Dec. 20, 2004 - Present Hochschuldozent at the Institute of Crystallography of the Department of Chemistry, Pharmacology, and Geosciences at the Albert-Ludwigs-Universität Freiburg, level C2. Teaching of 3-4 courses/semester.
Jan. 1, 2000 - Present Leader of Service Group of Materials Characterization & Detector Tech-nology at the Freiburger Materialforschungszentrum of the Albert-Ludwigs-Universität Freiburg The research group consists of 12 employees including graduate, PhD stu-dents, and scientists. Research activities: • Development of radiation detectors for X- and gamma rays based on semiconductors such as CdZnTe and GaAs • Processing and flip-chip bonding of pixel detectors for X-ray with a Photon Counting Readout electronic (Medipix) • Crystal growth of detector grade CdTe and (Cd,Zn)Te materials by the Bridgman method • Development of UV detectors by growth of epitaxial films of (Al,Ga)N by Molecular Beam Epitaxy • Material characterization of inorganic and organic solids with x-rays, optical and electrical methods
Jan. 1, 1999 - Dez. 19, 2004 Leader of Service Group of Materials Characterization & Detector Technology at the Freiburger Materialforschungszentrum of the Albert-Ludwigs-Universität Freiburg The research group consists of 12 employees including graduate, PhD students, and scientists. Research activities: • Development of radiation detectors for X- and gamma rays based on semiconductors such as CdZnTe and GaAs • Processing and flip-chip bonding of pixel detectors for X-ray with a Photon Counting Readout electronic (Medipix) • Crystal growth of detector grade CdTe and (Cd,Zn)Te materials by the Bridgman method • Development of UV detectors by growth of epitaxial films of (Al,Ga)N by Molecular Beam Epitaxy • Material characterization of inorganic and organic solids with x-rays, optical and electrical methods
University Assistant (level C1, equivalent to an assistant professor position) at the Crystallographic Institute of the University of Freiburg. Activities included supervision of Ph.D. and masters students and technicians, teaching of 1-2 courses/semester, as well as application for funding and management of several international research projects of crystal growth and detector development. Research topics: • Characterization and processing of 3D detector structures • Crystal growth of CdZnTe crystals • Investigation of defects in CdZnTe and CdTe crystals
Abstract: The quality and availability of room temperature CdTe sensor material for X-ray detection has improved significantly in the last years. A CdTe sensor with different pixel pitches and electrode pad sizes was bump-bonded to a Medipix2-MXR ASIC. With this photon counting detector we were able to investigate the influence of pixel pitch and electrode pad size on the energy response functions. The accurate knowledge of the energy response is crucial for energy resolving X-ray imaging. Therefore we exposed the detector to gamma rays of Am-241 and Co-57. In the following analysis of the energy response spectra we determined the number of events in the photo peak, the energy resolution and the threshold energy where the photo peak is found (photo peak position) caused by the absorption of the 59.5 keV photons of Am-241. For the energy calibration we used the 59.5 keV photons of Am-241 and the 122 keV photons of Co-57. Concerning energy resolution, energy threshold calibration and photo peak position our measurements show good agreement with the expected behaviour. The number of events in the photo peak strongly depends on the pixel pitch and the electrode pad configuration.
Abstract: The Medipix3 chip is a hybrid pixel detector readout chip working in Single Photon Counting Mode. It has been developed with a new front-end architecture aimed at eliminating the spectral distortion produced by charge diffusion in highly segmented semiconductor detectors. In the new architecture charge deposited in overlapping clusters of four pixels is summed event-by-event and the incoming quantum is assigned as a single hit to the summing circuit with the biggest charge deposit (this mode of operation is called Charge Summing Mode (CSM)). In Single Pixel Mode (SPM) the charge reconstruction and the communication between neighbouring pixels is disabled. This is the operating mode in traditional detector systems. This paper presents the results of the characterization of the chip with electrical stimuli and radioactive sources.
Abstract: We have a 1-mm-thick cadmium telluride (CdTe) sensor bump-bonded to a Medipix2 readout chip. This detector has been characterized using a poly-energetic x-ray beam. Open beam images (i.e. without an attenuating specimen between the x-ray source and the detector) have been acquired at room temperature using the MARS-CT system. Profiles of various rows and columns were analyzed for one hundred, 35-ms exposures taken with a bias voltage of -300 V (operating in electron collection mode). A region of increased sensitivity is observed around the edges of the detector. A reasonably periodic, repeatable variation in pixel sensitivity is observed. Some small regions with very low sensitivity and others with zero signals are also observed. Surrounding these regions are circular rings of pixels with higher counts. At higher flux (higher tube current in the x-ray source) there is evidence of saturation of the detector assembly. In this paper we present our understanding of the origin of these features and demonstrate the improved image quality obtained after correcting for these variations.
