n-and p-type doping of CdTe grown heteroepitaxially on GaAs substrate by radical assisted metal organic chemical vapor deposition technique were studied. n-type doping was studied by the gas phase n-butyliodine doping during the film growth, whereas p-type doping was carried by treating the undoped layers with alkaline metal compound (Na2Te) and excimer laser radiation. Highly conductive n-and p-layers were thus obtained which formed good ohmic contact with aluminum and gold electrodes, respectively. Using this technique, n-and p- layers were formed on intrinsic CdTe substrate to form p-i-n diode. This p-i-n structure showed a good diode characteristics and good sensitivity to X-ray radiation with dark current in the order of nA/mm 2 at room temperature.

In recent years the performance of room-temperature semiconductor detectors such as CdTe are improved and they are now suitable candidates for several applications. However, some key parameters that can severely affect such perfomances have not been measured yet. Thus we have studied the damaging of a set of CdTe detectors irradiated in a 60Co gamma-cell in a wide range of doses and dose-rates. A full characterization of the performance of irradiated detectors has been obtained by means of spectroscopic, electrostatic, photo-induced current transient spectroscopy and photo-deep level transient spectroscopy measurements to quote the energy resolution, the leakage current, the activation energy and capture cross-section of deep level defects, respectively.

This paper deals with a comparison between experimental gamma-ray spectra recorded with CdTe and CdZnTe detectors and a Monte Carlo code simulating the spectrum, in order to extract realistic values of the transport properties of both electrons and holes in these materials. In a second section, these values are compared to that obtained by others using other methods, in order to evaluate the accuracy of the various experimental procedures.

The electron-hole pair production energy (∼ 4.43 eV) in cadmium telluride semiconductor based detector would allow, in principle, to achieve spectroscopic performance close to germanium. In fact the material impurities, crystal lattice defects and low charge mobility, mainly for holes, can severely degrade the spectroscopic properties through charge trapping/detrapping and recombination phenomena, limiting the usable distance between the electrodes. The charge collection efficiency depends, following the Hecht relation, on the distance between the charge production location and the collecting cathode; we have studied the dependence of some CdTe spectroscopic parameters (energy resolution, photo-peak gain and efficiency) from this distance. A set of experimental tests were performed using a well collimated photon beam. The beam spot, obtained from a 57Co radioactive source through a thick (20 mm) tungsten collimator having a 0.2×2 mm window, was positioned between the two electrodes of a CdTe crystal of size 3×5×2 mm3, with the electrodes deposited on the 3×5 mm2 sides.

The detector was irradiated on the face having the area of 2×3 mm2, in the arrangement with the collecting field orthogonal to the incoming radiation (PTF configuration), and the largest side of the spot parallel to the electrode plane. Using a micro-positioning system a fine scanning of the interelectrodic region was performed.

In order to evaluate the detector behavior in a wider energy range, further tests using energies between 14 keV to 662 keV have been performed.

The results of these measurements are presented and discussed in order to emphasize a method to improve CdTe detector spectroscopic capabilities and to understand the detection behavior

Newly designed HgI2 two-dimensional arrays based on resistive charge division, obtained by depositing a Ge surface resistive layer between the readout Pd strips, have been fabricated. These detectors, coupled with very low noise preamplifiers, have shown a high detection efficiency over a large active area and an energy resolution of 15% for X-ray energy Ex =60 keV (241Am), becoming ≈ 9% at Ex = 122 keV (57Co). Further, a spatial sensitivity of ≈ 5 μm and a spatial resolution of ≈ 40 μim in both the directions parallel to the anode and to the cathode strips have been obtained at an energy equivalent to ≈ 40 keV, by making use, for a fine spatial characterization of the devices, of a laser beam spot with wavelength tuned to match the crystal bandgap (582 nm). These devices, with optimized cathode layer resistances, could be successfully employed in basic research (for example Bragg X-ray spectrometry) and for imaging in radiological and space applications, at least in the important energy range 40 <Ex <120 keV.

Preparation of polycrystalline mercuric iodide very thin (1 μm) films using laser ablation and thick films (100–600μm), using hot pressing, hot wall vapor deposition and screen printing methods, fabricated as radiation detector plates are briefly described. X-ray diffraction, photoluminescence and optical microscopic measurements as well as response to nuclear radiation will be given. Finally, recent results obtained with a large area imaging pixel detector will be shown.

