CdZnTe Research Articles

Passivation Effect and Stability of Diamond-Like Carbon Film on CdZnTe Devices

Leakage currents of a CdZnTe (CZT) detector are desired to be as low as possible for a better signal-to-noise ratio in the acquisition of x-ray/gamma-ray spectra. Surface passivation plays a major role in reduction of the surface leakage current and thereby improves the detector performance. In this paper, diamond-like carbon (DLC) film has been introduced as the passivation layer for a CZT detector. Passivation effect and stability of the DLC film on CZT devices have been investigated by the Raman spectrum, x-ray photoelectron spectroscopy (XPS), atomicforce microscopy (AFM) and current–voltage characteristic. High-quality DLC films were effectively fabricated on CZT devices measured by the Raman spectrum. The XPS in-depth profiling of CZT passivated with DLC film demonstrated that no interface reaction occurred, and DLC effectively prevented the oxidation reaction to produce the state Te4+ in Te3d spectra. Moreover, the surface leakage current evidently dropped after the DLC film passivation. In addition, properties of the DLC films with different thickness were investigated. It was shown that the effect of this passivation over stability with time is not perfect and an optimum thickness of the DLC film is proposed for reaching the best stability for CZT devices.

Using the HgxMg(1−x)Te ternary compound as a room temperature photodetector: The electronic structure, charge transport, and response function of the energetic electromagnetic radiation

In the present work, firstly, a first-principles study of the structural, electronic, and electron transport properties of the HgxMg1−xTe (HMT) ternary compound is performed using the ABINIT package and the results are compared with Cd0.9Zn0.1Te (CZT) as a current room-temperature photodetector. Next, the response functions of Hg0.6Mg0.4Te and Cd0.9Zn0.1Te under electromagnetic irradiation with 0.05 MeV, 0.2 MeV, 0.661 MeV and 1.33 MeV energies are simulated by using the MCNP code. According to these simulations, the Hg0.6Mg0.4Te ternary compound is suggested as a good semiconductor photodetector for use at room temperature.

Molecular statics simulation of CdTe grain boundary structures and energetics using a bond-order potential

The structure and energetics of coincidence site lattice grain boundaries (GB) in CdTe are investigated by mean of molecular statics simulations, using the Cd–Zn–Te bond-order potential (second iteration) developed by Ward et al (2012 Phys. Rev. B 86 245203; 2013 J. Mol. Modelling 19 5469–77). The effects of misorientation (Σ value) and interface plane are treated separately, complying with the critical need for full five-parameter characterization of GB. In addition, stoichiometric shifts, occurring between the inner interfaces and their adjacent atomic layers, are also predicted, revealing the energetic preference of Te-rich boundaries, opening opportunities for crystallography-based intrinsic interface doping. Our results also suggest that the intuitive assumption that Σ3 boundaries with low-indexed planes are more energetically favorable is often unfounded, except for coherent twins developing on {111} boundary planes. Therefore, Σ5, 7 or 9 boundaries, with lower interface energy than that of twin boundaries lying on different facets, are frequently encountered.

Effect of Intense Optical Excitation on Internal Electric Field Evolution in CdTe Gamma-Ray Detectors

The time-of-flight (TOF) transient currents in radiation detectors made of CdTe and Cd0.9Zn0.1Te (CZT) have been measured at several optical excitation intensities to investigate the effect of drifting carriers on the internal field. Both detectors show so-called space-charge-perturbed (SCP) current under intense optical excitation. A Monte Carlo (MC) simulation combined with an iterative solution of Poisson’s equation is used to reproduce the observed currents under several bias voltages and excitation intensities. The SCP theory describes well the transient current in the CZT detector, whereas injection of holes from the anode and a corresponding reduction of the electron lifetime are further required to describe that in the CdTe detector. We visualize the temporal changes in the charge distribution and internal electric field profiles of both detectors.

Preparation and surface defect regulation of CdZnTe films based on GaN substrates

High quality polycrystalline CdZnTe (CZT) films were prepared on GaN substrates via close-spaced sublimation (CSS). Au electrodes were deposited on the surface of CZT films to form CZT/Au structure. XRD, SEM, EDAX and UV-Vis spectra were adopted to investigate the structure, composition and morphology of CZT films. XPS and I-V characterizations were used to investigate the surface defects and electrical properties of CZT films. The quantity of the surface defects is decreased with the extended growth times and remains stable when the deposition times exceed 20 min. The minimum resistivity of CZT thick films grown on GaN substrates is 1.12 × 109 Ω-cm, which is about two orders of magnitude higher than that on FTOs. The high resistivity can significantly reduce the leakage currents of CZT films and thus improve the signal-to-noise ratio of the radiation detector based on CZT films.

