Semiconductor materials for x-ray detectors

We report on the continued development and testing of unique types of Cadmium Zinc Telluride (CZT) detectors.Using large volume (10 × 20 × 20 mm 3 ) CZT crystals, we contact variou s dual anodeŽ detector designs. Weincorporate a segmented cathode with “ve regions so that the charge on all seven contacts can be used todetermine the energy and the 3-D interaction location of detected X-ray and gamma-ray photons. We describethe status of the detector development program, emphasize strengths and weaknesses of the dierent contactcon“gurations, and discuss possible applications of Dual Anode Detectors in radiation detection applications.Keywords: CZT, X-ray Detector , Contact Geometry 1. INTRODUCTION The high average atomic number ( 50) and room temperature operation of Cadmium Zinc Telluride (CZT)detectors make them desirable for Homeland Security a pplications where high eciency of detecting nuclearartifacts is required, such as a stand o detector at a border crossing. We currently are working to developlarge-volume CZT detectors with few re adout channels by exploring novel contact designs. Such designs presentcertain advantages while avoiding the complications, limitations, and requirements of using the standard contactpatterns.The most common CZT detector designs in use today employ either pixel contacts or a coplanar grid (CPG)for the anode.

Cadmium(1−x)zinc(x)telluride thin films deposited by sequential pulsed laser deposition

Cadmium zinc telluride (CZT) is the material of choice for high-energy room-temperature X-ray and γ-ray detectors. However, the performance of pixelated detectors is greatly influenced by the quality of CZT. Crystal defects and impurities are one source of shallow and deep level traps for charge carriers. Fluorescence lifetime of the recombination of optically excited charges may indicate the presence and type of defects and impurities in CZT. Fluorescence lifetime imaging microscopy (FLIM) is used to examine the excited-state lifetime in CZT fabricated by different growth methods and conditions. The FLIM set-up analyzes luminescence emitted from the sample following photo excitation. Samples were optically excited above band gap with a pulsed laser (590 nm) for raster scanning a 220 × 165 µm2 sample area. In-situ room-temperature photoluminescence (PL) and FLIM were recorded simultaneously. In order to analyze the FLIM data, two dominant charge carrier decay processes (τ1, τ2) were identified. The luminescence signal decays with a rapid lifetime of τ1 ≈ 50–200 ps, and a large variety of long-lifetime components τ2 were found in the range of 225–900 ps. CZT grown by multi-tube physical vapor transport (MTPVT) showed extremely long-lived recombination decay times up to 3.5 ns in the vicinity of the interface at growth start. Further away from this interface, the recombination lifetime was in the typical range of fast transitions similar to those found in detector-grade CZT fabricated by travelling heater method. Crystalline material quality strongly influences FLIM lifetime. Time-resolved transients of MTPVT-grown CZT compared with industry-leading detector grade CZT (dots: measured data; lines: fitted exponential decay curves).

Cadmium Zinc Telluride (CD1-xZnxTe has become a crucial material for x-ray and gamma ray detection due to its wide band-gap, high atomic number and high density, which offer high efficiency and sharp spectroscopic resolution at room temperature. In addition, due to being lattice matched, it can also be used as substrate for the epitaxial growth of HgCdTe that can be used for infrared detection with high resolution. Hence, increasing the single crystal yield of CdZnTe from the grown ingot gained importance for the development of such detectors. In this study, a combination of modeling and experimental approaches has been developed in order to obtain high quality CdZnTe bulk crystals with good single crystal yield. A multi-zone Vertical Gradient Freeze (VGF) furnace was used for CdZnTe growth experiments. A global temperature model of the multi-zone furnace including complete geometry was employed using CrysMAS crystal growth modeling software. The correlation studies between the model and experimental behavior of the furnace are discussed in order to create a reliable model for temperature predictions. Temperature models were also included solid-liquid interface study in order to observe the interface shape at various stages of growth which would provide valuable insight about the quality and the yield of the ingot. Growth parameters and crucible geometries were estimated by CrysMAS simulations and interchanged between experiments. Effectiveness of temperature models and simulations was supported by experimental results such as single-crystalline yield, grain evolution and crystalline quality comparison by DCRC measurements of five successful crystal growths with moderate single crystal grain sizes.