Abstract: CdTe is a promising material for the detection of ó- and X-rays as 1 mm thick CdTe sensors offer an absorption efficiency that is higher than 50 % for photon energies up to 120 keV. Therefore 1mm thick CdTe from Acrorad has been flip-chipped as the sensor material on a Timepix readout-electronics ASIC at the Freiburg Materials Research Center (FMF). The transport properties of the CdTe material are investigated by determining the ÃÅÃÂ-product of the charge carriers by illuminating the non-collecting electrode with alpha particles. The method commonly used to determine the ÃÅÃÂ-product for non-pixelated devices was adapted for the Medipix pixelated readout-electronics. The position of the alpha particles on the pixel matrix provides spatial information, which was used to create a mapping of the ÃÅÃÂ-product. The energy resolutions and the positions of the noise edge of two CdTe-Timepix assemblies, one with a pixel pitch of 110 x 110 ÃÅm 2 and another one with a pixel pitch of 55 x 55 ÃÅm 2 were investigated by performing a threshold scan of a 241 Am source. A comparison of the energy spectra obtained with the two assemblies with different pixel pitches confirms the strong dependence of charge sharing on the energy spectra, as expected.
Abstract: Technological advances have made possible the development of pixelized photon counting semiconductor detectors, many of which are used in X-ray imaging to resolve the spectral composition of the incident photons. Here, in a so-called Hexa detector, we employ a 3 ÃÂ 2 array of Medipix2 MXR readout chips, bump bonded to a cadmium telluride sensor of 1 mm thickness with a pixel pitch of 165 ÃÅm. Each pixel in this assembly offers two variable energy thresholds, which enables counting of only those photons within an energy range of interest. As manufacturing tolerances cause deviations in each of the pixelâs responses, the two thresholds can be calibrated for every pixel to render their response to radiation more homogeneous. In this work, we compare various methods that we chose to equalize the lower thresholds: a) the noise edge of the detector electronics, the characteristic X-rays from b) silver and c) tantalum foils as well as flat fields obtained at d) 40 and e) 120 kVp. It will be shown that the energy dependence in the resulting adjustment bit maps are only small, whereas the question as to which strategy to choose (peak position vs. image homogeneity) will have a greater influence on the resulting corrections. Additionally, we observed a decrease in the mean adjustment values with increasing distance from the central axis of the Hexa detector under study.
Abstract: The temperature dependences of the resistivity of detector-grade semi-insulating CdTe and Cd0.9Zn0.1Te single crystals were investigated. The investigations have revealed that the thermal activation energy can be higher than E-g/2 at T -> 0 K or considerably less than this value, although the Fermi level is located near the middle of the band gap. It is shown that such an "anomalous" behavior of the electrical characteristics is explained in detail by the features of the compensation of deep acceptor levels in the semiconductor band gap. A method based on the electroneutrality equation is proposed for the determination of the ionization energy and compensation degree of the impurity (defect), which is responsible for the conductivity of the material. The results extracted with the use of this method lead to the prediction that the inversion of the conductivity type of the semiconductor under certain conditions can occur as the temperature varies during operation of a Cd(Zn)Te-based device.
Abstract: Polycrystalline CdTe layers were directly deposited on the Medipix2 readout chip by physical vapor deposition using a Riber molecular beam epitaxy 2300. Sb2Te3 was evaporated as backside contact material, resulting in an M-pi-p diode structure comprising the pixel contact material, the intentionally undoped CdTe, and Sb2Te3. It was possible to achieve x-ray images at 60 kVp. In addition, thermally oxidized Si wafers were coated with a comparable M-pi-p structure grown under identical conditions, to investigate electrical properties of the films. The specific resistance of the layers was found to be 3.85 x 10(8) Omega cm. (C) 2010 American Institute of Physics. [doi:10.1063/1.3492694]
Abstract: Epitaxial zinc oxide layers were grown by plasma-enhanced molecular beam epitaxy (P-MBE) on Al2O3 substrates. The zinc oxide films grown on c-plane (0 0 1) Al2O3 showed the formation of well-known 30 degrees rotation domains. By variation of the growth parameters, basically the II/VI ratio, the formation of rotation domains was successful suppressed and the expected relationships between substrate and film shaped. The influence of other growth parameters, such as growth temperature and film thickness, on the formation of rotation domains was found to be insignificant. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: Gamma-ray spectroscopic systems with high efficiency and energy resolution have become important in many fields. Conventional systems with analog shaping are more and more replaced by digital systems that can provide higher throughput, better energy resolution and better stability. We present the development of our new versatile real time Multi Channel Analyzer with digital signal processing, the GMCA. The device has two independent channels and is compatible with various detectors like Coplanar Grid (Cd,Zn)Te, HPGe, LaBr3, NaI and CsI. Different sensors for temperature, pressure and humidity provide additional information and allow the correction of thermal drifts. We show energy resolution measurements and compare spectra measured with different detectors connected to our system and to commercial systems both analog and digital.
