Changes in the Electrical Parameters of CdTe-based Crystals during Isothermal Annealing

We observed a novel unanticipated effect in CdTe, {\bf Cd}{1-{\bf x}}{\bf Zn}{\bf x}{\bf Te} and {\bf Cd}{1-{\bf x}}{\bf Mn}{\bf x}{\bf Te} crystals whilst we were measuring the dependences of several electrical properties (e.g., specific conductivity and free-carrier density) over time. During isothermal annealing under constant thermodynamic conditions (temperatures of 450-500^\circ {\rm C} and under maximal cadmium-vapor pressure), we recorded a jump-like increase in the conductivity after some \sim 1 - 2 hours of heating required to stabilize the sample's electric parameters. The values of specific conductivity and free-carrier density suddenly increased by up to tenfold, and they persisted at those levels during further ageing. At the same time, the sample's conductivity became insensitive to stoichiometric changes in the crystal. We explain this effect as reflecting a sudden reformatting of the sample's native/foreign point-defect structure. This transformation is evaluated and mathematically approximated within the framework of our model of the melting of Te-containing second-phase particles; this process releases impurities from within the particles. The respective diffusion 'clouds' grow, and at the moment of their mutual percolation (infiltration), a peculiar 'short circuit' is observed with striking changes in the crystals' electrical parameters.