Doping and electrical properties of Cd-doped crystals and LPE layers of Pb_1-xSn_xTe

The doping characteristics and electrical properties of Cd-doped bulk crystals and LPE layers of Pb_1-xSn_xTe, 0<or=x<or=0.25, were studied using Hall-effect, resistivity and Cd-solubility measurements. The bulk crystals were doped by Cd diffusion in a two-temperature-zone furnace and the LPE layers by adding a small percentage of Cd to the growth solution. The behaviour of Cd in Pb_0.8Sn_0.2Te depends on the Cd concentration. At low Cd concentrations the Cd atoms compensate the acceptor metal vacancies of the undoped material, most probably by occupying the metal vacancy sites. At medium Cd concentrations most of the Cd atoms incorporate in the crystal as an electrically inactive impurity and only a small fraction act as active donors. At high Cd doping the electron concentration n saturates to a constant value independent of the thermal history of the sample prior to diffusion, independent of the carrier type and concentration of the doped material, independent of the doping procedure and independent of the doping temperature. Possible explanations for the saturation in n are discussed. At high Cd doping the low-temperature mobility is found to be limited by scattering of electrons by neutral Cd centres distributed uniformly throughout the crystal lattice. For low Cd doping, ionised native defect scattering with the inclusion of electrical compensation is the dominant scattering mechanism below 40K.