Henry Chen

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.

Characterization of Traveling Heater Method (THM) Grown

High-performance semi-insulating single crystals of n-type (CZT) were grown using the traveling heater method (THM). X-ray and -ray detector configurations fabricated from this material have a room-temperature mean energy resolution of 4.3% FWHM for a source (122 keV) and uniform pixel-to-pixel response on monolithic 20205 pixellated detectors. Energy resolution of 1% FWHM for (662keV) has been measured on virtual Frisch-grid 4411 devices useful for homeland security applications. Additional characterization techniques including mobility-lifetime measurements, infrared microscopy, X-ray topography, and OPTICAL Deep Level Transient Spectroscopy (ODLTS) have demonstrated the superior quality of this THM CZT.

{\hbox{Cd}}_{0.9}{\hbox{Zn}}_{0.1}{\hbox{Te}}

Low‐temperature photoluminescence (PL) spectra of detector grade Cd1−xZnxTe (x=0.1) have been measured to obtain information about shallow level defect concentration introduced during mechanical polishing and chemical etching processes. We present here a comparative PL study of Cd0.9Zn0.1Te crystals treated by different chemical solutions used for nuclear detector surface treatment. The results show that the 5% Br–MeOH+2%Br–20% lactic acid in ethylene glycol treatment combines the advantages of bromine and lactic acid for chemical etching and results in the best surface condition, as evidenced by the largest I(D0,X)/Idef intensity ratio and the narrowest full width at half‐maximum of the main peak (D0,X). Changes in the surface morphology were also analyzed by atomic force microscopy and correlated with the PL results. Current–voltage (I–V) curves and the room‐temperature 55Fe spectral response of the sample etched by the best treatment are also presented and discussed.

Crystals

Over 25 1-cm3 CdZnTe crystals produced using the Traveling Heater Method at Redlen Technologies have been characterized. The charge carrier mobility and lifetime, and charge carrier transport uniformity of each crystal were measured using alpha particles. Some of the crystals were made into coplanar-grid detectors and their performance characterized using 662 keV gamma rays. The average electron mobility-lifetime product for these crystals was found to be a factor of about five times greater than that measured from crystals obtained over the last decade from two other commercial crystal growers. The coplanar-grid detectors produced from the material typically achieved an energy resolution at 662 keV near 2% FWHM when operated at room temperature. This is comparable to the best coplanar-grid detectors commercially produced today.

Low‐temperature photoluminescence of detector grade Cd1−xZnxTe crystal treated by different chemical etchants

A comparative study of four different metal deposition methods on detector grade Cd/sub 1-x/Zn/sub x/Te crystal has been done using current-voltage (I-V) characterization and /sup 133/Ba gamma spectroscopy. An electroless Au contact and thermal evaporated Au contact have been found to yield a smaller leakage current and a more linear/symmetric I-V curve than that of sputtered Pt and Au contact. An optimum detector performance can be achieved by combining the thermal evaporated Au deposition method with a chemical surface treatment consisting of 5% Br MeOH followed by 2% Br-20% lactic acid in ethylene glycol. Low temperature photoluminescence spectra of Cd/sub 1-x/Zn/sub x/Te samples corresponding to these different metallization schemes are also presented and discussed.

Evaluation of THM-Grown CdZnTe Material for Large-Volume Gamma-Ray Detector Applications

The spectral response of several crystals grown by the Traveling Heater Method (THM) were investigated. An energy resolution of 0.98% for a Pseudo Frisch-Grid of 4 times 4 times 9 mm3 and 2.1% FWHM for a coplanar-grid of size 11 times 11 times 5 mm3 were measured using 137Cs-662 keV. In addition a 4% FWHM at 122 keV has also been measured on 20 times 20 X 5 mm3 monolithic pixellated devices. The material shows great potential toward producing large-volume detectors with spectral performance that meets the requirement for high-resolution gamma-ray spectroscopy.

Investigation of electrical contacts for Cd/sub 1-x/Zn/sub x/Te nuclear radiation detector

In an effort to optimize the spectroscopic performance of large volume Cadmium Zinc Telluride (CZT) radiation detectors grown by the Traveling Heater Method (THM), especially for higher energy gamma detection applications, bulk material and pixellated detector performance of THM CZT crystals at 15mm-thick and 10mm-thick were compared to that at 5mm-thick. We demonstrate that the outstanding performance of monolithic pixellated THM CZT detectors was thickness-scalable. For the same monolithic 20×20 mm2 “Compton Camera” pixellated detector configuration, detector performance can certainly be significantly improved as the thickness of the detector increases from 5mm to 10mm and thicker as demonstrated via both 57Co 122 keV and 137Cs 662 keV spectral responses. Depth of interaction information obtained from 3D depth sensing measurement system, IR microscopy and x-ray topography were used to study the correlation between structural quality and detector performance. Additional material characterizations and detector performances of various sizes and configurations were also presented and discussed.

Spectral Response of THM Grown CdZnTe Crystals

While charge-sharing in pixellated CdZnTe (CZT) room temperature radiation detectors has been studied numerically and experimentally in the last 5-10 years, there are no analytical models that will guide the optimization of CZT detector operations and enable efficient use of the correction algorithms. In this paper, we propose a new analytical model to provide an effective framework to study these effects over a wide range of detector geometries, biasing conditions, and material properties. The proposed model correctly predicts the amount of charge-sharing events versus photon energy and detector pixel size which are confirmed experimentally and numerically at the Danish National Space Center.

Thickness scalability of large volume cadmium zinc telluride high resolution radiation detectors

Abstract This paper reviews some of the progress obtained in the understanding of defects in detector grade cadmium zinc telluride material (CZT). Several techniques have been utilized to elucidate some of the issues related to compositional uniformity, effects of precipitates, grain boundaries, and surface defects related to mechanical and chemical treatments. In few cases, special mapping capabilities had to be developed to allow correlations with detector performance.

Modeling charge-sharing effects in pixellated CZT detectors

Electrode contacting on semiconductor radiation detectors has been a topic of active interest in many recent investigations. Research activities have focused on the morphology and chemistry of modified surfaces using sophisticated preparation techniques and employing characterization methods that are able to discriminate between surface and bulk effects. From an applied point of view, the detector fabrication technology involves a series of fabrication steps which can be optimized. Results of an ongoing effort to improve the performance of high resolution CdxZn1-xTe spectrometers by addressing wafer surface preparation, electrode deposition and contact passivation are described.