The availability of large-size, detector-grade CdZnTe crystals in large volume and at affordable cost is a key to the further development of radiation-detector applications based on this II-VI compound. The high pressure Bridgman technique that supplies the bulk of semiinsulating CdZnTe crystals used in X-ray, γ-ray detector and imaging devices at present is hampered by material issues that limit the yield of large-size and high-quality crystals. These include ingot cracking, formation of pipes, material homogeneity and the reproducibility of the material from growth to growth. The incorporation of macro defects in the material during crystal growth poses both material quality limitations and technological problems for detector fabrication. The effects of macro defects such as Te inclusions and pipes on the charge-transport properties of CdZnTe are discussed in this paper. Growth experiments designed to study the origin and formation of large defects are described. The importance of material-crucible interactions and control of thermodynamic parameters during crystal growth are also addressed. Opportunities for growth improvements and yield increases are identified.

A comparative analysis of different Cd1−xZnxTe (CZT) crystals grown by a vertical high pressure Bridgman (VHPB) method is reported. The results of several analytical techniques, such as triple axis x-ray diffraction (TAD), rocking curves, low temperature photoluminescence (PL), scanning electron microscopy (SEM), and proton induced x-ray emission (PIXE) are discussed. Segregated carbon inclusions were identified by SEM and energy dispersive x-ray fluorescence (EDXRF), and the results are interpreted in terms of constitutional supercooling of the solidifying CZT melt. The carbon inclusions significantly decrease the resistivity, and in some cases, the noise is too large for the fabrication of radiation detectors. Combining these results with other measurements reported by our research team [1], we identify correlations between the growth and the defects identified in these CZT crystals.

We have completed a series of experiments where we realized Cd(1−x)ZnxTe by modified Vapor Phase Growth in which the CdTe and ZnTe components were separated in the growth ampoule, with their vapors passing through channels and openings designed to mix them stoichiometrically at the growth surface. We have obtained polycrystalline CZT boules 50 mm in diameter and 10 mm high whose structure is strikingly different from that obtained by standard High Pressure Bridgman methods. Our mVPG properties relevant for detector applications include resistivity of∼40 GΩ-cm when voltage is first applied, rising to ∼66 GΩ-cm after ∼20 min when the traps are filled, and electron mobility-lifetimes of∼2 × 10−3 cm2/ν. Here we present some structural and operational characterizations of our mVPG CZT.

Cadmium zinc telluride crystals were grown by vertical Bridgman processes using in situ compounding from high purity elements into pyrolytic boron nitride crucibles within sealed fused quartz ampoules containing cadmium vapor at a pressure of roughly one atmosphere. These conditions produce material having the low etch pit density, low precipitate density, high infrared transmission and high purity required for use as substrates for infrared focal plane detector arrays fabricated in epitaxial mercury cadmium telluride. Similar processes should be satisfactory for producing cadmium zinc telluride for gamma ray detectors.

Grain boundaries and twin boundaries in commercial Cd1−xZnxTe, which is prepared by a high-pressure Bridgeman technique, have been investigated with transmission electron microscopy, scanning electron microscopy, infrared-light microscopy and visible-light microscopy. Boundaries inside these materials were found to be decorated with Te precipitates. The shape and local density of the precipitates were found to depend on the particular boundary. For precipitates that decorate grain boundaries, their microstructure was found to consist of a single, saucer-shaped grain of hexagonal Te (space group P3121). Analysis of a Te precipate by selected-area diffraction revealed the Te to be aligned with the surrounding Cd1−xZnxTe grains. This alignment was found to match the (111) Cd1−xZnxTe planes with the (0111) planes of hexagonal Te. Crystallographic alignments between the Cd1−xZnxTe grains were also observed for a high-angle grain boundary. The structures of the grain boundaries and the Te/C1−xZnxTe interface are discussed.

Cadmium Zinc Telluride (CZT) shows great promise as a semiconductor radiation detector material. CZT possesses advantageous material properties over other radiation detector materials in use today, such as a high intrinsic resistivity and a high cross-section for x and γ-rays. However, presently available CZT is not without limitations. The hole transport properties severely limit the performance of these detectors, and the yield of material possessing adequate electron transport properties is currently much lower than desired. The result of these material deficiencies is a lack of inexpensive CZT crystals of large volume for several radiation detector applications. One approach to help alleviate this problem is to measure the spatial distribution (or map) the electrical properties of large area CZT wafers prior to device fabrication. This mapping can accomplish two goals: identify regions of the wafers suitable for detector fabrication and correlate the distribution of crystalline defects with the detector performance. The results of this characterization can then be used by the crystal manufacturers to optimize their growth processes. In this work, we discuss the design and performance of apparatus for measuring the electrical characteristics of entire CZT wafers (up to 10 cm × 10 cm). The data acquisition and manipulation will be discussed and some typical data will be presented.

