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 Brookhaven's 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.
Pulse shape analysis is proved to be a powerful tool to characterize the performance of CdZnTe devices and understand their operating principles. It allows one to investigate the device configurations, electron transport properties, effects governing charge collection, electric-field distributions, signal charge formation, etc. This work describes an application of different techniques based on the pulse shape measurements to characterize pixel, coplanar-grid, and
virtual Frisch-grid devices and understand the electronic properties of CZT material provided by different vendors. We report new results that may explain the performance limits of these devices.
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.
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.
CdZnTe detectors demonstrated great potentials for detection of gamma radiation. However, energy resolution of CdZnTe detectors is significantly affected by uncollected holes which have low mobility and short lifetime. To overcome this deleterious effects upon energy resolution special detector designs have to be implemented. The most practical of them are the small pixel effect device, the co-planar grid device, and the virtual Frisch-grid device. We routinely use a highly collimated high-intensity X-ray beams provided by National Synchrotron Light Source (NSLS) facility at Brookhaven National Laboratory to study of CdZnTe material and performances of the different types of devices on the micron-scale. This powerful tool allows us to evaluate electronic properties of the material, device performance, uniformity of the detector responses, effects related to the device's contact pattern and electric field distribution, etc. In particular, in this paper we present new results obtained from the performance studies of 15 x 15 x 7.5 mm3 coplanar-grid devices coupled to readout ASIC. We observed the effect of the strip contacts comprising the grids on the energy resolution of the coplanar-grid device.
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.