CdZnTe Crystals Research Articles

CdZnTe像素阵列探测器成像评价模型及实验

A flexible induced charge model of CdZnTe pixellated detector is presented. This model takes into account the charge trapping effect in the detection process. The distribution of the trapped carriers in CdZnTe crystal is derived based on the continuity equations. In consequence, an evaluation model based on the presampling modulation transfer function (presampling MTF) is obtained. As shown through the numerical simulation, the detector imaging quality decreases obviously with the increase in incident photon energy. When the electron- and hole-mobility lifetime products are 0.510-3 to 5.010-3 cm2/V and 2.010-5 to 7.510-5 cm2/V, respectively, the model signal is almost due to the electron

Measurement of the drift mobilities and the mobility-lifetime products of charge carriers in a CdZnTe crystal by using a transient pulse technique

In this work we present results on the measurement of the drift mobility and the mobility-lifetime product of charge carriers in a 16-pixellated CdZnTe detector. For the determination of an interaction position based on the pulse rise-time method in a CZT detector, it is necessary to characterize the transport properties governed by drift mobility and lifetime for electrons and holes. In order to extract the transport properties of an electron and a hole, we bombarded 5.5-MeV alpha particles from a 241Am source and 81-keV gamma rays emitted from a 133Ba source on the negatively biased contact of the CZT detector. A time-of-flight (TOF) method was used to measure the electron drift mobility at room temperature whose value turned out to be 906.4 cm2/V⋅s. With the Hecht's equation, the electron mobility-lifetime product was also determined from the bias-dependent alpha response and was equal to (9.88 ± 2.33) × 10−3 cm2/V. On the other hand, the hole mobility-lifetime product was evaluated by a model based on the average charge collection efficiency which accounts for the absorption probability with a given photon energy. By using a single parameter fitting of the model, we obtained the hole mobility-lifetime product of (8.28 ± 0.17) × 10−4 cm2/V.

Intensifying process of polarization ef fect within pixellated CdZnTe detectors for X-ray imaging

The intensifying process of polarization effect at room temperature in a pixellated Cadmium zinc telluride (CdZnTe) monolithic detector is studied. The process is attributed to the increase in build up space charges in the CdZnTe crystal, which causes an expansion of the space charge region under the irradiated area. The simulations of electric potential distributions indicate that the distorted electric potential due to the high X-ray flux is significantly changed and even deteriorated due to increasing space charges within the irradiated volume. An agreement between the space charge distribution and electric potential is discussed.

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.

Morphology evolution of micron-scale secondary phases in CdZnTe crystals grown by vertical Bridgman method

The morphology and evolution of micron-scale Te-rich secondary phases (SP) are discussed based on IR contrast images. Well-defined, hexagonal or triangular shaped, incoherent Te-rich SP particles were generally identified in as-grown CdZnTe crystals grown with excess Te. The creation of these particles is consistent with the Te-rich droplets encapsulated within the CdZnTe matrix, which grown by accumulation of the point defects. The surrounding CdZnTe crystal dominates the morphology of Te-rich SP particles orienting themselves along crystallographic directions. Star-shaped Te-rich particles and needle-like Te-rich SP clusters were normally revealed in as-grown CdZnTe crystals, obtained under Cd-rich conditions. The former is related to the dislocation loop punching associated with SP particles, which attributed to the internal stress relief. The latter is constituted by numerous small irregular Te precipitates, which is likely produced by the aggregation of Te precipitates along specific paths through thermo-migration during the in situ annealing and cooling processes.

Three-dimensional mapping of tellurium inclusions in CdZnTe crystals by means of improved optical microscopy

One of the main problems connected with the exploitation of CdTe and CdZnTe crystals for the preparation of X-ray imaging devices is the presence in the matrix of inclusions, mainly due to tellurium excess. It is important to set up characterization methods that permit one to map in three dimensions the presence and the diameter of inclusions. This is a key point for (both) a better understanding of the inclusion-formation mechanism and for the evaluation of crystal quality. For this purpose, we developed a system that can be implemented on standard transmission optical microscopes. The use of this tool in the characterization of Bridgman grown CdZnTe samples allowed us to uncover interesting features in the tellurium inclusion distribution.

