R. B. James

Effect of Te precipitates on the performance of CdZnTe detectors

Measurements using the National Synchrotron Light Source provided a detailed comparisons of the microscale detector response and infrared microscopy images for CdZnTe Frisch-ring x-ray and gamma detectors. Analysis of the data showed conclusively that local deteriorations of the electron charge collection and x-ray device response fully correlate with the presence of Te precipitates as seen in the IR images. Effects of the surface processing conditions on the detector performance were also clearly observed.

Single-charge-carrier-type sensing with an insulated Frisch ring CdZnTe semiconductor radiation detector

Performance optimization of an insulated Frisch ring design was investigated for a 3×3×6 mm CdZnTe planar semiconductor detector. The Frisch ring was composed of copper and was insulated from the detector surface with Teflon. Optimization variables included the Frisch ring length and the bias voltage. Optimized overall device performance was found using a 5 mm long Frisch ring extending from the cathode toward the anode, leaving a 1 mm separation between the Frisch ring and the anode. The best energy resolution observed was 1.7% full width at half maximum at 662 keV with the ring extending 4 mm from the cathode toward the anode.

Characterization of large cadmium zinc telluride crystals grown by traveling heater method

The focus of this paper is to evaluate thick, 20×20×10 and 10×10×10mm3, cadmium zinc telluride (CZT), Cd0.9Zn0.1Te, crystals grown using the traveling heater method (THM). The phenomenal spectral performance and small size and low concentration of Te inclusions/precipitates of these crystals indicate that the THM is suitable for the mass production of CZT radiation detectors that can be used in a variety of applications. Our result also proves that with careful material selection using IR and high-quality fabrication processes, the theoretical energy resolution limit can be achieved.

Characterization of Traveling Heater Method (THM) Grown

High-performance semi-insulating single crystals of n-type (CZT) were grown using the traveling heater method (THM). X-ray and -ray detector configurations fabricated from this material have a room-temperature mean energy resolution of 4.3% FWHM for a source (122 keV) and uniform pixel-to-pixel response on monolithic 20205 pixellated detectors. Energy resolution of 1% FWHM for (662keV) has been measured on virtual Frisch-grid 4411 devices useful for homeland security applications. Additional characterization techniques including mobility-lifetime measurements, infrared microscopy, X-ray topography, and OPTICAL Deep Level Transient Spectroscopy (ODLTS) have demonstrated the superior quality of this THM CZT.

{\hbox{Cd}}_{0.9}{\hbox{Zn}}_{0.1}{\hbox{Te}}

We studied the effects of small, <20 mum, 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.

Crystals

A review of growth methods used to produce Cd 1-x Zn x Te (CZT) (0.0<x<0.20) crystals for radiation detector applications is presented. Most of the results emphasize the high-pressure Bridgman (HPB) method. For selected melt-grown HPB ingots, the liquid/solid segregation coefficients of some impurities were measured. The correlation of the impurity content and nuclear detector performance will be discussed. Extended defects and surface and bulk crystallinity were measured using triple and double axis X-ray diffraction techniques (TAD and DAD-XRD), X-ray topography, and infrared microscopy. X-ray diffraction maps and IR images were generated and compared to gamma-ray detector tests to correlate macroscopic defects with the nuclear detector responses. Defects states of CZT were also investigated using low-temperature photoluminescence spectroscopy. Comparisons between the material and detector properties for different CZT growth methods will be discussed.

Performance-limiting Defects in CdZnTe Detectors

The ideal operation of CdZnTe devices entails having a uniformly distributed internal electric field. Such uniformity especially is critical for thick long-drift-length detectors, such as large-volume CPG and 3-D multi-pixel devices. Using a high-spatial resolution X-ray mapping technique, we investigated the distribution of the electric field in real devices. Our measurements demonstrate that in thin detectors, < 5 mm, the electric field-lines tend to bend away from the side surfaces (i.e., a focusing effect). In thick detectors, > 1 cm, with a large aspect ratio (thickness-to-width ratio), we observed two effects: the electric field lines bending away from or towards the side surfaces, which we called, respectively, the focusing field-line distribution and the defocusing field-line distribution. In addition to these large-scale variations, the field-line distributions were locally perturbed by the presence of extended defects and residual strains existing inside the crystals. We present our data clearly demonstrating the non-uniformity of the internal electric field.

Study of impurity segregation, crystallinity, and detector performance of melt-grown cadmium zinc telluride crystals

In the past few years, significant progress has been achieved in the development of room-temperature semiconductor detectors, particularly those based on CdZnTe (CZT) crystals. Several types of electron-transport-only detectors have been developed: pixel, coplanar-grid, cross-strip, drift-strip, orthogonal coplanar strip, and virtual Frisch grid, many of which are now commercially available. Despite all these varieties in the detector designs, they have many common features and problems. This review summarizes the common detector design constraints and related factors limiting performance of CZT detectors: bulk and surface leakage currents, surface effects, properties of Schottky contacts and surface interfacial layers, charge sharing and loss in multielectrode devices, charge transport nonuniformities, and fluctuations in the pulse height for long-drift-length devices. We also describe unique capabilities at Brookhaven National Laboratory, Upton, NY, for CZT device characterization and recent progress utilizing these tools.

Internal Electric-Field-Lines Distribution in CdZnTe Detectors Measured Using X-Ray Mapping

The surface recombination velocity of a Cd1−xZnxTe (CZT) radiation detector treated by mechanical polishing and by a standard 5% bromine in methanol chemical etch is reported. The light power dependence of the surface recombination velocity was measured using dc photoconductivity. The results reveal that the surface recombination velocity is a function of the electron generation rate, which can be described by a Shockley–Read one-center model. It was observed that the surface recombination velocity of the CZT detector treated by polishing only is much larger than that treated with polishing followed by chemical etching. The correlation of dc photoconductivity and low-temperature photoluminescence measurements of the CZT detector is also discussed.

Factors limiting the performance of CdZnTe detectors

We report on the recharging of the neutral state of a deep-donor layer that increases the efficiency of charge collection in detector-grade CdTe:In. Measurements with photoinduced current transient spectroscopy and thermoelectric effect spectroscopy revealed positively charged energy level at EC−0.65 eV. Photoluminescence measurements identified this level being responsible for the 0.68 eV emission band. Its positive charge is converted into a neutral one by the upward displacement of Fermi level. We discuss the nature of this deep defect based on the latest ab initio calculations.