The optical, electrical and surface properties of mercuric iodide platelets grown from solution in a HgI2-HI-H2O system were investigated by comparing them with Physical Vapor Transport (PVT) grown crystals. The absence of bulk imperfections and the uniformity of the as-grown surfaces and the KI solution etched surfaces were confirmed by optical microscopy. The as-grown surface uniformity is higher for solution grown crystals than that of PVT crystals, since the platelets do not have to be cleaved or polished. AFM studies show that the roughness for the cleaved, aged and etched surfaces were 0.06 nm, 0.48 nm and 0.3 nm respectively. Low temperature photoluminescence properties were measured for the two kind of crystals and will be discussed. However, I-V curves give higher current density and lower apparent resistivity values for the solution grown than for PVT grown crystals. Correlations between optical and surface quality as well as the electrical properties of the crystals grown from both solution and PVT methods are also discussed.

The hole-mobility•carrier-lifetime product of α mercuric iodide high energy radiation detectors has been enhanced through vapor crystal growth in microgravity. This improvement is closely correlated with specific characteristics of the crystal lattice, which have been identified by high resolution synchrotron x-ray diffraction imaging. These structural features and the associated performance are now being approached in terrestrial growth of α mercuric iodide.

Gravity may affect the uniformity of this crystal lattice in two distinct ways: 1) directly through deformation that it imposes on the lattice during growth and 2) indirectly through convection, which mixes any extraneous material. Inclusions associated with these processes harden the lattice and facilitate lattice folding. These changes affect the electronic parameters of detectors made from the crystals. As purification procedures are optimized, the incorporation of extraneous material is curtailed, enhancing electronic properties in spite of lattice flexing through loss of precipitation hardening.

These studies provide insight into the contribution of various aspects of crystalline order in α-mercuric iodide crystals to property improvement. This knowledge has led to modification of requirements for starting materials, adjustment of physical vapor growth procedures, and change in crystal handling procedures. As a result, the electronic performance of terrestrially grown radiation detectors has been improved, and we provide evidence that further enhancement is still possible.

This paper describes our recent research in developing vacuum sublimed lead iodide films for X-ray imaging. Lead iodide films are promising for this application due to their low dark current, high stopping efficiency, reasonably good charge transport, low cost, and relatively easy scale-up. Lead iodide films (up to 5 × 5 cm2 area) have been grown and characterized by measuring their X-ray imaging properties such as spatial resolution, and contrast transfer function. Excellent spatial resolution (> 10 lp/mm with high CTF ≈50%) has been recorded with PbI2 films. Relevant detection properties such as signal amplitude for given X-ray energy has also been measured and was found to be about 10 times larger as compared to standard phosphor screens used for X-ray imaging. Charge transport and timing characteristics of these films have been measured and the results indicate that these films should be capable of real-time operation. Application of these films for X-ray imaging such as mammography, fluoroscopy, and X-ray diffraction is addressed.

In this study we 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. We show that this zone refining method is very efficient in removing impurities from lead iodide, and we 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.

Lead iodide starting materials have been purified by vertical zone refining process. The purest materials of the zone-refined ingot, middle zone, were used for single crystal growth by Bridgman method. Differential scanning calorimetry, current-voltage and low temperature photoluminescence measurements were employed to investigate the effectness of zone refining process and as-grown crystal. The results showed that the materials from the middle section of zone-refined ingot is the purest, followed by first-to-freeze and last-to-freeze materials. No second phase and deviation from stoichiometry have been observed after zone refining process and Bridgman crystal growth method. The resistivities increase as the purity and crystallinity of materials increases. Cleavage of as-grown crystal did damage the surface and caused additional emission at 2.0 eV. This emission band was not observed after a 10% NaI solution etching.

In this paper we report the success in fabricating FTO/Si surface-barrier photodiodes produced by spray pyrolysis deposition technique, under ambient conditions. Three types of photodetectors for low-voltage-bias operation were developed based on high-resistivity silicon: 1. X-Ray detectors with energy resolution of 16.5% at 661.5 keV (137Cs source), consisting of surface-barrier PIN photodiode with an active area of 50 mm2 operating at 5V reverse bias, scintillator based on monocrystalline Bi4Ge3O12 and preamplifier (noise of 250 e− RMS.); 2. Fastresponse surface-barrier FTO/I n−-n+ silicon epitaxial photodiodes, operating at 10 V bias with rise times of 2 ns at λ = 0.85 μm; 3. Radiation-resistant drift epitaxial surface-barrier PIN photodiodes for unbiased operating conditions, with an exponential impurity distribution in a 8 μm thick epitaxial layer. A built-in electrical field due to the carrier concentration distribution in the epitaxial layer provides a considerable improvement in the “critical fluence” value (3× 1014 cm−2 ) for neutron irradiation.

The paper reports radiation damage experiments which influence properties of image sensors using pin photodiodes and thin film transistors. The effect of 60Co radiation at about 1.2MeV on the noise and drain-source current characteristics of hydrogenated amorphous and nanocrystalline silicon film transistor was examined as a function of the γ dose and hydrogen content of the films.