Abstract: Results on the properties of the known impurities, Ge, Sn, V and Bi, and the lattice imperfections, V-Cd and Te-Cd are summarized. We discuss their role in compensation, and in buffering the variations in shallow electronic levels in the grown ingot. We demonstrate that (similar to 2-3 kT) variations of the Fermi energy increases carrier trapping to the deep levels. Trapping is manifest in a photoconductivity signal that can be studied by photoconductivity methods, thus allowing to monitor the spectroscopic-grade material before fabricating the detectors. Our approach could be important in preventing the after-glow effect and polarization.
Abstract: Experiments of deposition of CdTe films have been carried out under microgravity in the Russian Foton M3 mission. The influence of gravity has been studied with these experiments and compared to the results of simulations. The measured deposition rate could be confirmed by the theoretical results for lower temperatures. For higher temperatures the measured thickness of the deposited films was larger compared to the theoretical data.
Abstract: The phenomenon of "dewetting" associated with the Vertical Bridgman (VB) crystal growth technique leads to the growth of a crystal without contact with the crucible. One dramatic consequence of this modified VB process is the reduction of structural defects within the crystal. It has been observed in several microgravity experiments for different semiconductor crystals. This work is concentrated on the growth of high resistivity (Cd, Zn) Te: In (CZT) crystals by achieving the phenomenon of dewetting under microgravity condition and its application in the processing of CZT detectors. Two Cd0.9Zn0.1Te In crystals were grown in space on the Russian FOTON satellite in the POLIZON-M facility in September 2007 (mission M3). At the end of the preliminary melting phase of one crystal, a Rotating Magnetic Field (RMF) was applied in order to reduce the typical tellurium clusters within the melt before the pulling. The other crystal was superheated with 20 K above the melting point before the pulling. A third reference crystal has been grown on the ground in similar thermal conditions. Profiles measurements of the space grown crystals surface gave the evidence of a successful dewetting during the crystal growth. Characterization methods such as IR microscopy and CoReMa have been performed on the three crystals. CZT detectors have been processed from the grown part of the different crystals. The influence of the dewetting on the material quality and the detector properties completes the study.
Abstract: The ATLAS detector is now installed at the CERN LHC and a precise evaluation of the radiation environment within and around the detector is required to understand the performance of the various detector systems, and to predict their useful lifetime. Furthermore, validation of earlier Monte Carlo predictions about the radiation field in ATLAS is necessary. In particular, it is important to perform these measurements in the early phases of the LHC operation. Many other radiation measuring devices lack sensitivity to low doses, and usually do not provide information on the composition of the radiation during this low intensity period. A network of 15 ATLAS-MPX devices has been installed at various positions in the ATLAS detector. These devices are capable of providing quantitative real-time information on the fluxes and flux distributions of the main radiation types in the experiment, including slow and fast neutrons. The technical description of the ATLAS-MPX detector network as well as the principles of the data analysis is presented. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: 1-Dimensional halocuprate(I) chains [(Cu2X4)(2-)](n) (= [(CuX2)(-)](n,) X = Cl, Br, I) have been synthesized under hydrothermal conditions through in-situ reduction of (CuX2)-X-II with (FeX2)-X-II or as phase pure materials through comproportionation of (CuX2)-X-II or (CuO)-O-II with Cu-0 metal in the presence of the respective aqueous hydrogen halide HX and a templating amine. Chains of trans edge-sharing tetrahedra are obtained with piperazinium or ethylenedi-ammonium dications, while the 4,4'-bipyridinium dication gave chains of cis edge-sharing tetrahedra. Two monoprotonated piperazinium groups act as cationic ligands (Hpipz(+)) towards copper atoms in a molecular [Cu-4(mu-Br-6)(Hpipz)(2)] cluster. Electrical crystal conductivities of the halocuprate [(Cu2X4)(2-)](n) (= [(CuX2)(-)](n)) chains (X = Cl, Br, I) are around 10(-8) S.cm(-1) at room temperature.
Abstract: CdTe thick films were prepared by vacuum deposition on amorphous substrates using MBE technique. The growth was performed at different temperatures to investigate the development of the growth rate, surface morphology, structure and optical properties. Properties of films deposited with a single CdTe source are compared with films grown with an additional Cd source. The growth experiments are discussed with regard to demands for X-ray detector applications.