For epitaxial growth of compound Zn1−xCd1−XTe by metal organic chemical vapor deposition (MOCVD), it is difficult to obtain a high composition ratio x. In this study, we have adopted a remote plasma enhanced (RPE) MOCVD method for the epitaxial growth. Cd1−xZnxTe with the composition ratio x in the range of 0 to 1 has been obtained while varying the ratio of dimethylcadmium (DMCd) to diethylzinc (DEZn) from 0 to 20%. The crystallinity of the epitaxial films was about 400 to 700 arcsec FWHM defined by X ray diffiraction measurements.

The internal electric field distributions of the CdZnTe (CZT) detectors under bias were characterized by optical polarized transmission at a 952 nm illumination utilizing the Pockels electro-optic effect. Two-dimensional (2D) images mapping the internal electrical field intensity changes were obtained to study the performance of CZT room-temperature radiation detectors. Planar and a P-I-N structured CZT detectors were investigated under different operating bias voltages. Analysis of optical profiles from a planar single crystal detector provides a quantitative nondestructive description of the electric field or voltage distributions inside a radiation detector. The P-I-N structured CZT detector showed a nearly uniform electric field in a width which varied with the operating bias voltage. An energyband model of a semiconductor junction with a depletion layer was employed to understand the results.

We have used low-temperature photoluminescence spectroscopy and photo-induced current transient spectroscopy to study the properties of copper-doped Cd1−xZnxTe with x=0.1 and chlorine-doped Cd1−xZnxTe with x=0.2, 0.35 and 0.5. The current-voltage characteristics and detector response were also measured. We observed variations in charge collection and resistivity in the Cu-doped samples which was correlated with variations in PICTS spectra. The Cl-doped material was found to have insufficient resistivity for detector operation.

The effect of surface oxidation of Cd1−xZnxTe (x = 0.1) detectors by chemical etching in hydrogen peroxide aqueous solution (H2O2) at different concentrations and etching times was investigated by low temperature photoluminescence (PL). The treatment resulted in better surface condition evidenced by the larger I(D0, X)/Idef intensity ratio and the narrower full width at half maximum of the main peak (D0, X). Peak shifts in the PL spectra associated with bound exciton lines and free to bound transition were also observed and attributed to the oxide layer. These surface effects were found to be dependent on H2O2 concentration and etching time. The significance of this surface oxidation on device passivation and the subsequent improvements in the detector performance are also discussed.

Cadmium zinc telluride (CdZnTe) is an emerging material for room-temperature x-ray and gamma ray detectors. The identification and control of point defects and charge compensators are currently important issues. Low-temperature photoluminescence (PL) and electron paramagnetic resonance (EPR) spectroscopies have been used to characterize point defects in CdZnTe crystals grown by the high-pressure Bridgman technique. Luminescence due to shallow donors, shallow acceptors, and deeper acceptors was monitored for a series of samples. An isotropic EPR signal attributed to shallow hydrogenic donors is observed in all samples, and the concentration of shallow donors has been determined. The nature of the defect centers (impurities, vacancies, vacancy-impurity complexes), and the correlation between defect concentration and device performance is discussed.

The electrical and charge collection properties of a semiconductor detector play an important role in a spectrometer's final performance. However, the studies of these properties often concentrate on only a few samples. In this work over 100 CdZnTe detectors from 12 different growth boules were characterized with one of the following test methods. The composition uniformity was evaluated with low temperature photoluminescence (PL) measurements. From the current-voltage characteristics the differences in CdZnTe detector resistivities were investigated. Charge collection properties, μτ-products, and energy resolutions were characterized with spectroscopic methods using an alpha and isotopic sources. A wide selection of test results are presented indicating the variety of CdZnTe material.

The understanding of the electric contact formation process is critical in exploiting the full potential of Cd1−xZnxTe(CZT) material for room temperature detection applications. The metalsemiconductor electrical characteristics were shown in this study to be strongly affected by the surface preparation steps prior to metallization (polishing, and chemical etching), the choice of the metal and contact deposition technique, and by the subsequent surface passivation of CdZnTe. In this paper, we also present the implementation new detector fabrication processing stepsresulting in a significant lowering of the dark leakage current and an improvement in the detector performance.

This paper describes some novel techniques developed for directly measuring the electron mobility μe and mean free drift time Te in wide band gap semiconductors. These methods are based on a newly-developed digital data analysis system, in conjunction with single carrier charge sensing and depth sensing techniques. Compared with conventional methods, the new techniques are easier to implement, do not involve curve fitting, allow the use of high energy γ-rays and are not sensitive to variations in pulse rise time.

