CdZnTe Material Research Articles

X-Ray Beam Studies of Charge Sharing in Small Pixel, Spectroscopic, CdZnTe Detectors

Recent advances in the growth of CdZnTe material have allowed the development of small pixel, spectroscopic, X-ray imaging detectors. These detectors have applications in a diverse range of fields such as medical, security and industrial sectors. As the size of the pixels decreases relative to the detector thickness, the probability that charge is shared between multiple pixels increases due to the non zero width of the charge clouds drifting through the detector. These charge sharing events will result in a degradation of the spectroscopic performance of detectors and must be considered when analyzing the detector response. In this paper charge sharing and charge loss in a 250 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu$</tex> </formula> m pitch CdZnTe pixel detector has been investigated using a mono-chromatic X-ray beam at the Diamond Light Source, U.K. Using a 20 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu$</tex> </formula> m beam diameter the detector response has been mapped for X-ray energies both above (40 keV) and below (26 keV) the material <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$K$</tex></formula> -shell absorption energies to study charge sharing and the role of fluorescence X-rays in these events.

Correlations Between Crystal Defects and Performance of CdZnTe Detectors

Poor crystallinity remains a major problem affecting the availability and cost of CdZnTe (CZT) detectors. Point defects are responsible for small gradual charge loss and correlated with the electron clouds' drift times, which allows electronic correction of the output signals to achieve high spectral-resolution even with large-volume CZT detectors. In contrast, extended defects causes significant charge losses, which typically are uncorrelated, and, thus, result in much greater fluctuations of the output signals that cannot be corrected. Although extended defects do not affect all the interaction events, their fraction rapidly increases with the crystal's thickness and volume. In this paper, we summarize our recent results from testing CZT material and detectors that emphasize the particular roles of two types of extended defects, and their contributions to the device's overall performance.

Characteristics of the dislocations in CdZnTe crystals revealed by etch pits

The configurations and distributions of triangular pyramid etch pits on the (1 1 1)B face of CdZnTe revealed by Everson etch were studied using a scanning electron microscope (SEM), an optical microscope and a confocal laser scanning microscope (CLSM). Six types of triangular pyramid etch pits with different configurations were observed, one for the first time. These etch pits correspond to threading dislocations in the materials. By measuring the depths and orientations of the pit tips as well as the side lengths of the triangular pits on the surface, the orientations of the dislocations were determined. They are 〈2̄ 1̄ 1〉, 〈1̄ 3̄ 1̄〉, 〈1̄ 1̄ 1〉, 〈1 5̄ 1〉, 〈0 3̄ 1〉 and 〈1̄ 1̄ 0〉 for the six types of triangular pyramid etch pits. Among them, dislocations with 〈2̄ 1̄ 1〉, 〈1̄ 1̄ 1〉, 〈1 5̄ 1〉 and 〈0 3̄ 1〉 orientations have higher densities in CdZnTe materials. It was also found that the proportion of triangular pyramid etch pit to total etch pits is variable for different CdZnTe wafers and related to the stoichiometry of CdZnTe melt. These results show that Everson etch can reveal more information of CdZnTe materials than etch pit density (EPD).

Effects of dislocations and sub-grain boundaries on X-ray response maps of CdZnTe radiation detectors

ABSTRACTThe imperfect quality of CdZnTe (CZT) crystals for radiation detectors seriously diminishes their suitability for different applications. Dislocations and other dislocation-related defects, such as sub-grain boundaries and dislocation fields around Te inclusions, engender significant charge losses and, consequently, cause fluctuations in the detector’s output signals, thereby hindering their spectroscopic responses. In this paper, we discuss our results from characterizing CZT material by using a high-spatial-resolution X-ray response mapping system at BNL’s National Synchrotron Light Source. In this paper, we emphasize the roles of these dislocation-related defects and their contributions in degrading the detector’s performance. Specifically, we compare the effects of the sub-grain- and coherent twin-boundaries on the X-ray response maps.

