M. Bugár

High temperature optical absorption edge of CdTe single crystal

The optical absorption edge of bulk CdTe single crystal was measured by infrared transmission under saturated Cd pressure in the temperature interval 295–1223 K. The absorption coefficient was directly determined up to the value of 100 cm−1. For higher values, it was estimated by extrapolating the spectra according to the Urbach exponential rule. It was observed that the common temperature-independent intersection of extrapolated Urbach absorption edge, the “Urbach focus,” does not exist in CdTe. The temperature dependence of band-gap energy Eg defined by Eg(300 K) = 1.518 eV and dEg/dT = − 4.4 × 10−4 eV/K was established, postulating linear temperature dependence of Eg by fitting the temperature dependent absorption coefficient at the band edge αg(T) = 6600 – 4T (K) (cm−1).

Theory of Deep Level Spectroscopy in Semi-Insulating CdTe

Numerical simulations of thermoelectric effect spectroscopy (TEES) and photo-induced transient current spectroscopy (PICTS) in CdTe were performed with the aim to interpret experimental results and to look for possibilities to obtain credible information about trap capture cross-sections. The TEES and PICTS spectra were calculated by solving kinetic equations. Trap energy and capture cross section were determined by variable heating rate and double gate methods and compared with input data. Deviation of the established capture cross-section from the input value was demonstrated for the strong retrapping. It was concluded that the trap capture cross-section, especially if obtained anomalously low, might be incorrect due to invalid premises of models, which neglect retrapping. The possibility to acquire an information about trap properties from the shape or magnitude of glow curves was checked. It was demonstrated that the magnitude of TEES signal corresponding to slow or fast retrapping depends unequally on the initial optical excitation.

Chemical diffusion in CdTe:Cl

High-temperature in situ galvanomagnetic measurements are reported in chlorine-doped CdTe, [Cl] ≈4 × 1018 cm−3 at temperatures T = 600–700 °C near Cd saturation. The chemical diffusion coefficient is determined by means of relaxation of electrical conductivity after a step-like change of ambient Cd pressure (PCd) and approximated well by a trial function . Both and equilibrium conductivity are calculated on the basis of a defect model in which donors ClTe are compensated by divalent acceptors Cd vacancies (VCd) and monovalent acceptor complexes (ClTeVCd). The properties of native point defects are consistent with undoped CdTe. It is shown that can be fit well assuming different diffusion coefficients for singly and doubly charged VCd.

Electromigration of Mobile Defects in C d T e

Electromigration of mobile charged defects in external electric field is investigated at various temperatures and biases in conductive undoped and semiinsulating In-doped CdTe, respectively. A set of electric contacts as potential probes arranged linearly along the sample was used for the detection of the drift of the local resistance modulation. The observed modulation drifting along the sample always from the positive toward negative contact after step-like bias polarity reversion points to the migration of positively charged point defects. Mobility and diffusion coefficient of mobile defects at 100degC and 600degC, respectively are determined. Electromigration of point defects is also tested by low temperature photoluminescence and a model explaining migration of charged defects is suggested.

Semi insulating CdTe:Cl after elimination of inclusions and precipitates by post grown annealing

We present in this contribution results of two-step annealing, when the CdTe:Cl doped samples are at first annealed under Cd overpressure to remove inclusions and the re-annealed under Te overpressure to restore the high resistivity state. Investigation of samples after Cd rich annealing by infrared microscope has proven, that all inclusions are removed. Also Te nano precipitates were strongly influenced by the annealing process. The resistivity of the samples after Te-rich annealing was restored to values ( ~ 108-109Ωcm). We observed, however, decrease of mobility-lifetime product of electrons from 10−3cm2/Vs to 10−4cm2/Vs. In order to understand the reason of this decrease we performed a study of point defects before and after annealing by thermoelectric effect spectroscopy. It shows a decrease of concentrations of most deep levels after two-step annealing. This behavior is completely different compared to past annealing studies, where concentration of deep levels strongly increased after annealing. The only level with an increased concentration in the current study is the midgap level (E ~ 0.8 eV). At the same time we observed increase of micro-twins in the samples investigated by transmission electron microscopy. The decrease of charge collection efficiency after two-step annealing may be therefore connected with re-arrangement of near midgap levels due to increase of concentrations of structure defects (micro twins, dislocations) that accumulate in their surroundings point defects with energy ~ 0.75 eV.

Multi-Species Diffusion in CdTe

We studied theoretically chemical self-diffusion and the diffusion of extrinsic atoms in CdTe. We compiled a general model describing the multi-species diffusion of arbitrary amounts of elements in a form optimized for numerical calculations and applied it to a model system of CdTe doped with slow- or fast-diffusing elements. The diffusion of slowly diffusing atoms was analyzed and compared with experimental findings. We uncovered possible drawbacks in the experimental data that might affect the ensuing analysis by researchers, so generating incorrect conclusions. We suggest a method of purifying CdTe from fast-diffusing impurities based on a proper annealing/etching process.

Preparation of Inclusion and Precipitate Free Semi-Insulating CdTe

Annealing conditions used for the preparation of semi-insulating CdTe after elimination of inclusions and precipitates are discussed. Second phase defects are eliminated by post-growth stoichiometric annealing and the semi-insulating CdTe is prepared by re-annealing under Te or Cd overpressure. In case of In-doped samples with high In concentration, semi-insulating CdTe with resistivity approximately 109 Omegacm was prepared by annealing under Te overpressure. In-situ high temperature measurement of the electrical conductivity and Hall coefficient is used to find annealing conditions for undoped and slightly In-doped CdTe. The dominant extrinsic acceptor level with a concentration of NA = 4 times 1015 cm-3 was determined and Cd-rich annealing was used for the preparation of semi-insulating CdTe.