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EFFECTIVENESS OF ELECTROSTATIC SHIELDING AND ELECTRONIC SUBTRACTION TO CORRECT FOR THE HOLE TRAPPING IN CDZNTE SEMICONDUCTOR DETECTORS.

Description: CdZnTe (CZT) is a very promising material for nuclear-radiation detectors. CZT detectors operate at ambient temperatures and offer high detection efficiency and excellent energy resolution, placing them ahead of high-purity Ge for those applications where cryogenic cooling is problematic. The progress achieved in CZT detectors over the past decade is founded on the developments of robust detector designs and readout electronics, both of which helped to overcome the effects of carrier trapping. Because the holes have low mobility, only electrons can be used to generate signals in thick CZT detectors, so one must account for the variation of the output signal versus the locations of the interaction points. To obtain high spectral resolution, the detector's design should provide a means to eliminate this dependence throughout the entire volume of the device. In reality, the sensitive volume of any ionization detector invariably has two regions. In the first, adjacent to the collecting electrode, the amplitude of the output signal rapidly increases almost to its maximum as the interaction point is located farther from the anode; in the rest of the volume, the output signal remains nearly constant. Thus, the quality of CZT detector designs can be characterized based on the magnitude of the signals variations in the drift region and the ratio between the volumes of the driR and induction regions. The former determines the ''geometrical'' width of the photopeak i.e., the line width that affects the total energy resolution and is attributed to the device's geometry when all other factors are neglected. The latter determines the photopeak efficiency and the area under the continuum in the pulse-height spectra. In this work, we describe our findings from systematizing different designs of CZT detectors and evaluating their performance based on these two criteria.
Date: August 26, 2007
Creator: BOLOTNIKOV,A.E.; CAMARDA, G.S.; HOSSAIN, A.; CUI, Y. & JAMES, R.B.
Partner: UNT Libraries Government Documents Department

Polarization Studies of CdZnTe Detectors Using Synchrotron X-Ray Radiation.

Description: New results on the effects of small-scale defects on the charge-carrier transport in single-crystal CdZnTe (CZT) material were produced. We conducted detailed studies of the role of Te inclusions in CZT by employing a highly collimated synchrotron x-ray radiation source available at Brookhaven's National Synchrotron Light Source (NSLS). We were able to induce polarization effects by irradiating specific areas with the detector. These measurements allowed the first quantitative comparison between areas that are free of Te inclusions and those with a relatively high concentration of inclusions. The results of these polaration studies will be reported.
Date: July 1, 2007
Creator: Camarda, G. S.; Bolotnikov, A. E.; Cui, Y.; Hossain, A. & James, R. B.
Partner: UNT Libraries Government Documents Department

EFFECT OF TE PRECIPITATES ON THE PERFORMANCE OF CDZNTE DETECTORS.

Description: A recent study of long-drift CdZnTe (CZT) Frisch-ring detectors showed that fluctuations of the collected charge (and device response) depend on the device dimensions and the concentration of Te precipitates in the material. This observation, which could be explained as the cumulative effect of precipitates, led to the investigation of thin (1 mm) planar detectors, where the effects of precipitates can be more clearly ascertained. To perform the investigation, a measurement facility was developed that allowed for high-resolution spatial mapping of the performance of CZT devices. New measurements emerging from this facility provided the first detailed comparisons of the micro-scale X-ray maps and infrared microscopy images for thin CZT samples. Analysis of the data showed conclusively that local deteriorations of device response fully correlate with Te precipitates seen in the IR images. Effects of surface processing conditions on the detector response were also clearly observed.
Date: September 20, 2005
Creator: CARINI, G.A.; BOLOTNIKOV, A.E.; CAMARDA, G.S.; WRIGHT, G.W.; LI, L. & JAMES, R.B.
Partner: UNT Libraries Government Documents Department

IMPROVEMENT OF CdMnTe DETECTOR PERFORMANCE BY MnTe PURIFICATION

Description: Residual impurities in manganese (Mn) are a big obstacle to obtaining high-performance CdMnTe (CMT) X-ray and gamma-ray detectors. Generally, the zone-refining method is an effective way to improve the material's purity. In this work, we purified the MnTe compounds combining the zone-refining method with molten Te, which has a very high solubility for most impurities. We confirmed the improved purity of the material by glow-discharge mass spectrometry (GDMS). We also found that CMT crystals from a multiply-refined MnTe source, grown by the vertical Bridgman method, yielded better performing detectors.
Date: April 25, 2011
Creator: Kim, K. H.; Bolotnikov, A. E.; Camarda, G. S.; Tappero, R.; Hossain, A.; Cui, Y. et al.
Partner: UNT Libraries Government Documents Department

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

Description: Large-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.
Date: April 13, 2009
Creator: Yang, G.; Bolotnikov, A.E.; Camarda, G.S.; Cui, Y.; Hossain, A.; Yao, H.W. et al.
Partner: UNT Libraries Government Documents Department

CDZNTE ROOM-TEMPERATURE SEMICONDUCTOR GAMMA-RAY DETECTOR FOR NATIONAL-SECURITY APPLICATIONS.

