Csaba Szeles

Advances in the crystal growth and device fabrication technology of CdZnTe room temperature radiation detectors

The performance of CdZnTe room-temperature X-ray and gamma-ray detectors is determined by material and device defects that govern carrier transport trough the device. In this contribution, we review common bulk, interface, and surface defects and their effects on charge transport, charge transport uniformity, and device performance. We note that pure CdZnTe grown under Te-rich conditions has an excess of Cd-vacancies and other Te-related native defects and must be electrically compensated in order to obtain high resistivity material. Through the critical analysis of the various compensation schemes it is shown that deep level defects must be introduced with donor doping elements in order to achieve a practical compensation technique. The role of carrier trapping and limitations on detector performance with increasing crystal size are discussed. Based on typical measured carrier lifetimes and the available literature data on carrier capture cross sections, we estimate that the residual acceptor concentration in CdZnTe detector crystals is much lower than widely thought, about 10/sup 11/ cm/sup -3/ instead of 10/sup 15/ cm/sup -3/. The deleterious effects of structural defects within single crystals are also discussed. We also provide a brief overview of the progress in CdZnTe crystal growth and device fabrication technologies aiming at reducing the concentration of the detrimental defects and improving CdZnTe detector performance.

CdZnTe Semiconductor Detectors for Spectroscopic X-ray Imaging

Next generation high-flux X-ray imaging technology is expected to advance towards multi-color or spectroscopic imaging and will significantly expand the capabilities of the technique in a multitude of applications. Spectroscopic X-ray imaging will require energy-sensitive detector arrays. In this work we evaluated the applicability of pulse-mode CdZnTe detector arrays to high-flux spectroscopic imaging. To study the material and device performance limitations of currently available CdZnTe detectors under high-flux X-ray irradiation we designed a 2D monolithic CdZnTe test array and associated test system. The detector arrays were 16 times 16 pixel devices with 0.4 mm times 0.4 mm area pixels on a 0.5 mm pitch and were fabricated using 8.7 mm times 8.7 mm times 3.0 mm CdZnTe single crystals. We measured the high-flux performance of over 1200 such arrays with various bulk CdZnTe crystal properties using a 120 kVp X-ray source and our custom built test system. We studied the various static and dynamic charge collection effects typically not observed in low-flux applications. These included dynamic polarization, static charge steering and dynamic lateral polarization and charge steering. In parallel with the experimental effort we developed a dynamic charge transport and trapping model to describe the experimentally observed static, dynamic and transient phenomena. For the first time we demonstrated > 15 times 106 counts/s/mm2 count-rate for several hundred such CdZnTe detector arrays. In addition we demonstrated good < 1% short term count-rate stability of the detector arrays.

Accurate measurement of electrical bulk resistivity and surface leakage of CdZnTe radiation detector crystals

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.

Advances in the crystal growth of semi-insulating CdZnTe for radiation detector applications

The growth of large-volume semi-insulating CdZnTe single crystals with improved structural perfection has been demonstrated by the electro-dynamic gradient (EDG) technique and active control of the Cd partial pressure in the ampoule. The EDG technique nearly completely eliminates the uncontrolled radiative heat transport commonly encountered in traditional Bridgman systems where the charge and furnace move relative to each other. The control of the Cd partial pressure allowed the solidification and cool-down of the ingots close to the stoichiometric composition. As a result, the formation and incorporation of large size (/spl ges/1 /spl mu/m diameter) Te inclusions was avoided during crystallization and ingots with high structural perfection were achieved. CdZnTe crystals with 10/sup 9/-10/sup 10/ /spl Omega/cm electrical resistivity, good detector performance and electron mobility-lifetime product as high as /spl mu/T/sub e/=1.2/spl times/10/sup -3/ cm/sup 2//V were obtained using the EDG technique.

Interfacial chemistry and the performance of bromine-etched CdZnTe radiation detector devices

The interfacial chemistry and composition of Pt electrodes sputter deposited on bromine-etched CdZnTe surfaces was studied by X-ray photoelectron spectroscopy. The interfacial composition of a functioning and a nonfunctioning 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. Most of the oxide present at the interface was found to be TeO/sub 2/. The results suggest that the interdiffusion of the atoms and associated charges contribute to stoichiometric variations at the metal-semiconductor interface and influence the electrical performance of the devices.

