Morphology and seebeck coefficients of electrodeposited Bi2Se3 films grown onto Au(111)/Si substrates

Abstract

Polymorphism in Bi2Se3 allows it to be tuned for unique electrical, thermal, and optical properties. The commonly reported rhombohedral structure is a topological insulator, a narrow gap semiconductor with a bandgap of 0.2–0.3 eV, and has been widely studied for thermoelectric applications. The alternative orthorhombic structure is a semiconductor with a larger bandgap of 0.9–1.2 eV. The opportunity to fabricate a mixture of these orthorhombic and rhombohedral structures provides a chance for materials engineering to optimize its electrical and thermal properties. Here we report the morphology and the Seebeck coefficient of mixed-phase, Se-rich, n-type Bi2Se3 films prepared by electrodeposition using an acidic bath. Post-nucleation formation of smooth films was observed to be followed by the emergence of crystals and continued growth through the coalescence of the newly formed crystals. The room temperature Seebeck coefficients of the films were also observed to vary as a function of the film thickness, increasing from -90 µV/K (0.74 µm thickness) to a maximum of -184.4 µV/K (1.84 µm thickness) and gradually decreasing to -100.8 µV/K (5.72 µm thickness). Analysis of XRD patterns for the Bi2Se3 films showed that the thickness dependence of Seebeck coefficients was related to the transition from pure rhombohedral to a mixture of orthorhombic and rhombohedral phases. The thickness dependent Seebeck effect was further discussed by the computational study of the Bi2Se3 band structures and Seebeck coefficients of pure rhombohedral and pure orthorhombic structures.