Optical And Thermoelectric Properties Of Nanocrystalline Bi2(Se1-xTex)3 Films

Abstract

ABSTRACTGranular nanocrystalline Bi2(Se1-xTex)3 films were prepared from highly crystalline bulk alloys used as source materials by means of thermal evaporation under vacuum (2 × 10−6 mbar). The nanofilms were deposited onto highly cleaned glass substrates of BK7 type. The structural characteristics, including surface morphology, roughness, and crystallite size, were studied by X-ray diffraction (XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM) analysis. Perfect crystallinity and nanoscalability of the prepared samples were asserted. Transmittance and reflectance spectra of the prepared nanofilms were investigated over the wavelength range of 400–2,700 nm. In the visible light region, all of the films showed an opaque behavior; however, as the wavelength increased into the infrared region, a growing transparency was observed. Based on the measured transmission and reflection spectra, the coefficients of absorbance and extinction as well as the optical band gap were determined. The refractive index was deduced from the reflectance measurements using the determined extinction coefficient. Direct allowed band transitions were confirmed in all films with a band gap energy of 0.85–1.05 eV, depending on the Te content. The possibility of an indirect band gap in the lower photon energy region was discussed. The Seebeck coefficient and electrical conductivity were measured in the temperature range of 300–500 K. Very large values of the power factor of 80 and 131 were obtained for Be2Se3 films at room temperature and 473 K, respectively; the latter is three times larger than best previously reported values for similar film systems.