Abstract: Fifteen ATLAS-MPX devices are installed in the ATLAS detector and cavern by the ATLAS-MPX collaboration. This network of sensors provides a real-time measurement of the composition of the radiation field inside the ATLAS detector and its cavern. This article reports calibration measurements of the thermal and fast neutron detection efficiency of the ATLAS-MPX devices. In order to detect neutrons, the 300-mu m-thick silicon sensor of each device was covered with mask of (LiF)-Li-6 and polyethylene (PE) converters. Efficiency measurements were performed with calibrated natural thermal neutron source, calibrated Cf-252 and (AmBe)-Am-241 sources. Fast neutrons with monochromatic energy of 14MeV were delivered by a Van de Graaff (VDG) accelerator beam. The devices showed a mean efficiency of 1.02 X 10(-2) for thermal neutrons ((LiF)-Li-6-Si region), 3.17 x 10(-4) for Cf-252, 7.51 X 10(-4) for (AmBe)-Am-241 and 1.81 X 10(-3) for 14MeV neutrons (PE-Si region) while all background signals generated by X-rays, gamma-rays and electrons were fully suppressed by appropriate discrimination level. These data allow the estimation of the neutron fluxes in ATLAS and provide initial data for precise simulation of the detector environment at the ATLAS-MPX devices locations. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: Spectroscopic X-ray imaging enhances image contrast and provides advanced object information due to energy resolution. The Medipix-2 chip is a photon counting semiconductor detector and features two energy thresholds for energy selective imaging. The aim of this study is to present the development of optimized threshold adjustment masks with small energy windows of about 3 keV width using a monochromatic X-ray source for equalization and to demonstrate the benefits of spectroscopic X-ray imaging using an integrated circuit as object.
Abstract: We present the results of a comprehensive study of distribution of zinc, resistivity, and photosensitivity in a Cd1-xZnxTe ingot grown by the vertical Bridgman method. We used several complementary methods, viz., glow discharge mass spectroscopy, photoluminescence, resistivity-, and photosensitivity-mapping, along with photo-induced current transient spectroscopy to characterize the material. We identified electronic levels in the bandgap responsible for compensation, recombination, and photosensitivity. (c) 2008 Elsevier B.V. All rights reserved.
Abstract: For the few decades, II-VI compound semiconductors are gaining attention because of its numerous applications in the field of detector technology, photovoltaic, nuclear medicine, astronomy etc. In the recent past, materials scientists focused their attention for the growth of CdTe/CdZnTe single crystals because it doesn't require any specialized cooling and detects higher energy photos as in comparison with the existing Ge, Si and Hgl(2) detectors. In the present study, we are going to discuss five main approaches in order to get good quality CZT crystal and we have successfully grown the CZT crystal by adopting these approaches. They are: i) oscillatory Bridgman technique previous to the growth process, ii) modifying the thermal environments in a Bridgman geometry using a Pt tube as a cold finger in order to reduce the growth velocity iii) growth from the vapour phase using Bridgman geometry with a pyrolitic boron nitride (PBN) crucible to locate the feed material, and with a special temperature profile, iv) microgravity experiments in the FOTON M3 mission using magnetic field prior to the growth process and v) growth by a boron oxide encapsulation. The detailed discussions are given in the following sections.
Abstract: Real-time measurement of the composition and spectroscopic characteristics of the radiation field inside the ATLAS detector and cavern will be carried out with MediPix2 devices. In view of this application, a calibration of the low energy threshold was performed by illuminating the devices with low energy photons, using Am-241 sources and X-rays. A procedure was thereafter established for the calibration of all MediPix2 devices to be used in ATLAS. Using X-ray tubes with filters at the Czech Metrology Institute, photons of energies between 8 and 250 keV were obtained and the detection efficiency of the MediPix devices to these radiations was determined. An exponential decrease of the photon intensity as a function of detector depth was observed, illuminating the detector with the beam almost parallel to the surface. The properties of the decrease of intensity were used to determine the exact incident angle of the beam. (c) 2008 Elsevier B.V. All rights reserved.
Abstract: We studied the resistivity, photosensitivity, photoluminescence, and surface photovoltage of CdTe crystals doped with Ge or Sri to extend our knowledge of the influence of the deep-donor level on compensation and afterglow effects. We demonstrated a strong correlation between photosensitivity caused by photoelectrons with Fermi-level variations near the Ge-Cd(0/2+) or Sn-Cd(0/2+) energy levels. Surface photovoltage measurements confirmed that when the concentration of residual acceptors varied along the direction of growth, then trapping conditions dramatically changed as a defect was converted from a neutral state to doubly charged positive one.
Abstract: We show that the conduction band structure of dilute AlxGa1-xAs1-yNy with x <= 0.37 and y <= 0.04 can be described consistently by the experimentally motivated band anticrossing model. The interband transition energies E-, E-+Delta(0), and E+ have been derived from a full line shape fit to photomodulated reflectance (PR) spectra recorded at room temperature. The PR data were taken (a) from a series of Al0.06Ga0.94As1-yNy samples with y <= 0.04 and (b) from a set of AlxGa1-xAs0.99N0.01 layers with x <= 0.37. The latter series covers the range of Al concentrations where the AlxGa1-xAs band gap energy E-M is expected to cross the nitrogen-induced energy level E-N. The resulting nitrogen- and Al-concentration dependent interband transition energies are described by the band anticrossing model using a matrix element for the coupling between the nitrogen-induced states and the extend lowest conduction band states of C-MN=2.32 eV and a nitrogen level energy E-N=(1.625+0.069x) eV, the latter measured with respect to the GaAs valence band edge. (C) 2008 American Institute of Physics.
