Observation of a power factor enhancement in MBE-grown Bi_1-xSb_x alloy thin films

Bulk Bi_1-xSb_x is a direct (indirect) gap semiconductor for 0.07<x<0.15(0.15<x<0.22) with a maximum bandgap of 14-20 meV at x≈0.12-0.15. Small bandgap Bi_1-xSb_x alloys have been used as the n-type thermoelement for coolers operating around 80 K. The optimum operating temperature strongly depends on the magnitude of the bandgap of the material. We have grown BiSb alloy thin films on CdTe(111)B over a wide range of Sb concentrations using molecular beam epitaxy and have observed a maximum bandgap of 40 meV at x = 0.09, which is larger than that observed in bulk alloys. Thermoelectric power (TEP) measurements showed that a maximum TEP value occurred in a thin film with x = 0.09. This thin-film TEP value was 16% larger than bulk at room temperature. The power factor (S²σ) results show that the BiSb alloy thin films peak at a significantly higher temperature (250 K) than previously reported for the bulk alloy (80 K), which is possibly due to the enhanced bandgap. This bandgap enhancement may be associated with strain effects. The results show significant promise for use of band-engineered materials in thermoelectric devices.