Fast Neutron Damage of a Pixelated CdZnTe Gamma Ray Spectrometer

This study describes the damage to a pixelated 1 cm times 1 cm times 1 cm CdZnTe detector caused by fast neutrons in the energy range 1-7 MeV. Measurements of electron mutau product were performed before and after irradiation and also following thermal annealing. Spectroscopic information was acquired with a 133Ba gamma source. Before neutron irradiation, sharp peaks were observed for the 133Ba lines, with combined pixel energy resolution for the 356 keV line of 2.1% FWHM. After irradiation, clear deterioration of the spectral response is observed, with combined energy resolution of the 356 keV line of about 6% FWHM. After thermal annealing, the spectral response shows significant recovery, with combined energy resolution of 2.3% FWHM. We successfully simulate these results with a theoretical model. It is concluded that the fast neutrons cause displacement damage in the lattice of the bulk CdZnTe material.

Extended Defects in CdZnTe Radiation Detectors

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Large-volume CdZnTe (CZT) single crystals with electron lifetime exceeding 10 <formula formulatype="inline"><tex Notation="TeX">$\mu{\hbox {s}}$</tex> </formula> have recently become commercially available. This opened the opportunity for making room temperature CZT gamma-ray detectors with extended thicknesses and larger effective areas. However, the extended defects that are present even in the highest-quality material remain a major drawback which affects the availability and cost of large CZT detectors. In contrast to the point defects that control electron lifetime and whose effects on the charge collection can be electronically corrected, the extended defects introduce significant fluctuations in the collected charge, which increase with a crystal's thickness. The extended defects limit the uniformity in the electrons' drift distance in CZT crystals, above which electron trapping cannot effectively be corrected. In this paper, we illustrate the roles of the extended defects in CZT detectors with different geometries. We emphasize that the crystallinity of commercial CZT materials remains a major obstacle on the path to developing thick, large-volume CZT detectors for gamma-ray imaging and spectroscopy. </para>

Comparison of the x-ray spectroscopy response and charge transport properties of semi-insulating In/Al doped CdZnTe crystals

The x-ray spectroscopy performance of In/Al doped CdZnTe planar detectors based on as-grown crystals were investigated at room temperature, using a Tb x-ray source with a principle energy of 44.2 keV. The observed broadening in the photopeak resolution was attributed to incomplete charge carrier collection due to carrier trapping and scattering by the defects in the crystal. Alpha particle spectroscopy and pulse shape rise time analysis were used to measure the electron mobility lifetime product (μτ), as well as the mobility (μ) of the CdZnTe material grown with different dopant concentrations. To further clarify the role of the dopant and associated trapping states, temperature dependent alpha particle spectroscopy and pulse shapes were investigated at various applied bias fields over a temperature range from 200 to 300 K. CdZnTe doped with 1.5 ppm In exhibits excellent x-ray spectral resolution and charge transport properties, which implies a lower density of trapping centers in the crystal. The deep levels associated with Cdi2+ have been tentatively recognized as electron trapping centers and this is confirmed by the observed reduction in electron lifetime in CdZnTe crystal with 15 ppm In. Additionally, a shallower electron detrapping defect, with an activation energy of 0.14±0.02 eV, was also discovered to be simultaneously present in the crystal. In 30 ppm Al doped CdZnTe, however, the carrier mobility was significantly degraded due to scattering of the ionized centers attributed to the aluminum interstitial Ali.

Effects of Te Inclusions on the Performance of CdZnTe Radiation Detectors

Te inclusions existing at high concentrations in CdZnTe (CZT) material can degrade the performance of CZT detectors. These microscopic defects trap the free electrons generated by incident radiation, so entailing significant fluctuations in the total collected charge and thereby strongly affecting the energy resolution of thick (long-drift) detectors. Such effects were demonstrated in thin planar detectors, and, in many cases, they proved to be the dominant cause of the low performance of thick detectors, wherein the fluctuations in the charge losses accumulate along the charge's drift path. We continued studying this effect using different tools and techniques. We employed a dedicated beam-line recently established at BNL's National Synchrotron Light Source for characterizing semiconductor radiation detectors, along with an IR transmission microscope system, the combination of which allowed us to correlate the concentration of defects with the devices' performances. We present here our new results from testing over 50 CZT samples grown by different techniques. Our goals are to establish tolerable limits on the size and concentrations of these detrimental Te inclusions in CZT material, and to provide feedback to crystal growers to reduce their numbers in the material.