Abstract: CdTe films with a thickness of 100Â [mu]m were grown by molecular beam epitaxy (MBE) on semi-insulating LEC GaAs (0Â 0Â 1) substrates. The films were characterized by X-ray diffraction (XRD), photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), I/V characteristics and detector measurements using a 241Am source. The grown layers were highly oriented as revealed from X-ray pole figure measurements and high-resolution XRD experiments. Low temperature PL measurements showed the formation of A-center complexes including Ga and In acting as donors. Furthermore, the distribution of these impurities in the films was determined by spatially resolved PL measurements. XPS demonstrates the formation of a TeO2 overlayer due to Cd depletion at the surface of the films. The electrical measurements including I/V characteristics and detector measurements resulted in resistivity values of around 3Ã108Â [Omega]Â cm and a [mu]t product of electrons of 10-5Â cm2/V.
Notes: the Proceedings of the 15th International Conference on Crystal Growth (ICCG-15) in conjunction with the International Conference on Vapor Growth and Epitaxy and the US Biennial Workshop on Organometallic Vapor Phase Epitaxy
Abstract: The deep levels in semi-insulating CdTe crystals grown by vertical gradient freezing technique were studied using thermoelectric effect spectroscopy (TEES) and photo-induced current transient spectroscopy (PICTS). The measurement of TEES spectra in the temperature range 90âÂÂ400 K was performed at different heating rates. The positions of the levels and their capture cross-sections were obtained by using the heating rate method. It was found that near midgap levels in samples doped with shallow donors (Cl, In) have a low value of capture cross section ( ##IMG## [http://ej.iop.org/icons/Entities/ap.gif] â 10 âÂÂ15 â 10 âÂÂ17 cm 2 ) and are hole traps. Samples doped with a deep donor (Ge, Sn) have a much higher capture cross section of the midgap level ( ##IMG## [http://ej.iop.org/icons/Entities/ap.gif] â 10 âÂÂ13 cm 2 ), which acts as an electron trap. Comparison of the TEES results with the PICTS method has shown that while the evaluated values of ionization energies are comparable using both methods, the PICTS technique gives systematically approximately two orders of magnitude higher capture cross sections than TEES.
Abstract: We explored the growth and characteristics of CdTe doped with Sn to heighten our understanding of the role of deep levels on electrical compensation and trapping. We demonstrated, for the first time, the strong dependence of the Sn-Cd charge state on the Fermi-level variation (2-3kT) in high-resistivity CdTe. The concentration of deep traps for electrons was determined by the number of doubly positively charged Sn2+ atoms. Thermoelectric-effect spectroscopy and photovoltage measurements revealed the conversion of the SnCd defect from the electron Sn-Cd(2+) trap to the hole Sn-Cd(0) trap. The results agree well with the existence of a negative U-center in the Sn-Cd(0/2+) defect. We also showed that the neutral Sn defect is responsible for the near midgap C-band -> bound hole radiative transitions band with a maximum at 0.76 eV.
Abstract: The deep levels in semi-insulating CdTe crystals grown by vertical gradient freezing technique were studied using thermoelectric effect spectroscopy (TEES) and photo-induced current transient spectroscopy (PICTS). The measurement of TEES spectra in the temperature range 90-400 K was performed at different heating rates. The positions of the levels and their capture cross-sections were obtained by using the heating rate method. It was found that near midgap levels in samples doped with shallow donors (Cl, In) have a low value of capture cross section (approximate to 10(-15)-10(-17) cm(2)) and are hole traps. Samples doped with a deep donor (Ge, Sn) have a much higher capture cross section of the midgap level (approximate to 10(-13) cm(2)), which acts as an electron trap. Comparison of the TEES results with the PICTS method has shown that while the evaluated values of ionization energies are comparable using both methods, the PICTS technique gives systematically approximately two orders of magnitude higher capture cross sections than TEES.
Abstract: (Cd,Zn)Te crystals were grown from the melt by the Bridgman Method with diameters of 25 and 75 mm. The material was doped with indium. The structural quality of the crystals was constantly improved to increase the single crystalline areas. These improvements reduced the number of grains in the 25 mm and 75 mm diameter, respectively. The crystals were characterized by electrical and optical methods to evaluate the relation between structural quality and detector performance. A reduction of the etch pitch density down to 2 x 10(3) cm(-2) was achieved by the optimization of the crystal growth The resistivity of the crystals was in the range of 2 x 10(9) up to 5 x 10(10) Omega cm. The detector performance was tested with different radiation sources. The product of mobility-lifetime of charge carriers was 3 X 10(-3) cm(2)/V. The energy resolution for different radiation energies were measured for detector thickness of 1 mm and 10 mm.
