Cadmium Zinc Telluride Radiation Detectors Research Articles

Study of the electrical characteristics of CdZnTe Schottky diodes

Cadmium zinc telluride (CZT) is a promising material for the room temperature detection and spectroscopy of ionizing radiations. Two sets of samples, exhibiting good and unacceptable performance of CZT radiation detectors, have been studied in order to resolve difference in the electrical and structural parameters which govern operational characteristics of CZT detectors. I–V, barrier capacitance, carrier lifetime, and drift velocity measurements have been combined to reveal the main differences in electrical characteristics which determine crucial difference in spectral performance of CZT radiation detectors.

Utility of HACOR score in predicting failure of noninvasive mechanical ventilation and mortality in cancer patients

Commercial detector-grade cadmium zinc telluride (CZT) crystals still suffer from various types of extended defects, e.g., dislocations, micro-grains, grain boundaries, and Te-rich secondary phases. Most of these defects cannot readily be identified and characterized using conventional techniques, though they are believed to be the dominant factor causing non-uniformity in the detector response. In this work, we revealed and characterized these secondary-phase defects in CZT crystals by employing multiple advanced techniques, e.g. X-ray diffraction topography, micro-scale X-ray response mapping, chemical etching and infrared microscopy. We then evaluate the detector performance of the crystals by recording high spatial-resolution raster scans of the charge collection and spectral response. We directly correlated the influence of the secondary-phase defects on the performance of the detector responses. The experimental results exhibit clear evidence of the undesirable effects of extended defects on the performance of commercial CZT radiation detectors.

Direct observation of influence of secondary-phase defects on CZT detector response

Commercial detector-grade cadmium zinc telluride (CZT) crystals still suffer from various types of extended defects, e.g., dislocations, micro-grains, grain boundaries, and Te-rich secondary phases. Most of these defects cannot readily be identified and characterized using conventional techniques, though they are believed to be the dominant factor causing non-uniformity in the detector response. In this work, we revealed and characterized these secondary-phase defects in CZT crystals by employing multiple advanced techniques, e.g. X-ray diffraction topography, micro-scale X-ray response mapping, chemical etching and infrared microscopy. We then evaluate the detector performance of the crystals by recording high spatial-resolution raster scans of the charge collection and spectral response. We directly correlated the influence of the secondary-phase defects on the performance of the detector responses. The experimental results exhibit clear evidence of the undesirable effects of extended defects on the performance of commercial CZT radiation detectors.

An XPS study of bromine in methanol etching and hydrogen peroxide passivation treatments for cadmium zinc telluride radiation detectors

The performance of single crystal CdZnTe radiation detectors is dependent on both the bulk and the surface properties of the material. After single crystal fabrication and mechanical polishing, modification of the surface to remove damage and reduce the surface leakage current is generally achieved through chemical etching followed by a passivation treatment. In this work, CdZnTe single crystals have been chemically etched using a bromine in methanol (BM) treatment. The BM concentrations employed were 0.2 and 2.0 (v/v) % and exposure times varied between 5 and 120s. Angle resolved XPS and sputter depth profiling has been employed to characterize the surfaces for the different exposure conditions. A Te rich surface layer was formed for all exposures and the layer thickness was found to be independent of exposure time. The enriched Te layer thickness was accurately determined by calibrating the sputter rate against a CdTe layer of known thickness. For BM concentrations of 0.2 (v/v) % and 2 (v/v) %, the Te layer thickness was determined to be 1.3±0.2 and 1.8±0.2nm, respectively. The BM etched surfaces have subsequently been passivated in a 30wt.% H2O2 solution employing exposure time of 15s. The oxide layer thickness has been calculated using two standard XPS methodologies, based on the Beer–Lambert expression. The TeO2 thickness calculated from ARXPS data are slightly higher than the thickness obtained by the simplified Beer–Lambert expression. For BM exposures of 30–120s followed by a passivation treatment of 30wt. % H2O2 solution employing an exposure time 15s, the ARXPS method gave an average TeO2 thickness value of 1.20nm and the simplified Beer–Lambert expression gave an average thickness value of 0.99nm.

