Engineering Education

UV Zn V Te (0.04(x(0.24) detector crystals grown in the Ukraine and Russia have been evaluated and compared to US-grown materials. Various analytical techniques were used to study the materials for trace impurities, precipitates, crystallinity, and electrical transport properties. Relatively, high concentrations of carbon and trace impurities such as Se, Nd and Si have been detected in the crystals. In most cases, the crystals showed lower resistivity than US-grown CZT. However, recent crystals grown in Russia exhibited better detector performance than those grown previously, and a good response to an Am radioactive source was found. Electron lifetimes below 1 s were measured in crystals having signi"cant numbers of micro-defects, compared to lifetimes of 5}15 s found in spectrometer grade materials produced in the US. Furthermore, the zinc composition along the growth axis showed better homogeneity in comparison with the US material. perfection of the "nal product. The widespread desire for portable gamma-ray spectrometers based on solid-state detectors has triggered the present investigation of CZT grown by di!erent suppliers. In each case the crystals were grown using the vertical high-pressure Bridgman. Crystals grown in the US, Russia, and the Ukraine were compared in terms of their ability to produce high-quality detectors [1}10]. The results have 0168-9002/99/$ } see front matter 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 -9 0 0 2 ( 9 8 ) 0 1 5 7 7 -0

A method for passivating the surface of Cd1−xZnxTe (CZT) x-ray and gamma ray detectors using relatively simple dry processing techniques has been developed. Leakage currents were significantly reduced for several processing methods. CZT samples were exposed to an oxygen plasma and/or coated with a reactively sputtered silicon nitride layer. Several parameters of the oxygen plasma step were found to be important for achieving enhanced surface resistivity. SiNX has been previously characterized and was used because of its high dielectric quality and low deposition temperature. Reduction in leakage current after passivation by a factor of as much as twenty is demonstrated. Results are also presented which give a measure of the long-term stability of the passivating layers.

We have applied several techniques, including photoluminescence, proton-induced x-ray emission, photocurrent, and alpha particle response mapping, for mapping micron- and millimeter-scale variations in cadmium zinc telluride. We have correlated the degree of inhomogeneity determined by these techniques with performance of gamma-ray spectrometers fabricated from the material.

We have employed both low-temperature photoluminescence (PL) spectroscopy and high-spatial-resolution, roomtemperature PL mapping to determine com~sition variation in Cd, _ .Zn,Te grown by high-pressure Bridgman. Com~sition variations AX of approximately 5-10% are observed between tip and heel of an approximately 13 cm long bottle, while fluctuations of l-2% are observed within a single, detector-scale sample. We also show that there are great discrepancies in the calculated zinc concen~ation, depending on which expression for E,(x) is chosen from the literature. We have performed high-resolution, triaxial X-ray analysis on selected samples to determine which of the published relations is most accurate for our material. We have also examined the relationship between the low-temperature PL spectrum and detector performance, and found that the line width of the donor-fund-exciton peak can be used as a predictor of performance.

We present a comprehensive review of the material properties of cadmium zinc telluride (CZT, Cd 1Àx Zn x Te) with zinc content x 0X1±0.2. Particular emphasis is placed on those aspects of this material related to room temperature nuclear detectors. A review of the structural properties, charge transport, and contacting issues and how these are related to detector and spectrometer performance is presented. A comprehensive literature survey and bibliography are also included. #

We have used room temperature photoluminescence to correlate radiation detector performance with Cadmium Zinc Telluride (CdZnTe) material quality in previous work. Enhancements in the experimental apparatus and the analysis software now allow us to achieve higher spatial resolution along with PL lineshape analysis. We have examined several CdZnTe crystals previously characterized as radiation detectors with the new apparatus and report on the results of this analysis method are presented along with experimental results.

We have demonstrated a new amplifier topology for coplanar-grid detectors that provides true differential readout with a single, fully-differential, charge-sensitive preamplifier. A prototype, multi-detector system with adjustable gain for each detector has been demonstrated using this fully differential approach. In its initial implementation using general-purpose amplifier chips, this system produces comparable noise performance to the traditional two-amplifier readout with specialized, charge-sensitive preamplifiers. In lieu of a differential gain to correct for electron trapping, the circuit uses a differential sampling scheme. This method enables a symmetrical photopeak to be obtained, but introduces some undesired filtering that limits energy resolution.

The material showing the greatest promise today for production of large-volume gamma-ray spectrometers operable at room temperature is cadmium zinc telluride (CZT). Unfortunately, because of deficiencies in the quality of the present material, high-resolution CZT spectrometers have thus far been limited to relatively small dimensions, which makes them inefficient at detecting high photon energies and ineffective for weak radiation signals except in near proximity. To exploit CZT fully, it will be necessary to make substantial improvements in the material quality. Improving the material involves advances in the crystallinity, purity, carrier lifetimes, and control of the electrical compensation mechanism. A more detailed understanding of the underlying material problems limiting the performance of CZT gamma-ray detectors is required; otherwise, problems with supply, delivery times, and unit cost of large-volume (>5 cm3 active volume) CZT spectrometers are expected to continue. A variety of analytical and numerical techniques have been employed to quantify crystallinity, strain, impurities, compositional and stoichiometric variations, bulk and surface defect states, carrier mobilities and lifetimes, electric field distributions, and surface passivation. Data from these measurements were correlated with spatial maps of the gamma-ray and alpha particle spectroscopic response, and feedback on the effectiveness of crystal growth and detector fabrication procedures has been generated. The results of several of these analytical techniques will be presented in this paper.

The RF/photonics module in Comsol Multiphysics provides complete flexibility in specifying the dielectric function or refractive index as a function of electric field. It is therefore a relatively simple matter to incorporate second-order (second harmonic generation) or third-order (four-wave mixing) nonlinear effects in a qualitative way. However, to move towards practical device modeling, it is essential to include phase mismatch effects, which depend crucially on dispersion. In this paper we show how to incorporate an auxiliary differential equation into a time-dependent EM wave propagation mode to model four-wave mixing in the presence of dispersion. Special cases of this model are used to realize both Lorentz and Drude dispersion models.

While Comsol Multiphysics has great flexibility for modeling photonic devices, the computational demands of the general finite element method make it most suitable for small devices, on the order of a few microns in extent. Many integrated optical devices are much larger, having lengths of hundreds of microns or millimeters.