Electrical properties of CdTe near the melting point

A new experimental setup for the investigation of electrical conductivity (σ) in liquid and solid CdTe was built for a better understanding of the properties near the melting point (MP). The temperature dependence of σ was studied, within the interval 1,050-1,130°C, at defined Cd-partial pressures 1.3-1.6 atm, with special attention to the liquid-solid phase transition. We found that the degree of supercooling decreases with increasing Cd overpressure and reaches the lowest value at 1.6 atm without change of the melting temperature during heating

High temperature mobility of CdTe

The Hall mobility of electrons μH is measured in CdTe in the temperature interval 450-1050°C and defined Cd overpressure in near-intrinsic conditions. The strong decrease of μH above 600°C is reported. The effect is explained within a model of multivalley conduction where both electrons in �1c minimum and in L1c minima participate. The theoretical description is based on the solution of the Boltzmann transport equation within the relaxation time approximation including the polar and acoustic phonon intravalley and intervalley scatterings. The �1c to L1c separation �E=0.29 - 10-4T (eV) for the effective mass in the L valley mL=0.35m0 is found to best fit the experimental data. Such �E is about four times smaller than it is predicted by first-principle calculations. © 2001 American Institute of Physics

Defect structure of CdZnTe

(Cd,Zn)Te single crystals fabricated in our laboratory by Vertical Gradient Freeze Method were subjected to two types of experiments. In the first experiment, samples were annealed at temperatures 600-900 °C under different Cd overpressures. They were quenched after annealing to room temperature and Hall effect and conductivity measurements were performed. In the second experiment, in situ Hall effect and conductivity measurements on neighbor samples were done also at temperatures 700-900 °C. We determined equilibrium concentrations of defects at temperatures 700-900 °C and the stoichiometry intrinsic line in the p-T diagram comparing the experiments and the theoretical model presented by Berding. Results of the presented analysis can be used to find optimal growth and annealing conditions to reduce precipitation of Cd/Te or to produce intrinsic material suitable for fabrication of (CdZn)Te gamma-ray detectors

High-temperature conductivity of solid and liquid CdTe

Conductivity of CdTe single crystals fabricated in our laboratory by vertical gradient freeze method was studied at temperatures 400-1200 degreesC. Two clear slopes are seen - 0.7 eV in the solid and 4.6 eV in the liquid. Experimental value of electrical conductivity at the melting point 10 Omega (-1)cm(-1) was observed. A model explaining the experimental data is suggested. (C) 2000 Elsevier Science B.V. All rights reserved

Photoluminescence of deep levels in (CdZn) Te-correlation with diffusion length measurement

Photoluminescence of deep levels near mid-gap in P-(Cd1-xZnx)Te (x�0.03-0.05) grown by the vertical gradient freezing method was investigated. Correlation with recombination properties manifested by temperature dependence of diffusion length of minority electrons and the corresponding mobility-lifetime product was studied. Low-temperature photoluminescence (PL) in the spectral range 0.68-1.3 eV and a temperature dependence of diffusion length of minority electrons (DL) measured by EBIC in the temperature range 60-300 K on both as-grown and annealed samples (800 °C and 900 °C) were investigated. We observed, that while as-grown samples exhibited a steep increase in DL with decreasing temperature in the temperature range 60-140 K, annealing and subsequent quenching resulted in a significant decrease of DL at these temperatures. Photoluminescence band with a peak at approx. 0.84 eV in annealed samples with increased recombination and therefore low DL was observed, while no such peak was detected in the as-grown samples with high DL. Luminescence in the �0.8 eV band is usually attributed to a VCd related defect which is supposed to act as a recombination centre. It can be concluded, that a correlation between effects of increased recombination manifested by decreased values of DL at temperatures 60-140 K and detection of the�0.8 eV PL band was observed. Reduction of the mobility-lifetime product by increased recombination in (CdZn)Te single crystals is therefore probably connected with a presence of a VCd-related defect

Semiinsulating CdTe

Experimental conditions for the growth of near stoichiometric high-resistivity CdTe single crystals with a minimal concentration of point defects are investigated. The position of the stoichiometric line in the pressure-temperature (P-T) phase diagram is evaluated from high-temperature in situ galvanomagnetic 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 dependence on the deep defect density NDD, PCd, and annealing temperature T shows, that the range of optimal PCd, at which high resistivity can be reached, broadens with increasing NDD or decreasing T. It is shown that at low T <450°C the deep defect density < 1015 cm-3 is sufficient to grow the high-resistivity CdTe. © 2002 Elsevier Science B.V. All rights reserved

Pre- and postmelting of cadmium telluride

A stepwise character of cadmium telluride melting is shown by using differential thermal analysis and conductivity measurements in the 1323 to 1473 K temperature range. According to the results of differential thermal analysis the parameters of CdTe melting are determined by the premelting processes that are related to defect production in crystal lattice. The crystallization processes are controlled with the melt state (structure) that depends on its maximum temperature

Determination of energy gap in Cd1-xZnxTe (x = 0-0.06)

A dependence of energy gap on composition x for Cd1-xZnxTe (x = 0-0.06) was found combining x-ray diffraction and low temperature photoluminescence measurements (free exciton energy) on Cd1-xZnxTe single crystals. The use of this dependence for evaluation of Zn content (x) in the crystals shows very good agreement with the near infrared absorption method. Published dependences Eg(x) are reviewed and compared with the dependence Eg(x, T = 4 K) = 1.606 + 0.520x + 0.254x2 derived in this work

The influence of growth conditions on the quality of CdZnTe single crystals

Experimental conditions were investigated for growth of inclusion-free near-stoichiometric CdZnTe single crystals with a minimized concentration of native point defects. The positions of the stoichiometric line pS = 8 � 105 exp(-1.76 � 104/T) (atm) and the room-temperature and high-temperature p-n lines were evaluated from high-temperature in situ galvanomagnetic measurements. The Cd pressure at the congruent melting point was estimated at � 1.15-1.20 atm from analysis of the total inclusion volume of five single crystals fabricated at Cd pressures in the range of 1-1.3 atm. An inclusion-free single crystal was prepared at PCd � 1.2 atm. Calculations based on a model of two major defects, the Cd vacancy and the Cd interstitial, show that a very small deviation of PCd from PS results in a large generation of the native defects. Thus a reproducible production of a high-resistivity material by a slow cooling along the PS seems to be very difficult