Behavior of Ag admixtures in Sb2Te3 and Bi2Te3 single crystals

Single crystals of Sb2Te3 doped with Ag (c(Ag) = (0-9) x 10(19) cm(-3)) were prepared from the melt Sb2-xAgxTe3 (denoted by a) or the melt Sb2AgxTe3 (denoted by b). The reflectivity in the IR region, electrical conductivity, and Hail coefficient were determined for these crystals. From the reflection spectra values of the high-frequency dielectric constant, optical relaxation time, and plasma resonance frequency were obtained for crystals with various Ag contents. The dependencies of the real (epsilon(1)) and imaginary (epsilon(2)) parts of the dielectric function and the imaginary part of the energy loss function (Im(-1/epsilon)) on the wavenumber were also determined. From the epsilon(2)(nu) dependence at room temperature, it was concluded that in SbzTe3(Ag) crystals the scattering mechanism of free carriers on the acoustical phonons prevails, but the participation of scattering on ionized impurities is also probable. The a- and b-type crystals of Sb2Te3(Ag) manifested practically the same values of transport coefficients and optical properties in the TR region. The determined values of the Hall coefficient showed that Ag atoms doping the Sb,Te, crystal structure behave as accepters. The incorporation of 1 Ag atom into the crystals prepared from Sb2-xAgxTe3 and Sb2AgxTe3 melts increases the hole concentration by about 1.7 holes. This result is interpreted as the incorporation of Ag atoms into the crystal structure in the form of substitutional defects of Ag"(Sb) with two negative charges. Part of the Ag atoms can form defects of the type [Ag"(Sb) + h(.)], i.e., Ag'(Sb), or be incorporated in the four-layer lamellae [Te-Ag0.5Sb0.5-Te-Sb0.05Ag0.5] as uncharged defects. The Ag atoms incorporated into the Bi2Te3 structure always behave as donors. The formation of Ag-i(.) interstitials is proposed as well. The different behavior of Bg atoms in Sb2Te3 and Bi2Te3 crystals is probably associated with different parameters of the chemical bonds in these crystals, which is supported by the results of a semiempirical calculation, (C) 1998 Academic Press.