A pixelated cadmium zinc telluride (CZT) detector has been characterised for the purpose of developing a quantitative single photon emission computed tomography (SPECT) system for dosimetry of molecular radiotherapy (MRT). This is the aim of the Dosimetric Imaging with CZT (DEPICT) project, which is a collaboration between the University of Liverpool, The Royal Marsden Hospital, The Royal Liverpool and Broadgreen University Hospital, and the commercial partner Kromek. CZT is a direct band gap semiconductor with superior energy resolution and stopping power compared to scintillator detectors used in current SPECT systems. The inherent detector properties have been investigated and operational parameters such as bias voltage and peaking time have been selected to optimise the performance of the system. Good energy resolution is required to discriminate γ-rays that are scattered as they are emitted from the body and within the collimator, and high photon throughput is essential due to the high activities of isotopes administered in MRT. The system has an average measured electronic noise of 3.31 keV full width at half maximum (FWHM), determined through the use of an internal pulser. The energy response of the system was measured across the energy region of interest 59.5 keV to 364.5 keV and found to be linear. The reverse bias voltage and peaking time producing the optimum FWHM and maximum photon throughput were 600 V and 0.5 μs respectively. The average dead time of the system was measured as 4.84 μs and charge sharing was quantified to be 0.71 % at 59.5 keV . A pixel sensitivity calibration map was created and planar images of the medical imaging isotopes 99mTc and 123I were acquired by coupling the device to a prototype collimator, thereby demonstrating the suitability of the detector for the DEPICT project.

High-resolution Schottky CdTe diode for hard X-ray and gamma-ray astronomy

Semiconductor photon-counting detectors based on high atomic number, high density materials [cadmium zinc telluride (CZT)/cadmium telluride (CdTe)] for x-ray computed tomography (CT) provide advantages over conventional energy-integrating detectors, including reduced electronic and Swank noise, wider dynamic range, capability of spectral CT, and improved signal-to-noise ratio. Certain CT applications require high spatial resolution. In breast CT, for example, visualization of microcalcifications and assessment of tumor microvasculature after contrast enhancement require resolution on the order of [Formula: see text]. A straightforward approach to increasing spatial resolution of pixellated CZT-based radiation detectors by merely decreasing the pixel size leads to two problems: (1)fabricating circuitry with small pixels becomes costly and (2)inter-pixel charge spreading can obviate any improvement in spatial resolution. We have used computer simulations to investigate position estimation algorithms that utilize charge sharing to achieve subpixel position resolution. To study these algorithms, we model a simple detector geometry with a [Formula: see text] array of [Formula: see text] pixels, and use a conditional probability function to model charge transport in CZT. We used COMSOL finite element method software to map the distribution of charge pulses and the Monte Carlo package PENELOPE for simulating fluorescent radiation. Performance of two x-ray interaction position estimation algorithms was evaluated: the method of maximum-likelihood estimation and a fast, practical algorithm that can be implemented in a readout application-specific integrated circuit and allows for identification of a quadrant of the pixel in which the interaction occurred. Both methods demonstrate good subpixel resolution; however, their actual efficiency is limited by the presence of fluorescent [Formula: see text]-escape photons. Current experimental breast CT systems typically use detectors with a pixel size of [Formula: see text], with [Formula: see text] binning during the acquisition giving an effective pixel size of [Formula: see text]. Thus, it would be expected that the position estimate accuracy reported in this study would improve detection and visualization of microcalcifications as compared to that with conventional detectors.

We describe a stacked detector made of thin cadmium telluride (CdTe) diode detectors. By using a thin CdTe device, we can overcome the charge loss problem due to the small mobility and short lifetime of holes in CdTe or cadmium zinc telluride (CdZnTe) detectors. However, a CdTe detector with a thickness of more than 5 mm is needed for adequate detection efficiency for gamma-rays of several hundred keV. Good energy resolution and good peak detection efficiency are difficult to obtain using such a thick CdTe detector. The stacked detector enabled us to realize a detector with both high-energy resolution and good efficiencies for gamma rays up to several hundred keV. In order to verify this concept, we constructed a prototype made of ten layers of a 0.5-mm-thick CdTe diode detectors with a surface area of 21.5 mm/spl times/21.5 mm. With this, we have achieved 5.3-keV and 7.9-keV energy resolution [full width at half maximum (FWHM)] at 356 keV and 662 keV, respectively, at the temperature of -20/spl deg/C.