Description: One important mission of the Department of Energy's National Nuclear Security Administration is to develop reliable gamma-ray detectors to meet the widespread needs of users for effective techniques to detect and identify special nuclear- and radioactive-materials. Accordingly, the Nonproliferation and National Security Department at Brookhaven National Laboratory was tasked to evaluate existing technology and to develop improved room-temperature detectors based on semiconductors, such as CdZnTe (CZT). Our research covers two important areas: Improving the quality of CZT material, and exploring new CZT-based gamma-ray detectors. In this paper, we report on our recent findings from the material characterization and tests of actual CZT devices fabricated in our laboratory and from materials/detectors supplied by different commercial vendors. In particular, we emphasize the critical role of secondary phases in the current CZT material and issues in fabricating the CZT detectors, both of which affect their performance.
Date: May 4, 2007
Creator: Camarda, G. S.; Bolotnikov, A. E.; Cui, Y.; Hossain, A.; Kohman, K. T. & James, R. B.
Partner: UNT Libraries Government Documents Department

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

Description: The behavior of the internal electric-field of nuclear-radiation detectors substantially affects the detector's 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 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 1/6 to 1/2 of the total depth of the detector.
Date: October 26, 2010
Creator: Yang, G.; Bolotnikov, A.E.; Camarda, G.S.; Cui, Y.; Hossain, A.; Kim, K.H. et al.
Partner: UNT Libraries Government Documents Department

Te Inclusions in CZT Detectors: New Method for Correcting Their Adverse Effects

Description: Both Te inclusions and point defects can trap the charge carriers generated by ionizing particles in CdZnTe (CZT) detectors. The amount of charge trapped by point defects is proportional to the carriers’ drift time and can be corrected electronically. In the case of Te inclusions, the charge loss depends upon their random locations with respect to the electron cloud. Consequently, inclusions introduce fluctuations in the charge signals, which cannot be easily corrected. In this paper, we describe direct measurements of the cumulative effect of Te inclusions and its influence on the response of CZT detectors of different thicknesses and different sizes and concentrations of Te inclusions. We also discuss a means of partially correcting their adverse effects.
Date: October 25, 2009
Creator: Bolotnikov, A.E.; Babalola, S.; Camarda, G.S.; Cui, Y.; Egarievwe, S.U.; Hawrami, R. et al.
Partner: UNT Libraries Government Documents Department

Distribution of Te inclusions in a CdZnTe wafer and their effects on the electrical properties of fabricated devices

Description: We quantified the size and concentration of Te-inclusions along the lateral- and the growth-directions of a {approx}6 mm thick wafer cut axially along the center of a CdZnTe ingot. We fabricated devices, selecting samples from the center slice outward in both directions, and then tested their response to incident x-rays. We employed, in concert, an automated IR transmission microscopic system and a highly collimated synchrotron X-ray source that allowed us to acquire and correlate comprehensive information on Te inclusions and other defects to assess the material factors limiting the performance of CdZnTe detectors.
Date: May 24, 2010
Creator: Hossain , A.; Xu, L.; Bolotnikov, A.E.; Camarda, G.S.; Cui, Y.; Yang, G. et al.
Partner: UNT Libraries Government Documents Department

Internal electric-field-lines distribution in CdZnTe detectors measured using X-ray mapping

Description: 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.
Date: October 19, 2009
Creator: Bolotnikov, A. E.; Camarda, G. S.; Cui, Y.; Hossain, A.; Yang, G.; Yao, H. W. et al.
Partner: UNT Libraries Government Documents Department

PERFORMANCE-LIMITING DEFECTS IN CDZNTE DETECTORS.

Description: We studied the effects of small, <20 {micro}m, 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 hound the materials-related problem and predict the enhancement in performance expected by reducing the size and number of Te inclusions within the crystals.
Date: October 29, 2006
Creator: BOLOTNIKOV, A.E.; CAMARDA, G.S.; CUI, Y.; KOHMAN, K.T.; LI, L.; SALOMON, M.B. et al.
Partner: UNT Libraries Government Documents Department

TE INCLUSIONS AND THEIR RELATIONSHIP TO THE PERFORMANCE OF CDZNTE DETECTORS.

