Haim Hermon

Comparison of PbI2 and HgI2 for direct detection active matrix x-ray image sensors

The factors determining the x-ray sensitivity of HgI2 and PbI2 as direct detector materials for large area matrix addressed x-ray image sensors are described, along with a model to explain their different properties. The imaging studies are made on test arrays with 512×512 pixels of size 100 μm. The x-ray sensitivity and spatial resolution are reported, along with measurements of the various mechanisms that influence the sensitivity, such as charge collection, x-ray absorption, fill factor, and image lag. The spatial resolution of PbI2 decreases with increasing film thickness, but this effect is not observed in HgI2. The x-ray response data are used to compare the sensitivity to the theoretical values for the ionization energy and to identify the various loss mechanisms. We find that the sensitivity of HgI2 can be explained by a few small and well characterized loss factors. This material exhibits good spatial resolution, high fill factor, and high charge collection. PbI2 films exhibit lower sensitivity, ...

Thick films of X-ray polycrystalline mercuric iodide detectors

Polycrystalline HgI 2 thick film detectors are among the leading semiconductor materials to be used as direct converters in X-ray digital radiography. Their properties along with a survey of the properties of alternative materials, such as PbI 2 or A-Se, will be given. The preparation of HgI 2 detector plates, both by direct evaporation (Physical vapor deposition, (PVD)) and by binding the individual crystallites with polymeric glue, forming screen-printed (SP) detector plates, will be described. The microstructure of the PVD thick films showing a columnar morphology, as determined by SEM measurements, will be shown. The X-ray response to radiological X-ray generator of 85 kVp using the current integration mode will be reported for both PVD and SP films. Finally, some X-ray images taken at Xerox-Parc using HgI 2 polycrystalline detectors will be shown.

Polycrystalline mercuric iodide detectors

The fabrication of polycrystalline HgI2 thick film detectors using the hot wall physical vapor deposition, method is described. The X-ray response of these detectors to a radiological X-ray generator of 60 kVp has been studied using the current integration mode. The response expressed in (mu) A, the dark current expressed in pA/cm2 and sensitivity expressed in (mu) C/R(DOT)cm2 are given for these detectors for several thickness and grain sizes. The optimal sensitivity is compared with published data on the response to X-rays by polycrystalline PbI2 and A-Se detectors.

High-resolution direct-detection x-ray imagers

We report on a-Si direct detection x-ray image sensors with polycrystalline PbI2, and more recently with HgI2. The arrays have 100 micron pixel size and, we study those aspects of the detectors that mainly determine the DQE, such as sensitivity, effective fill factor, dark current noise, noise power spectrum, and x-ray absorption. Line spread function data show that in the PbI2 arrays, most of the signal in the gap between pixels is collected, which is important for high,DQE. The leakage current noise agrees with the expected shot noise value with only a small enhancement at high bias voltages. The noise power spectrum under x-ray exposure is reported and compared to the spatial resolution information. The MTF is close to the ideal sinc function, but is reduced by the contribution of K-fluorescence in the PbI2 film for which we provide new experimental evidence. The role of noise power aliasing in the DQE and the effect of slight image spreading are discussed. Initial studies of HgI2 as the photoconductor material show very promising results with high x-ray sensitivity and low leakage current.

Comparative study of Pbl2 and Hgl2 as direct detector materials for high-resolution x-ray image sensors

X-ray imaging properties are reported for HgI2 and PbI2, as candidate materials for future direct detection x- ray image sensors, including the first results from screen- printed HgI2 arrays. The leakage current of PbI2 is reduced by using new deposition conditions, but is still larger than HgI2. Both HgI2 and PbI2 have high spatial resolution but new data shows that the residual image spreading of PbI2 is not due to k-edge fluorescence and its possible origin is discussed. HgI2 has substantially higher sensitivity than PbI2 at comparable bias voltages, and we discuss the various loss mechanisms. Unlike PbI2, HgI2 shows a substantial spatially non-uniform response that is believed to originate from the large grain size, which is comparable to the pixel size. We obtain zero spatial frequency DQE values of 0.7 - 0.8 with PbI(subscript 24/ under low energy exposure conditions. A model for signal generation in terms of the semiconducting properties of the materials is presented.