Cadmium zinc telluride (CZT) pixel detectors show very good spectral and spatial resolution and are suitable for use in compact hard-X-ray sensors operated without cryogenics. One of the most appealing astrophysical applications is their use as focal-plane detectors for multilayer hard-X-ray telescopes operating in the (10-80) keV energy band. This paper reports experimental results obtained from two 16 x 16 pixellated CZT detectors (10 x 10 x 1 mm 3 and 10 x 10 x 2 mm 3 single crystals) with 450 μm pixel operated at room temperature using standard commercial read-out electronics. We have investigated about energy resolution and material properties of both detectors. Both arrays gave similar results showing an energy resolution less than 5% at 59.5 keV and a good material uniformity. The good energy resolution of both detectors demonstrates as the pixellated anode structure reduces (small pixel effect) the incomplete charge collection due to severe hole trapping typically present in CZT crystals. A calibration curve, obtained using three different energies, shows a linear response of the detectors.

RÖNTEC’s UHV Dewar detectors have established new standards for high resolution, lowmaintenance, low operating cost, and reliability in Si(Li) X-ray detectors. Now, the recently introduced XFlash® series X-ray detectors are enabling new methodologies for microanalysis and element imaging in the SEM. These detectors are compact, liquid-nitrogen-free semiconductor Xray detectors that are based on Silicon Drift Diode (SDD) technology. XFlash detectors produce extraordinarily high count rates with excellent energy resolution and have introduced ultra-fast microanalysis and element mapping to the SEM world. The addition of color to SEM images enables easy visualization of element distributions and allows the microstructural features and compositional variations of different materials to be more readily identified. Persons unfamiliar with electron microscopy can more readily interpret color images than black and white or gray scale images. This new technology has great potential to revolutionize electron microscopy.RÖNTEC’s UHV Dewar Detector offers the highest long-term stability and best energy resolution ever specified for a commercial Si(Li) detector (less than 129 eV). The UHV design leads to small size and weight (for reduced column loading) along with extremely low nitrogen consumption and low susceptibility to microphonics. The UHV detector never ices up and thus never requires defrosting or warm-ups. It is available with a variety of entrance windows for light element analysis.

Scintillators based on ZnSxSe1–x are promising materials for X-ray and γ-ray detection. For optoelectronic devices, it is better to use semiconductor compounds with a direct-zone energy structure with its spectral range lying in the fundamental absorption region. The band gap in such semiconductors is an important parameter that affects the energy resolution, ionization energy, dark current and other scintillation characteristics. The effect of sulfur content on the optical width of the band gap in mixed crystals ZnSxSe1–x is investigated in this paper. The test samples for this study were grown by Bridgman-Stockbarger in graphite crucibles with the diameter of 25 mm in the Ar atmosphere (PAr = 2∙106 Pa) at a temperature from 1870 to 2000 K, depending on the composition of the initial raw materials. Six samples with different content of components were obtained: ZnS0.07Se0.93, ZnS0.15Se0.85, ZnS0.22Se0.78, ZnS0.28Se0.72, ZnS0.32Se0.68, ZnS0.39Se0.61. The transmittance of the samples in the range from 61 to 67% at 1100 nm (sample thickness 4 mm) indicates a high optical quality of the crystals. It was established that the optical width of the band gap for mixed crystals ZnSxSe1–x increases from 2.59 to 2.78 eV with increasing sulfur content from 0.07 to 0.39 for direct transitions and from 2.49 to 2.70 eV for indirect transitions. A comparison was made between theoretical and experimentally obtained values of the band gap width. It is shown that no new (defective) levels appear in the band gap. The smooth dependence of the optical band gap on the composition indicate a possibility of growing ZnSxSe1–x mixed crystals by directional solidification techniques for X-ray and γ-ray detectors. The wider band gap and higer atomic mass ratio of ZnSxSe1–x crystals, as compared to ZnSe(Te) or/and ZnSe(Al) crystals, extend application areas of such semiconductor material

