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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. Index Terms--CdZnTe, crystal defects, 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. Index Terms--CdZnTe, radiation detectors, Te inclusions.

We studied the effects of small, Index Terms--CdZnTe, semiconductor detectors, Te inclusions.

High-performance semi-insulating single crystals of n-type [Cd.sub.0.9][Zn.sub.0.1] Te (CZT) were grown using the traveling heater method (THM). X-ray and [gamma]-ray detector configurations fabricated from this material have a room-temperature mean energy resolution of 4.3% FWHM for a [sup.57]Co source (122 keV) and uniform pixel-to-pixei response on monolithic 20 x 20 x 5 [mm.sup.3] pixeilated detectors. Energy resolution of Index Terms--CdZnTe, CZT, homeland security, medical imaging, Optical Deep Level Transient Spectroscopy (ODLTS), pixellated detectors, Te inclusion, Traveling Heater Method.

Commercial 15 x 15 x 7.5 [mm.sup.3] coplanar grid CdZnTe detectors were studied on the micron-scale using a collimated high-energy X-ray beam provided by Brookhaven's National Synchrotron Light Source. This powerful tool enables simultaneous studies of detector response uniformity, electronic properties of the material, and effects related to the device's contact pattern and electric field distribution. The availability of a front-end Application Specific Integrated Circuit, developed at Brookhaven's Instrumentation Division, providing low noise amplification of grids and cathode signals, corresponding timing signal and adjustable relative gain, allowed to correlate performance mapping and fluctuations in collected charge. We observed the effect of the strip contacts comprising the coplanar grids on the energy resolution of the coplanar-grid device.

The dependence of the energy gap in Bridgman-grown [Cd.sub.1-x][Mn.sub.x]Te crystals on the Mn mole fraction and temperature is measured. New equations are formulated that have expressed the energy gap as a function of Mn mole fraction and temperature, where the energy gap has increased linearly with an increase in the Mn content in the crystal and with a decrease in temperature.

The analysis of photoconductive decay curves has helped in comparing the surface recombination velocities of semi-insulating CdMnTe:In crystals grown by the vertical Bridgman method with or without surface passivation. The studies have shown that not only the size but also the spatial configuration of the Te precipitates has modulated the carrier-transport properties.

Larger volume cadmium zinc telluride (CZT) crystals are developed by using high-resolution radiation detectors based on the traveling heater method. The results have shown that the theoretical energy resolution limit can be achieved with careful material selection by using infrared and high-quality fabrication processes.

We grew Cd 1− x Mn x Te crystals with a nominal Mn concentration of 5% by the vertical Bridgman growth technique. The structural quality of the crystal was evaluated by white beam X-ray topography in the National Synchrotron Light Source (NSLS) facility at Brookhaven National Laboratory (BNL). We observed that the crystal was free from a sub-grain boundary network, as revealed by X-ray topography and verified by our etching study. The concentration of the secondary phases, averaged over the entire ingot, was 2–3 times lower than in conventional Bridgman grown cadmium zinc telluride (CZT) crystals. [ABSTRACT FROM AUTHOR]

Abstract: Material homogeneity is critical in achieving high-performance in all types of radiation detectors. This requirement is not inevitably satisfied in today's commercial detector-grade CdZnTe (CZT) material because it contains high concentrations of extended defects, in particular, Te inclusions, dislocation networks, and twin- and subgrain-boundaries that affect the energy resolution and the efficiency of the devices. Defects, such as grain boundaries and cracks that completely block charge-carrier transport are impermissible in CZT radiation-detectors at concentrations exceeding certain threshold values. Our group in Brookhaven National Laboratory (BNL) conducts systematic studies, detailing the roles of crystal defects in CZT detectors and the mechanisms underlying their formation and effects. We employ infrared transmission microscopy, white beam X-ray diffraction topography, and high-spatial-resolution X-ray response mapping to identify particular types of defects and reveal their relationship with the devices' performances. In this article, we summarize some of the most important results that our group obtained over the past 5 years. [Copyright &y& Elsevier]