Preparation of indium tin oxide contact to n-CdZnTe gamma-ray detector

The nonmetal electrode material Indium Tin Oxide (ITO) has advantages of excellent conductivity, higher adhesion, and interface stability, showing potential to replace the metallic contacts for fabrication of CdZnTe (CZT) X/γ-ray detectors. In this work, high quality ITO electrodes for n-type CZT crystals were prepared by magnetron sputtering under a sputtering power of 75 W and a sputtering pressure of 0.6 Pa. A low dark current of ∼1 nA is achieved for the 5 × 5 × 2 mm3 ITO/CZT/ITO planar device under 100 V bias. The characteristics of Schottky contact are presented in the room temperature I-V curves, which are similar to those of the Au contact detectors. Based on the thermoelectric emission theory, the contact barrier and resistance of ITO electrodes are evaluated to be 0.902–0.939 eV and 0.87–3.56 × 108 Ω, respectively, which are consistent with the values of the Au electrodes. The ITO/CZT/ITO structure detector exhibits a superior energy resolution of 6.5% illuminated by the uncollimated 241Am @59.5 keV γ-ray source, which is comparable to the CZT detector with Au electrodes.

Measurement of air kerma rate and ambient dose equivalent rate using the G(E) function with hemispherical CdZnTe detector

Since the room-temperature detector CdZnTe (CZT) has advantages in terms of detection efficiency, energy resolution, and size, it has been extensively used to detect X-rays and gamma-rays. So far, nuclear radiation detectors such as cerium chloride doped with lanthanum bromide (LaBr $$_3$$ (Ce)), thallium doped with cesium iodide (CsI (Tl)), thallium doped with sodium iodide (NaI (Tl)), and high-purity germanium (HPGe) primarily use the spectroscopy-dose rate function (G(E)) to achieve the accurate measurement of air kerma rate ( $$\dot{K}_{a}$$ ) and ambient dose equivalent rate ( $$\dot{H}^*(10)$$ ). However, the spectroscopy-dose rate function has been rarely measured for a CZT detector. In this study, we performed spectrum measurement using a hemispherical CZT detector in a radiation protection standards laboratory. The spectroscopy-dose rate function G(E) of the CZT detector was calculated using the least-squares method combined with the standard dose rate at the measurement position. The results showed that the hemispherical CZT detector could complete the measurement of air kerma rate ( $$\dot{K}_{a}$$ ) and ambient dose equivalent rate ( $$\dot{H}^*$$ (10)) by using the G(E) function at energies between 48 keV and 1.25 MeV, and the relative intrinsic errors were, respectively, controlled within ± 2. 3 and ± 2. 1%.

Analysis of Deep and Shallow Traps in Semi-Insulating CdZnTe

Trap levels which are deep or shallow play an important role in the electrical and the optical properties of a semiconductor; thus, a trap level analysis is very important in most semiconductor devices. Deep-level defects in CdZnTe are essential in Fermi level pinning at the middle of the bandgap and are responsible for incomplete charge collection and polarization effects. However, a deep level analysis in semi-insulating CdZnTe (CZT) is very difficult. Theoretical capacitance calculation for a metal/insulator/CZT (MIS) device with deep-level defects exhibits inflection points when the donor/acceptor level crosses the Fermi level in the surface-charge layer (SCL). Three CZT samples with different resistivities, 2 × 104 (n-type), 2 × 106 (p-type), and 2 × 1010 (p-type) Ω·cm, were used in fabricating the MIS devices. These devices showed several peaks in their capacitance measurements due to upward/downward band bending that depend on the surface potential. Theoretical and experimental capacitance measurements were in agreement, except in the fully compensated case.