Internal Electric Field Behavior of Cadmium Zinc Telluride Radiation Detectors Under High Carrier Injection

The behavior of the internal electric field of nuclear-radiation detectors substantially affects detector performance. We investigated the distribution of the internal field in cadmium zinc telluride (CZT) detectors under high carrier injection. We noted the build-up of a space-charge region near the cathode that produces a built-in field opposing the applied field. Its presence entails the collapse of the electric field in the rest of the detector, other than the portion near the cathode. Such a space-charge region originates from serious hole trapping in CZT. The device’s operating temperature greatly affects the width of the space-charge region. With increasing temperature from 5°C to 35°C, its width expanded from about one-sixth to one-half of the total depth of the detector.

Chemical etching and surface oxidation studies of cadmium zinc telluride radiation detectors

AbstractCdZnTe (CZT) is commonly used as a radiation detector material and the surface properties are important as they influence detector performance. The surface chemistry is generally controlled through chemical etching and oxidation processes. In this paper, X‐ray photoelectron spectroscopy (XPS) is employed to investigate changes in the surface composition of single‐crystal Cd0.95Zn0.05Te samples after exposure to bromine in methanol (BM) chemical etching treatments and subsequent oxidation in air or 30% H2O2. BM treatment of CZT is found to result in a graded Te‐rich surface layer which increases in thickness as a function of BM concentration. Room‐temperature air oxidation of 0.2% BM‐treated CZT follows a logarithmic rate law. BM‐treated CZT exposed to 30% H2O2 for 30 s shows a linear increase in the TeO2 oxide thickness with increasing BM concentration up to a BM concentration of 1.5%. Above 2% BM concentration, the Te enrichment in the CZT surface region has reached saturation and is effectively pure Te. Copyright © 2010 John Wiley & Sons, Ltd.

Opto-electrical characterization and X-ray Mapping of large-volume cadmium zinc telluride radiation detectors

AbstractLarge-volume cadmium zinc telluride (CZT) radiation detectors would greatly improve radiation detection capabilities and, therefore, attract extensive scientific and commercial interests. CZT crystals with volumes as large as hundreds of centimeters can be achieved today due to improvements in the crystal growth technology. However, the poor performance of large-volume CZT detectors is still a challenging problem affecting the commercialization of CZT detectors and imaging arrays. We have employed Pockels effect measurements and synchrotron X-ray mapping techniques to investigate the performance-limiting factors for large-volume CZT detectors. Experimental results with the above characterization methods reveal the non-uniform distribution of internal electric field of large-volume CZT detectors, which help us to better understand the responsible mechanism for the insufficient carrier collection in large-volume CZT detectors.

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.

Surface passivation of cadmium zinc telluride radiation detectors by potassium hydroxide solution

The spectral resolution of cadmium zinc telluride (CZT) room temperature nuclear radiation detectors is often limited by the presence of conducting surface species that increase the surface leakage current. Surface passivation plays an important role in reducing this surface leakage current and thereby decreasing the noise of the detectors and improving the spectral energy resolution. Chemical etching with a Br-MeOH solution leaves CZT surfaces rich in Te and is considered as one of the primary causes of the increased surface leakage current. Previous studies have shown that hydrogen peroxide (H2O2) forms oxides of tellurium on the CZT surface and thus acts as a good passivating agent. In this study we will present results on the use of potassium hydroxide (KOH) as an alternative passivating agent. The KOH aqueous solution leaves a more stoichiometric (evaluated from the trends in the surface Cd:Te ratio) and smoother CZT surface. The passivation effects of KOH solution on the surface of the CZT have been characterized by current-voltage measurements for different KOH concentrations and etching times for both parallel strip electrodes as well as a metal-semiconductor-metal configuration. The surface chemical composition and its morphology were studied by scanning x-ray photoelectron spectroscopy and atomic force microscopy. The comparison and demonstration of improvements in the spectral resolution of the CZT detectors (based on 241Am spectra) with and without the KOH treatment are presented.

Ion microbeam studies of cadmium zinc telluride radiation detectors by IBICC

Ion beam induced charge collection (IBICC) and time resolved IBICC (TRIBICC) techniques were used for imaging electronic properties of cadmium zinc telluride (CZT) room temperature radiation detectors. The detectors were bombarded with a scanned 5.4 MeV He microbeam and the detector response was analyzed at each point. The electron mobility (μe) and lifetime (τe), and charge collection efficiency maps were calculated from the data. In order to determine the radiation damage to the detectors, the signal deterioration was measured as the function of dose.