P. Fochuk

High-temperature point defect equilibrium in CdTe modelling

High-temperature (673-1173 K) electrical property (σ, R H ) measurements in undoped and In-doped (up to 1 × 10 20 at/cm 3 ) CdTe were performed undcr different stoichiometric conditions. Cadmium (tellurium) vapor pressure or temperature electron (hole) density dependences were obtained. The results are described by a computed defect structure model, which optimizes the defect reaction constants. A new In incorporation defect reaction is proposed, explaining the dopant self-compensation mechanism.

Defect chemistry in CdTe〈In〉 crystals

High-temperature electrical measurements are explained in the framework of the theory of quasi-chemical reactions between defects in solids using solubility data for In in CdTe. The free carrier density is dependent on the solubility of dopant and the electrons are compensated by both native point defects (Cd vacancies) and the In-containing associate. Calculated point defect diagrams give electron densities in good agreement with the measured values.

Array of virtual Frisch-grid CZT detectors with common cathode readout and pulse-height correction

We present our new results from testing 15-mm-long virtual Frisch-grid CdZnTe detectors with a common-cathode readout for correcting pulse-height distortions. The array employs parallelepiped-shaped CdZnTe (CZT) detectors of a large geometrical aspect ratio, with two planar contacts on the top and bottom surfaces (anode and cathode) and an additional shielding electrode on the crystals sides to create the virtual Frisch-grid effect. We optimized the geometry of the device and improved its spectral response. We found that reducing to 5 mm the length of the shielding electrode placed next to the anode had no adverse effects on the devices performance. At the same time, this allowed corrections for electron loss by reading the cathode signals to obtain depth information.

Dopant Content and Thermal Treatment of

We measured, in the 873-1173 K temperature range, the temperature- and Cd vapor-pressure-dependences of the free electron density in single CdTe(In) crystals with different In contents. Increasing the cooling rate of the crystals and/or decreasing the well-defined Cd vapor pressure reduced the free-electron density. We interpreted and modelled these phenomena and the crystals high-temperature electrical properties within the framework of Krogers point-defect theory. Our experiments demonstrated the possibility of controlling the free-electron density in CdTe(In) crystals by changing the cooling rate. We supplemented a point-defect structural study of CZT(In) crystals by low-temperature (80-420 K) electrical measurements. These findings allowed us to identify the nature of the point defects responsible for free-carrier scattering, which is an important parameter influencing the mutau-product value in detector-grade material.

{\rm Cd} _{1-{\rm x}} {\rm Zn} _{\rm x} {\rm Te} \langle {\rm In}\rangle

The evolution of defect structure occurring during the cooling of Tellurium-rich CdTe is studied theoretically and experimentally. Classical nucleation theory is applied to comprehend the precipitation of excess Te and a formation of Te precipitates both in pure lattice and on extended defects is taken into account. Numerical simulations demonstrate significant effect of the cooling process on the room temperature precipitate magnitude and density. Theoretical results are compared with high temperature in-situ measurements of Te-rich CdTe:In. It is shown that the cooling rate significantly affects the temperature dependence of the conductivity. Characteristic temperatures, at which excess Te precipitates, are identified as apparent bows in measured dependencies. Optimum annealing-cooling treatment to obtain detector grade CdTe is suggested.

: Effects on Point-Defect Structures

We investigated the influence of the ratio of the electron and hole mobility-lifetime products, (μτ)e,h and (μτ)e/(μτ)h, on the resolution of CdZnTe planar radiation detectors via Monte-Carlo simulations. Preliminary results show that this ratio exercises a larger effect than that of any other parameter on the detector’s peak-to-valley ratio and resolution. We determined the range of values of the ratio (μt)e/(μτ)h where the fast degeneration of the photopeak in CdZnTe detectors takes place at a gamma-ray energy 661.7 keV (137Cs). We offer an explanation, based on the results of some of our experimental data, on the spectrometric performance of CdZnTe detectors.

Dynamics of Point Defects in Tellurium-Rich CdTe

We studied the electrical properties of Cd0.9Zn0.1Te:In (CZT) single crystals with [In]=3*1015 at/cm3 at its high-temperature point-defect (PD) equilibrium state under a Cd overpressure (PCd). We detailed the influence of thermal treatment and the deviation of stoichiometry on the electron concentration, observing unexpectedly high conductivity and an increase in free-electron density ( ~ 1.5-2 orders of magnitude) when annealing the sample at 770 K under a Cd vapor pressure (0.01 atm.). Prolonged exposure of the samples under these conditions lowered the electron density by two approximately orders-of-magnitude until it approached the intrinsic value. The electron mobility after such treatment increased to CZTs maximal values at ~ 460 K (650-700 cm2/(V*s)). Therefore, such annealing can be effective in assuring high-resistive CZT detectors after crystal growth, or by special treatment, thereby eliminating the inclusions. We analyzed these data in the framework of Krögers theory of quasi-chemical reactions, and compared the findings to those obtained for undoped CdTe.

Transport properties and spectrometric performances of CdZnTe gamma-ray detectors

Electrical properties of CdTe single crystals doped by Sb were studied in situ at high temperature point defect equilibrium under well defined Cd and Te vapour pressure. Up to samples revealed p-type conductivity both under Cd and Te saturation. The position of the deep acceptor level was determined using hole density temperature dependency at 350-650 K. The point defects responsible for hole density are supposed to be acceptors. The n-type conductivity above is determined by intrinsic point defects: electrons under Te saturation due to native disorder and Cd interstitials under Cd saturation.

Effect of

This paper reports a study of photooxidation and photomodification processes of the CdTe/CdS quantum dots embedded in a polymer matrix under ambient condition. During the first few minutes of irradiation, the quasi-inverse increase in photoluminescence intensity has been observed indicating the passivation of the nanocrystal surface traps by water molecules. A prolonged irradiation of the polymer film containing CdTe/CdS quantum dots leads to a significant decrease in the photoluminescence intensity together with the “blue shift” of the photoluminescence peak energy associated with quantum dot photooxidation. The mechanisms of the CdTe/CdS core/shell quantum dot photooxidation and photomodification in a polymer matrix are discussed. We have found a correlation between the photostability of the quantum dots and the CdS shell thickness as well as the ratio of core elements.

{\rm Cd}_{0.9}{\rm Zn}_{0.1}{\rm Te\!:\!In}

We discussed the design implementation and results from testing 2 × 2-, 3 × 3-, and 2 × 4-arrays of 6×6 ×15 mm3 CdZnTe virtual Frisch-grid detectors. In these measurements we employed a data acquisition system based on the H3D ASIC developed by BNLs Instrumentation Division in collaboration with the University of Michigan for 3D position-sensitive detectors. We used CZT crystals with a range of performance attributes to evaluate practical array configurations and detector-assembling procedures. The detector ratings were assigned based on the pulse-height spectra and correlated with data from X-ray diffraction topography measurements and X-ray response mapping obtained at BNLs National Synchrotron Light Source. The results helped us to better understand the performance limits of these detectors, and to identify future improvements in the arrays design and requirements for the new readout ASIC.