2, Materials Science and Engineering, University of Virginia, Charlottesville, Virginia, United States
Polymorphism in Bi2Se3 allows it to be tuned for unique electrical, thermal and optical properties. The commonly reported rhombohedral structure exhibits semi-metallic properties corresponding to a band gap of 0.32 eV and has been widely studied for thermoelectric applications and as topological insulators. The alternative orthorhombic structure is more semiconducting and has been reported to have a band gap close to 1.2 eV. The opportunity to fabricate a mixture of these orthorhombic and rhombohedral structures provides a chance for materials engineering with the aim of optimizing its electrical and thermal properties. Here we report the room temperature Seebeck coefficient and electrical resistance of mixed phase n-type Bi2Se3 films. Bi2Se3 films with Bi:Se atomic fraction of 38:62 was prepared by electrodeposition using an acidic bath. The XRD pattern of the electrodeposited Bi2Se3 films confirmed the existence of a mixed phase structure where the orthorhombic phase was found to be improved upon emergence and subsequent coverage of the film surface with large crystals, as seen from the surface morphology for these films. The n-type conductivity of Bi2Se3 was confirmed from XPS and Mott-Schottky analysis. A maximum room temperature Seebeck coefficient of -229.3 μV-1K-1 was observed upon characterizing the films having varying thicknesses. The cross-plane resistance was found to improve from increasing film thickness of 0.24 μm to 1.82 μm suggesting an optimum charge transport through these films at 1.84 μm thickness. The measured Seebeck coefficient values are consistent with calculated values based on first principles calculations.