An unmeasurable effect upon the noise characteristics of the TFT was observed. The predominant drain-source current with increasing dose was in the form of various shifts of the transfer characteristics toward negative gate voltage and decrease of the transfer characteristic subthreshold slope from 5at% to 26at% of hydrogen content in the films. These, in turn, resulted in a large increase in leakage current for gate voltages at the periods the TFTs were not conducting. The radiation damage of the pin diode has much less influence on operation of array than the damage of the TFT.

Annealing for 3h at 180°C restored TFT threshold voltage properties, particularly in the case of amorphous films with greater contents of hydrogen.

Results are presented for the first time of a study on the degradation of the electrical performance of MOSFET's processed on SIMOX substrates and subjected to a 220-MeV carbon irradiation. For the n-MOSFETs an unstable increase of the drain current in linear operation is found, while for the p-MOSFETs, a drastic reduction is observed, both in linear operation and in saturation. The radiation damage is also compared to the results for 1-MeV electrons, 1-MeV fast neutrons and 20-MeV alpha rays. The differences in the damage coefficients are explained by the differences in the number of knock-on atoms and the nonionizing energy loss (NIEL). The recovery behavior of the device performance by isochronal annealing is also reported.

This paper presents results from our on-going efforts to characterize semiconductor thin films for direct x-ray conversion. We deposit these thin films onto an amorphous silicon (a-Si:H) readout array with the overall goal of developing a large area x-ray detector for protein crystallography, and for other x-ray imaging fields.

Large area amorphous silicon image sensor arrays are important for x-ray medical imaging and document scanning as well as a variety of other applications where large sensor size is required. The paper first summarizes the present state of the flat panel x-ray imager technology, and compares the two main approaches for x-ray detection. We then describe the performance of a new, large area, high resolution, radiographic imager based on a single amorphous silicon array with 2304×3200 pixels, and an active area of 30×40 cm (12×1 6”).

The new generation of elementary particle and nuclear physics experiments demand instrumentation with a more precise spatial resolution and a better and faster energy response. Nuclear physics and space experiments need position sensitive pad detectors having very thin entrance windows while high energy physics and medical applications use fast microstrip or drift detectors. Silicon pixel detectors can be improved by implementing integrated electronics on it. They allow a better X-ray energy resolution and are also used in hybrid photocathode tubes for faster timing and larger dynamic range.

High-energy particles incident to a semiconductor sensitively produce electron-hole pairs and the excited current has been used for radiation detectors. Although semiconductors have advantages such as high sensitivity and fast response, a drawback is weakness in radiation damage. To improve the radiation resistance, effects of shallow-impurity doping was explored for Si. Specimens of CZ-Si doped with P or B were bombarded with 17 MeV protons. The radiation-induced current of doped Si has been evaluated, as a function of proton fluence. The signal of particle-induced conductivity showed a fairly constant response to a proton flux, up to 1015 ions/cm2. The fluence range applicable as Si sensors was extended 103-104 times as much as that of non-doped Si, instead of a lower signal-to-noise ratio. Shallow impurities passivate the deep centers of radiation-induced defects, and the radiation tolerance continues until the pre-existent carriers are exhausted. The tolerant fluence is also usable to detect the integrated fluence, by controlling the initial impurity concentration.

Results are presented of a study on the degradation of the electrical and optical performance of n+p Si avalanche photodiodes, subjected to 1-MeV fast neutrons and to a 220-MeV carbon irradiation. The dark current increases after irradiation, while the photo current decreases. Two dominant hole capture levels, which are responsible for the degradation of performance, are after irradiation observed by DLTS (Deep Level Transient Spectroscopy). The degradation caused by neutron irradiation is smaller than that for carbon irradiation. The differences in the radiation damage are explained by the differences in the number of knock-on atoms and the nonionizing energy loss (NIEL). The recovery behavior of the device performance by isochronal annealing is also reported.

Results are presented of a study on the degradation of the electrical performance of Fe contaminated n+p Si diodes, subjected to a 220-MeV carbon irradiation. The reverse current of the diodes increases after irradiation, while the capacitance and hence the doping concentration decreases. The areal and peripheral components of the leakage current are extracted from diodes with different area to perimeter ratios. Both the generation and the recombination lifetime calculated from I/V and C/V characteristics also decrease. The deep levels in the Si substrate induced by the irradiation are mainly responsible for the degradation of the diode performance. The radiation damage is also studied for 1 -MeV electrons and 1 -MeV fast neutrons. The performance degradation for carbon irradiation is three orders of magnitude larger than that for electron irradiation. The differences in the radiation damage are explained by the differences in the number of knock-on atoms and the nonionizing energy loss (NIEL), which is attributed to the difference of mass and the possibility of nuclear collision with target Si atoms