CdZnTe Crystals Research Articles

Synchrotron radiation studies of spectral response features caused by Te inclusions in a large volume coplanar grid CdZnTe detector

We report preliminary results from a synchrotron radiation study of Te inclusions in a large volume single crystal CdZnTe (CZT) coplanar-grid detector. The experiment was carried out by probing individual inclusions with highly collimated monochromatic X- and γ-ray beams. It was found that for shallow X-ray interaction depths, the effect of an inclusion on measured energy loss spectra is to introduce a ∼10% shift in the peak centroid energy towards lower channel numbers. The total efficiency is however not affected, showing that the net result of inclusions is a reduction in the Charge Collection Efficiency (CCE). For deeper interaction depths, the energy-loss spectra shows the emergence of two distinct peaks, both downshifted in channel number. We note that the observed spectral behaviour shows strong similarities with that reported in semiconductors, which exhibit polarization effects, suggesting that the underlying mechanism is common.

Response function of the CdZnTe SPEAR detector

The response function of the SPEAR CdZnTe detector developed by eV Microelectronics was studied in detail. This compact detector includes a CdZnTe crystal and a preamplifier. It is operational at room temperature. The energy range of the detector is between 10 keV and 1 MeV, and its relative resolution is less than 4% at 122 keV. The goal of our study was to establish an empirical analytical approximation of the response function to be used in Monte Carlo simulations (e.g. for spectrum unfolding). Various radionuclides were used for the study. The energy spectrum obtained from the detector not only showed smooth peaks but also had a tail on the low‐energy side of the peak. This tail part was more prominent at higher energies. By using the experimental spectrum for various nuclides such as 241Am, 109Cd, 57Co and 137Cs, the parameters required to replicate the experimental response with a peak plus exponential model were evaluated. Copyright © 2011 John Wiley & Sons, Ltd.

Effect of sub-bandgap illumination on the internal electric field of CdZnTe

Post-growth manipulation of the internal electric field in CdZnTe crystals using sub-bandgap illumination is measured as a function of temperature through infrared (IR) transmission measurements. Using near sub-bandgap IR illumination, both the optical de-trapping of charge carriers and the reduction in carrier recombination increased the mobility lifetime in the crystal. The increased carrier transport is a direct result of decreased hole and electron trapping in addition to other underlying mechanisms. Concentration of the electric field near the cathode is also observed. We measured the electric field distribution with sub-bandgap illumination as a function of temperature via the Pockels effect.

Growth and Characterization of CZT Crystals by the Vertical Bridgman Method for X-Ray Detector Applications

CdZnTe crystals were grown by the vertical Bridgman method in closed quartz ampoules. The crystalline quality and the impurity content of these crystals were studied. Several X-ray detectors were cut out of these crystals. The resistivity, emission spectra, μτ product, and spectroscopic characteristics of these detectors were extensively measured and compared with the characteristics of detectors obtained from CdZnTe crystals grown by the boron oxide encapsulated vertical Bridgman technique. The detectors prepared from crystals grown without boron oxide show good μτ value, spectroscopic resolution, and higher reproducibility. The influence of growth method on impurity content and on detector response was discussed.

The COCAE Detector: An Instrument for Localization—Identification of Radioactive Sources

The COCAE project develops an instrument for localization and identification of radioactive sources. For the localization task it will exploit the Compton scattered photons within its detecting layers. It is based on pixilated Cd(Zn)Te matrices in a stacked configuration. Progress has been achieved in all the components necessary for this technology. CdZnTe crystals have been grown of up to 75 mm in diameter. A wafer level pixelization process has been tested and its results will be presented. Angular resolution for directional identification of the order of 6 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> requires energy resolution better than 5%. For this reason we have investigated planar metal-p-n diodes fabricated by using laser-induced solid phase doping and Schottky diodes created by Ar-ion etching of the Cd(Zn)Te surfaces before deposition of Ni electrodes. The energy resolution achieved is better than 1% FWHM @662 KeV. The development of a pixel CMOS readout integrated circuit has been undertaken. It outputs in digital format address, energy and time information for the pixels which have collected charge above a given threshold. Simulation studies of the minimum detection limits, of the Compton sequencing algorithms, of the angular resolution and of the reconstruction of the radioactive source position have been performed. Experiments with a precursor setup using an existing hybrid detector are underway.

