Behavior of Ag Admixtures in Sb2Te3and Bi2Te3Single Crystals

Abstract

Single crystals of Sb2Te3doped with Ag (cAg=(0–9)×1019cm−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 Hall 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 (ε1) and imaginary (ε2) parts of the dielectric function and the imaginary part of the energy loss function (Im(−1/ε)) on the wavenumber were also determined. From theε2(ν) dependence at room temperature, it was concluded that in Sb2Te3(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 IR region. The determined values of the Hall coefficient showed that Ag atoms doping the Sb2Te3crystal structure behave as acceptors. The incorporation of 1Ag atom into the crystals prepared from Sb2−xAgxTe3and Sb2AgxTe3melts 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″Sbwith 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–Sb0.5Ag0.5] as uncharged defects. The Ag atoms incorporated into the Bi2Te3structure always behave as donors. The formation of Agiinterstitials is proposed as well. The different behavior of Ag atoms in Sb2Te3and Bi2Te3crystals is probably associated with different parameters of the chemical bonds in these crystals, which is supported by the results of a semiempirical calculation.