Heteroepitaxial growth of highly anisotropic Sb2Se3 films on GaAs.

The epitaxial integration of anisotropic materials with mainstream cubic semiconductors opens new routes to advanced electronic and photonic devices with directional properties. In this work, we synthesize heteroepitaxial thin films of orthorhombic "quasi-1D" Sb2Se3 on cubic GaAs(001) using molecular beam epitaxy. Traditionally, the synthesis of anisotropic films with low symmetry materials is challenging due to multiple grain orientations that form. On a macroscopic scale, such a film tends towards isotropic properties, even if individual grains possess anisotropic responses. We achieve epitaxial Sb2Se3 grains on pristine homoepitaxial GaAs templates at low temperatures of 180-200 °C. With the Sb2Se3 1D axis aligned in-plane to GaAs [110] and the primary van der Waals direction lying out-of-plane, we find a birefringence of 0.2 between in-plane orthogonal directions and a giant out-of-plane birefringence greater than 1 at telecom wavelengths. Growth at higher temperatures up to 265 °C yields Sb2Se3 of an unusual in-plane rotated texture that further enhances the in-plane optical index anisotropy to 0.3.