Characterization of etch pit formation via the Everson-etching method on CdZnTe crystal surfaces from the bulk to the nanoscale

Abstract A combination of atomic force microscopy, optical microscopy, and mass spectrometry was employed to study CdZnTe crystal surface and used etchant solution following exposure of the CdZnTe crystal to the Everson etch solution. We discuss the results of these studies in relationship to the initial surface preparation methods, the performance of the crystals as radiation spectrometers, the observed etch pit densities, and the chemical mechanism of surface etching. Our results show that the surface features that are exposed to etchants result from interactions with the chemical components of the etchants as well as pre-existing mechanical polishing.

Coexistence of para and ferromagnetic phases of [formula omitted] in undoped CdZnTe ( [formula omitted]) crystals

The signatures of the coexistence of para and ferromagnetic phases for the Fe 3+ charge state of iron have been identified in the low temperature electron spin resonance (ESR) spectra in undoped CdZnTe ( Zn ∼ 4 % ) crystals and independently verified by superconducting quantum interference device (SQUID) and AC susceptibility measurements. In the paramagnetic phase the inverse of AC susceptibility follows the Curie–Weiss law. In the ferromagnetic phase the thermal evolution of magnetization follows the well-known Bloch T 3 / 2 law. This is further supported by the appearance of hysteresis in the SQUID measurements at 2 K below T C which is expected to lie in between 2 and 2.5 K.

Fluctuations in induced charge introduced by Te inclusions within CdZnTe radiation detectors

Recently, homogenization theory based on a multiple-scale perturbation of the electron transport equation has been used to derive a mathematical framework for modeling the excess charge lost to Te inclusions within radiation detectors based on semi-insulating cadmium zinc telluride (CdZnTe). In that theory, the heterogeneous material is mathematically replaced by a homogenized CdZnTe crystal whose effective electron attenuation length incorporates the additional uniform electron trapping caused by the inclusions. In this paper, the homogenization theory is extended to incorporate fluctuations in the induced charge (i.e., charge collection nonuniformities) introduced by the random position and size distributions of a noncorrelated population of small (i.e, <20 μm) Te inclusions. Analysis of the effective parameters derived within the homogenized framework is used to develop a probability distribution of effective electron attenuation lengths, and therefore effective mobility-lifetime products, as a function of both the position and size distribution of Te inclusions. Example distributions are detailed for the case of an exponential size distribution at various number densities. Further, it is demonstrated that the inclusion-induced material nonuniformities derived in this paper can be numerically sampled efficiently, making them applicable to Monte Carlo device simulation of realistic CdZnTe detectors. Simulated charge induction maps and pulse-height spectra are presented and compared to recently published measurements.

Characterization of CZT crystals grown by the boron oxide encapsulated vertical Bridgman technique for the preparation of X-ray imaging detectors

CdZnTe crystals for the preparation of X-ray imaging detectors have been grown by the boron oxide encapsulated vertical Bridgman method. The homogeneity of the crystals has been studied by photoluminescence mapping, energy-dispersive X-ray analysis, and resistivity mapping. The zinc distribution follows an anomalous behavior that deviates from the normal freezing equation. The wafers cut perpendicular to the growth direction show a homogeneous resistivity distribution, suggesting the possible exploitation of these crystals for the production of large volume imaging detectors.

Study on the Microstructure of Au/CdZnTe Interface by HRTEM

As a new type of detector material, the CdZnTe (CZT) crystal has the advantages such as a wide energy gap, a large average atomic number, the strong ability to stop X- ray and high resistivity. The CZT is not only the best epitaxial substrate to the growth process of infrared detector material—HgCdTe, but also can be widely used in the detector production of X-ray fluorescence analysis, nuclear waste monitoring, airport and port security testing, industrial detection, medical diagnosis, astrophysics research, etc. So it has very broad prospects. Compared with the traditional NaI detector, the CZT detector has a better energy resolution. Compared with Si and Ge detector, it can work at room temperature. So it absorbed great attention in recent years. The structure of the interface between the metal electrode and CZT has great effect on the performance of detector. Au electrode film was prepared on high resistance CZT single crystal by using vacuum evaporation technology in this research. The interface of Au/CZT was investigated by high resolution transmission electron microscope (HRTEM). The results showed that there existed a disordered layer with the thickness of 5-10nm in the interface between the Au electrode and CZT wafer which was only mechanical polished (as opposed to polished and etched) in our present experimental condition. That could be approved as an amorphous layer through diffraction pattern. On the other hand, the interface was flat, smooth and no amorphous layer on which CZT wafer was polished and etched.