High-flux x-ray response of composite mercuric iodide detectors

A theological model is presented which analyses the sensitivity of composite detectors to a flux of x-rays emerging form a radiological x-ray generator. The model describes the many factor which influenced the x-ray response, for the case where the detector is composed of several layers of crystallites separated by a polymeric glue as is the case of composite HgI2 detectors fabricated by the screen print method. The model also describes the variation of the sensitivity with grain size and dielectric constant, taking into account the dielectric constant of the binder showing also the experimental result. Finally, the experimental result of the sensitivity vs. the voltage is shown for single crystal and composite HgI2 detectors and these results are compared with polycrystalline PbI2 and a-Se, which are the main material candidates for medical digital radiology.

Mercuric iodide thick films for radiological X-ray detectors

For the first time polycrystalline HgI2 photoconductor material directly evaporated on a-Si array for direct conversion of x-rays for imaging purposes, were successfully imaged at Xerox-Palo Alto Research Center. The initial results are very promising and show a high x-ray sensitivity and low leakage current. Since Ti-W alloys are used as pixel electrodes, an intermediate passivation layer must be used to prevent a chemical reaction with the detector plate. The thickness that these Polycrystalline HgI2 thick film detectors have been fabricated until now is up to 1,800 micrometers , which makes them useful also for high energy applications. The characterization of the Polycrystalline HgI2 thick films deposited with or without the passivation layers by measuring their dark currents, sensitivity to 65 and 85 kVp x-rays and residual signals after 1 minute of biasing, will be shown for several detectors. Some preliminary results will be shown for some novel screen-printed HgI2 detectors.

Radiological x-ray response of polycrystalline mercuric-iodide detectors

A first image of some tiny screws were obtained for the first time with polycrystalline HgI2 acting as the photoconductor material deposited on a-Si direct conversion X- ray image sensors, produced by Xerox -- Palo Alto Research Center. The initial results are very promising and show a high X-ray sensitivity and low leakage current. The response of these detectors to a radiological X-ray generator of 65 kVp has been studied using the current integration mode. Already its sensitivity expressed in (mu) C/R*cm2, is very high, values of 20 (mu) C/R*cm2 have been measured for films of 100 - 250 microns thickness and bias of 50 - 200 volts respectively, which is superior to the published data for competing materials such as polycrystalline PbI2 and a-Se detectors. The fabrication and characterization measurements of the Polycrystalline HgI2 thick film detectors will be given. The characterization data which will be reported here consists of: (a) sensitivity, (b) dark currents, (c) stability of sensitivity dependence on the number of exposure, (d) X-ray response dependence on dose energy and (e) signal decay dependence on the number of exposures.

Stoichiometry and electrical charge transport in HgI2 crystals

Abstract Stoichiometry of vapor-grown HgI 2 (mercuric iodide) crystals has been studied quantitatively using Raman spectroscopy and liquid solution spectrophotometry. Evaluation of mercury excess is based on differentiation between the Raman spectra of HgI 2 and Hg 2 I 2 (mercurous iodide). Iodine excess is determined by dynamic dissolution of HgI 2 in CC1 4 (tetrachloromethane) and measurements of the characteristic I 2 absorption intensity at 517 nm. The threshold for stoichiometric analysis has been lowered to about 10 ppm, which is an order of magnitude better than that achieved by other methods. Crystals grown from starting materials subjected to the standard purification by repeated sublimations, melting and additional sublimation have been found nearly stoichiometric regardless of the synthesis method of the compound. It is shown that both mercury and iodine rich crystals can be grown by controlling the composition of the starting material or the growth atmosphere. Large excess of either mercury or iodine ( > 100 mole ppm) causes a noticeable deterioration in the energy resolution of HgI 2 nuclear radiation detectors, which is discussed in terms of trapping of radiation-induced charge carriers by nonstoichiometric defects. The kinetics of trapping have been studied using the transient charge technique.

Comparison of cadmium zinc telluride crystals grown by horizontal and vertical Bridgman and from the vapor phase

Characterization studies of Cd1−xZnxTe (0<x<0.24) crystals, CZT, grown by high pressure vertical Bridgman (HPVB), low pressure (LPB) vertical modified Bridgman (VB), horizontal modified Bridgman (HB), and physical vapor deposition (PVD) methods were performed. For selected melt-grown ingots, the liquid/solid segregation coefficients of some of the impurities were established. For most of the crystals, the surface and the bulk crystallinity were determined using triple and double axis X-ray diffraction techniques (TAD and DAD XRD). X-ray topography maps were also used to study macroscopic defects. The difference in properties of CZT grown by these methods are discussed.