Amlan Datta

Study of Different Cool Down Schemes During the Crystal Growth of Detector Grade CdZnTe

Cd0.9Zn0.1Te crystals were grown using a modified vertical Bridgman growth technique to produce radiation detector grade material. Motivated by the importance of the retrograde solubility problem in II-VI compound semiconductors, particularly CZT, different cool down techniques were used to observe the effects on the secondary phase (SP) size, distribution, density and resulting detector properties. Presented are four of the cool down schemes performed and the results in terms of the secondary phases and charge transport characteristics of the grown crystals. The cooling of the grown ingot to ambient temperatures was done over various lengths of time. The differences between the cool down methods are in the rates of cooling through the retrograde solubility phase. Apart from these cool downs, quenching studies were also performed on some crystal growths giving unique results in terms of the secondary phase distribution and characteristics as radiation detectors.

Characterization of Stress in Thallium Bromide Devices

Thallium bromide (TlBr) is a wide bandgap, compound semiconductor with high gamma-ray stopping power and promising physical properties. Several surface modification techniques have been demonstrated to increase the lifetime of TlBr devices at room temperature. However, absence of reproducibility in the performance of TlBr detectors (even with low ionic conduction at -20°C) suggests presence of unexplored bulk phenomena. Stress in the TlBr crystal due to various intrinsic (e.g. grain boundaries and dislocations networks) in conjunction with external factors such as thermal, mechanical, and electrical loadings explains detector-to-detector variations. Photoelasticity and opto-electrical techniques were applied to visualize and qualitatively correlate the device performance with stress. Changes in stress patterns with variations in ambient temperature were clearly demonstrated. Electric field fluctuations in TlBr detectors with time were for the first time observed using the Pockels effect.

Variations in μτ measurements in cadmium zinc telluride

A number of commonly employed experimental methods used to determine the μτ product in the semiconductor CdZnTe were investigated. The objective was to determine possible differences in results inherent in the techniques. A pixelated and two planar devices were studied using various distinct methods together with different excitation sources and amplifier shaping times. Variations in the results up to a factor of 5 were found. Variations due to shaping times, particle type, and energy were evident.

Performance limiting processes in room temperature thallium bromide radiation detectors

Thallium Bromide (TlBr) is a promising room-temperature radiation detector candidate with excellent charge transport properties. However, several critical issues are needed to be addressed before deployment of this material for long-term field applications. In this paper, the relevance and, scientific and technological progress made towards solving these challenges for TlBr have been discussed. The possible research pathways to mitigate the concerns related to this material have been analyzed and clearly established. Findings from novel experiments performed at CapeSym have revealed that the most significant factors for achieving long-term performance stability for TlBr devices involve physical and chemical conditions of the surface, residual stress, and choice of metal contacts. Palladium electrodes on TlBr devices resulted in a 20-fold improvement in the device lifetime when compared to its Br-etched Pt counterpart. Electron and hole contributions towards the spectroscopic response of the TlBr detector significantly depend on the interaction position of the incoming radiation and was clearly observed in this study. TlBr device fabrication techniques need significant improvement in order to attain reliable, repeatable, and stable, long-term performance.

The Influence of Cation Impurities on the Scintillation Performance of

To better identify the influence of cation impurities on the scintillation performance of SrI2(Eu), SrI2 crystals were grown, each co-doped with 4 mol% Eu^{2 +} and 0.2 mol% of one of the following: Mg^{2 +}, Ba^{2 +}, Cs⁺, Ca^{2 +}, Fe^{2 +}, Cu⁺, Na⁺, and Sn^{2 +}. Four 10 mm diameter crystals were grown at a time by the vertical Bridgman-Stockbarger method. The segregation behavior and the scintillation performance of 10 mm dia.×6 mm cylinders and 7 mm×6 mm×2 mm cuboids were characterized. Mg^{2 +}, Cs⁺, Fe^{2 +}, and Cu⁺ impurities did not adversely affect scintillation properties, and segregated during growth. However, Na⁺, Ba^{2 +}, and Ca^{2 +} did not segregate well and degraded light yield and energy resolution, especially at the larger sample size. Sn^{2 +} proved to be the most detrimental to light yield and produced a secondary emission peak at 600 nm, but did not affect the non-proportionality response. The results of this study suggest that SrI2(Eu) can tolerate a surprisingly large amount of cation impurities. These findings suggest that the purity requirements for starting materials can be relaxed, and purification efforts may be adjusted to target only the most harmful impurities.