Hand-Held Gamma-Ray Spectrometer Based on High-Efficiency Frisch-Ring CdZnTe Detectors

Frisch-ring CdZnTe detectors have demonstrated both good energy resolution, <1% FWHM at 662 keV, and good efficiency in detecting gamma rays, highlighting the strong potential of CdZnTe materials for such applications. We are designing a hand-held gamma-ray spectrometer based on Frisch-ring detectors at Brookhaven National Laboratory. It employs an 8 times 8 CdZnTe detector array to achieve a high volume of 19.2 cm3, so greatly improving detection efficiency. By using the front-end application-specific integrated circuits (ASICs) developed at BNL, this spectrometer has a small profile and high energy-resolution. It includes a signal processing circuit, digitization and storage circuits, a high-voltage module, and a universal serial bus (USB) interface. In this paper, we detail the system's structure and report the results of our tests with it.

Chemical mechanical polishing and nanomechanics of semiconductor CdZnTe single crystals

(1 1 1), (1 1 0) Cd0.96Zn0.04Te and (1 1 1) Cd0.9Zn0.1Te semiconductor wafers grown by the modified vertical Bridgman method with dimensions of 10 mm × 10 mm × 2.5 mm were lapped with a 2–5 µm polygonal Al2O3 powder solution, and then chemically mechanically polished by an acid solution having nanoparticles with a diameter of around 5 nm, corresponding to the surface roughnesses Ra of 2.135 nm, 1.968 nm and 1.856 nm. The hardness and elastic modulus of (1 1 1), (1 1 0) Cd0.96Zn0.04Te and (1 1 1) Cd0.9Zn0.1Te single crystals are 1.21 GPa, 42.5 GPa; 1.02 GPa, 44.0 GPa; and 1.19 GPa, 43.4 GPa, respectively. After nanocutting is performed by the Berkovich nanoindenter, the surface roughness Ra of the (1 1 1) Cd0.9Zn0.1Te single crystal attains a 0.215 nm ultra-smooth surface. The hardness and elastic modulus of three kinds of CdZnTe single crystals decrease with the increase of indentation load. When the nanoindenter departs the surface of the crystals, the adherence effects are obvious for the three kinds of single crystals. This is attributed to the plastic sticking behavior of CdZnTe material at a nanoscale level. When the indentation load of the three kinds of CdZnTe single crystals is in the range of 4000–12 000 µN, the adhered CdZnTe material on the nanoindenter falls onto the surface and accumulates around the nanoindentation.

Identification of deep trap levels from thermally stimulated current spectra of semi-insulating CdZnTe detector material

Deep trap levels in the semi-insulating (SI) CdZnTe detector material were characterized by simultaneous multiple peak analysis based on thermally stimulated current (TSC) measurements. In our TSCs that have been published previously electron hole pairs were created through the use of proton beam irradiation. Charge carriers were captured in deep traps and afterward released by thermal emission, which was recorded in the 90–300 K range. We showed that the obtained TSC spectra could be well fitted with a unique set of 14 different deep traps, which were all simultaneously and completely characterized. The obtained trap data are in good accordance with earlier deep trap characterizations of the other authors obtained on similar SI CdZnTe materials using different methods.

Surface leakage current control with heterojunction-type passivation in semi-insulating CdZnTe material

Inter-pixel resistance in the planar structure X-ray or gamma-ray detector has severe effects on signal cross-talk and the signal integration time. Generally, Br-MeOH etching in Cd-based compounds leaves a Te-rich or Cd-rich surface resulting from selective etching. The etching then converts the Te-rich or Cd-rich surface in air into TeO 2 or CdO oxides, which exhibit low resistance. To reduce the surface recombination, we adopted (NH 4) 2S for surface passivation in the polycrystalline CdZnTe:Cl grown by thermal evaporation method. The optimization of passivation was confirmed by I–V measurement and X-ray photoemission spectroscopy (XPS) analysis. From the I–V curve, we confirmed that the surface resistance was considerably increased after passivation with (NH 4) 2S. XPS data showed that (NH 4) 2S passivation removed conductive TeO 2 layers and induced formation of insulating CdTeO 3 and CdS layers. A heterojunction by the thin CdS layer and CdZnTe was formed.

