Sb ₂Te ₃ and Bi ₂Te ₃ thin films grown by room-temperature MBE

Sb ₂Te ₃ and Bi ₂Te ₃ thin films were grown on SiO ₂ and BaF ₂ substrates at room temperature using molecular beam epitaxy. Metallic layers with thicknesses of 0.2 nm were alternately deposited at room temperature, and the films were subsequently annealed at 250°C for 2 h. x-Ray diffraction and energy-filtered transmission electron microscopy (TEM) combined with high-accuracy energy-dispersive x-ray spectrometry revealed stoichiometric films, grain sizes of less than 500 nm, and a texture. High-quality in-plane thermoelectric properties were obtained for Sb ₂Te ₃ films at room temperature, i.e., low charge carrier density (2.6 × 10 ¹⁹ cm ^-3), large thermopower (130 μV K ^-1), large charge carrier mobility (402 cm ² V ^-1 s ^-1), and resulting large power factor (29 μW cm ^-1 K ^-2). Bi ₂Te ₃ films also showed low charge carrier density (2.7 × 10 ¹⁹ cm ^-3), moderate thermopower (-153 μV K ^-1), but very low charge carrier mobility (80 cm ² V ^-1 s ^-1), yielding low power factor (8 μW cm ^-1 K ^-2). The low mobilities were attributed to Bi-rich grain boundary phases identified by analytical energy-filtered TEM.