Granular 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 102μW=m K2 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