The deviation from a stoichiometry and the amphoteric behaviour of Ga in PbTe/Si films

Abstract

The experimental results, which have been received during the examination of the crystal structure and electronic properties of PbTe/Si heterostructures doped with Ga by means of two different techniques, are systematised in this study. The first doping procedure is based on the two-zone annealing of previously formed by modified HWE technique PbTe/Si heterostructures in the saturated vapour which corresponds to the three-phase equilibrium L1-GaTe(S)-V in Ga-Te binary system. The second method of preparation of PbTe〈Ga〉/Si heterostructures offers the direct one stage synthesis, in which the doping and the layer condensation processes proceed simultaneously. The experimental results clearly demonstrate the different character in the charge carrier densities evolutions with the Ga impurity concentration for PbTe〈Ga〉/Si films prepared by these techniques. The measurements of conductivity and Hall coefficient have shown that Ga atoms behave as the donor impurities only in PbTe〈Ga〉/Si films with little excess of Te, which were fabricated by vapour phase doping. On the contrary, in PbTe〈Ga〉 films prepared by a direct one-stage synthesis the character of electrical activity of Ga impurities depends upon both the concentration and the deviation from stoichiometry. It has been found that the Pb1−yGayTe1+δ films with yGa>0.015, which are heterogeneous and consist of two (PbTe+GaTe) or three phases (PbTe+GaTe+Ga2Te3) have the best sensitivity to IR radiation. In the scope of quasi chemical approach it is possible to explain the complicated amphoteric behaviour of Ga atoms by different mechanisms of substitution (GaPb×, GaPb1− and GaPb1+) or implantation (Gai3+) of impurity atoms in the PbTe crystal structure.