Effect of Mechanical Anisotropy on Grinding of CdZnTe Wafers

In this study, nanoindentation and nanoscratching were employed to research on the mechanical anisotropy of CdZnTe (110) and (111) planes, and their effects on the grinding of CdZnTe wafers were studied. The results suggest that the primary slip system is responsible for the anisotropy of hardness and frictional coefficient. It is easy to slip along directions on (110) plane and directions on (111) plane during scratching, hence the frictional coefficient is the lowest compared to that of other directions, and high surface quality could be obtained during grinding along these directions. The problem of embedded abrasives, which is due to the soft nature of CdZnTe crystal, could be solved when using the grinding process instead of the lapping process.

Analysis of In and Al doped high resistivity CdZnTe crystal

AbstractIn this paper, the vertical Bridgman method was used to grow In‐doped, Al‐doped and In‐Al codoped CdZnTe crystals. The temperature‐dependent Hall effects, X‐ray diffraction (XRD), dislocation density were applied to study the energy level of chief defects and the possible existence of compensatory mechanism in the doped CdZnTe crystal. The XRD results showed that lattice distortion existed in the three doped samples, and the lattice distortion in Al‐doped crystal was the highest among them. The phenomenon may attribute to the stress caused by the occupation of In and Al atoms at Cd vacancies which make atoms depart from their equilibrium positions. The Hall effect test showed that the n‐type In doped CdZnTe samples were estimated to have shallow donors at 15.6 meV below the conduction band, which is associated with [InCd], and the In‐Al codoped samples have shallow donors at 15.1 meV below the conduction band, which is associated with [InCd]and [AlCd]. While, for the p‐type Al doped CdZnTe sample, shallow acceptors were deduced at 59.9 meV above the valence band, which is considered to be [VCd‐2AlCd] (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence and electric spectroscopy of dislocation-induced electronic levels in semi-insulated CdTe and CdZnTe

We studied deformation-induced defects in semi-insulating CdTe and CdZnTe by infrared photoluminescence (PL), contact less photoconductivity and resistivity. Plastic deformation increased the concentrations of grown-in defects, namely, those of an important midgap level E C−0.74 eV in CdTe and Cd 1− x Zn x Te ( x<0.1), the materials of choice in today’s X-ray and gamma ray detector technology. We confirmed the direct correlation between Y-emission and the dislocation density in both compounds. The Y-band intensified near an indenter deformation or near a scribing line, but was barely visible in low-dislocation areas (etch pit density <2×10 5 cm −2). Our results correlate with recent findings that dislocation-induced defects and their clusters degrade charge collection in radiation detectors. Photoluminescence of midgap levels can serve as a tool to identify areas of degraded performance in semi insulated CdTe and CdZnTe crystals for radiation detectors.

Extended defects in CdZnTe crystals: Effects on device performance

We explored some unique defects in a batch of cadmium zinc telluride (CdZnTe) crystals, along with dislocations and Te-rich decorated features, revealed by chemical etching. We extensively investigated these distinctive imperfections in the crystals to identify their origin, dimensions, and distribution in the bulk material. We estimated that these features ranged from 50 to 500 μm in diameter, and their depth was about ∼300 μm. The density of these features ranged between 2×10 2 and 1×10 3 per cm 3. We elaborated a model of them and projected their effect on charge collection and spectral response. In addition, we fabricated detectors with these defective crystals and acquired fine details of charge-transport phenomena over the detectors’ volume using a high-spatial resolution (25 μm) X-ray response mapping technique. We related the results to better understand the defects and their influence on the charge-transport properties of the devices. The role of the defects was identified by correlating their signatures with the findings from our theoretical model and our experimental data.