Ion Beam (RBS) and XRF Analysis of Metal Contacts Deposited on CdZnTe and CdTe Crystals

Rutherford Backscattering Spectrometry (RBS) using 6 MeV alpha particles and X-Ray Fluorescence (XRF) with a Pd-anode X-Ray generator were performed to characterize Au and Pt contacts deposited by electroless technique and thermal evaporation on differently treated surfaces of CdZnTe and CdTe crystals. The aim of this study is to understand and improve the structure of the material-electrode interface. The thickness, the stoichiometry and the concentration profiles of platinum, gold, cadmium, zinc, tellurium and oxygen present at the surface layers were determined. The distribution of Cd deficiency at the interface layers was profiled using simulations and showed complex profiles in the samples, which can greatly affect the electrical quality of the detectors.

Treatment of the surface of blanks for fabrication of CdZnTe-based detectors

The depth of the damaged layer forming on the surface of the CdZnTe crystals during standard mechanical treatment of them is determined by the microhardness measurement and chemical selective etching. The rates of chemical etching of the surface of the CdZnTe crystals in the solutions of bromine in methanol with various bromine concentrations are determined.

Effect of mechanical anisotropy on material removal rate and surface quality during polishing CdZnTe wafers

The mechanical characters of CdZnTe crystal were investigated by nanoscratch tests, and the effects of mechanical anisotropy on the material removal rate and surface quality were studied by polishing tests. There is a peak of frictional coefficient at the early stage of scratch, and increasing the vertical force will result in the increase of peak value correspondingly. The fluctuation phenomenon of frictional coefficient is generated at high vertical force. The lateral forces show the apparent twofold and threefold symmetries on (110) and (111) planes, respectively. To obtain high surface quality, low polishing pressure and hard direction (〈 \(\bar 1\)10 〉 directions on (110) plane and 〈 11\(\bar 2\) 〉 directions on (111) plane) should be selected, and to achieve high material removal rate, high polishing pressure and soft direction (〈 001 〉 directions on (110) plane and 〈 \(\bar 1\)2\(\bar 1\) 〉 directions on (111) plane) should be selected.

Photoconductivity Mapping of Semi-Insulating CdZnTe

Semi-insulating CdZnTe crystals were studied by photoconductivity mapping using both the contactless method and measurement with evaporated Au contacts. The contact quality was tested by measurement of voltampere characteristics by a three-point method. We evaluated the distribution of space charge analyzing the slope of lux-ampere characteristics. Mobility- lifetime (μτ) product maps were extracted fitting the Hecht relation to voltampere characteristics measured with a weak light at wavelength 750 nm. Correlation analysis of contactless resistivity and photoconductivity maps shows, that both these parameters are anticorrelated. This fact can be explained by a shift of Fermi level changing the average occupation of a midgap level. A decrease of occupation with an increasing resistivity results in an increase of electron trapping and decreased photoconductivity. This qualitative explanation is supported by simulating the dependence of the photocurrent density on the Fermi level position near the midgap using parameters of midgap levels assumed in state-of-the art radiation detectors. The map of electron mobility-lifetime product correlates with the maps of photocurrent intensity measured with and without the Au contact. The comparison of maps displaying slopes of lux-ampere characteristics with the map of μτ shows, that areas with a higher accumulation of space charge have a smaller μτ .

Flight performance of an advanced CZT imaging detector in a balloon-borne wide-field hard X-ray telescope—ProtoEXIST1

