Cd1-xZnxTe nanopowder with the average particle size 10 nm was produced through vapor
deposition. Dense ceramic material was compacted from the nanopowder at room temperature.
The effect of annealing on grain growth, phase transitions and some physical properties was
studied.
Wide-gap II-VI semiconductor crystalline materials are conventionally used in laser optics, light emitting devices, and nuclear detectors. The advances made in the studies of nanocrystals and in the associated technologies have created great interest in the design of semiconductor devices based on these new materials. The objectives of this work are to study the microstructure and the properties of the new material produced through CdTe nanopowder compression and to consider the prospects of its use in the design of ionizing-radiation detectors and in laser optics. Highly dense material produced of 7-10 nm CdTe particles under pressure of 20-600 MPa at temperatures from 20 to 200°C was analyzed using x-ray diffractometry, texture analysis, light and scanning electron microscopy, and optical spectrophotometry. The mechanical and electrical properties of the compacted material were measured and compared with similar characteristics of the conventionally grown single crystals. Phase transformation from metastable to stable crystal structure caused by deformation was observed in the material. Sharp crystallographic texture {001} that apparently affects specific mechanical, electrical and optical characteristics of compacted CdTe was observed.
The specific resistivity calculated from the linear current-voltage characteristics was about 1010 Ohm×cm, which is aapromisingly high value regarding the possibility of using this material in the design of semiconductor radiation detectors. The optical spectra show that the transmittance in the infrared region is sufficient to consider the prospects of possible applications of CdTe ceramics in laser optics.
High-pressure Bridgman (HPVB) and vertical zone melting (HPVZM) growth has been applied for manufacturing Cd1-xZnxTe (x = 0.04 - 0.2), CdSe and ZnSe crystal tapes with sizes up to 120×120×12 mm. The influence of the technological parameters of the growth process on the crystal quality and some properties is discussed. The dependence of the inclusion (bubbles) content on deviation from the melt stoichiometry is determined. The method for growing plates with low content of the inclusions is described. High-resistivity crystal tapes of undoped CdZnTe (1010 Ohm×cm), CdSe (1011 Ohm×cm) and ZnSe (>1011 Ohm×cm) were prepared. Possibility of the tape growth on the oriented seed is shown on example of CdSe. The difference between HPVB and HPVZM results is described. Main HPVZM advantage for II-VI compound crystal growth is possibility of obtaining crystals with stoichiometric composition or with controlled deviation from stoichiometry. Hence HPVZM is preferable for growing high-resistivity II-VI crystals with low inclusion content.
The c-axis optical properties of cuprate superconductors have been studied by grazing angle reflectivity technique. We tested the interlayer tunneling model of high temperature superconductivity and showed that the basic relation between the condensation energy of the superconducting state and the interlayer Josephson coupling does not hold for Tl2Ba2CuO6. Measuring the reflectivity at an oblique angle of incidence on the ab-surface of Tl2Ba2CuO6, we found the c-axis plasma resonance at 28 cm-1. The corresponding Josephson coupling energy is at least an order of magnitude lower than the condensation energy determined for the specific heat measurements. In the double layer compound Tl2Ba2CaCu2O8 the c-axis plasma resonance was observed at 26 cm-1. The c-axis penetration depth and the c-axis conductivity follow the Ambegaokar-Baratoff or 'dirty limit' superconductor relation if a very small energy gap is assumed.
Various types of precipitates and grain boundaries have been studied in Cd1-xZnxTe (CZT). In this study we used elemental analysis methods such as scanning electron microscopy (SEM), microprobe analysis, inductively coupled plasma mass spectroscopy (ICP/MS) and the new laser ablation ICP/MS methods. Transient charge technique was applied for the first time of CZT crystals for evaluating the electrical transport properties of semiconductors. Another method, IR transmission spectroscopy, enables us to evaluate the microstructure defects and then to correlate this with impurity level and electrical properties in order to have a better understanding of the requirements to improve the yield for large volume CZT spectrometers. We have evaluated crystals from the former Soviet Union, which have high concentration of defects. Precipitates and grain boundaries rich with carbon were observed in CZT crystals. Electrical transport properties such as (mu) (tau) (mobility-lifetime product) were measured and correlated with the chemical physical defects, as observed by IR transmission. On crystals rich with many microstructures, as shown by IR transmission, lifetimes below 1 microsecond(s) were measured, compared with 5 - 15 microsecond(s) on the detector grade materials. SEM and microprobe analysis performed on the precipitates gave high values of carbon. However, using laser ablation ICP/MS, a value in the range of 200 - 800 ppm for carbon was measured.
More recent Russian grown single crystals of Cd0.8Zn0.2Te (CZT) were evaluated using proton induced x ray emission (PIXE), x ray diffraction (XRD), photoluminescence (PL), infra red (IR) transmission microscopy, leakage current measurements and response to nuclear radiation. Whereas in the past the Russian grown samples were not acceptable for gamma ray detectors application, the present samples had a somewhat better crystallinity and a higher resistivity, and did even show distinct photopeaks for an 241Am spectrum. Differences in the material properties between various Russian (n- and p-type) and U.S. (n-type) CZT are described.
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