Low-temperature characterization and micropatterning of coevaporated Bi₂Te₃ and Sb₂Te₃ films

Thermoelectric (TE) properties of the coevaporated Bi2Te3 and Sb2Te3 films are measured from 100 to 300 K for Seebeck coefficient αS and from 5 to 300 K for electrical resistivity ρe, mobility μe, and Hall coefficient RH. For the low-temperature characterization of TE films, the conditions for coevaporation deposition of Bi, Te, and Sb to form Bi2Te3 and Sb2Te3 films are also investigated, including substrate material, substrate temperature Tsub, and elemental flux ratio (FR). The resublimation of Te occurring above 473 K significantly affects the film composition and quality. Our optimal deposition conditions for Bi2Te3 films are Tsub=533 K and FR=2.4, and those for Sb2Te3 films are Tsub=503 K and FR=3.0. The TE properties of both films are strongly temperature dependent, while Bi2Te3 films show a stronger temperature dependence than Sb2Te3 films due to different major scattering mechanisms. αS of both the coevaporated films are close to or higher than those of bulk materials, but ρe is much higher (due to lower carrier concentrations for Sb2Te3 films and lower μe for Bi2Te3 films). Also, no freeze-out regime is found for both Bi2Te3 and Sb2Te3 films at low temperatures. The room-temperature power factors of αS²/ρe for Bi2Te3 and Sb2Te3 films are 2.3 and 2.0 mW/K² m, and the maxima are 2.7 mW/K² m for Bi2Te3 at T=220 K and 2.1 mW/K² m for Sb2Te3 at T=280 K. Shadow mask technique is successfully used for the micropatterning (20 μm) of TE films with no significant change in properties.