Structural, electrical, optical and thermoelectric properties of e-beam evaporated Bi-rich Bi2Te3 thin films

Bi-rich Bi2Te3 thin films are prepared at 300 K using e-beam evaporation technique. A source power of 45 W for e-beam is used. Post deposition, these as-deposited Bi-rich Bi2Te3 (Bi-BT-AD) films are annealed at 100 degrees C (Bi-BT100), 200 degrees C (Bi-BT-200) and 300 degrees C (Bi-BT-300) for 1 h under a pressure of 3 x 10(-4) Pa. X-ray diffraction measurements reveal the presence of Bi phase together with crystalline Bi2Te3 indicating the possible presence of Bi-rich Bi2Te3 phase in the Bi-BT-AD film. The broad peaks from Bi2Te3 (015) plane indicate nanocrystalline nature of particles. With annealing, no change in diffraction pattern is observed for Bi-BT-100. However, Bi-BT200 and Bi-BT-300 films show the emergence of x-ray reflection from unknown phases around 20 - 20 degrees and 47 degrees. This indicates Bi related secondary phase segregation and the thermodynamic instability for the presence of Bi in Bi2Te3 lattice. From Raman studies it is discerned that Bi secondary phase coexists along with the Bi-rich Bi2Te3 nanocrystalline grains. On vacuum annealing Bi-rich Bi2Te3 phase in thin films prevails as evidenced from the ptype electrical characteristics, while excess Bi disappears and converts into an unknown minor phase. The resistivity of all the annealed films are - 0.9 x 10(-4) Qcm. The Seebeck coefficients also do not show any change and remain around 33 to 36 mu V/K. Thermoelectric properties of Bi-BT-100 exhibit high power factors when measured at different AT with a maximum of - 17.5 x 10(-4) W/K(2)m for AT = 100 degrees C. Thus, unlike the nearstoichiometric thin films, Bi-rich thin films require low temperature annealing (similar to 100 degrees C) to achieve optimized parameters. Bi-rich Bi2Te3 thin films also show higher power factor compared to the near-stoichiometric thin films. Thus, favourable thermoelectric properties can be achieved at 300 K for temperature sensitive device fabrication using Bi-rich Bi2Te3 thin films.