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Mercuric iodide sensor technology

Description: This report describes the improvement in the performance and the manufacturing yield of mercuric iodide detectors achieved by identifying the dominant impurities, carrier traps, and processing steps limiting device performance. Theoretical studies of electron and hole transport in this material set fundamental limits on detector performance and provided a standard against which to compare experimental results. Spectroscopy techniques including low temperature photoluminescence and thermally stimulated current spectroscopy were applied to characterize the deep level traps in this material. Traps and defects that can be introduced into the detector during growth, from the contact, and during the various steps in detector fabrication were identified. Trap energy levels and their relative abundances were determined. Variations in material quality and detector performance at the micron scale were investigated to understand the distribution in electric field in large volume detectors suitable for gamma-ray spectroscopy. Surface aging and contact degradation was studied extensively by techniques including atomic force microscopy, transmission electron microscopy, and variable angle spectroscopic ellipsometry. Preferred handling and processing procedures for maximizing detector performance and yield were established. The manufacturing yield of high resolution gamma-ray detectors was improved from a few percent to more than 30%.
Date: September 1, 1996
Creator: James, R.B.; Anderson, R.J. & Schlesinger, T.E.
Partner: UNT Libraries Government Documents Department

Defects and impurities in mercuric iodine processing

Description: In the fabrication of mercuric iodide HgI{sub 2} room temperature radiation detectors, as in any semiconductor process, the quality of the final device is very sensitive to the impurities and defects present. Each process step can change the effects of existing defects, reduce the number of defects, or introduce new defects. In HgI{sub 2} detectors these defects act as trapping and recombination centers, thereby degrading immediate performance and leading to unstable devices. In this work we characterized some of the defects believed to strongly affect detector operation. Specifically, we studied impurities that are known to be present in typical HgI{sub 2} materials. Leakage current measurements were used to study the introduction and characteristics of these impurities, as such experiments reveal the mobile nature of these defects. In particular, we found that copper, which acts as a hole trap, introduces a positively charged center that diffuses and drifts readily in typical device environments. These measurements suggest that Cu, and related impurities like silver, may be one of the leading causes of HgI{sub 2} detector failures.
Date: March 1, 1996
Creator: van Scyoc, J.M.; James, R.B.; Schlesinger, T.E. & Gilbert, T.S.
Partner: UNT Libraries Government Documents Department

Correlation between nuclear response and defects in CZT

Description: Vertical high pressure Bridgman (VHPB) was considered until now to be the most successful crystal growth method to produce Cd{sub 1{minus}x}Zn{sub x}Te (CZT), (0.04 < x < 0.24), for X- and gamma-ray detector crystals. Recently Horizontal Bridgman (HB) Cd{sub 1{minus}x}Zn{sub x}Te crystals produced by IMARAD Co. have also been successfully fabricated into nuclear spectroscopic radiation detectors. In view of the database of many years' study of the electrical properties of VHPB CZT grown and obtained from various sources, the authors also studied the HB CZT crystals in order to compare the defects present in both different kinds of crystals grown by different methods. The VHB-grown samples were examined using thermoelectric emission spectroscopy (TEES), X- and gamma ray spectroscopy and laser induced transient charge technique (TCT). The surface and the bulk crystalline homogeneity were mapped using triaxial double crystal x-ray diffraction (TADXRD) and infrared transmission spectroscopy (IR). They have found a correlation between crystallinity, IR transmission microstructure and trapping times. Spectrometer grade VHPB CZT crystals exhibit trapping times of 20 {micro}s for electrons and 7 {micro}s for holes, however, regions, which were opaque to IR transmission, had trapping times shorter by one order of magnitude. The trapping times of HB CZT for electrons, were 10--15 {micro}s. A similar trend has been observed on VHPB CZT crystals with poor crystallinity. The HB CZT crystals that they measured in this study had a crystallinity that was inferior to that of the best spectroscopic grade VHPB crystals.
Date: July 19, 1999
Creator: Hermon, H.; Schieber, M.; James, R. B.; Lee, E.; Cross, E.; Goorsky, M. et al.
Partner: UNT Libraries Government Documents Department