Abstract: 100 mu m thick films of CdTe were grown on semi-insulating (100) GaAs substrates by Physical Vapor Transport (PVT) in a modified Molecular Beam Epitaxy facility. The grown layers were highly oriented as revealed from X-ray pole figure measurements. Temperature- and intensity-dependent photoluminescence measurements were taken before and after the chemical removal of the substrate to determine the effect of the GaAs substrate and to estimate the crystallographic quality of the layers. Current-voltage characteristics were performed to obtain the resistitivity of the layers with 10(8) Omega cm.
Abstract: For applications with high-energy X-ray photons (30-120 keV) semiconductors with high Z like CdTe and GaAs are required. In comparison with silicon detectors, the absorption of GaAs is higher. GaAs wafers had been prepared and flip-chip bonded to the Medipix2 readout chip at the Freiburger Materialforschungszentrum (FMF) using a low-temperature process. For this work, 300-mu m-thick wafers were chosen for first measurements. The pixel and backside contacts were processed with Ti-Pt-Au to form Schottky contacts. The surface of the samples was passivated with BCB. The pixel pitch was 55 mu m (standard Medipix) and 110 mu m. The performance of these GaAs detectors was studied for different radiation sources and X-ray tubes. The efficiency of the GaAs assemblies is compared to 700-mu m-thick Si assemblies. The number of detected quanta in 300 mu m GaAs is 10 times higher than in 700 mu m Si for gamma-rays at 44 and 60 keV. This can be explained by the higher absorption of the GaAs. (C) 2007 Elsevier B.V. All rights reserved.
Abstract: The performance characteristics of Frisch-ring CdZnTe (CZT) detectors are described and compared with other types of CZT devices. The Frisch-ring detector is a bar-shaped CZT crystal with a geometrical aspect ratio of similar to 1 : 2. The side surfaces of the detector are coated with an insulating layer followed by a metal layer deposited directly upon the insulator. The simple design operates as a single-carrier device. Despite the simplicity of this approach, its performance depends on many factors that are still not fully understood. We describe results of testing several detectors fabricated from CZT material produced by different vendors and compare the results with numerical simulations of these devices.
Abstract: The growth of AlN on c-plane sapphire substrates is reported for a growth temperature of 1100 degrees C in a radio frequency plasma assisted molecular beam epitaxy system. The high growth temperature in combination with Al-rich growth conditions resulted in a high crystalline quality (FWHM of the 0002 x-ray reflection of 650") and low O incorporation. Furthermore, the incorporation of Si and Mg as n- and p-type dopants, respectively, has been studied for these growth conditions. For Si doping the corresponding cell temperature was varied between 1300 and 1350 degrees C. Secondary ion mass spectrometry (SIMS) showed a homogeneous Si depth profile up to a concentration of 7 x 10(20) cm(-3), but the Si doped layers remained highly resistive. Incorporation of Mg was observed only at a low growth temperature of 830 degrees C; at higher growth temperatures SIMS revealed a strong surface segregation effect while the amount of Mg incorporated into the AlN layer remained below the detection limit. The build-up of a Mg accumulation layer at the growth surface due to segregation was found to cause a significant reduction in growth rate.
Abstract: Four CdTe ingots with gradually increased concentration of the Sn impurity have been grown by the vertical gradient freeze method and were characterized with glow discharge mass spectroscopy, photoinduced current transient spectroscopy, resistivity, photoconductivity, and photoluminescence techniques. It was shown that the Sn impurity strongly influences resistivity and photoconductivity of the material. Concentration of Sn must be higher than the total concentration of residual acceptors to reach strong compensation. The middle-gap donor level pins the Fermi-level. Photoconductive high resistivity material can be prepared with Sn concentrations in the melt in the range 10(18)-10(19) cm(-3). In total, 6 electron traps and 3 hole traps were identified in the band gap by several complementary techniques.
Abstract: The growth of AlN on c -plane sapphire substrates is reported for a growth temperature of 1100 ðC in a radio frequency plasma assisted molecular beam epitaxy system. The high growth temperature in combination with Al-rich growth conditions resulted in a high crystalline quality (FWHM of the 0002 x-ray reflection of 650â) and low O incorporation. Furthermore, the incorporation of Si and Mg as n- and p-type dopants, respectively, has been studied for these growth conditions. For Si doping the corresponding cell temperature was varied between 1300 and 1350 ðC. Secondary ion mass spectrometry (SIMS) showed a homogeneous Si depth profile up to a concentration of 7 à10 20 cm âÂÂ3 , but the Si doped layers remained highly resistive. Incorporation of Mg was observed only at a low growth temperature of 830 ðC; at higher growth temperatures SIMS revealed a strong surface segregation effect while the amount of Mg incorporated into the AlN layer remained below the detection limit. The build-up of a Mg accumulation layer at the growth surface due to segregation was found to cause a significant reduction in growth rate.
