Search

The large mismatch of the coefficients of thermal expansion (CTE) between the metal contact and CdZnTe exerts thermal stress at the metal/CZT interface, which causes mechanical degradation of the contact in addition to the poor adhesion of the metallic thin film to CZT. To form a reliable and stable interface, the contact material should have better adhesion and a close match of the coefficients of thermal expansion with CZT/CdTe. Here, we report on our investigations of a novel non-metallic contact layer for use in radiation detector applications. The proposed ZnO:Al contact layer offers better adhesion due to the oxide interface, higher hardness and better matching of the CTE with CZT. It has high prospects for a reliable and stable device structure that can serve as a replacement to the common metallic electrodes used today. We evaluated AZO contacts to CZT and extracted the electronic characteristics, such as resistivity and mobility-lifetime product of electrons, and compared the results of the same characterization measurements for CZT with gold contacts. The present observations showed that the characteristics of CZT detectors with AZO contacts are nearly identical to the same detectors with gold contacts. [ABSTRACT FROM AUTHOR]

We investigated the feasibility of long-drift-time CdZnTe (CZT) gamma-ray detectors, fabricated from CZT material produced by Redlen Technologies. CZT crystals with cross-section areas of 5 × 5mm² and 6 × 6mm² and thicknesses of 20-, 30-, 40-, and 50-mm were configured as 3D position-sensitive drift detectors and were read out using a front-end ASIC. By correcting the electron charge losses caused by defects in the crystals, we demonstrated high performance for relatively thick detectors fabricated from unselected CZT material. [ABSTRACT FROM AUTHOR]

The longer electron lifetime of today’s CdZnTe (CZT) crystals allows for free carriers to travel longer distances in the crystals, which means that, in principle, thicker devices could be fabricated. These thicker CZT devices would offer greater detection efficiency for high-energy gamma-ray detectors. However, up to now, the thicknesses and sizes of actual detectors have still been limited by the nonuniform detector response, and the biggest devices reported in the literature are (20\times 20\times 15)\-mm^3 pixelated detectors with a drift distance of 15 mm. Although thicker and bigger single crystals are becoming available today, the high requirements on their crystal quality drastically reduce their acceptance yield and increase their cost. Fortunately, in many cases, the inhomogeneity in response can be corrected by segmenting the active volumes of the detectors and correcting the responses generated from each of the voxels. Such high-granularity position-sensitive detectors open up the opportunity for using thicker and less-expensive CZT crystals. The goal of this work is to demonstrate that today’s commercial high electron mobility-lifetime CZT material is suitable for a new class of detectors with 20-25-mm drift distances and even larger in the near future, provided that the detectors’ response nonuniformities can be corrected on a scale comparable to or larger than the sizes of the electron clouds, which is \sim 100~\mu\m. [ABSTRACT FROM AUTHOR]

CdZnTe (CZT) is a promising medium for room-temperature gamma-ray detectors. However, the low production yield of acceptable quality crystals hampers the use of CZT detectors for gammaray spectroscopy. Significant efforts have been directed towards improving quality of CZT crystals to make them generally available for radiation detectors. Another way to address this problem is to implement detector designs that would allow for more accurate and predictable correction of the charge loss associated with crystal defects. In this work, we demonstrate that high-granularity position-sensitive detectors can significantly improve the performance of CZT detectors fabricated from CZT crystals with wider acceptance boundaries, leading to an increase of their availability and expected decrease in cost. [ABSTRACT FROM AUTHOR]

The presence of Te inclusions degrades the quality of today's CdZnTe (CZT) crystals used for X- and gamma-ray detectors; both their sizes and concentrations densities must be reduced. Over the past years, many researchers proposed using long-term annealing (>24 h) under Cd vapor pressure to reduce or even eliminate the inclusions visible under IR microscopes. We annealed detector-grade CZT samples for periods of 15 to 60 min under Cd-, Zn-, or Te-overpressure or in vacuum at 1000–1200 K. We determined the optimal temperature, duration, and the vapor atmosphere for such high-temperature annealing, typically at \sim1100~K for 0.5–1.0 h. The results were very promising in eliminating Te-rich inclusions, even on twins where the inclusions are more stable than in the unperturbed lattice; indeed, we saw almost no inclusions whatsoever by IR transmission microscopy after such annealing. We note that eliminating inclusions at lower temperatures takes much longer. However, annealing under a Cd vapor pressure at temperatures above \sim1170~K generates a large quantity of irregular Cd inclusions. The samples' resistance after annealing was estimated by I-V curves. [ABSTRACT FROM AUTHOR]