Integration of electro-optic lenses and scanners on ferroelectric LiTaO{sub 3}

Description: An integrated electro-optic lens/scanner device was fabricated on a ferroelectric LiTaO{sub 3} wafer. This was done using lithographically defined domain-inverted regions extending through the crystal thickness. A lens power of 0.233 cm{sup {minus}1}kV{sup {minus}1} and a scanner deflection of 12.68 mrad{sup {minus}1}kV{sup {minus}1} was observed. The authors also demonstrate an electro-optic lens stack collimator which collimates an input beam focused to a 5{micro}m waist diameter at {minus}2.3kV.
Date: March 13, 1999
Creator: Gopalan, V.; Gahagan, K.T.; Jia, Q.X.; Tobinson, J.M.; Mitchell, T.E.; Schlesinger, T.E. et al.
Partner: UNT Libraries Government Documents Department

Fine-scale spatial response of CdZnTe radiation detectors

Description: Several studies have suggested that the uniformity of Cadmium Zinc Telluride (CZT) detectors play an important role in their performance when operated as gamma-ray spectrometers. However the detailed gamma response of simple planar detectors as a function of position over the device area is largely unknown. To address this issue the authors have built a system capable of measuring the detector response with a resolution of {approximately}250 {micro}m. The system consists of a highly collimated ({approximately}200 {micro}m) photon source (<150 kev) scanned over the detector using a computer controlled two-axis translation stage. Fifteen samples configured as planar detectors were examined with the new apparatus. The material grade of the detectors examined varied from counter to select discriminator. Two classes of spatial response variation were observed and are presented here. Infrared (IR) transmission images were also acquired for each sample and correlation between features in the pulse height spectrum and crystalline defects were observed.
Date: November 7, 1998
Creator: Brunett, B. A.; Scyoc, J. M. Van; Hilton, N. R.; Lund, J. C.; James, R. B. & Schlesinger, T. E.
Partner: UNT Libraries Government Documents Department

Fabrication and characterization of high-speed integrated electro-optic lens and scanner devices

Description: Uniaxial strain impact experiments have been performed to obtain shock compression and release response of a 0.22 g/cm{sup 3} polyurethane foam in a configuration where the foam impacts a thin target witness plate. Wave profiles from a suite of ten experiments have been obtained, where shock amplitudes range from 40 to 500 MPa. A traditional P-{alpha} porous material model generally captures the material response. A fully three-dimensional explicit representation of the heterogeneous foam structure modeled with numerical simulations recovers some of the high frequency aspects of the particle velocity records.
Date: February 1, 1999
Creator: Gahagan, K.T.; Gopalan, V.; Jia, Q.X.; Kawas, M.J.; Mitchell, T.E.; Robinson, J.M. et al.
Partner: UNT Libraries Government Documents Department

Evaluation of CZT crystals from the former Soviet Union

Description: Vertical high pressure Bridgman (VHPB) Cd{sub 1{minus}x}Zn{sub x}Te (0.04 < x < 0.24) detector crystals grown in the Ukraine and Russia have been evaluated and compared to US-grown materials. Various analytical techniques were used to study the materials for trace impurities, precipitates, crystallinity, and electrical transport properties. Relatively high concentrations of carbon and trace impurities such as Se, Nd and Si have been detected in the crystals. In most cases, the crystals showed lower resistivity than US-grown CZT. However, recent crystals grown in Russia exhibited better detector performance than those grown in prior years, and good response to an {sup 241}Am radioactive source was found. Electron lifetimes below 1 {micro}s were measured in crystals having significant numbers of micro-defects, compared to lifetimes of 5--15 {micro}s found in spectrometer grade materials produced in the US. Furthermore, the zinc composition along the growth axis showed better homogeneity in comparison with the US material.
Date: January 26, 1998
Creator: Hermon, H.; Schieber, M.; James, R. B.; Antolak, A. J.; Morse, D. H.; Brunett, B. et al.
Partner: UNT Libraries Government Documents Department

Multi-parameter high-resolution spatial maps of a CdZnTe radiation detector array

Description: Resistivity results from a 48x48 pixelated CdZnTe (CZT) radiation detector array are presented alongside X-ray topography and detector mapping with a collimated gamma-ray beam. By using a variety of measurements performed on the same sample and registering each data set relative to the others, the spatial dependence of relationships between them was examined. The local correlations between resistivity and one measure of detector performance were strongly influenced by the positions of grain boundaries and other gross crystal defects in the sample. These measurements highlight the need for material studies of spatially heterogeneous CZT to record position information along with the parameters under study.
Date: November 7, 1998
Creator: Hilton, N. R.; Barber, H. B.; Brunett, B. A.; Eskin, J. D.; Goorsky, M. S.; James, R. B. et al.
Partner: UNT Libraries Government Documents Department