We successfully carried out the first high-altitude balloon flight of a wide-field hard X-ray coded-aperture telescope ProtoEXIST1, which was launched from the Columbia Scientific Balloon Facility at Ft. Sumner, New Mexico on October 9, 2009. ProtoEXIST1 is the first implementation of an advanced CdZnTe (CZT) imaging detector in our ongoing program to establish the technology required for next generation wide-field hard X-ray telescopes. The CZT detector plane in ProtoEXIST1 consists of an 8 x 8 array of closely tiled 2 cm x 2 cm x 0.5 cm thick pixellated CZT crystals, each with 8 x 8 pixels, covering a 256 cm^2 active area with 2.5 mm pixels. A tungsten mask, mounted at 90 cm above the detector provides shadowgrams of X-ray sources in the 30 - 600 keV band for imaging, allowing a fully coded field of view of 9 Deg x 9 Deg with an angular resolution of 20 arcmin. To reduce the background radiation, the detector is surrounded by semi-graded (Pb/Sn/Cu) passive shields on the four sides all the way to the mask. On the back side, a 26 cm x 26 cm x 2 cm CsI(Na) active shield provides signals to tag charged particle induced events as well as >~ 100 keV background photons from below. The flight duration was only about 7.5 hours due to strong winds (60 knots) at float altitude (38-39 km). Throughout the flight, the CZT detector performed excellently. The telescope observed Cyg X-1 for ~ 1 hour at the end of the flight. Despite a few problems with the pointing and aspect systems that caused the telescope to track about 6.4 deg off the target, the analysis of the Cyg X-1 data revealed an X-ray source at 7.2 sigma in the 30-100 keV energy band at the expected location from the optical images taken by the onboard daytime star camera. The success of this first flight is very encouraging for the future development of the advanced CZT imaging detectors (ProtoEXIST2, with 0.6 mm pixels).

On the Role of Boron in CdTe and CdZnTe Crystals

It is well known that group III elements act as donors if they play a substitutional role at the metallic site in II-tellurides; nevertheless, several studies report both on the creation of complexes with vacancies, named A-centers, and on the involvement in self-compensation mechanisms, especially for indium. The boron concentration in II-tellurides is negligible, and its contribution to transport mechanisms has not been studied yet. For the last few years the authors have been developing a new technique to grow CdZnTe by the vertical Bridgman technique, taking advantage of encapsulation by means of boron oxide. In this way, crystals characterized by large single grains, low etch pit density, and high resistivity have been obtained. Recently, x-ray detectors with state-of-the-art performance have been produced from such crystals. Boron contamination, as a consequence of this growth method, is quite low but at least one order of magnitude above values obtained with other growth techniques. Besides being a low-cost technique which is also suitable for large-scale mass production, the encapsulated vertical Bridgman technique is quite useful to prevent dislocations, grain boundaries, and stacking faults; for these reasons, careful characterization was performed to understand the effect of boron both on the electrical properties and on the spectroscopic performance of the final crystals. Our characterization is mainly based on low-temperature photoluminescence in addition to electrical current–voltage measurements, photostimulated current, and x-ray spectroscopy. The results indicate that boron behaves like other group III elements; in fact, boron forms a complex that does not affect the good performance of our x-ray detectors, even if it shows some properties which are typical of A-centers.

Effect of ionizing radiation in ultrasmall doses on dielectric properties of CdZnTe crystals with anomalously high polarizability

The effect of ultrasmall exposure doses (10–45 R) of gamma-radiation on both real and imaginary parts of the complex dielectric permittivity of CdZnTe crystals in the low-frequency range has been discovered. It is shown that the observed effect is related to (i) changes in the system of intrinsic structural defects, which cause a deviation of the crystal composition from stoichiometry and (ii) growth-related large-scale fluctuations of the electric potential.

Correlation between Te-rich Particles in CdZnTe Crystals and Corresponding PL Spectra

Micron-scale Te-rich particles presented in CdZnTe crystals were directly observed using infrared transmission microscopy(IRTM).The generation of various Te-rich particles was discussed in conjunction with the growth conditions.In order to evaluate the impurities and defects behaviors,and to examine the crystalline quality,photoluminescence(PL) spectra were obtained at 10 K.The corresponding resistivity was evaluated simultaneously.The correlation between different shaped Te-rich particles in CdZnTe crystals and the corresponding PL spectra at 10 K was concluded compared with the characterized photo-peak in PL spectra.Well-defined sphere,hexagonal and triangular shaped incoherent Te-rich particles were generally presented in CdZnTe crystals grown from the charge with excess Te,which the(D0,X) peak usually dominated in the PL spectra.Cross-shaped Te-rich particles were found in CdZnTe crystals grown under stoichiometric condition where the DAP peak dominated in the PL spectra.Star-shaped Te-rich particles were normally obtained in CdZnTe crystals under Cd-rich conditions where the FWHM of(D0,X) peak was wide.