Observations on Nanoscale Te Precipitates in CdZnTe Crystals Grown by the Traveling Heater Method Using High Resolution Transmission Electron Microscopy

Te precipitates in CdZnTe (CZT) crystals grown by the traveling heater method (THM) are investigated using high-resolution transmission electron microscopy (HRTEM). The results show that in THM-grown CZT crystals, Te precipitates are less than 10 nm in size—much smaller than those in Bridgman-grown CZT. They have hexagonal structure and form a coherent interface with zinc blend structure CZT matrix in the orientation relationship [ 1 ¯ 12 ] M / / [ 0001 ] P and ( 1 1 ¯ 1 ) M / / ( 1 ¯ 100 ) P . A ledge growth interface with the preferred orientation along the [ 1 1 ¯ 1 ] M and [ 110 ] M was found near Te precipitates. The growth and nucleation mechanism of Te precipitates are also discussed.

Establishment of CZT pixellated radiation detection system and pulse-height correction

11 × 11 × 3 mm3 and 4 × 4 pixel array CdZnTe (CZT) detector system based on the principle of CZT pixel array detector and correction algorithm has been established. The leakage current of CZT pixellated crystal is tested and analyzed. The pulse amplitude spectrum of the CZT detector has been measured using 137 Cs 662 keV Gamma source. The average energy resolution of pixellated detector system is 3.3% and full energy peak FWHM is 21.85 keV. The corrected algorithm of pulse-height is applied for improving spectroscopy response. The average energy resolution of corner pixels has been improved. The best energy resolution of pixels is 2.5% after correction. The energy resolution of the 4 × 4 pixellated CZT detector system is excellent. The spectrum performance of edge pixels is obvious improved by pulse-height correction. The resolution of edge pixels is improved efficiently.

Modification of growth interface of CdZnTe crystals in THM process by ACRT

The accelerated crucible rotation technique (ACRT) was introduced in the traveling heater method (THM) growth process of detector-grade CdZnTe (CZT) crystals to regulate the convection in the melt and to modify the growth interface morphology. Several ingots with the diameter of 53 mm were grown by THM with/without ACRT. The ingots were quenched during the growth to show both macroscopic and microscopic morphologies of the growth interfaces. The results show that by using ACRT the growth interface can be changed from a concave one to the flat or even convex one depending on the ACRT parameters, which is favorable for reducing nucleation in the melt to get larger CZT grains. Meanwhile, by using ACRT in THM process, the microscopic interface was changed from a diffused one to cellular or even planar one (at suitable ACRT parameters), through which the trapped Te inclusions was decreased for one order. An ingot grown by THM with constant rotation rate of 40 rpm was also grown, which have also reduced the interface curvature in macro-scale and Te inclusions to some extent, but the effects are not as significant as ACRT with high crucible rotation rate.

High-Temperature Annealing of CdZnTe Detectors

The electrical properties of CdZnTe(CZT) above the melting point of tellurium (Te) inclusions were determined during in situ annealing. The thermal annealing cycles of the CZT detectors were 490 °C, 530 °C, and 570 °C continuously, which were higher than the melting points of elemental Te and Te inclusions and lower than the sublimation temperature of CZT. Unexpectedly, the CZT detectors exhibited very low leakage current at room temperature after the thermal annealing cycles due to the formation of rectifying contacts. The activation energy of high-resistivity CZT was 0.81 eV indicating pinning of Fermi level nearly in the middle of bandgap. At room temperature, CZT detectors with rectifying contacts showed clearly the 59.5-keV gamma-ray peak of Am-241. Observed fluctuations of the leakage current at about 470 °C might have originated from a mixed conductivity of liquid and solid CZT due to the melting of Te inclusions.

HRTEM Analysis of Crystallographic Defects in CdZnTe Single Crystal

In recent years, CdZnTe has attracted much attention due to its superior electrical and structural properties for room-temperature operable gamma and x-ray detectors. However, CdZnTe (CZT) material has often suffered from crystallographic defects encountered during the growth and post-growth processes. The identification and structural characterization of these defects is crucial to synthesize defect-free CdZnTe single crystals. In this study, Cd0.95 Zn0.05 Te single crystals were grown using a three-zone vertical Bridgman system. The single crystallinity of the material was ensured by using x-ray diffraction measurements. High-resolution electron microscopy (HRTEM) was used to characterize the nano-scale defects on the CdZnTe matrix. The linear defects oriented along the ⟨211⟩ direction were examined by transmission electron microscopy (TEM) and the corresponding HRTEM image simulations were performed by using a quantitative scanning TEM simulation package.

