O. Kopach

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

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.

: Effects on Point-Defect Structures

We detail our new results from testing an array of 15-mm long virtual Frisch-grid CdZnTe detectors with a cathode signal readout-scheme intended to improve spectral response by correcting for electron trapping. We designed a novel electrode configuration for these long-drift detectors that ensures an energy resolution close to the statistical limit, and high detection efficiency. However, in reality, the quality of the crystals limits the performance of this type of device. Here, we describe the characterization of the array, show our preliminary results obtained with gamma-ray sources, and expound on their relation to our material-characterization data.

Design Considerations and Testing of Virtual Frisch-Grid CdZnTe Detector Arrays Using the H3D ASIC

Using the differential thermal analysis we investigated parameters of melting and crystallization processes of the CdTe based phase in CdTe-Al system near the CdTe side (CdTe + 2 mol. % Al, CdTe + 4 mol. % Al and CdTe + 6 mol. % Al). Varying temperatures of the melts intermediate isothermal holding for 10-, 30- and 60 minutes during their heating up to 1423 K we determined conditions of the melts full homogenization. It was concluded about change of the CdTe phase melting mechanism with Al content rise.

Characterization of a 15-mm-long virtual Frisch-grid CZT detector array

Peculiarities of Cd0.95-xMnxZn0.05Te (x=0.05-0.25) alloys melting and crystallization kinetics were investigated by the differential thermal analysis at various heating/cooling rates. We synthesized Cd0.95-xMnxZn0.05Te alloys from elemental materials in a double-zone furnace. Their melting-crystallization rate was 5- and 10-K/min with a melt-dwell time of 1-, 10-, 20-, and 30-minutes. We found that the melting temperature, Tm, declined with increasing “x”, while the crystallization temperature, Ts, rose with increasing “x”. We detailed the dependencies of the crystallization rate versus the melt’s crystallization temperature. In most cases, the dependencies dropped with the rise of the crystallization temperature, while the crystallization temperature fell with decreasing melt-dwell time.

Regularities in the melting and crystallization of CdTe-Al alloys

We measured, in the 873-1173 K temperature range, the temperature- and Cd vapor-pressure-dependences of the free electron density in single Cd<inf>1-x</inf>Zn<inf>x</inf>Te≪In≫ (0 ≤ × ≤ 0.1) 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 crystal’s high-temperature electrical properties within the framework of Kröger’s point-defect theory. Our experiments demonstrated the possibility of controlling the free-electron density in Cd<inf>1-x</inf>Zn<inf>x</inf>Te≪In≫ crystals by changing the cooling rate. We supplemented a point-defect structural study of these 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 μτ -product value in detector-grade material.