OPTICAL-PROPERTIES OF PB-DOPED BI2TE3 SINGLE-CRYSTALS

The reflectance and the transmittance spectra in the IR region were me asured on Pb-doped Bi2Te3 single crystal samples grown by a modified B ridgman technique. The plasma resonance frequency, the optical relaxat ion time, and the high-frequency permittivity were determined by fitti ng the Drude-Zener formulas to the reflectance spectra. It vias found that Pb impurities in Bi2Te3 behave as accepters. A part of incorporat ed Pb atoms behaves as electrically inactive. This effect is explained as due to the fact that electrically active Pb-atoms form substitutio nal defects, Pb-Bi', and the others form inactive two-dimensional defe cts - seven-layer lamellae Te-Bi-Te-Pb-Te-Bi-Te. The transmittance spe ctra were used for the determination of the dependence of the absorpti on coefficient K on the energy of incident photons. The optical width of tl-ie energy gap increases with increasing Pb content. The values o f the exponent alpha from the relation of K similar to lambda(alpha) f or the long-wavelength absorption edge range within the interval of 1. 6 to 2.8, i.e. the dominant scattering mechanism of free current carri ers in PE-doped Bi2Te3 crystals is the scattering by acoustic phonons. By comparison of the effect of doping atoms of the IV.B group of the periodic table on the concentration of holes in Bi2Te3 crystal lattice was concluded that the tendency to form substitutional defects Me-Bi' (Me = Ge. Sn, Pb) in these crystals increases from Ge to Pb, whereas the tendency to the formation of seven-layer lamellae Te-Bi-Te-Me-Te-B i-Te decreases.