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.