In this paper, the composition, morphology and electrical properties of the passivation layer for CdZnTe detectors were investigated. A two-step passivation was performed under different conditions, including passivation time, temperature and NH 4 F/H 2 O 2 concentration. The obtained passivation layers were characterized by XRD, SEM, XRF and I-V methods. The results showed that the best passivation condition was with the NH 4 F/H 2 O 2 concentration of 10wt%, passivation time of 40min and temperature of 20oC. Under this condition, the passivation layer was purely oxidized and compact, and the surface leakage current of CZT crystals was most effectively reduced. Keywords: CdZnTe, passivation process, surface treatment 1. INTRODUCTION The Cadmium zinc telluride (CZT) had shown great promise as a material for large-volume room-temperature X-ray and gamma-ray spectrometers because of its wide band-gap and high absorption coefficients [1-3]. However, the performance of CZT X-ray and gamma-ray spectrometers was of ten limited by its surface l eakage currents which was a critical factor determining the noise and an alysis efficiency of the detectors. Th us, reducing the surface leakage currents and the noise effects was very important for the fabrication of CdZnTe detectors. It was well known that the surface processing, such as polishing, etching and passivation, had great effects on the surface leakage currents of CZT wafers[4-6]. Usually, the 2–5% Br–MeOH (BM) solution was used to re move the surface damaged laye r introduced by cutting and mechanical polishing to obtain a clean su rface before electrode deposition, which would in return produce Te enriched surfaces[7]. The enrichment of Te on the surface was favorable for Ohmic contacts, but also leaded to great surface leakage. The passivation treatment was an important step in the production of CdZnTe detectors, in order to reduce the surface leakage. The passivation was classified to wet chemi cal methods and dry methods [5,6], with the wet chemical methods more popular, which produced sulfide or oxide films on the CdZnTe surface to restrict the surface leakage currents [8-10]. In this paper, a two-step passivation of the CZT detector was investigated under different conditions including passivation time, NH

The state-of-the-art power switching devices made from SiC and GaN semiconductors contain a high density of crystal defects. Most of these defects are present in starting wafers and some are generated during device processing. There is little conclusive evidence so far on the exact role that the crystal defects play on device performance, manufacturing yield, and more importantly, long-term field-reliability especially when devices are operating under extreme stressful environments. This paper provides a review of the current state-of-the-art of SiC and GaN power semiconductor material technology, and the potential impact crystal defects may have on high-density power switching electronics. Group III-Nitride materials suffer from both extended and point defects, each of which will challenge the material’s application in both vertical and lateral power devices. The extended defects include vertical threading dislocations of both edge and screw type. The latter defects have been shown to be correlated to leakage in vertical two terminal device structures while the influence of the former is still undetermined and remains a critical research issue. Channel surfaces in vertical three terminal devices will also degrade due to vertical threading dislocations. These extended defects occur in all epitaxial layers grown on c-plane substrates (the predominant and largest area substrate type) and are the result of the lack of a high quality substrate bulk material as well as substrate surface. The present substrate technology after multiple efforts (either GaN, SiC or Si) still have a high defect density indicating that low-defect seeds may be non-existing. Point defects influence the background carrier concentration in low-doped layers and various recombination processes. The intrinsic nature of III-N materials makes nitrogen-vacancies a predominant point defect that automatically dopes the crystal n-type, but there are impurities (oxygen and carbon) from most growth environments that also contribute to conductivity. These defects are not well understood and not well controlled. Such defects must be minimized in order to realize thick epitaxial layers with drift regions that will support both high blocking voltages and low on-state resistances. A wide range of materials characterization methods have been applied to further understand these impurities. The semiconductor and dielectric materials used in the construction of power electronics switching devices employed for energy efficiency applications experience extreme electrical and thermal stresses during the on-state as well as when switching high voltages and high currents. For example, in a bipolar type power semiconductor switching device in order to achieve significant conductivity modulation and reduce the drift-region resistance, high-level injection of minority carriers is typical. If the semiconductor contains a high density of crystal defects, as is the case with WBG semiconductors, minority carrier recombination phenomenon can be adversely affected. For example, it has been shown more than a decade ago [3, 4] that basal plane dislocations (BPDs) present in SiC are excited by the energy released from minority carrier recombination process; the result is excess BPD generation and its glide within the semiconductor material that leads to an increase in the drift-region resistance and thermal run-away in a forward biased bipolar junction diode The reported experimental data for high-voltage SiC semiconductor strongly suggests that material defects have profound impact on its characteristics under extreme environments. Since GaN semiconductor has even higher density of material defects, this problem is further exacerbated especially in thick epitaxial GaN layers which are required for power switches beyond 5kV. The integration of the pillar junction scheme with the trench-gate GaN MOSFET processes developed at the University of Maryland will be reported. The thick pillar junction epitaxial layers have been grown by a commercial vendor, using a windowed GaN growth process and/or selective deep etching, followed by GaN regrowth to fabricate columns with opposite carrier polarities. The doping density of the n--pillars under the gate has been specified with background carrier concentration n ≤ 1x1016 cm-3, while the p-pillars will have a higher doping density (p ≥ 1x1018 cm-3) to maximize depletion in the n--pillar. The p-body and n+-source layers are grown by subsequent epitaxial growth across the whole structure. Our current research into optimized trench gate fabrication and gate dielectric processing has been used to fabricate gate trenches into the structure. These devices can be compared to traditional trench MOSFETs with extremely thick blocking layers of comparable thickness to the pillar junction layer in order to determine the role of defects in thick GaN epi layers in terms of limiting device performance and reliability.