Ultra High Flux 2-D CdZnTe Monolithic Detector Arrays for X-Ray Imaging Applications

The performance of 2-D CdZnTe monolithic detector arrays designed for high flux X-ray imaging applications was studied. For the first time we have obtained 5 times 106 counts/s/mm2 count-rate for a CdZnTe pixelated detector array. This count-rate is more than twice the highest count-rate ever achieved using a CdZnTe detector array. Such excellent performance was demonstrated for more than 600 individual CdZnTe detector arrays. The 2-D CdZnTe monolithic arrays were 16 x 16 pixel devices with 0.4 mm times 0.4 mm area pixels on a 0.5 mm pitch and were fabricated using 8.7 mm times 8.7 mm times 3.0 mm CdZnTe single crystals grown by the high-pressure, electro-dynamic gradient freeze technique. The CdZnTe detector arrays were bonded to a ceramic substrate with the Z-bondtrade technique. This enabled performance testing of the individual detector arrays before bonding to the read-out ASIC chip. The detector arrays were characterized in a custom designed test system. The measurement and data acquisition system consisted of a 16 times 16 pin probe head and 256-channel read-out electronics controlled by a host PC. We utilized our 8-channel fast bipolar ASIC chip and computer controlled 120 kVp X-ray source. In order to measure the true throughput of the CdZnTe devices a counts correction method was developed and implemented that compensates for the counting system non-linearity caused by pile-up and amplifier shaping time effects. Survey of detector array performance as a function of CdZnTe charge transport properties showed that the maximum achievable count-rate of these detectors strongly depends on the hole charge transport properties of the crystals.

Current issues of high-pressure Bridgman growth of semi-insulating CdZnTe

The availability of large-size, detector-grade CdZnTe crystals in large volume and at affordable cost is a key to the further development of radiation-detector applications based on this II-VI compound. The high pressure Bridgman technique that supplies the bulk of semiinsulating CdZnTe crystals used in X-ray, γ-ray detector and imaging devices at present is hampered by material issues that limit the yield of large-size and high-quality crystals. These include ingot cracking, formation of pipes, material homogeneity and the reproducibility of the material from growth to growth. The incorporation of macro defects in the material during crystal growth poses both material quality limitations and technological problems for detector fabrication. The effects of macro defects such as Te inclusions and pipes on the charge-transport properties of CdZnTe are discussed in this paper. Growth experiments designed to study the origin and formation of large defects are described. The importance of material-crucible interactions and control of thermodynamic parameters during crystal growth are also addressed. Opportunities for growth improvements and yield increases are identified.

Growth and properties of semi-insulating CdZnTe for radiation detector applications

The growth and properties of semi-insulating CdZnTe for nuclear radiation detector applications are reviewed. The current state of the high-pressure Bridgman growth and the potentials of the conventional vertical and horizontal Bridgman techniques to grow radiation detector material are discussed. The characteristic macroscopic and microscopic defects of high-pressure Bridgman grown CdZnTe ingots, such as cracks, pipes, inclusions, precipitates, grain boundaries and their effect on the electrical and charge trapping properties of the material are reviewed.

Gamma-ray and neutron spectrometer for the Dawn mission to 1 Ceres and 4 Vesta

We present the design of the gamma-ray and neutron spectrometer (GR/NS) for Dawn, which is a NASA Discovery-class mission to explore two of the largest main-belt asteroids, 1 Ceres and 4 Vesta, whose accretion is believed to have been interrupted by the early formation of Jupiter. Dawn will determine the composition and structure of these protoplanetary bodies, providing context for a large number of primitive meteorites in our sample collection and a better understanding of processes occurring shortly after the onset of condensation of the solar nebula. The Dawn GR/NS design draws on experience from the successful Lunar Prospector and Mars Odyssey missions to enable accurate mapping of the surface composition and stratigraphy of major elements, radioactive elements, and hydrogen at both asteroids. Here, we describe the overall design of the GR/NS and compare the expected performance of the neutron spectrometer subsystem to the neutron spectrometer on Mars Odyssey. We also describe radiation damage studies carried out on CdZnTe detectors, which will be components of the primary gamma-ray spectrometer on Dawn. We conclude that provisions for annealing at moderate temperatures (40/spl deg/C to 60/spl deg/C) must be made to ensure that the spectrometer will function optimally over the nine-year mission.

Fast High-Flux Response of CdZnTe X-Ray Detectors by Optical Manipulation of Deep Level Defect Occupations

We experimentally investigate the possible correlation between high hole-trap concentrations in wide-bandgap semiconductors and delayed temporal response of high-flux x-ray detector devices to changing photon fluxes. We show that fast photo-current response can be achieved with (1) CdZnTe detectors with high hole mobility-lifetime products, (2) temperature increased detrapping, and (3) constant below-bandgap energy light illumination that modifies the dark defect occupation towards a steady-state with a reduced concentration of active hole traps. This way, the detector signal stabilizes immediately upon flux onset, independent of details of the semiconductors point defect structure. Quasi-instantaneous response stabilization (<; 3 ms) to x-ray flux changes > 107 photons mm-2 s-1 is demonstrated.