Abstract: In an ideal single-carrier-type gamma ray detector, the amplitudes of the signals and the carrier drift times are correlated variables. However, if the charges produced by an incident photon are not fully collected, as is the case in CdZnTe detectors containing crystal defects, the above correlation does not hold. This permits the application of an event recognition algorithm to identify these incomplete charge collection (ICC) events, caused by the “bad” regions inside a detector, so that they can be removed from pulse height spectra. The ICC events primarily contribute to the Compton continuum and the low-energy tail of the photopeak. Thus, rejecting such events should not affect significantly the photopeak efficiency, but should improve the spectral response, e.g., the peak-to-Compton ratio, for a detector fabricated from material with relaxed crystal quality requirements. Such crystals are those currently available from vendors. The use of stronger ICC correlation-function rejection criteria can improve the energy resolution of these lower-quality crystals, but at the price of a loss in photoefficiency. [Copyright &y& Elsevier]

In this letter, we have designed and fabricated low-loss ultrasmall corner mirrors and T-branches on thin silicon-on-insulator material system. The measured performance of the devices agrees with simulations using finite-difference time-domain calculations. These devices demonstrate the potential of this high-refractive-index-contrast material system for high-density planar optical integrated circuits [ABSTRACT FROM PUBLISHER]

The availability of large volume crystals with the same energy gap in melt-grown CdZnTe (CZT) is restricted due to the Zn segregation in CdTe hosts. We observed the migration of Zn in the solid phase along the positive temperature gradient direction both in situ and post-growth temperature gradient annealing (TGA) of CZT. Diffusivity of Zn obtained from the in situ TGA was approximately 10 −5 cm 2 /s order and completely different mechanism with that of post-growth. The CZT ingots obtained through in situ TGA have uniform Zn and resistivity of 10 10 Ω cm orders. The CZT detectors fabricated from in situ TGA applied ingots exhibit 10% of energy resolution for 59.5 keV peak of 241 Am. [ABSTRACT FROM AUTHOR]

We grew CdTe x Se 1− x crystals with nominal Se concentrations of 5%, 7%, and 10% by the vertical Bridgman technique, and evaluated their compositional homogeneity and structural quality at the NSLS’ X-ray fluorescence and white beam X-ray topography beam lines. Both X-ray fluorescence and photoluminescence mapping revealed very high compositional homogeneity of the CdTe x Se 1− x crystals. We noted that those crystals with higher concentrations of Se were more prone to twinning than those with a lower content. The crystals were fairly free from strains and contained low concentrations of sub-grain boundaries and their networks. [ABSTRACT FROM AUTHOR]

Abstract: The origin and nature of star-shaped prismatic punching defects (PPDs) were analyzed by white-beam X-ray diffraction topography (WBXDT) and photoluminescence (PL) measurements. PPDs were observed in CdTe-based compounds that had Te-rich secondary phase defects and were annealed only under Cd overpressure. The evolutional features of the PPDs during annealing were similar to those of a diffusion process, that is, the in-diffusion of Cd vapor via Cd vacancies. PL mapping around the PPDs reveals that the stoichiometry of PPDs is the same as that of a normal matrix, and that PPDs are just structural defects with non-radiative emission induced by the exothermic reaction between Cd and Te. We find that Te-rich secondary phase defects can be removed without the generation of star-shaped PPDs, if the exothermic reaction rate can be slowed down to reduce the thermal gradient between the Te-rich region and the surrounding matrix. [Copyright &y& Elsevier]

Abstract: We characterized the structural quality of CdTe x Se1−x crystals grown by the Traveling Heater Method (THM) from a Te-rich solution using Synchrotron White Beam X-ray Diffraction Topography in the reflection mode. Structural defects were also studied by chemical etching of the crystal surfaces. The crystals were found to be fairly free from strains, and they had very few sub-grain boundaries and dislocation/sub-grain boundary networks. [Copyright &y& Elsevier]

We grew CdTe x Se1−x (CTS) crystals from a Te-rich solution by using the traveling heater method (THM). The average size distribution and concentration of Te inclusions/precipitates in these as-grown samples were measured to be ~7×104 cm−3, which is much lower than values typical for the present state-of-the-art commercial CdZnTe (CZT) material. Their low-temperature photoluminescence measurement indicates high quality of the material; however the resistivity obtained via I–V curve measurements was ~5×108 Ω-cm, which is low in comparison to that required for gamma detectors. A well-resolved alpha response peak can be detected for both planar- and hemispherical detector geometry. The (μτ)e value for our as-grown sample at room temperature was found to be ~4×10−3 cm2/V. [ABSTRACT FROM AUTHOR]