Gomez W. Wright

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.

Performance characteristics of Frisch-ring CdZnTe detectors

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 /spl sim/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.

Factors limiting the performance of CdZnTe detectors

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.

Development of large single crystal (3-inch ingot) CdZnTe for large-volume nuclear radiation detectors

Further progress has been made in the development of the Modified Vertical Bridgman method for the growth of CdZnTe crystals for fabrication of x-ray and gamma-ray detectors to operate at room temperature. Specifically, the diameter of the grown ingots has been increased from 2 to 3 inches. High quality, large volume (up to 6 in3) twin-free single crystals have been produced. Detectors fabricated with this material show sharp energy resolution and good uniformity.

New Results from Performance Studies of Frisch-Grid CdZnTe Detectors

New data regarding performance studies of Frish-grid CdZnTe (CZT) detectors are presented. The Frisch-grid detector configuration under investigation is a bar shaped CZT crystal with teh side surfaces coated with an insulating layer. A Frisch grid is fashioned by inserting the CZT bar into a metallic sleeve, or by depositing the metal directly upon the insulator; hence the semiconductor material does not come in contact with the metal grid. The simple design operates well as a single-carrier-sensitive device. Despite the simplicity of this device, its performance depends on the balanced combinations of several factors, including the bulk and surface conductivity, μτ product, and geometrical aspect ratio. Described are several effects that determine charge collection in such drift devices and, consequently, the performance of the non-contacting Frisch-grid configuration.

Effects of surface roughness on large-volume CdZnTe nuclear radiation detectors and removal of surface damage by chemical etching

This study investigates the effectiveness of chemical etchants to remove surface damage caused by mechanical polishing during the fabrication of Cd0.9Zn0.1Te (CZT) nuclear radiation detectors. We evaluate different planar CZT devices fabricated from the same CZT crystals. All detectors used electroless Au for the metal contacts. Different polishing particle sizes ranging from 22.1-μm SiC to 0.05-μm alumina were used, which caused different degrees of surface roughness. Current-voltage measurements and detector testing were used to characterize the effects of surface roughness and etching on the material and detector properties.

The effects of precipitates on CdZnTe device performance

A high-intensity X-ray beam collimated down to a 10-micrometer spot size, available at Brookhavens National Synchrotron Light Source (NSLS), was employed to perform X-ray mapping to measure the correlation between microscopic defects (precipitates) and variations in the collected charges in long-drift CdZnTe (CZT) detectors. First, we use X-ray diffraction topography (XDT) measurements at the high-energy beamline and IR microscopy to identify the defects distribution and strains in the bulk of CZT crystals. Then, we perform X-ray raster scans of the CZT detectors to measure their responses with 10-micrometer spatial resolution. The brightness of the source allows for good statistics in very short times. Precipitates that were singled out with X-ray scans are locally investigated by applying pulse-shape analysis. The presentation discusses how precipitates affect the device performance.

New results from testing of coplanar-grid CdZnTe detectors

New results from studies of coplanar-grid CdZnTe (CZT) detectors are presented. The coplanar-grid detectors were investigated by using a highly collimated X-ray beam available at Brookhavens National Synchrotron Light Source and by applying a pulse-shape analysis. The coplanar-grid detector operates as a single-carrier device. Despite the fact that its operational principle is well known and has been investigated by many groups in the past, we found some new details that may explain the performance limits of these types of devices. The experimental results have been confirmed by extensive computer modeling.

Study of growth defects in CZT and their influence on detector uniformity

The main problems involved in applying Cadmium Zinc Telluride (CZT) to detectors are the crystal perfection required and the difficulty in making reliable surface electrical contacts to the material. Our efforts have focused on the development of interconnect techniques and testing methods which will allow us to explore the interaction of defects with detector properties. Local stoichiometry variations and other local disordering make it very hard to find a systematic correlation between performance and material defects in the macroscopic scale. In order to understand the factors limiting the energy resolution of CZT detectors, our efforts were directed to the area of material characterization and detector testing using the National Synchrotron Light Source (NSLS). NSLS provides us with a highly collimated high intensity X-ray beam, which is employed to investigate micron-scale detector performance mapping and the correlation between microscopic defects and fluctuations in collected charge. Some results were already published and more are presented and correlated to X-ray diffraction topography (XDT) measurements. XDT at the beamline X17B1 is used to investigate more systematically the origins of the mosaicity that can give us information about the defect distribution and strains in bulk CZT crystals.