CdZnTe Single Crystals Research Articles

OPTIMIZATION OF CLINICAL RESEARCH AT MUCOSA-PERIOSTEAL INJURY

The actual problem of modern osteology as diagnosis of bone architectonics in the area of bone periosteum is presented in the article. On the base of clinical studies the architectonics features of bone structures after injury were revealed. The efficiency of the proposed technique by applying the Cd(Zn)Te single crystal as a filter for radiovisiography was established. The combination of radiovisiographic research and clinical observation as a single diagnostic process in dentistry was presented.

Machined immersion grating with theoretically predicted diffraction efficiency.

An immersion grating composed of a transmissive material with a high refractive index (n>2) is a powerful device for high-resolution spectroscopy in the infrared region. Although the original idea is attributed to Fraunhofer about 200 years ago, an immersion grating with high diffraction efficiency has never been realized due to the difficulty in processing infrared crystals that are mostly brittle. While anisotropic etching is one successful method for fabricating a fine groove pattern on Si crystal, machining is necessary for realizing the ideal groove shape on any kind of infrared crystal. In this paper, we report the realization of the first, to the best of our knowledge, machined immersion grating made of single-crystal CdZnTe with a high diffraction efficiency that is almost identical to that theoretically predicted by rigorous coupled-wave analysis.

Bulk Growth of CdZnTe: Quality Improvement and Size Increase

We report the bulk growth of single-crystal CdZnTe and characterization of material associated with large-area wafers produced from the CdZnTe ingots. Our experimental vertical gradient freeze set-up enables accurate detection of the beginning and end of the crystallization step by careful monitoring of the thermal cycle. Single crystal, (111)-oriented ingots with a diameter of 80 mm were routinely obtained without grain boundary or twin. The size of the CdZnTe ingots was extended to 115 mm in diameter, enabling production of large-dimension substrates suitable for infrared focal-plane arrays with megapixel-resolution. Crystal quality was investigated by double-crystal x-ray rocking curve mapping and by chemical revelation of etch pits. Typical mean values for the rocking curve full width at half maximum were in the range 20–40 arcs. Evaluation of etch pit density on the (111)Te face furnished values in the low 104/cm2.

Migration of Te inclusions in CdZnTe single crystals under the temperature gradient annealing

The migration of Te inclusions in CdZnTe single crystals has been investigated under temperature gradient fields in saturated Cd/Zn vapor. The migration velocity of Te inclusion upwards temperature gradient is calculated in consideration of the ordinary and thermal diffusion during annealing. The critical annealing conditions for eliminating Te inclusions are supposed based on the calculations. However, the necessary annealing conditions (annealing temperature and time) are always much larger than the suggested ones. The obtained annealing results can be summarized into three categories based on the elimination efficiency η of Te inclusions. Excluding the influence of insufficient annealing conditions, η always tends to be larger than 70% but with a restricted average value around 85%. Further discussions indicate that the interface kinetics, such as the size and void effects, have great influence on η and the migration behaviors of Te inclusions. The size effect likely results in longer annealing time, while the void effect leads to the density of Te inclusions (or voids) with diameter of 5–25μm increase after long-time annealing.

Dislocation-mediated coupling mechanism between the microstructural defects and Te inclusions in CdZnTe single crystals

The dislocation-mediated coupling mechanism between the microstructural defects and Te inclusions in CdZnTe crystal have been investigated through the defect-selective etching and cathodoluminescence (CL). The enrichment of well-arranged dislocation-related defects was found around the Te inclusions. A dislocation-mediated defects interaction model was proposed and native deep-level defects were considered to be associated with induced dislocation motions, showing as a dark CL contrast. The spatial arrangement of the microstructural defects were confined to a stellated octahedron.

Study on Te inclusions migration in CdZnTe single crystals during annealing under Cd/Zn vapor

The variation of density and diameter of Te inclusions in indium-doped Cd0.9Zn0.1Te(CZT)single crystals during annealing under Cd/Zn vapor were studied via IR microscopy observation.The results indicated that the density of Te inclusions distributed homogeneously before annealing,while after annealing,the density of Te inclusions increased for about one order near the surface,but decreased for one order inside,and the density increased gradually along the direction of temperature gradient.The diameter of Te inclusions was mainly between 1μm and 25μm before annealing,while those with the diameter45μm near the wafer's surface and those5μm and25μm inside increased notably after annealing.The reason for the above variation was determined by Te inclusions migration along the direction of temperature gradient.And during the migration,Te inclusions grew via incorporating with those with similar diameter and Ostwald ripening among those with comparatively different diameter.But the ripening was not sufficient so that many smaller Te inclusions remained in the annealed samples.