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Thallium bromide (TlBr) is a wide bandgap, compound semiconductor with high gamma-ray stopping power and promising physical properties. However, performance degradation and the eventual irreversible failure of TlBr devices can occur rapidly at room temperature, due to “polarization”, caused by the electromigration of Tl+ and Br- ions to the electrical contacts across the device. Using the Accelerated Device Degradation (ADD) experiment, the degradation phenomena in TlBr devices have been visualized and recorded. This paper focuses on “ageing” of the device cathode at various temperatures. ADD is a fast and reliable direct characterization technique that can be used to identify the effects of various growth and post-growth process modifications on device degradation. Using this technique we have identified cathode degradation with the migration of Br- ions and an associated generation and growth of Thallium-rich fractal “ferns” from the cathode. Its effect on the radiation response of the device has also been discussed in this paper. The chemical changes in the cathode were characterized using Energy-dispersive X-ray spectroscopy.

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Achieving a high yield of detector grade CdZnTe single crystals is one of the greatest challenges in CdZnTe crystal growth. Over 85, 3.5-4 inch long CdZnTe crystals were grown in a 43 zone modified low pressure Vertical Bridgman Electro-Dynamic Gradient (EDG) Freeze 3 in. Mellen Sunfire furnace. The diameter of the grown ingots has been increased using a new Electro-Dynamic Gradient (EDG) Freeze 4 in. Mellen Sunfire Platinum coil furnace to visualize the effects of increased ampoule diameter on the grain structure and spectroscopic properties of CdZnTe. The reproducibility of detector grade crystals using the 4 in. furnace is discussed and some properties of the fabricated detectors are presented.

Cathode Degradation in Thallium Bromide Devices

The scintillation radiation detection community is on the cusp of a major breakthrough with the potential deployment of Europium-activated Strontium Iodide (SrI2:Eu) detectors for medical imaging and homeland security applications. Compared to the traditional scintillators (such as NaI), SrI2 provides much better energy resolution and light output. The crystal growth of SrI2:Eu has been impaired for a long time due to cracking problems, which makes it highly unreliable and nonreproducible. This significantly increases the cost of the material which in turn impedes wide-scale deployment and limits its advantages over other scintillators. In this paper, we demonstrate a technique of growing crack-free SrI2:Eu crystals by monitoring the stoichiometry of the melt atmosphere during processing and crystal growth. Using the feedback information from the in situ monitoring technique, the stoichiometry of the melt was corrected and multiple crack-free SrI2:Eu crystals of diameters 1.5 inches were repeatedly grown using Bridgman configuration with no visible inclusions, bubbles or defects whatsoever.

Modified vertical bridgman growth of Cd 1−x Zn x Te detector grade crystal in a 4 inch EDG furnace

To better identify the influence of light anion impurities on the scintillation performance, small boules of SrI₂(Eu) were grown by the vertical Bridgman-Stockbarger method, each co-doped with 0.2% of one of the following: C⁰, CO3^2-, N^3-, O^2-, OH^-, PO4^3-, S^2-, SO4^2-, Cl^- and Br^-. Residual impurity concentrations were measured, and the scintillation performance of resulting detectors was characterized. Oxygen was tolerated up to 0.2% on a molar basis. Sulfur proved to be highly detrimental to both crystallinity and scintillation performance. Nitrogen produced additional emission near 480 nm. This study suggests that SrI₂(Eu) readily incorporates anion impurities, which may substitute for iodine, but these may also be removed before and during growth by volatilization. Purity metrics for starting materials should include sulfur and carbon, as well as oxygen and H₂O.

Crystal growth of large diameter strontium iodide scintillators using in Situ stoichiometry monitoring

Cadmium Zinc Telluride (CdZnTe/CZT) crystals were grown using a modified vertical Bridgman growth technique with 10 % Zn concentration at Washington State University (WSU). Analyses of the effects of volume (vol.) %, number density (cm-3), mean diameter (μm) of secondary phases (SPs) and thickness (mm) of the CZT crystals on single crystal properties such as carrier mobility lifetime (μτe) and resistivity (ρ) were performed. Some correlations were observed between μτe values of different CZT crystals and vol. %, number density, mean diameter of SPs and thickness of the crystals. High μτe and lower SP vol. % values were obtained for the ingots grown with rapid cool down times and with no intentional amounts of excess Te/Cd. For the selected samples, the effects of the SPs on the μτe values were established for the SPs whose mean diameters were ≤4 μm and >4 μm. These studies indicate vol. % and mean diameter of SPs are the important parameters for CZT crystal performance as a radiation detector.