O. Khakhan

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.

Polycrystalline BN and LiF Based Semiconductor Alpha Particle and Neutron Detectors

Composite, polycrystalline, semiconductor, hexagonal BN alone or mixed with B₄C or BMg₂ and cubic LiF were embedded in an insulating matrix which acts as a binder and tested as alpha particle and neutron detectors. The boron containing semiconductors have the natural abundance of ¹⁰B which is ~20%. In the case of LiF is the natural content of ⁶Li only 7.4%. The matrix binder further reduces the contents of ¹⁰B and ⁶Li to about one half. For alpha particles from a ²⁴¹Am alpha source of ~10⁵ cm^-2s^-1, 5.5 MeV gives the detector a wide peak in the spectrum. In the case of the thermal neutron detection, if the sources are very weak of only ~100 cm^-2sec^-1, the neutron counting is possible by subtracting the total signal from the noise. The resulting a signal to noise (S/N) ratio is of only ~2 and one needs a larger collection time of hours for a larger number of pulses to be collected. With a source of a higher neutron flux such as a nuclear reactor, which can offer flux of 10⁷ cm^-2s^-1, is the S/N ratio of ~33. LiF composites with only 3.7% of ⁶Li could also detect very weak neutron sources, but the very low amount of ⁶Li produced a S/N ratio of ~1.4. Compared with widely used ³He detectors is the thermal neutron detection efficiency of the BN and LiF based detectors significantly higher for the same sizes of detectors. Even at a pressure of 3 atmospheres of an inch in diameter ³He detector is the detection efficiency 64% compared with ~72% of a 1 mm thick composite BN detector. Results of enriched ⁶LiF and mixed BN composite detector measurements with stronger neutron sources will be also reported.

Microstructure and energy resolution of 59.6 keV /sup 241/Am gamma absorption in polycrystalline HgI/sub 2/ detectors

Polycrystalline HgI/sub 2/ layers prepared by different modifications of physical vapor deposition (PVD) exhibit different microstructure. Under some fabrication procedures, the samples exhibit a columnar structure, with columns highly oriented in the [001] direction (c-axis) normal to the layer surface. Differences in manufacturing procedures manifest themselves in different average column length, different porosity, and different average material density. The most nonporous, dense, thick HgI/sub 2/ layers are obtained by activating the preferential growth along the c-axis perpendicularly to the substrate plane. The microstructure correlates to the material electrical conduction properties: dark current, mobility, and trapping time. For a sufficiently pure starting material, and grain length approaching the layer thickness, the layer may exhibit electron mobility as high as /spl mu//sub n/=87 cm/sup 2//V/spl middot/s, electron trapping time as long as /spl tau//sub n/=18 /spl mu/s, hole mobility /spl mu//sub p/=4.1 cm/sup 2//V/spl middot/s, and hole trapping time of /spl tau//sub p/=3.5 /spl mu/s. These values are quite close to those of a single crystal. Nuclear detectors fabricated using such layers exhibit energy resolution of gamma absorption, as demonstrated for the 59.6 keV emission of /sup 241/Am.

Semiconductor polycrystalline alpha detectors

In order to check possible novel neutron detectors based on composite semiconductor detectors containing nuclides with large cross sections for neutron, we tested their response to alpha particles. In the present paper we describe results obtained with composite samples made of hexagonal Boron Nitride particles bound with Polystyrene or Nylon-6. The samples were tested under 5.5 MeV alpha particle radiation emitted from 241Am source and 4.8MeV alpha particle of 226Ra source. Some of the responses of these composite detectors to thermal neutrons were already reported and here we shall show some newer results obtained with thermal neutrons, from a low intensity 241Am - 9Be and also from a medium intensity 252Cf source, which were thermalized using 10 cm thick paraffin. The Alpha detection experiments show that all the tested samples, regardless of the binder, show a well-defined peak around the 270 energy channel. There was very little polarization of the alpha radiation, since the amplitude of the alpha peak is reduced after ~ 2min from start of the irradiation, from 100% to 95% and it stayed stable at this level for another 10 minutes. The alpha spectrum detected from a PbI2 single crystal is also shown for comparison. The neutron spectrum obtained by the composite BN samples showed an apparent peak around the 150 energy channel. The Signal to noise ratio for neutron detection from radionuclide shown here is about 2 only, whereas recent results to be published later, obtained with our composite BN detectors from a neutron beam of about 107 sec-1cm-2 is ~2 5. The 1.4 and 1.7 MeV alpha peaks resulting from the nuclear reaction of thermal neutrons with 10B of the boron nitride detector are not buried in the noise range. The capacitance noise requires small contact areas, therefore for large area detectors it is necessary to produce an electronic read-out device which can add up a multitude of small (less than 10sq.mm) pixilated contacts.

Progress in polycrystalline HgI/sub 2/ used for X-ray imaging detectors

Use of physical vapor deposition (PVD) of polycrystalline HgI/sub 2/ films on Si-TFT arrays brought about a breakthrough in the use of HgI/sub 2/ for large area pixellated X-ray imaging. Latest advances in the deposition process led to full-texture high-density films, with highly orientated crystallites, as evidenced for example by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The good structural data also yielded excellent electrical charge transport properties, which approached those of single crystals. Transient charge transport (TCT) with alpha-particle near-surface absorption was used to measure carrier mobility, trapping time, and surface recombination velocity for each sample. Typical electron and hole mobility of high quality polycrystalline HgI/sub 2/ films 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 cong/ 18 /spl mu/s and /spl tau//sub p/ /spl cong/ 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. The performance of these detectors as spectrometers in a standard nuclear spectroscopy system was evaluated. We used a gamma source of /sup 241/Am with the characteristic 59.6 keV gamma photo-peak. The full width at half maximum (FWHM) of the detector photo peak depended on its charge transport properties. High quality polycrystalline HgI/sub 2/ film detectors yield a peak of approximately 38 keV FWHM, while lower quality ones yield a much broader peak of FWHM > 70 keV. Such widths are still inferior to those of a single crystal (typically /spl sim/5 keV), yet the results suggest that further improvement through optimization of manufacturing conditions is possible. The talk reviews our past efforts, recent new results, and plans for the future.

Nuclear spectroscopy with polycrystalline-PbI 2 radiation detectors

Polycrystalline PbI2 films were deposited by Physical Vapor Deposition on ITO and gold coated glass substrates. The structural characterization of the samples was studied by using X-ray diffraction and Scanning Electron Microscopy. Two different preferred orientations - [00l] and [110] directions were found as a function of substrate temperatures, and type of substrate (ITO or Gold). The [110] direction is perpendicular to the c-axis in the hexagonal system. Four types of crystalline preferred orientations were found and named as A, B, C, and D type. The different samples were also annealed at different temperatures to improve their densities. The nuclear spectroscopic data for single crystal and polycrystalline samples were taken with 5.64 MeV Alpha particles emitted from 241Am source. The samples were also measured for their response to a blue light LED irradiation. Both polarities were studied for all samples. In most measurements it was confirmed that holes are the major carriers. The correlation between microstructure and electrical properties as affected by annealing and crystalline orientation is discussed.