A multi-technique characterization of electroless gold contacts on single crystal CdZnTe radiation detectors

Cadmium zinc telluride (CdZnTe) is now established as a popular choice of sensor for the detection of γ-rays and hard x-rays, leading to its adoption in security, medical and scientific applications. There are still many technical challenges involving the deposition of high-quality, uniform metal contacts on CdZnTe. A detailed understanding of the interface between the bulk CdZnTe and the metal contacts is required for improvements to be made. To understand these complex interfaces, a range of complementary materials characterization techniques have been employed, including x-ray photoelectron spectroscopy depth profiling, focused ion beam cross section imaging and energy dispersive x-ray spectroscopy. In this paper a number of Redlen CdZnTe detectors with asymmetric anode/cathode contacts have been investigated. The structures of the contacts were imaged and their compositions identified. It was found that the two stage electroless indium/electroless gold deposition process on ‘polished only’ surfaces formed a complex heterojunction on the cathode, incorporating compounds of gold, gold–tellurium, tellurium oxide (of varying stoichiometry) and cadmium chloride up to depths of several 100 nm. Trace amounts of indium were found, in the form of an indium–gold compound, or possibly indium oxide. At the surface of the CdZnTe bulk, a thin Cd depleted layer was observed. The anode heterojunction, formed by a single stage electroless gold deposition, was thinner and exhibited a simpler structure of gold and tellurium oxide. The differing (asymmetric) nature of the anode/cathode contacts gave rise to asymmetric current–voltage (I–V) behaviour and spectroscopy.

Photoluminescence analysis of high-resistivity CdZnTe:In single crystals obtained by annealing

High-resistivity In-doped CdZnTe (CZT:In) single crystals were obtained by annealing under Te atmosphere. Photoluminescence spectrum was used to investigate annealed CZT:In single crystals. The results indicated that (D0, X) peak which is related to the crystal quality appeared after annealing. The donor–acceptor pair peak DAP mainly originated from the complex consisted of the donor In+Cd and the acceptor [In+CdV2−Cd]−. The intensity of DAP peak decreased, which might suggest that the impurities were removed. Moreover, Dcomplex peak containing two peaks for as-grown CZT:In consisted of Cd vacancy-related (D1) and dislocation-related (D2) defects. However, the intensity of D2 peak was very low because of the elimination of Cd inclusions after annealing. And the intensity of D1 peak increased because of the increase of Cd vacancies by evaporating for a long time.

MOVPE Growth of Thick Single Crystal CdZnTe Epitaxial Layers on Si Substrates for Nuclear Radiation Detector Development

Details about the MOVPE growth of thick single crystal CdZnTe layers on (211)Si substrates are presented. The growth was carried out at a substrate temperature of 650°C, using dimethylcadmium, dimethylzinc, and diethyltellurium precursors. Control of Zn-concentration in the range from 0 to 0.2 was performed by controlling the precursors' flow-rates and ratio. Results from the XRD showed grown layers were single crystalline with no phase separation observed. The 4.2 K PL results show high intensity bound-exciton peaks which shifted to higher energies with increasing Zn-concentrations. A p-CdZnTe/p-CdTe/n-CdTe/n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -Si hetero-junction diode was fabricated and evaluated for its possible application in nuclear radiation detector development, which exhibited good rectification property.