Synchrotron X-ray Based Characterization of CdZnTe Crystals

Synthetic CdZnTe (CZT) crystals can be used for the room temperature-based detection of gamma radiation. Structural/morphological heterogeneities within CZT, such as secondary phases (namely, precipitates and inclusions), can negatively affect detector performance. We used a synchrotron-based x-ray technique, specifically extended x-ray absorption fine-structure (EXAFS) spectroscopy, to determine whether there are differences on a local structural level between intact CZT of high and low radiation detector performance. These studies were complemented by data on radiation detector performance and transmission infrared (IR) imaging. The EXAFS studies revealed no detectable local structural differences between the two types of CZT materials.

Cumulative effects of Te precipitates in CdZnTe radiation detectors

High-quality radiation detector-grade CdZnTe material is free from large-scale defects, such as grain boundaries, twins, and large Te or Cd inclusions (>50 μm), although it usually contains high concentrations of uniformly distributed Te inclusions and precipitates, typically of ∼20-μm-diameter size or smaller. We address the effects of the small-size Te precipitates on charge collection in CZT detectors, the significance of which is not yet well characterized. The strong correlation that we earlier found between the high-resolution X-ray maps and IR images proved that even small Te precipitates can trap substantial fractions of charge from the electron cloud. In this work, we modeled the transport of an electron cloud across idealized CZT devices containing Te precipitates to demonstrate that their cumulative effect can explain the degradation of energy resolution and the detection efficiency losses observed in actual CZT devices. Due to lack of experimental data on how the Te precipitates interact with an electron cloud, we developed a simplified (phenomenological) model based on the geometrical aspects of the problem. Despite its simplicity, the model correctly reproduced many experimental facts and gave quantitative predictions on the extent to which the presence of Te precipitates and inclusions can be tolerated. The broadening of the electron cloud due to repulsion and diffusion is at the core of the problem, making even low concentrations of small precipitates important in the device's performance.

Temperature Study of CdZnTe Coplanar-Grid Detectors

The coplanar-grid (CPG) and other electron-only detection techniques have made possible the use of CdZnTe-based detectors for gamma-ray spectroscopy when high efficiency, good energy resolution, and near room temperature operation are required. Despite the demonstrated potential of the technologies, widespread use remains hampered in part by the limited availability of the highly uniform CdZnTe material required for high-resolution spectroscopy. However, it has been recently shown that mild cooling of CdZnTe CPG detectors can result in a significant improvement in the energy resolution of the detectors thereby allowing a wider range of material to be used for high-resolution applications. In this paper, we show that improved spectroscopic performance can consistently be achieved through a combination of detector cooling and increased detector bias. Energy resolutions of about 1% FWHM at 662 keV for detector volumes up to 2.3 cm <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^3$</tex> have been obtained at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$-20^circ$</tex> C. With the electronic noise subtracted, this amounts to an intrinsic resolution of 0.8%. We also show that further cooling of the detectors to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$-30^circ$</tex> C leads to field polarization and a loss of spectroscopic performance.

Examination of the effects of high-density plasmas on the surface of HgCdTe

High-density argon-hydrogen plasmas have been demonstrated to be very effective as etchants of CdTe, CdZnTe, and HgCdTe materials for focal plane array applications. Understanding the physical, chemical, and electrical characteristics of these surfaces is critical in elucidating the mechanisms of processing Hg1−xCdxTe. The ways in which these plasmas interact with HgCdTe, such as etch rates and loading, have been studied.1–11 However, little is known on how these plasmas affect the first few atomic layers of HgCdTe. In this study, the effects of high-density plasmas on the surface of HgCdTe were examined. The combination of argon and hydrogen plasma etch leaves a well-ordered, near-stoichiometric surface determined by both x-ray photoelectron spectroscopy and reflection high-energy electron diffraction (RHEED). Starting with Hg0.78Cd0.22Te, we were able to produce surfaces with x=0.4 and a RHEED pattern sharp enough to measure 2×1 reconstruction.

Performance characteristics of Frisch-ring CdZnTe detectors

The performance characteristics of Frisch-ring CdZnTe (CZT) detectors are described and compared with other types of CZT devices. The Frisch-ring detector is a bar-shaped CZT crystal with a geometrical aspect ratio of /spl sim/1:2. The side surfaces of the detector are coated with an insulating layer followed by a metal layer deposited directly upon the insulator. The simple design operates as a single-carrier device. Despite the simplicity of this approach, its performance depends on many factors that are still not fully understood. We describe results of testing several detectors fabricated from CZT material produced by different vendors and compare the results with numerical simulations of these devices.

