[en] A general overview of the investigation of aluminium and some aluminium alloys by using Moessbauer spectroscopy and positron annihilation spectroscopy is presented. The detection of iron and cobalt impurities in aluminium, and the determination of Fe-Al bond by Moessbauer technique is described in detail. The various possibilities of positron annihilation spectroscopy, including 2γ angular correlations, Doppler broadening, positronium detection, electronic structure studies, various lattice defect studies, investigations of Al-Zn, Al-Mg, Al-Mn, Al-Si, Al-Ge, Al-Sn alloys, vacancies, and surface studies are also presented. (R.P.)

[en] In this paper, we theoretically investigate the mechanism of polarization in wide-bandgap semiconductor radiation detectors under high-flux x-ray irradiation. Our general mathematical model of the defect structure within the bandgap is a system of balance laws based on carrier transport and defect transition rates, coupled together with the Poisson equation for the electric potential. The dynamical system is self-consistently evolved in time using a high-resolution wave propagation numerical algorithm. Through simulation, we identify and present a sequence of dynamics that determines a critical flux of photons above which polarization effects dominate. Using the experience gained through numerical simulation of the full set of equations, we derive a reduced system of conservation laws that describe the dominant dynamics. A multiple scale perturbation analysis of the reduced system is shown to yield an analytical dependence of the maximum sustainable flux on key material, detector, and operating parameters. The predicted dependencies are validated for 16x16 pixel CdZnTe monolithic detector arrays subjected to a high-flux 120 kVp x-ray source

[en] Good detection efficiency and high energy-resolution make Cadmium Zinc Telluride (CdZnTe) and Cadmium Telluride (CdTe) detectors attractive in many room temperature X-ray and gamma-ray detection applications such as medical and industrial imaging, industrial gauging and non-destructive testing, security and monitoring, nuclear safeguards and non-proliferation, and astrophysics. Advancement of the crystal growth and device fabrication technologies and the reduction of bulk, interface and surface defects in the devices are crucial for the widespread practical deployment of Cd_1-xZn_xTe-based detector technology. Here we review the effects of bulk, interface and surface defects on charge transport, charge transport uniformity and device performance and the progress in the crystal growth and device fabrication technologies aiming at reducing the concentration of harmful defects and improving Cd_1-xZn_xTe detector performance. (copyright 2004 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

[en] Under intense x-ray irradiation, wide band gap semiconductor radiation detectors fabricated from crystals with low hole transport properties develop a steady-state space charge distribution that results from a dynamic equilibrium between charge carrier dynamics and the incident photon field. At a high enough x-ray flux, this space charge can collapse the electric field within the detector, resulting in the paralyzation of photon counting (i.e., high-flux polarization). However, well before polarization causes a catastrophic device failure, there can be enough space charge present to significantly modify the electric field. A modified field affects the electron transport and, therefore, signal generation within the sensor, which can ultimately degrade the performance of high-rate photon counting electronics. In this study, we analytically solve the fundamental equation of charge conservation to derive the modified electron transport in the presence of an exponential space charge distribution that results from the incident x-rays. We use these space-time solutions to calculate and study the time dependence of the resulting charge-induced signals. The predicted induced signals are compared throughout with numerical solutions of the full charge transport equation. In addition, we present analogous closed-form signals for a uniform distribution relevant to a broader class of γ-ray applications. Finally, we use these solutions to derive a two-parameter family of modified Hecht curves that naturally predict a voltage offset that appears due to the space charge.

[en] A classical method for the accurate measurement of the bulk resistivity and a quantitative separation of bulk and surface leakage currents in semi-insulating CdZnTe radiation detectors is evaluated. We performed an extensive set of experiments on CdZnTe single-crystal test devices to confirm the reliability and reproducibility of the measurements and the validity of the underlying assumptions for data analysis and parameter extraction. The experiments included temperature dependent dual current-voltage measurements on devices with guard electrodes as a function of device thickness, surface preparation, surface passivation, and electrode deposition conditions. We also evaluated the temperature dependence of the bulk resistivity and implemented a general temperature normalization routine to allow a reliable comparison between various crystal samples

[en] The effect of temperature and composition dependences of deep level ionization energies in semi-insulating compound semiconductors is investigated and illustrated in the example of the ternary Cd_1-xZn_xTe system. Those dependences are determined by the behavior of the band extrema in an absolute energy scale and the actual nature of a particular defect, which controls its relation to the host crystal states. Examples for the interpretation of experimental temperature and composition dependences of critical charge transport parameters are given

[en] It has been observed that pixillated CdZnTe detectors fabricated from crystals with low hole transport properties (μ_hτ_h<10^-5 cm² V^-1) experience a dynamic lateral polarization when exposed to a high flux of x-rays. In this effect, counts are transferred from pixels near the edge of the irradiated region to pixels in the interior. In this letter, we propose a mechanism capable of explaining the observed dynamical effect. The mechanism is based on a transverse electric field that is generated due to space charge that builds within the material. This transverse field, in turn, is responsible for the altered carrier trajectories toward the center of the irradiated region

[en] The interfacial chemistry and composition of Pt electrodes sputter deposited on bromine-etched CdZnTe surfaces was studied by XPS, SIMS, AES, NRA and RBS. The interfacial composition of a functioning and a non-functioning CdZnTe detector shows significant differences. The degree of cation out-diffusion into the Pt overlayer and the in-diffusion of Pt into the CdZnTe correlate with the degree of oxidation found at the metal-semiconductor interface. Practically all the oxide present at the interface was found to be TeO2. The results suggest that the inter-diffusion of the atoms and associated charges contribute to stoichiometric variations at the metal-semiconductor interface and influence the electrical performance of the devices

[en] Tl_6SI_4 is a promising material for room-temperature semiconductor radiation detection applications. The history of the development of semiconductor radiation detection materials has demonstrated that impurities strongly affect the carrier transport and that material purification is a critically important step in improving the carrier transport and thereby the detector performance. Here, we report combined experimental and theoretical studies of impurities in Tl_6SI_4. Impurity concentrations in Tl_6SI_4 were analyzed by glow discharge mass spectrometry. Purification of the raw material by multi-pass vertical narrow zone refining was found to be effective in reducing the concentrations of most impurities. Density functional theory calculations were also performed to study the trapping levels introduced by the main impurities detected in experiments. We show that, among dozens of detected impurities, most are either electrically inactive or shallow. In the purified Tl_6SI_4 sample, only Bi has a significant concentration (0.2 ppm wt) and introduces deep electron trapping levels in the band gap. Lastly, improvement of the purification processes is expected to further reduce the impurity concentrations and their impact on carrier transport in Tl_6SI_4, leading to improved detector performance.