (210) to (200). By replacing MgO(001) substrates with a-plane sapphire substrates, which have rectangular lattice-matched symmetry that breaks the SnSe domain degeneracy, Mortelmans et al 188 realized SnSe films free of 90°twin defects.…”

“…Chang et al used high-purity SnSe granule source materials to grow highly oriented SnSe single layers on graphene substrates using a two-step growth and anneal process. SnSe nanoplates have been produced by optimizing the growth and annealing temperatures. ,, Maintaining stoichiometry is critical for the growth of SnSe. Chin et al used individual elemental source materials to study the self-limiting stoichiometry in SnSe and revealed that within a limited range, changing the Se:Sn flux ratio does not influence the film stoichiometry, but only rotates the primary crystallographic orientation from (210) to (200).…”