Electrical conduction behavior and hopping rates estimate of cadmium zinc telluride single crystal

CdZnTe (CZT) single crystal was grown by a modified vertical Bridgman method. It was used to study the electrical conduction behavior of CZT crystal by impedance spectroscopy in the temperature range over 303–573K. The imaginary part vs real part of complex impedance plots was considered to be a semicircle indicating only bulk contributions. The value of activation energy derived from the peak of the imaginary part of electrical impedance was found to be 0.08±0.005eV, which was assigned to relate the migration and hopping of ionized Cd vacancies and Te antisites. The discrepancy between activation energy of dc electrical conductivity (0.35±0.03eV) and the activation energy (0.20±0.04eV) of hopping rates was attributed to the high ion hopping rates and low attempt frequencies. The mechanism of electrical conduction was mainly ascribed to a short range migration or hopping of charge carriers including ionized Cd vacancies and Te antisites in the CZT crystal system.

Observation, morphology evolution and elimination of Cd inclusions in indium-doped CdZnTe single crystals

In this paper, several interesting kinds of Cd inclusions were observed in indium-doped CdZnTe (CZT:In) crystals. Characterizations revealed that all inclusions were actually clusters of many irregular Cd precipitates with a size lower than 1 μm. The needle-like Cd inclusion only had three orientations (11), (11) and (211) along which dislocations and stacking faults lie. The cross-like Cd inclusion consisted of two crossed needle-like inclusions with a 120° included angle. And the star-like Cd inclusion consisted of three crossed needle-like inclusions with a 120° included angle to each other. The mechanism of morphology evolution of Cd inclusions is proposed for the first time. Moreover, all kinds of Cd inclusions could be completely eliminated by appropriate post-growth annealing under a Te atmosphere.

An XPS study of bromine in methanol etching and hydrogen peroxide passivation treatments for cadmium zinc telluride radiation detectors

The performance of single crystal CdZnTe radiation detectors is dependent on both the bulk and the surface properties of the material. After single crystal fabrication and mechanical polishing, modification of the surface to remove damage and reduce the surface leakage current is generally achieved through chemical etching followed by a passivation treatment. In this work, CdZnTe single crystals have been chemically etched using a bromine in methanol (BM) treatment. The BM concentrations employed were 0.2 and 2.0 (v/v) % and exposure times varied between 5 and 120s. Angle resolved XPS and sputter depth profiling has been employed to characterize the surfaces for the different exposure conditions. A Te rich surface layer was formed for all exposures and the layer thickness was found to be independent of exposure time. The enriched Te layer thickness was accurately determined by calibrating the sputter rate against a CdTe layer of known thickness. For BM concentrations of 0.2 (v/v) % and 2 (v/v) %, the Te layer thickness was determined to be 1.3±0.2 and 1.8±0.2nm, respectively. The BM etched surfaces have subsequently been passivated in a 30wt.% H2O2 solution employing exposure time of 15s. The oxide layer thickness has been calculated using two standard XPS methodologies, based on the Beer–Lambert expression. The TeO2 thickness calculated from ARXPS data are slightly higher than the thickness obtained by the simplified Beer–Lambert expression. For BM exposures of 30–120s followed by a passivation treatment of 30wt. % H2O2 solution employing an exposure time 15s, the ARXPS method gave an average TeO2 thickness value of 1.20nm and the simplified Beer–Lambert expression gave an average thickness value of 0.99nm.

Concentration of extended defects in CdZnTe single crystals: Effects of cooling rate after growth

We analyzed two CZT crystals cut from as-grown CdZnTe (CZT) ingots, the only difference between them being the rate of cooling after the crystal growth process. Using White Beam X-ray Diffraction Topography (WBXDT) and Infrared (IR) Transmission Microscopy, we identified and quantified the extended defects, e.g., Te inclusions, dislocations, and sub-grain boundaries. The effects of cooling rate on the size distribution and concentration were studied. The WBXDT and IR images of the fast-cooled crystal revealed very high density of dislocations and sub-grain boundaries, crisscrossing throughout its entire volume, extending from deep inside almost to the surface. In addition, IR analyses showed that the concentration of Te inclusions in the fast-cooled crystal (106cm−3) was higher than that in the slow-cooled one (105cm−3). For the latter, both the WBXDT and the IR images were bright and clear with low concentration of defects. We concluded that slow cooling rate can greatly reduce the number of Te inclusions and inclusion-decorated extended defects in as-grown CZT ingots.