The control of the concentration of Zn and its fluctuation in the high pressure Bridgman grown CdZnTe crystals is part of our characterization work on the ternary grown ingots grown in house. In order to reach both high sensitivity and high position resolution, we have developed a new system consisting of a X-ray generator, coupled to a focusing X-ray capillary, delivering intense beams in the micron scale, since the intensity gain is around a factor of 100 compared to conventional methods.

The characteristic X-rays are measured through a high resolution CdZnTe detector (225 eV at 5.9 keV FWHM) cooled by a Peltier system. The results of our investigations on different kinds of crystals will be discussed.

Continued improvements in the manufacturing of Cd1−xZnxTe (CZT) material have resulted in a practical thermoelectrically cooled X-ray and gamma-ray detector of very high energy resolution. A high resolution spectroscopy system was used to measure the Fano factor in CZT at temperatures down to -40°C. The best resolution of the 5.9 keV 55Fe peak was measured to be 188 eV FWHM, while the best resolution of the 59.5 keV 241Am peak was measured to be 482 eV FWHM. The minimum measured Fano factor was 0.082, with several measurements yielding a value of 0.089±0.005. With a resolution of 4.2 keV FWHM for the 662 keV peak of 137Cs, these detectors demonstrate excellent performance in detecting X-rays and gamma rays.

We demonstrate that the regularization method due to Weese can resolve closely-spaced peaks in PICTS spectra for cadmium zinc telluride. We also show that electron and hole traps can be distinguished from each other by the bias dependence of the spectrum when using an excitation source that is primarily infrared but which contains a small component of visible light. Lastly we show that there are qualitative differences between PICTS spectra taken with infrared excitation and those taken with visible excitation. We attribute the surface-related levels to diffusion into the detector material of gold from electroless deposition of contacts.

Zinc composition variation and gross structural defects including grain and tilt boundaries, twins, and mechanical cracks in high pressure Bridgman Cd1−xZnxTe are characterized and correlated to various detector-related responses. Triple axis x-ray diffraction, double crystal x-ray topography, infrared microscopy, and etch pit density measurements are used to reveal and quantify the spatial distribution and the nature of the structural defects. Mechanical cracks in the material are found to act as conductive ‘shorting paths’, indicated by excessive leakage currents and reduced charge (electron) collection measured along these cracks. Reduced charge collection is also obtained across grain boundaries and in regions with poor crystallinity, indicating that they serve as carrier recombination sites. Finally, the effects of the zinc composition variation on the measured leakage current and the amount of electrons collected are found to be masked by gross structural defects. These characterization techniques provide a wealth of information which can be used not only to study the relationship between the structural and device properties of CdZnTe but also to screen production material for subsequent device fabrication.

The CZT boule was grown by a modified vertical Bridgman process using in-situ compounding, Cd over-pressure and a pyrolytic boron nitride crucible within a fused ampoule. During growth, the Cd vapor pressure was near 1 atmosphere. These growth conditions tend to give high purity, good stoichiometry, few precipitates, and dislocation densities in the low to mid-104 cm−2 range. The crystals, after polishing, were annealed in a nearly saturated Cd, Zn atmosphere to fill residual Cd-site vacancies and achieve high resistivity in the 1010 γ-cm range. Low temperature photoluminescence study shows very good crystalline quality and a very low concentration of deep level recombinations. Single crystal samples were diced into 1 cm squares for evaluation as gamma ray detectors. The best detector results (4.5% resolution at 60 keV) were achieved for a 2 hour anneal at 850°C.

CdTe and CdZnTe X-ray and gamma ray detectors in the form of single elements or as monolithic segmented arrays have been shown to be useful in imaging systems utilized in medical, research or industrial applications. These detectors possess inherently better energy resolution than scintillators coupled to either photodiodes or photomultipliers, and they may lead to compact imaging systems or to imaging systems of enhanced spatial resolution and better contrast resolution. Photopeak efficiencies of these detectors is greatly affected by relatively low hole mobility-lifetime product and therefore continuing efforts are still underway to improve the characteristics of both CdTe and CdZnTe materials in order to achieve reproducible detectors with higher photopeak efficiencies for either low or high energy gamma rays.

The following paper is divided into three parts: The first part compares the characteristics of planar CdTe and CdZnTe single elements nuclear detectors containing metal contacts. Characteristics include: Charge collection efficiencies for both electrons and holes indicated by the mobility-lifetime product, energy resolutions, leakage currents and robustness in field use. The second part describes excellent spectroscopic results using a lcm thick CdZnTe monolithic segmented pad detector array. This part also compares spectra for various gamma energies obtained by this segmented detector to that of a 1 cm 3 detector acting as a single element planar detector. The third part discusses the characteristics of a new generation nuclear gamma camera for medical diagnostics based on room-temperature CdTe and CdZnTe spectrometers and its advantages over an Anger type scintillating nuclear camera.