M. Schieber

Study of impurity segregation, crystallinity, and detector performance of melt-grown cadmium zinc telluride crystals

A review of growth methods used to produce Cd 1-x Zn x Te (CZT) (0.0<x<0.20) crystals for radiation detector applications is presented. Most of the results emphasize the high-pressure Bridgman (HPB) method. For selected melt-grown HPB ingots, the liquid/solid segregation coefficients of some impurities were measured. The correlation of the impurity content and nuclear detector performance will be discussed. Extended defects and surface and bulk crystallinity were measured using triple and double axis X-ray diffraction techniques (TAD and DAD-XRD), X-ray topography, and infrared microscopy. X-ray diffraction maps and IR images were generated and compared to gamma-ray detector tests to correlate macroscopic defects with the nuclear detector responses. Defects states of CZT were also investigated using low-temperature photoluminescence spectroscopy. Comparisons between the material and detector properties for different CZT growth methods will be discussed.

Near single-crystal electrical properties of polycrystalline HgI/sub 2/ produced by physical vapor deposition

Polycrystalline films of HgI/sub 2/ Prepared by Physical Vapor Deposition (PVD) exhibited electrical charge transport properties similar to those of single crystals. Transient charge transport (TCT) measurements were used to evaluate their electrical properties. The mobility /spl mu/, trapping time /spl tau/, and surface recombination velocity s of electrons or holes were determined by analyses of transient voltages developed across the sample in response to a drift of the corresponding charge carriers created by alpha particle absorption near one of the electrodes. Typical electron-, and hole mobilities were /spl mu//sub n/ = 88 cm/sup 2//V/spl middot/s and /spl mu//sub p/ = 4.1 cm/sup 2//V/spl middot/s, respectively; Trapping times were /spl tau//sub n/ >/spl tilde/16 /spl mu/s and /spl tau//sub p/ </spl tilde/ 3.5 /spl mu/s, and surface recombination velocities s/sub n/ /spl cong/ 1.4/spl times/10/sup 5/ cm/s and s/sub p/ /spl cong/ 3.7/spl times/10/sup 3/ cm/s. All parameters depend to a large extent on the material deposition technology. The effect of carriers being first generated in near-surface traps and then gradually released is observed.

Efficiency of composite boron nitride neutron detectors in comparison with helium-3 detectors

It has been clearly demonstrated that the composite boron nitride (BN) semiconductor polycrystalline bulk detectors with BN grains embedded in a polymer matrix operate as an effective detector of thermal neutrons even if they contain natural boron only. A reasonable signal to noise ratio has been achieved with detector thickness of about 1mm. A Monte Carlo simulation of neutron reactions in the BN detector was done to estimate the thermal neutron detection efficiency. The result was compared with widely used He3 based detectors to prove advantages of BN detectors, which are especially promising for neutron imaging and for large area sensors.

Theoretical and experimental sensitivity to X-rays of single and polycrystalline HgI2 compared with different single-crystal detectors

Abstract The response for medical X-ray generator of single and polycrystalline HgI2 to 65xa0kVp X-rays has been compared to the response of single-crystal CdZnTe and PbI2 detectors using the current integration mode. The dark current density expressed in pA/mm2 and sensitivity expressed in μC/R×cm2 are given for these detectors for several thicknesses. The polycrystalline HgI2 thick film detectors were fabricated using the hot wall physical vapor-phase deposition (PVD) method.

Improved technique for HgI2 crystal growth

Abstract A modified vapor growth furnace for HgI 2 and a new two-parts growth ampoule containing a removable seal has been developed. The new ampoule allows several growth sequences during the same or different experiments. The new furnace is also equipped with a 3D moving cold finger for the control of selective nucleation, which reduces the total time required for a given growth run. The new ampoule permits introduction of a large variety of substrates (up to 5×5 cm). Controlled amounts of doping materials are move easily charged into the new ampoule even in the middle of a growth run.