Arrays of Position-Sensitive Virtual Frisch-Grid CdZnTe Detectors: Results From a $4\times 4$ Array Prototype

Position-sensitive virtual Frisch-grid (VFG) CdZnTe (CZT) detectors offer a unique capability for correcting the response nonuniformities caused by crystal defects. This allowed us to achieve high energy resolution, while using typical-grade commercial CZT crystals with relaxed requirements to their quality, thus reducing the overall cost of detectors. Another advantage of the VFG detectors is that they can be integrated into arrays and used in small compact hand-held instruments or large-area gamma cameras that will enhance detection capability for many practical applications, including nonproliferation, medical imaging, and gamma-ray astronomy. Here, we present the results from testing small array prototypes coupled with front-end application-specified integrated circuit. Each detector in the array is furnished with 5-mm-wide charge-sensing pads placed near the anode. The pads signals are converted into XY coordinates, which combined with the cathode signals (for Z coordinates) provide 3-D position information of all interaction points. The basic array consists of a number of detectors grouped into $2\times 2$ subarrays, each having a common cathode made by connecting together the cathodes of the individual detectors. These features can significantly improve the performance of detectors while using typical-grade low-cost CZT crystals to reduce the overall cost of the proposed instrument.

Predicted performance of a PG-SPECT system using CZT primary detectors and secondary Compton-suppression anti-coincidence detectors under near-clinical settings for boron neutron capture therapy

A test version of a prompt-gamma single photon emission computed tomography (PG-SPECT) system for boron neutron capture therapy (BNCT) using a CdZnTe (CZT) semiconductor detector with a secondary BGO anti-Compton suppression detector has been designed. A phantom with healthy tissue region of pure water, and 2 tumor regions of 5 wt% borated polyethylene was irradiated to a fluence of 1.3×109n/cm2. The number of 478 keV foreground, background, and net counts were measured for each detector position and angle. Using only experimentally measured net counts, an image of the 478 keV production from the 10B(n,α)7Li* reaction was reconstructed. Using Monte Carlo simulation and the experimentally measured background counts, the reliability of the system under clinically accurate parameters was extrapolated. After extrapolation, it was found that the value of the maximum-value pixel in the reconstructed 478 keV γ-ray production image overestimates the simulated production by an average of 9.2%, and that the standard deviation associated with the same value is 11.4%.

Characterization of spectrometric photon-counting X-ray detectors at different pitches

There is growing interest in energy-sensitive photon-counting detectors based on high flux X-ray imaging. Their potential applications include medical imaging, non-destructive testing and security. Innovative detectors of this type will need to count individual photons and sort them into selected energy bins, at several million counts per second and per mm2. Cd(Zn)Te detector grade materials with a thickness of 1.5 to 3 mm and pitches from 800 μm down to 200 μm were assembled onto interposer boards. These devices were tested using in-house-developed full-digital fast readout electronics. The 16-channel demonstrators, with 256 energy bins, were experimentally characterized by determining spectral resolution, count rate, and charge sharing, which becomes challenging at low pitch. Charge sharing correction was found to efficiently correct X-ray spectra up to 40 × 106 incident photons.s−1.mm−2.

Architecture and characterization of the P4DI CMOS hybrid pixel sensor

Gamma ray imaging can be used for the extraction either of the activity map of a source or of the attenuation map of an object or both, as well as for the identification of the material composition of the emitting source or the object. All these imaging modalities can benefit from instruments giving the information of the energy of the converted photons and also the spatial and time coordinates of the conversion. The P4DI CMOS and hybrid provides the core technology for this task being a 2-D array based on Cd(Zn)Te material for the sensing layer. It consists of 1250 pixels with 400 μ m pitch. The energy resolution of the 241Am photopeak is 3.5 keV, time resolution is less than 12 μ s and power consumption is less than 100 mW. Architecture and characterization are described.