Abstract: Undoped CdSe monocrystals and CdSe nano-crystals films have been studied at various temperatures by continuous wave (cw) photoluminescence. We report on a characteristic deep level emission, which is consistently observed in the wurtzite bulk- and nanocrystalline forms of CdSe. Two broad luminescence bands, which are separated from the excitonic emission by 0.5 and 0.7eV occur in CdSe, prepared by quite different techniques. These bands experience, similar to the excitonic emission, a spectral shift to high energy enforced by the quantum confinement in nano-CdSe. The defects responsible for this luminescence are probably two different V-Cd-V-Se, divacancies: one is oriented along the hexagonal c-axis, the other is oriented along the basal Cd-Se bond directions. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: The excitonic and deep-level photoluminescence (PL) in CdSe nanocrystal (NC) films (wurtzite type) was studied under continuous-wave excitation as a function of excitation power, temperature, and time of photoaging. It was shown that the intensity-power dependencies are identical for excitonic and deep-level emissions in a wide temperature range. At low temperatures (80- 100 K), both emissions were saturated at the laser power used, which generates more than one exciton per nanocrystal. A transition point from the linear to the saturated region was dependent on the temperature, size, and quality of the NCs. A clear inverse dependency between the intensities of excitonic and deep-level emissions was revealed at 80 K over the entire sample area. At room-temperature, the quantum yield dropped significantly and a higher laser power was needed to reach PL saturation. An increase in temperature led to worsening of the reverse dependence between excitonic and deep-level emissions, and at room-temperature, they became uncorrelated. These results can be explained by Auger recombination and also by an increase of nonradiative recombination in the surface states with increasing temperature.
Abstract: The excitonic and deep-level photoluminescence (PL) in CdSe nanocrystal (NC) films (wurtzite type) was studied under continuous-wave excitation as a function of excitation power, temperature, and time of photoaging. It was shown that the intensity-power dependencies are identical for excitonic and deep-level emissions in a wide temperature range. At low temperatures (80-100 K), both emissions were saturated at the laser power used, which generates more than one exciton per nanocrystal. A transition point from the linear to the saturated region was dependent on the temperature, size, and quality of the NCs. A clear inverse dependency between the intensities of excitonic and deep-level emissions was revealed at 80 K over the entire sample area. At room-temperature, the quantum yield dropped significantly and a higher laser power was needed to reach PL saturation. An increase in temperature led to worsening of the reverse dependence between excitonic and deep-level emissions, and at room-temperature, they became uncorrelated. These results can be explained by Auger recombination and also by an increase of nonradiative recombination in the surface states with increasing temperature.
Abstract: CdTe and (Cd,Zn)Te crystals were grown to study the compensation mechanism and the influence on the transport properties. Undoped and doped crystals with Sn, In, and Ge were grown. The crystals showed resistivities up to 10(9) Omegacm and higher. The transport properties depended strongly on the dopant and the compensation mechanism. For the doping with a deep donor, the mobility-lifetime product of electrons were 2 x 10(-5) cm/V and 4 x 10(-4) M cm(2)/V for Ge and Sn doped, respectively. The highest values were obtained for In doped (Cd,Zn)Te with 3.3 x 10(-3) cm(2)/V.
Abstract: Two CdTe crystals had been grown in microgravity during the STS-95 mission. The growth configuration was dedicated to obtain dewetting of the crystals and to achieve high quality material. Background for the performed experiments was based on the theory of the dewetting and previous experience. The after flight characterization of the crystals has demonstrated existance of the dewetting areas of the crystals and their improved quality regarding the earth grown reference sample. The samples had been characterized by EDAX, Synchrotron X-ray topography, Photoluminescence and Optical and IR microscopy. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Abstract: A flip-chip bonding technology has been developed for CdTe and (Cd,Zn)Te X-ray detectors. The aim of this paper is to choose a suitable chip interconnection technique for the fabrication of X-ray pixel detectors to fit the material requirements of detector grade CdTe and (Cd,Zn)Te. The proposed method is based on a flexible process, which permits the use of an eutectic PbSn alloy with a photoresist mask. A photosensitive polymer acts as a passivation layer and solder stop mask on CdTe or (Cd,Zn)Te. The proposed process provides the opportunity to do all process steps "in-house."
Abstract: High resistive and photosensitive CdTe doped with In aimed for fabrication of X- and gamma-ray detectors was produced by Vertical Gradient Freeze method. A complex investigation of defects and compensation by a number of optical and photoelectrical mapping methods was performed. A model of energy levels dominating the recombination processes in the material was elaborated, where the role of In, and related complexes as well as native defects (Cd vacancy and its compexes) is discussed.
Abstract: Undoped and Ge-doped CdTe crystals have been grown using the dewetting phenomenon on the earth. A gap effected by the dewetting between the crystal and ampoule was created with a thickness up to 60 mum, but it was not stable for the complete growth process. The dewetting was stable for the first 25 mm of the growth, but than it got unstable. The main deteriorating factor was a change of a liquid-solid interface shape from the convex to concave. Structural, electrical, photoelectrical and optical characteristics of the studied samples showed better material quality in dewetted areas. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: We report a substantial reduction in the impurity concentration of semi-insulating CdTe:Ge grown by the vertical Bridgman method by using sublimation of the feed material. Specific resistivity (rho(dark)) values of up to 3 x 10(9) Omega cm were obtained for samples with a relatively high photosensitivity (PS) value and optimal compensation. Concentrations of impurities in the feed and as-grown crystals were determined by the glow discharge mass spectroscopy (GDMS) method. The energy levels in the band-gap were studied by photoluminescence (PL), and the data were correlated with the GDMS measurements. The highest values of rho(dark) and PS were observed in the regions where the PL bands via the deep levels of Ge and Te antisite were present.