This work summarizes findings from GATE (GEANT4 Applications for Emission Tomography) simulations with the incorporation of certain experimentally-measured, ‘realistic' parameters in the data analysis. The simulation models a large volume cadmium zinc telluride (CZT) preclinical PET system with sub 9% energy resolution and sub-mm spatial resolution. 2 of the 4 heads of the final design have been constructed, each consisting of a 4x3 matrix of dual-CZT detector modules. Each detector module comprises two 40x40x5 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> CZT crystals with 39 anode strips (100 um width, 1mm pitch each) on the top face and 8 cathode strips (4.9 mm width, 5 mm pitch each) on the bottom face. The crystals are stacked in an anode-cathode-cathode-anode configuration to form the detector module. Photons enter in an "edge-on" orientation, traversing a minimum of 40 mm of CZT which increases detection efficiency. A simulation of the system was constructed in GATE, and data were recorded as "hits," which are ideal, non-blurred interactions, to retain multiple interaction event information within the modeled detector modules. A preliminary energy system response function was generated using (a) photo-peak centroid vs FWHM for Ge-68, Ba-133, and Cs-137 point source data to form an energy-dependent normal distribution to randomly blur energy per simulation interaction, and (b) a Ge-68 point source-measured depth corrected energy vs cathode to anode (C/A) ratio energy plot which was used as a mask over a simulated C/A vs energy plot, isolating allowable C/A ratio-energy combinations. The energy resolution of the system measured with the Ge-68 point source was found to be 8.1%. After applying the measured energy system response function, the simulated energy resolution was found to be 6.3%. The extracted average number of interactions per 511 keV photon after application of the various experimental parameters was 1.05.

Cadmium manganese telluride (CdMnTe) is one of the tertiary compounds of cadmium telluride (CdTe) that has shown great promise in the detection of X-rays and gamma-rays at room temperature without cryogenic cooling. While cadmium zinc telluride (CdZnTe) wide band gap semiconductor has been widely researched and developed to high resolution detector, CdMnTe has comparatively received less effort in development. It is however expected that a more homogeneous CdMnTe crystal with lesser defects than CdZnTe will be easier to grow since the segregation coefficient of Mn in CdTe is closer to unity than that of Zn. This paper briefly reviews the material properties and growth techniques for CdMnTe and presents progress in the development of CdMnTe detectors. Three successive Bridgman growths of CdMnTe resulted in detector grade crystals.