I. Turkevych

High-temperature electron and hole mobility in CdTe

We have developed a novel technology for forming high-temperature stable ohmic contacts on CdTe, and we have measured the Hall mobilities of electrons at temperatures between 300–1300 K and holes between 225–600 K. The temperature interval also covers the region 300–600 K, where no data have yet been presented. The measured mobilities agree very well with theoretical calculations.

Chemical Self-Diffusion in CdTe

High-temperature in-situ galvanomagnetic measurements on CdTe are performed at temperatures T = 500-700°C in the interval of Cd pressures (P Cd ) one order of magnitude below and above the ideal stoichiometry line. The time evolution of samples after step-like change of P Cd is analyzed and the chemical diffusion coefficient D = 5 exp (-1.12 eV/k B T) (cm2/s) is evaluated. Neither magnitude of P Cd nor direction of the step of P Cd (increased/decreased) have manifested a distinguishable effect on D. Surface conduction below 600 °C dependent on P Cd is reported.

Defect-induced optical transitions in CdTe and Cd0.96Zn0.04Te

Within a model of the compensated shallow acceptor classical impurity band, we study the optical absorption of CdTe and Cd1−xZnxTe, at energies of 20–70 meV below the energy gap connected with acceptor–conduction band transitions. The shape of the absorption edge is fitted by Monte Carlo numerical simulations and we estimate the total density of charged impurities, which is hardly detectable by other experimental techniques. The activation energy ≈59 meV and density ~1014 cm−3 of the dominant shallow acceptor level are reported.

Preparation of semi-insulating CdTe by post-growth annealing

Thermodynamic conditions for a post growth annealing to prepare near stoichiometric semi-insulating (SI) of CdTe with a minimized concentration of point defects are looked for in undoped and Sn-doped single crystals. The high temperature (200-1000°C) in-situ conductivity σ and Hall effect measurements are used to control the native defect density and to find out the Cd pressure PCd at which shallow defects are compensated. We show, that contrary to the undoped samples, where the change of the type of conductivity by variations of PCd is easy, the Sn-doped samples exhibit due to the Sn self-compensation much more stable behavior. The temperature near 500°C is reported to be optimum for the real-time annealing of bulk samples. The chemical diffusion is sufficiently fast at this temperature, simultaneously the lower temperature is preferred because the native defect density can be tuned gently by changing PCd. The measurement of temperature dependencies of σ in annealed samples below 500°C is used to establish the position of Fermi level and to characterize the structure of both shallow and deep levels detected in the sample. The quasichemical formalism is used for evaluation of defect density and for analysis of nature of deep levels.

Pure and deep-level doped semi-insulating CdTe

Experimental conditions for a growth of near stoichiometric high resistive CdTe single crystals with a minimized concentration of point defects have to be defined. The position of the stoichiometric line in the pressure-temperature (P-T) phase diagram was evaluated from high-temperature in situ galvanomagntic measurements. Calculations based on a model of two major native defects (Cd vacancy and Cd interstitial) show, that a very small variation of Cd pressure P_{Cd} results in a strong generation of uncompensated native defects. Modelling of room temperature carrier density in dependence of the deep defect density NDD, PCd, and annealing temperature T shows, that the range of optimal PCd, at which the high resistivity can be reached, broadens with increasing NDD or decreasing T. It is shown, that at low T<450 degree(s)C the deep defect density <1015cm-3 is sufficient to grow the high resistive CdTe. CdTe doped with Vanadium is used as a model example.