Cadmium Zinc Telluride (CZT) is among the most promising materials for room-temperature X- and gamma-ray detectors. However, crystal defects such as Te inclusions and subgrain boundaries significantly hamper their performances. In this work, we evaluated CZT crystals grown by the modified low-pressure Bridgman technique at the IMEM Institute, Parma. We characterized the crystals by IR microscopy to identify the sizes and concentrations of the Te inclusions, along with high spatial resolution X-ray response mapping to measure the uniformity of their charge-transport properties. In addition, we employed white X-ray beam diffraction topography to analyze their crystalline structure. [ABSTRACT FROM AUTHOR]

Semi-insulating (Cd,Mn)Te crystals offer a material that may compete well with the commonly used (Cd,Zn)Te crystals for manufacturing large-area X- and gamma-ray detectors refid="ref1"/. The Bridgman growth method yields good quality, high-resistivity (10^9-10^10\ \Omega\cdotcm) crystals of (Cd,Mn)Te:V. Doping the as-grown crystals with the compensating agent vanadium (\approx 10^16\ cm^-3) ensures their high resistivity; thereafter, annealing them in cadmium vapors reduces the number of cadmium vacancies. Applying the crystals as detectors necessitates having satisfactory electrical contacts. Accordingly, we explored various techniques of ensuring good electrical contacts to these semi-insulating (Cd,Mn)Te crystals, assessing metallic layers, monocrystalline semiconductor layers, and amorphous (or nanocrystalline) semiconductor layers. We found that ZnTe heavily doped (\approx 10^18\ cm^-3) with Sb, and CdTe heavily doped (\approx 10^17\ cm^-3) with In, proved satisfactory semiconductor contact layers. They subsequently enabled us to establish good contacts (with only narrow tunneling barriers) to the Au layer that usually constitutes the most external contact layer. We outline our technology of applying electrical contacts to semi-insulating (Cd,Mn)Te, and describe some important properties. [ABSTRACT FROM AUTHOR]

CdMnTe offers several potential advantages over CdZnTe as a room- temperature gamma-ray detector, but many drawbacks in its growth process impede the production of large, defect-free single crystals with high electrical resistivity and high electron lifetimes. Here, we report our findings of the defects in several vanadium-doped as-grown as well as annealed Cd1− xMn xTe crystals, using etch pit techniques. We carefully selected single crystals from the raw wafer to fabricate and test as a gamma-ray detector. We describe the quality of the processed Cd1− xMn xTe surfaces, and compare them with similarly treated CdZnTe crystals. We discuss the characterization experiments aimed at clarifying the electrical properties of fabricated detectors, and evaluate their performance as gamma-ray spectrometers. [ABSTRACT FROM AUTHOR]

We studied the effects of small, <20 µm, Te inclusions on the energy resolution of CdZnTe gamma-ray detectors using a highly collimated X-ray beam and gamma-rays, and modeled them via a simplified geometrical approach. Previous reports demonstrated that Te inclusions of about a few microns in diameter degraded the charge-transport properties and uniformity of CdZnTe detectors. The goal of this work was to understand the extent to which randomly distributed Te-rich inclusions affect the energy resolution of CZT detectors, and to define new steps to overcome their deleterious effects. We used a phenomenological model, which depends on several adjustable parameters, to reproduce the experimentally measured effects of inclusions on energy resolution. We also were able to bound the materials-related problem and predict the enhancement in performance expected by reducing the size and number of Te inclusions within the crystals. [ABSTRACT FROM AUTHOR]

The performance characteristics of Frisch-ring CdZnTe (CZT) detectors are described and compared with other types of CZT devices. The Frisch-ring detector is a bar-shaped CZT crystal with a geometrical aspect ratio of ~1 : 2. The side surfaces of the detector are coated with an insulating layer followed by a metal layer deposited directly upon the insulator. The simple design operates as a single-carrier device. Despite the simplicity of this approach, its performance depends on many factors that are still not fully understood. We describe results of testing several detectors fabricated from CZT material produced by different vendors and compare the results with numerical simulations of these devices. [ABSTRACT FROM AUTHOR]

We conducted low-temperature annealing experiments at temperatures slightly above and below the melting point of Te to clarify the effects of the state of Te inclusions (solid or liquid) upon the formation of 'star-like' defects in Cd-annealed CdZnTe (CZT). We also carried out post-growth annealing experiments with and without using Cd vapor to clarify the mechanism of formation of such defects. We demonstrated that these 'star-like' defects are due to the reaction between in-diffused Cd atoms and the molten Te inclusions, but we found no observable 'one-to-one' correlation between 'star-like' defects and Te inclusions. The non-uniform distribution of Te inclusions in the CZT matrix could account for this phenomenon since the punching distance of the dislocations depends on the volume fraction of inclusions within the matrix. [ABSTRACT FROM AUTHOR]