Influence of Laser Radiation on Optical Properties and Surface Structure of CdZnTe Crystal

The change of surface of Cd0.9Zn0.1Te crystal morphology after irradiation by pulsed Nd:YAG laser has been studied. Thermogradient effect (TGE) has the main role in the interaction process of laser radiation with semiconductors. A change of the irradiated surface relief has been observed by atom force microscope. Self-organizing nano-cones are observed on the irradiated surface of Cd0.9Zn0.1Te crystal at intensity of the laser in the range of 4.0 – 12.0 MW/cm2. A graded band gap structure with optical window was formed at the top of nano-cones.

Effect of Te inclusions in CdZnTe crystals at different temperatures

CdZnTe crystals often exhibit nonuniformities due to the presence of Te inclusions and dislocations. High concentrations of such defects in these crystals generally entail severe charge-trapping, a major problem in ensuring the device’s satisfactory performance. In this study, we employed a high-intensity, high-spatial-resolution synchrotron x-ray beam as the ideal tool to generate charges by focusing it over the large Te inclusions, and then observing the carrier’s response at room- and at low-temperatures. A high spatial 5-μm resolution raster scan revealed the fine details of the presence of extended defects, like Te inclusions and dislocations in the CdZnTe crystals. A noticeable change was observed in the efficiency of electron charge collection at low temperature (1 °C), but it was hardly altered at room-temperature.

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).

Point Defects in CdZnTe Crystals Grown by Different Techniques

We studied, by current deep-level transient spectroscopy (I-DLTS), point defects in CdZnTe detectors grown by different techniques. We identified 12 different traps with energy levels from 7 meV to 1.1 eV. Although the levels of most of the identified defects were independent of the crystal growth techniques, nevertheless there were some associated differences in the traps energies and densities.

Size and distribution of Te inclusions in detector-grade CdZnTe ingots

AbstractTo observe the Te inclusions distribution along the axial direction of CdZnTe ingots, batches of as-grown detector-grade CdZnTe crystals grown by vertical Bridgman method, were investigated using IR transmission imaging. However, there is not a rigorous regularity for the micron-scale Te inclusions distribution along the axial direction of CdZnTe ingots according to the measurements of the present investigation. The size and distribution variations of Te inclusions are attributed to the growth velocity, furnace temperature situation and morphological changes at the liquid-solid interface during crystal growth. Electron mobility lifetime products were discussed in conjunction with Te inclusions. However, no strict correlations are identified between the sizes and concentrations of Te inclusions and the γ-ray spectroscopy response. Extended defects in CdZnTe crystals, which are not visible via IR transmission imaging, potentially attribute to CdZnTe detectors performance degrades.

Melt growth of high-resistivity CdZnTe crystals by controlling Cd over-pressures

In this study, Cd 1− x Zn x Te crystals, with x=0.20, were grown unseeded by closed-ampoule vertical directional solidification (Bridgman) technique under controlled Cd over-pressures. CdZnTe crystals of high electrical resistivity have been reproducibly obtained with the In dopant concentration of 4–6 ppm, atomic and a Cd reservoir temperature between 785 and 825 °C. A combination of three process modifications have contributed to the accomplishment: (1) homogenizing the ampoules of starting materials under the vacuum environment to minimize contamination, (2) performing growth under controlled Cd over-pressures with the optimal In dopant, and (3) controlling post-growth cooling procedures to maintain uniformity of electrical properties over the crystal.

Study of Structural Defects in CdZnTe Crystals by High Resolution Electron Microscopy

ABSTRACTWe investigated defects in CdZnTe crystals produced from various conditions and their impact on fabricated devices. In this study, we employed transmission and scanning transmission electron microscope (TEM and STEM), because defects at the nano-scale are not observed readily under an optical or infrared microscope, or by most other techniques. Our approach revealed several types of defects in the crystals, such as low-angle boundaries, dislocations and precipitates, which likely are major causes in degrading the electrical properties of CdZnTe devices, and eventually limiting their performance.