Performance comparison of small-pixel CdZnTe radiation detectors with gold contacts formed by sputter and electroless deposition

Recent improvements in the growth of wide-bandgap semiconductors, such as cadmium zinc telluride (CdZnTe or CZT), has enabled spectroscopic X/γ-ray imaging detectors to be developed. These detectors have applications covering homeland security, industrial analysis, space science and medical imaging. At the Rutherford Appleton Laboratory (RAL) a promising range of spectroscopic, position sensitive, small-pixel Cd(Zn)Te detectors have been developed. The challenge now is to improve the quality of metal contacts on CdZnTe in order to meet the demanding energy and spatial resolution requirements of these applications. The choice of metal deposition method and fabrication process are of fundamental importance. Presented is a comparison of two CdZnTe detectors with contacts formed by sputter and electroless deposition. The detectors were fabricated with a 74 × 74 array of 200 μm pixels on a 250 μm pitch and bump-bonded to the HEXITEC ASIC. The X/γ-ray emissions from an 241Am source were measured to form energy spectra for comparison. It was found that the detector with contacts formed by electroless deposition produced the best uniformity and energy resolution; the best pixel produced a FWHM of 560 eV at 59.54 keV and 50% of pixels produced a FWHM better than 1.7 keV . This compared with a FWHM of 1.5 keV for the best pixel and 50% of pixels better than 4.4 keV for the detector with sputtered contacts.

The effects of deep-level defects on the electrical properties of Cd0.9Zn0.1Te crystals**Project supported by the National Natural Science Foundation of China (No. 51502234) and the Scientific Research Plan Projects of Shaanxi Provincial Department of Education of China (No. 15JS040).

The deep-level defects of CdZnTe (CZT) crystals grown by the modified vertical Bridgman (MVB) method act as trapping centers or recombination centers in the band gap, which have significant effects on its electrical properties. The resistivity and electron mobility–lifetime product of high resistivity Cd0.9Zn0.1Te wafer marked CZT1 and low resistivity Cd0.9Zn0.1Te wafer marked CZT2 were tested respectively. Their deep-level defects were identified by thermally stimulated current (TSC) spectroscopy and thermoelectric effect spectroscopy (TEES) respectively. Then the trap-related parameters were characterized by the simultaneous multiple peak analysis (SIMPA) method. The deep donor level ( dominating dark current was calculated by the relationship between dark current and temperature. The Fermi-level was characterized by current–voltage measurements of temperature dependence. The width of the band gap was characterized by ultraviolet-visible-infrared transmittance spectroscopy. The results show the traps concentration and capture cross section of CZT1 are lower than CZT2, so its electron mobility–lifetime product is greater than CZT2. The Fermi-level of CZT1 is closer to the middle gap than CZT2. The degree of Fermi-level pinned by of CZT1 is larger than CZT2. It can be concluded that the resistivity of CZT crystals increases as the degree of Fermi-level pinned near the middle gap by the deep donor level enlarges.

ChromAIX2: A large area, high count-rate energy-resolving photon counting ASIC for a Spectral CT Prototype

Spectral CT based on energy-resolving photon counting detectors is expected to deliver additional diagnostic value at a lower dose than current state-of-the-art CT [1]. The capability of simultaneously providing a number of spectrally distinct measurements not only allows distinguishing between photo-electric and Compton interactions but also discriminating contrast agents that exhibit a K-edge discontinuity in the absorption spectrum, referred to as K-edge Imaging [2]. Such detectors are based on direct converting sensors (e.g. CdTe or CdZnTe) and high-rate photon counting electronics.To support the development of Spectral CT and show the feasibility of obtaining rates exceeding 10Mcps/pixel (Poissonian observed count-rate), the ChromAIX ASIC has been previously reported showing 13.5Mcps/pixel (150Mcps/mm2 incident) [3]. The ChromAIX has been improved to offer the possibility of a large area coverage detector, and increased overall performance. The new ASIC is called ChromAIX2, and delivers count-rates exceeding 15Mcps/pixel with an rms-noise performance of approximately 260 e-. It has an isotropic pixel pitch of 500µm in an array of 22×32 pixels and is tile-able on three of its sides. The pixel topology consists of a two stage amplifier (CSA and Shaper) and a number of test features allowing to thoroughly characterize the ASIC without a sensor. A total of 5 independent thresholds are also available within each pixel, allowing to acquire 5 spectrally distinct measurements simultaneously. The ASIC also incorporates a baseline restorer to eliminate excess currents induced by the sensor (e.g. dark current and low frequency drifts) which would otherwise cause an energy estimation error.In this paper we report on the inherent electrical performance of the ChromAXI2 as well as measurements obtained with CZT (CdZnTe)/CdTe sensors and X-rays and radioactive sources.