Mechanical characteristics of nanocrystalline layers containing nanotwins induced by nanogrinding of soft-brittle CdZnTe single crystals

Nanocrystalline layers containing nanotwins induced under nanogrinding of soft-brittle cadmium zinc telluride single crystals exhibit higher hardness, ductility and fracture resistance, as well as a lower friction coefficient. This is different from the damaged layer of hard-brittle silicon semiconductors and usually nanocrystalline metals. The formation mechanism of nanotwins is interpreted by the undeformed chip thickness induced grinding energy. Compressive stress is formed in the nanocrsytalline layer containing nanotwins, whereas tensile stress is generated in that without nanotwins.

Mechanical properties of nanocrystalline deformed layers induced by nanogrinding of CdZnTe single crystals

Nanocrystalline deformed layers were generated in cadmium zinc telluride (CZT) single crystals by nanogrinding using three different grit sizes. The mechanical properties of the deformed layers were measured using nanoindentation. The hardness of the deformed nanocrystalline layers in the soft-brittle CZT semiconductor was higher than that of the perfect single crystal. This result is different from those found for deformed layers of hard-brittle silicon semiconductors, where the hardness of the deformed layers is lower than those of the perfect single crystal.

Matrix-controlled morphology evolution of Te inclusions in CdZnTe single crystal

The fine morphologies of microscale Te inclusions in CdZnTe single crystal were investigated to reveal their shape evolution. Such inclusions from crystal ingots with different post-growth cooling rates were analyzed using scanning electron microscopy after surface treatment. A tetrakaidecahedron model embodying {1 0 0} and {1 1 1} matrix facets was developed to interpret the form of the Te inclusions. An entire shape evolution process was also proposed where the final configuration of the Te inclusions was a tetrahedron comprising {1 1 1}B facets.

Performance of a Large Volume 20&lt;formula formulatype="inline"&gt; &lt;tex Notation="TeX"&gt;$\times$&lt;/tex&gt;&lt;/formula&gt;20&lt;formula formulatype="inline"&gt;&lt;tex Notation="TeX"&gt;$\times$&lt;/tex&gt; &lt;/formula&gt;15 mm&lt;formula formulatype="inline"&gt;&lt;tex Notation="TeX"&gt;$^{3}$&lt;/tex&gt;&lt;/formula&gt; CPG Detector

Large-volume CdZnTe (CZT) single crystals with electron lifetimes exceeding 10 μs recently became available commercially for making large effective area gamma-ray arrays. However, significant variations were observed in the performance of detectors made from such arrays. We evaluated the spectroscopic performance of such a large-volume, 20 × 20 × 15 mm3, coplanar-grid (CPG) CdZnTe detector, intended for use in a handheld radioisotope-identifier, which unexpectedly showed a poor spectral performance with an energy resolution of 3.5-4% FWHM measured with 662-keV gamma-rays. To understand the factors affecting its performance, we applied several characterization techniques, viz., white X-ray diffraction topography measurements, IR microscopy, and micron-spatial resolution X-ray mapping. They allowed us to identify major crystal defects in the device that limit its energy resolution and detection efficiency.

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.

Low-temperature spatially resolved micro-photoluminescence mapping in CdZnTe single crystals

We utilized a low-temperature spatially resolved micro-photoluminescence mapping technique to investigate the spatial variation of photoluminescence- and electronic-defect states in areas of CdZnTe (CZT) single crystals containing structural-imperfections. Photoluminescence mapping of the donor-bound-exciton emission reveals an unexpected blue-shift of the CZT bandgap at Te inclusions, which indicates that for optical measurements the localized strain field needs to be considered for accurate calculation of Zn composition and energy levels near micro-scale defects. We observed that the line widths of the donor-bound-exciton peak and defect-related D band are broadened in regions with a high density of dislocations; in contrast, the donor-acceptor-pair peak is narrowed.

Transmission electron microscopy characterization of the deformation of CdZnTe single crystals induced by nanoscratching

The deformation characteristics of soft-brittle CdZnTe single crystals induced by nanoscratching were investigated using cross-sectional transmission electron microscopy. Significant pile-up was observed, but no amorphization phases existed. Twins, stacking faults and nanocrystals were found in the scratched subsurface. This is somewhat different from the deformation patterns obtained from our previous nanogrinding study, where twins were seldom observed. The deformation mechanisms were discussed.