Description: Te-rich secondary phases existing in CdZnTe (CZT) single crystals degrade the spectroscopic performance of these detectors. An unpredictable number of charges are trapped, corresponding to the abundance of these microscopic defects, thereby leading to fluctuations in the total collected charge and strongly affecting the uniformity of charge-collection efficiency. These effects, observed in thin planar detectors, also were found to be the dominant cause of the low performance of thick detectors, wherein the fluctuations accumulate along the charge's drift path. Reducing the size of Te inclusions from a virtual diameter of 10-20 {micro}m down to less than 5 {micro}m already allowed us to produce Frisch-ring detectors with a resolution as good as {approx}0.8% FWHM at 662 keV: Understanding and modeling the mechanisms involving Te-rich secondary phases and charge loss requires systematic studies on a spatial scale never before realized. Here, we describe a dedicated beam-line recently established at BNL's National Synchrotron Light Source for characterizing semiconductor detectors along with a IR system with counting capability that permits us to correlate the concentration of defects with the devices' performances.
Date: August 13, 2006
Creator: CARINI, G.A.; BOLOTNIKOV, A.E.; CAMARDA, G.S.; CUI, Y.; JACKSON, H.; BURGER, A. et al.
Partner: UNT Libraries Government Documents Department

HIGH SPATIAL-RESOLUTION IMAGING OF TE INCLUSIONS IN CZT MATERIAL.

Description: We present new results from our studies of defects in current single-crystal CdZnTe material. Our previous measurements, carried out on thin ({approx}1 mm) and long (>12 mm) CZT detectors, indicated that small (1-20 {micro}m) Te inclusions can significantly degrade the device's energy resolution and detection efficiency. We are conducting detailed studies of the effects of Te inclusions by employing different characterization techniques with better spatial resolution, such as quantitative fluorescence mapping, X-ray micro-diffraction, and TEM. Also, IR microscopy and gamma-mapping with pulse-shape analysis with higher spatial resolution generated more accurate results in the areas surrounding the micro-defects (Te inclusions). Our results reveal how the performance of CdZnTe detectors is influenced by Te inclusions, such as their spatial distribution, concentration, and size. We also discuss a model of charge transport through areas populated with Te inclusions.
Date: August 13, 2006
Creator: CAMARDA, G.S.; BOLOTNIKOV, A.E.; CARINI, G.A.; CUI, Y.; KOHMAN, K.T.; LI, L. et al.
Partner: UNT Libraries Government Documents Department

READOUT SYSTEM FOR ARRAYS OF FRISCH-RING CDZNTE DETECTORS.

Description: Frisch-ring CdZnTe detectors have demonstrated good energy resolution for identifying isotopes, <1% FWHM at 662 keV, and good efficiency for detecting gamma rays. We will fabricate and test at Brookhaven National Laboratory an integrated module of a 64-element array of 6 x 6 x 12 mm{sup 3} Frisch-ring detectors, coupled with a readout electronics system. It supports 64 readout channels, and includes front-end electronics, signal processing circuit, USB interface and high-voltage power supply. The data-acquisition software is used to process the data stream, which includes amplitude and timing information for each detected event. This paper describes the design and assembly of the detector modules, readout electronics, and a conceptual prototype system. Some test results are also reported.
Date: October 29, 2006
Creator: CUI, Y.; BOLOTNIKOV, A.E.; CAMARDA, G.S.; DE GERONIMO, G.; O'CONNOR, P.; JAMES, R.B. et al.
Partner: UNT Libraries Government Documents Department

Internal electric-field-lines distribution in CdZnTe detectors measured using X-ray mapping

Description: 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, 21 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.
Date: June 1, 2008
Creator: Bolotnikov, A. E.; Camarda, G. S.; Cui, Y.; Hossain, A.; Yang, G.; Yao, H. W. et al.
Partner: UNT Libraries Government Documents Department

Investigation of Charge Transport Properties of CdZnTe Detectors with Synchrotron X-ray Radiation

Description: Various internal defects, such as Te inclusions, twin boundaries, dislocation, etc., are prevalent in as-grown CdZnTe (CZT) crystals, which affect the charge transport properties of CZT crystals and, therefore, worsen the performance of CZT detectors. In order to develop high quality CZT detectors, it is imperative to clarify the effects of internal defects on the charge transport properties of CZT. Simple flood illumination with nuclear radiation source cannot reveal the nature of highly localized defects in CZT. Therefore, at Brookhaven's National Synchrotron Light Source (NSLS), we have developed a unique testing system for micro-scale defect investigation of CZT, which employs an X-ray beam collimated with the spatial resolution as small as 3 x 3 {micro}m{sup 2}, a microscopic size comparable to the scale of common defects in CZT. This powerful tool enables us to investigate the effect of internal defects on charge transport properties of CZT in detail.
Date: October 19, 2008
Creator: Yang, G.; Bolotnikov, A. E.; Camarda, G. S.; Cui, Y.; Hossain, A. & James, R. B.
Partner: UNT Libraries Government Documents Department