High-energy X-ray diffraction and topography investigation of CdZnTe

High-energy transmission x-ray diffraction techniques have been applied to investigate the crystal quality of CdZnTe (CZT). CdZnTe has shown excellent performance in hard x-ray and gamma detection; unfortunately, bulk nonuniformities still limit spectroscopic properties of CZT detectors. Collimated high-energy x-rays, produced by a superconducting wiggler at the National Synchrotron Light Source’s X17B1 beamline, allow for a nondestructive characterization of thick CZT samples (2–3 mm). In order to have complete information about the defect distribution and strains in the crystals, two series of experiments have been performed. First, a monochromatic 67 keV x-ray beam with the size of 300×300 µm2 was used to measure the rocking curves of CZT crystals supplied by different material growers. A raster scan of a few square centimeter area allowed us to measure the full-width at half-maximum (FWHM) and shift in the peak position across the crystal. The rocking curve peak position and its FWHM can be correlated with local stoichiometry variations and other local defects. Typically, the FWHM values ranging from 8.3 arcsec to 14.7 arcsec were measured with the best crystal used in these measurements. Second, transmission white beam x-ray topography (WBXT) was performed by using a 22 mm×200 µm beam in the energy range of 50 keV to 200 keV. These types of measurements allowed for large area, high-resolution (50 µm) scans of the samples. Usually, this technique is used to visualize growth and process-induced defects, such as dislocations, twins, domains, inclusions, etc. the difference in contrast shows different parts of the crystal that could not be shown otherwise. In topography, good contrast is indicative of a high quality of the sample, while blurred gray shows the presence of defects. Correlation with other techniques (e.g., infrared (IR) mapping and gamma mapping) was also attempted. Our characterization techniques, which use highly penetrating x-rays, are valid for in-situ measurements, even after electrical contacts have been formed on the crystal in a working device. Thus, these studies may lead to understanding the effects of the defects on the device performance and ultimately to improving the quality of CZT material required for device fabrication. It is important to study crystals from different ingot positions (bottom, center, and top); consequently, more systematic studies involving scans from center to border are planned.

Gamma spectroscopy with pixelated CdZnTe detectors

Excellent spectroscopy for higher energy gamma-rays is obtained with thick pixelated CdZnTe detectors, despite the large hole trapping inherent in the CdZnTe material. Our detector design was assisted by means of a theoretical Monte-Carlo code for optimizing the pixelated detector geometry for obtaining pad signals with minimum dependence on the depth of gamma interaction. We present spectra obtained for the 662-keV line of /sup 137/Cs taken with a 1 cm/spl times/1 cm/spl times/1 cm CdZnTe detector with the anode segmented to a 4/spl times/4 array of pad electrodes. For each gamma interaction, signals for all 16 pad of the electrodes and for the common cathode electrode were digitized to enable further correction for the depth of gamma interaction and for the reconstruction of Compton scatter interactions. For single pixel events, the summed pad spectrum yields an energy resolution of 1.0% FWHM. For the reconstruction of Compton events with signals in two pad electrodes, we obtain a summed peak with an energy resolution of 1.4% FWHM.

CdZnTe and CdTe materials for X‐ray and gamma ray radiation detector applications

AbstractGood detection efficiency and high energy‐resolution make Cadmium Zinc Telluride (CdZnTe) and Cadmium Telluride (CdTe) detectors attractive in many room temperature X‐ray and gamma‐ray detection applications such as medical and industrial imaging, industrial gauging and non‐destructive testing, security and monitoring, nuclear safeguards and non‐proliferation, and astrophysics. Advancement of the crystal growth and device fabrication technologies and the reduction of bulk, interface and surface defects in the devices are crucial for the widespread practical deployment of Cd1−xZnxTe‐based detector technology. Here we review the effects of bulk, interface and surface defects on charge transport, charge transport uniformity and device performance and the progress in the crystal growth and device fabrication technologies aiming at reducing the concentration of harmful defects and improving Cd1−xZnxTe detector performance. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)