Lead Iodide X-Ray and Gamma-Ray Spectrometers for Room and High Temperature Operation

In this study the authors report on the results of the investigation of lead iodide material properties. The effectiveness of a zone refining purification method on the material purity is determined by ICP-MS and ICP-OES and correlated to the electrical and physical material properties. They show that this zone refining method is very efficient in removing impurities from lead iodide, and they also determine the segregation coefficient for some of these impurities. Triple axis X-ray diffraction (TAD) analysis has been used to determine the crystalline perfection of the lead iodide after applying various cutting, etching and fabrication methods. The soft lead iodide crystal was found to be damaged when cleaved by a razor blade, but by using a diamond wheel saw, followed by etching, the crystallinity of the material was much improved, as observed by TAD. Low temperature photoluminescence also indicates an improvement in the material properties of the purified lead iodide. Electrical properties of lead iodide such as carrier mobility, were calculated based on carrier-phonon scattering. The results for the electrical properties were in good agreement with the experimental data.

Electrical charge transport properties of HgI2 intercalated with hydrocarbons and/or doped with excess mercury and iodine

X-ray diffraction (XRD) and transient charge technique (TCT) were applied to study the lattice parameters and the electronic properties of mercuric iodide (HgI2) crystals doped with hydrocarbons (HC) and excess of iodine or mercury. The lattice spacing along the c-axis is strongly influenced by HCs doping, and expansion of up to 4% has been measured. Smaller changes of lattice parameters have been measured on crystals with iodine or mercury excess. It is suggested that the incorporation of HCs into HgI2 lattice occurs mainly by intercalation, where excess of mercury or iodine are incorporated as interstitials or as vacancy-type defects which induce more traps for charge carriers as reflected by shorter trapping times.

Raman and infrared spectroscopy and transport properties of aged organometallic precursors used for the deposition of high-Tc superconducting films

Abstract Raman and infrared (IR) spectra of Ba, Sr and Ca 2,2,6,6-tetramethyl-3,5-heptanedionates (THD), volatile precursors used for organometallic chemical vapor deposition (OMCVD) of high- T c superconducting (HTCS) thin films, have been studied for freshly supplied commercial materials or those prepared in our laboratory and aged for several months in vacuum and in air. Some of the spectral lines present in freshly prepared material disappeared in precursors aged in air but were unchanged if aged in vacuum. Only freshly prepared or vacuum-stored Ba(THD) 2 could be 100% transported by sublimating at 210°C. Air aged material transported only 60% under the same conditions of evaporation. Mixing 20% aged Ba(THD) 2 with 80% naphthalene achieved 100% transport of Ba(THD) 2 .

Magnetic relaxation and pinning in Ag/Bi(Pb)SCCO tapes: deformation-induced effects

Magnetic hysteresis and magnetic relaxation in as-sintered and cold-pressed after sintering Ag/Bi-2223 tapes have been studied in the temperature range of 5-45 K for two field orientations: -axis and -plane. Significantly higher effective pinning energy, , for as compared to that for , is connected with the mechanism of intrinsic pinning for the vortex motion normal to the atomic planes. Cold pressing results in the reduction of magnetization and transport inter-granular currents due to the introduction of additional weak links and microcracks into a sample. For the case the drastic drop in relaxation rate as well as the increase in the field of the first large scale vortex penetration, , was observed after cold pressing, indicating the enhancement of intra-granular and surface pinning by generated defects. Accordingly , the increase of in the limit of low currents was observed after cold pressing. Warm pressing which normally generates significantly lower concentration of residual defects than cold pressing, does not result in any inhibition of relaxation. For in-plane field vortices penetration into a tape interior through microcracks is the dominating deformation-induced effect resulting in the acceleration of magnetic relaxation and the significant decrease of .

State of the art of wide-bandgap semiconductor nuclear radiation detectors

SummaryThe leading materials which operate as room temperature nuclear radiation detectors are HgI2, CdTe, and Cd1−xZnxTe (0.05>x>0.25). However, additional materials have also been developed, such as semi-insulating GaAs and PbI2. A comparison of the charge transport properties of all these materials will be made, followed by a discussion of each of the materials separately. Crystal growth methods of spectrometer-grade materials will be mentioned, and defects which limit their performance will be discussed. Nuclear spectra measurements with detectors fabricated from these materials, for low X-ray energies as well as for high-energy gamma-rays, will be shown. Polarization effects which occur in some detectors such as HgI2 will also be discussed. Correlation between crystalline perfection and detector performance will be shown. Results of quantitative chemical analysis of various detector materials and problems encountered in determining accurate values ofx in Cd1−xZnxTe and its homogeneity in the bulk will be presented. Finally, the present state of the art and developments for the near future will be discussed.