Te-rich Bi2Te3 thin films by electron−beam deposition: Structural, electrical, optical and thermoelectric properties

Abstract

Tellurium-rich Bi2Te3 thin films are deposited by electron−beam evaporation technique at 300 K. These as-deposited thin films are further annealed at 100 °C, 200 °C and 300 °C for 1 h at a pressure of 3 × 10−4 Pa. X-ray diffraction (XRD) patterns of as-deposited films distinctly show Te phase along with Bi2Te3. Peak intensity ratio suggests the polycrystalline nature of as-deposited Bi2Te3 films. On vacuum annealing Te-rich Bi2Te3 films exhibit improved crystallinity with a c-axis preferred orientation. In addition, structural features related to Te and Bi2Te3 composition change with Te fraction diminishing on annealing at 300 °C. From Raman spectral studies, the presence of distinct Te-rich regions, predominantly within the interlayers of Bi2Te3, are discerned. Te becomes structurally integrated within the quintuples of Bi2Te3 lattice as intergrown layers. Disordered planar structures, mostly concentrated on crystallite surfaces result from Te accumulations as evidenced in high-resolution transmission electron microscopy lattice images and energy dispersive X-ray spectroscopy mapping. These are consistent with the observations from XRD and Raman studies further confirming Te-rich Bi2Te3 characteristics. Electrical properties of Te-rich Bi2Te3 thin films exhibit n-type semiconductor behaviour. Seebeck coefficient for as-deposited film is ~ 32 μV/K, which increases to ~ 97 μV/K on 200 °C annealing. Resistivity increases from 1.39 × 10−4 Ωcm to 18.76 × 10−4 Ωcm and power factor changes from 7.4×10−4W/K2m to 27.17×10−4W/K2m going through a maximum at 200 °C upon systematic annealing. Te-rich Bi2Te3 thin films annealed at 200 °C exhibit high power factor (∼29×10−4W/K2m) for a wide range of temperature gradients ( ΔT from 30 °C to 165 °C).