Sb_2Te_3 and Bi_2Te_3 Thin Films Grown by Molecular Beam Epitaxy at Room Temperature

摘要

Nano-alloyed p-type Sb_2Te_3 and n-type Bi_2Te_3 thin films were grown on SiO_2/Si and BaF_2 substrates by molecular beam epitaxy (MBE) in two steps: (i) Repeated deposition of fivelayer stacks with sequence Te-X-Te-X-Te (X = Sb or Bi) with elemental layer thicknesses of 0.2 nm on substrates at room temperature, (ii) annealing at 250 °C for two hours at which phase formation of Sb_2Te_3 or Bi_2Te_3 occurred. The room temperature MBE deposition method reduces surface roughness, allows the use of non lattice-matched substrates, and yields a more accurate and easier control of the Te content compared to Bi_2Te_3 thin films, which were epitaxially grown on BaF_2 substrates at 290 °C. X-ray diffraction revealed that the thin films were single phase, poly-crystalline, and textured. The films showed grain sizes of 500 nm for Sb_2Te_3 and 250 nm for Bi_2Te_3, analyzed by transmission electron microscopy (TEM). The in-plane transport properties (thermopower S, electrical conductivity a, charge carrier density n, charge carrier mobility μ, power factor S~σ) were measured at room temperature. The nano-alloyed Sb_2Te_3 thin film revealed a remarkably high power factor of 29 μW cm~(-1) K~(-2) similar to epitaxially grown Bi_2Te_3 thin films and Sb_2Te_3 single crystalline bulk materials. This large power factor can be attributed to a high charge carrier mobility of 402 cm~2 V~(-1) s~(-1) similar to high-ZT Bi_2Te_3/Sb_2Te_3 superlattices. However, for the nano-alloyed Bi_2Te_3 thin film a low power factor of 8 μW cm~(-1) K~(-2) and a low charge carrier mobility of 80 cm~2 V~(-1) s~(-1) were found. Detailed microstructure and phase analyses were carried out by energy-filtered TEM in cross-sections. Quantitative chemical analysis by energy-dispersive x-ray spectroscopy (EDS) was also applied. In Bi_2Te_3 thin films, few nanometer thick Bi-rich blocking layers at grain boundaries and Te fluctuations by 1.3 at.% within the grains were observed. The small charge carrier densities are explained by a reduced antisite defect density due to the low temperatures to which the thin films were exposed during annealing.