Spectrally sensitive MOS photovaricaps based on CdZnTe and CdS crystals

Metal-oxide-semiconductor (MOS) heterostructures based on CdS and CdZnTe crystal substrates have been obtained by photothermal oxidation (PTO) in an oxygen-containing atmosphere. PTO regimes that ensure the formation of CdO films with various electrical characteristics on CdS substrates are determined. The obtained MOS structures exhibit high values of the light-induced capacitance variation. The position of the maximum spectral sensitivity of the capacitance of photovaricaps can be significantly changed by varying the stoichiometric composition of the initial semiconductor crystals and by doping them with electrically active impurities.

Nanomechanical behaviors of (110) and (111) CdZnTe crystals investigated by nanoindentation

The nanomechanical behaviors of (110) and (111) CdZnTe crystals were investigated by nanoindentation. It was found that the indenter tip was adhered by the removed materials in scanning testing area although the scanning force on the tested surface was very small (1000 nN), which would affect the testing result of nanoindentation, so the indenter was clean before nanoindentation test. The experimemtal results showed that the hardness and Young’s modulus decreased with the increase of indentation loads on the same plane. Because of the anisotropy of the CdZnTe crystal, the average hardness of (110) plane is 35% lower than that of (111) plane, and there are about 30% difference of the hardness along different crystallographic directions on the same plane. The hardness in 0° and 120° testing directions was the same due to the threefold symmetry of a Berkovich indenter. And the anisotropy affected the surface quality during machining of CdZnTe crystal.

Characterization of CdZnTe crystal grown by bottom-seeded Bridgman and Bridgman accelerated crucible rotation techniques

The growth of CdZnTe crystals with diameter up to 60 mm using bottom-seeded Bridgman method as well as Bridgman accelerated crucible rotation technique (ACRT–B) was investigated. Both ingots exhibit high yields, where single crystal with the volume exceeding 200 cm 3 is produced. The crystal properties of two ingots were compared in the aspects of yields, crystalline quality and composition uniformity. For CdZnTe ingot grown by bottom-seeded Bridgman method, the full width at half-maximum (FWHM) of X-ray rocking curve was determined to be 36?, indicating a better crystalline quality than ingot grown by ACRT-B method, which gave FWHM of 56?. The composition distribution of Zn and In in CdZnTe was determined by using electron probe microanalysis (EPMA) and inductively coupled plasma mass spectrometry (ICP-MS), respectively. The effective segregation coefficients of Zn k Zn and In k In in the two ingots were evaluated by fitting the experimental data with the Pfann equation.

Characterization of detector-grade CdZnTe crystals grown by traveling heater method (THM)

This work focuses on the 3. Resultsanddiscussioncharacterization of 10×10×10 mm 3 THM-grown CdZnTe detector-grade crystals that have been post-growth annealed to remove the secondary phases (SPs). All three detectors showed an average energy resolution of ∼1.63% for a small guarded pixel with 3.5 mm diameter, measured using 137Cs—662 keV with an average peak-to-Compton ratio of 2.7. The characterization showed vestiges of SPs and micro-twins present in some of the crystals indicating that the SPs prior to annealing were large and had size in the range of 100–500 μm. The various detectable structural features, such as micron twins, strains and sub-micron level of Te inclusions seemed to have little or no influence in the radiation spectrometer performance of the detectors; this is possibly because they are either having low density or electrically inactive.

Electric field induced local features in the photoresponse spectra of CdZnTe crystals

t is established that the photocurrent spectrum I = f(λ) of a CdZnTe crystal depends on the configuration and frequency of an external electric field applied to the sample. In particular, the application of periodic field pulses in the Haar wavelet form leads to modification of the spectrum at frequencies corresponding to certain maxima of the wavelet coefficients. It is demonstrated that, by representing the photore-sponse spectrum in the form of parametric signatures plotted as I(λ) versus dI/dλ, it is possible to separate and evaluate the partial contributions of various photosensitive centers in the given crystal.