Material properties limiting the performance of CZT gamma-ray detectors

Description: CdZnTe (CZT) nuclear radiation detectors are advanced sensors that utilize innovative technologies developed for wide band-gap semiconductor industry and microelectronics. They open opportunities for new types of room-temperature operating, field deployable instruments that provide accurate identification of potential radiological threats and timely awareness for both the civilian and military communities. Room-temperature radiation detectors are an emerging technology that relies on the use of high-quality CZT crystals whose availability is currently limited by material non-uniformities and the presence of extended defects. To address these issues, which are most critical to CZT sensor developments, we developed X-ray mapping and IR transmission microscopy systems to characterize both CZT crystals and devices. Since a customized system is required for such X-ray measurements, we use synchrotron radiation beams available at BNL's National Synchrotron Light Source. A highly-collimated and high-intensity X-ray beam supports measurements of areas as small as 10 x 10 {micro}m{sup 2}, and allowed us to see fluctuations in collected charge over the entire area of the detector in a reasonable time. The IR microscopy system allows for 3D visualization of Te inclusions and other extended defects. In this paper, we describe the experimental techniques used in our measurements and typical results obtained from CZT samples produced by different suppliers.
Date: March 16, 2009
Creator: Bolotnikov, A. E.; Babalola, S.; Camarda, G. S.; Cui, Y.; Egarievwe, S. U.; Hossain, A. et al.
Partner: UNT Libraries Government Documents Department

Electric Field Distribution of Cadmium Zinc Telluride (CZT)

Description: Cadmium Zinc Telluride (CZT) is attracting increasing interest with its promise as a room-temperature nuclear-radiation-detector material. The distribution of the electric field in CZT detectors substantially affects their detection performance. At Brookhaven National Laboratory (BNL), we employed a synchrotron X-Ray mapping technique and a Pockels-effect measurement system to investigate this distribution in different detectors. Here, we report our latest experimental results with three detectors of different width/height ratios. A decrease in this ratio aggravates the non-uniform distribution of electric field, and focuses it on the central volume. Raising the bias voltage effectively can minimize such non-uniformity of the electric field distribution. The position of the maximum electric field is independent of the bias voltage; the difference between its maximum- and minimum-intensity of electric field increases with the applied bias voltage.
Date: August 2, 2009
Creator: Yang, G.; Bolotnikov, A.; Camarda, G.S.; Cui, Y.; Hossain, A.; Kim, K. et al.
Partner: UNT Libraries Government Documents Department

Detector Performance of Ammonium-Sulfide-Passivated CdZnTe and CdMnTe Materials

Description: Dark currents, including those in the surface and bulk, are the leading source of electronic noise in X-ray and gamma detectors, and are responsible for degrading a detector's energy resolution. The detector material itself determines the bulk leakage current; however, the surface leakage current is controllable by depositing appropriate passivation layers. In previous research, we demonstrated the effectiveness of surface passivation in CZT (CdZnTe) and CMT (CdMnTe) materials using ammonium sulfide and ammonium fluoride. In this research, we measured the effect of such passivation on the surface states of these materials, and on the performances of detectors made from them.
Date: August 1, 2010
Creator: Kim, K. H.; Bolotnikov, A. E.; Camarda, G. S.; Marchini, L.; Yang, G.; Hossain, A. et al.
Partner: UNT Libraries Government Documents Department

Effect of chemical etching on the surface roughness of CdZnTe and CdMnTe gamma radiation detectors

Description: Generally, mechanical polishing is performed to diminish the cutting damage followed by chemical etching to remove the remaining damage on crystal surfaces. In this paper, we detail the findings from our study of the effects of various chemical treatments on the roughness of crystal surfaces. We prepared several CdZnTe (CZT) and CdMnTe (CMT) crystals by mechanical polishing with 5 {micro}m and/or lower grits of Al{sub 2}O{sub 3} abrasive papers including final polishing with 0.05-{micro}m particle size alumina powder and then etched them for different periods with a 2%, 5% Bromine-Methanol (B-M) solution, and also with an E-solution (HNO{sub 3}:H{sub 2}O:Cr{sub 2}O{sub 7}). The material removal rate (etching rate) from the crystals was found to be 10 {micro}m, 30 {micro}m, and 15 {micro}m per minute, respectively. The roughness of the resulting surfaces was determined by the Atomic Force Microscopy (AFM) to identify the most efficient surface processing method by combining mechanical and chemical polishing.
Date: August 11, 2008
Creator: Hossain,A.; Babalola, S.; Bolotnikov, A.E.; Camarda, G.S.; Cui, Y.; Yang, G. et al.
Partner: UNT Libraries Government Documents Department