Abstract: Knudsen cell mass spectrometry has been applied to study the quasi-binary system CdTe-ZnTe at 900 and 780K. Composition of the vapours, vapour pressures, activities, and free energy of: mixing were determined at 900 K. Activities of CdTe were shown to have a negative deviation from ideal behaviour, whereas those of ZnTe exhibited positive deviation. The free energy of mixing in CdTe-ZnTe is more negative than that of the ideal solution. P X section of the P-T-X phase diagram was constructed. and the relationship between the composition of the crystal, vapour pressure and composition of the vapour was determined in the whole concentration range. At 900 K, CdTe-ZnTe was found to be a continuous solid solution with no phase separation and without congruent sublimation. The shape of the CdTe activity isotherm at 780 K might suggest phase separation in Cd1-xZnxTe at this temperature. (C) 2003 Elsevier B.V. All rights reserved.
Abstract: We present a review of the vapor phase growth of cadmium telluride under conditions of reduced gravity in space. Particular emphasis is paid to the growth in a closed system by sublimation THM. Structural and electrical properties of the material grown under microgravity (mu-g) and earth gravity (1-g) conditions are reviewed and compared. A positive influence of mu-g conditions on CdTe crystal quality is reported. A brief literature survey of the vapor growth of other semiconductors (GeTe, HgI2, CdHgTe, ZnSe) in a p-gravity environment is also presented. (c) 2005 Elsevier Ltd. All rights reserved.
Abstract: Experimental results of vapor pressure measurement are presented for the whole temperature and pressure range for the existence of Cd0.95Zn0.05Te1+/-delta. Vapor pressure scanning approach was used to outline the maximum nonstoichiometry range and pressure-temperature-composition (P-T-X) phase arrangement of the solidus. Pronounced retrograde solubility on the Te side and rather weak temperature dependence of the metal solubility was registered. Position of the Cd0.95Zn0.05Te1+/-delta solidus in P-T-X phase space is represented in T-X and P(i)-T projections (i = Cd, Zn and Te-2). Isothermal sections of the sublimation region are presented in partial pressures, and from these, temperature dependence of the crystal composition along the vapor pressure minimum curve is derived. Cd(g), Zn(g) and Te-2(g) partial pressures as a function of temperature were deduced for constant crystal compositions Xs, which, on the one hand, constituted the thermodynamic basis of crystal growth of Cd0.95Zn0.05Te1+/-delta with fixed compositions, and on the other hand, led to evaluation of the partial thermodynamic functions for sublimation of Cd0.95Zn0.05Te1+/-delta. Thermodynamic aspects of Cd0.95Zn0.05Te1+/-delta sublimation are discussed. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: In this communication, the latest experimental results are presented on a composition of Cd1-xZnxTe1+/-delta, both x and delta, determined by the high-precision, vapor-pressure scanning method. The space arrangement of the single-phase solidus volume of Cd1-xZnxTe1+/-delta in the pressure-temperature-composition (P-T-X) phase space has been derived from direct vapor-pressure measurements in the temperature range up to 1,375 K. From the experimental data for x = 0.0, 0.05, 0.1, 0.15, 0.25, 0.5, 0.75, 0.8, 0.9, and 1, the complete phase diagram of Cd(1-x)Zn(x)Je(1+/-delta) was constructed. Transformation of the diagram was traced for the whole range of existence of the Cd1-xZnxTe1+/-delta solid solution as a function of ZnTe concentration. Increase of ZnTe in the solid solution results in the extension of the homogeneity range and a shift of the solidus toward Te, so that already for x greater than or equal to0.15 the solidus does not contain the stoichiometric, (n(Cd) + n(Zn)):n(Te) = 50 mol.% plane. From detailed experimental studies, maximum non-stoichiometry as a function of the temperature was determined in a wide temperature range for Te and metal solubility Temperature dependencies of the partial pressures of vapor-phase species for different crystal compositions, X-S = const, were deduced from the vapor-pressure experiment for the x = 0.05 section of the diagram. From these results, relationships are derived between crystal composition, vapor pressure, and composition of the conjugated vapor.
Abstract: Two different X-ray tube accelerating voltages (60 and 70 kV) are used for diagnosis of front teeth and molars. Different energy ranges are necessary as function of tooth thickness to obtain similar contrast for imaging. This technique drives the costs for the X-ray tube up and allows for just two optimized settings. Energy range selection for the detection of the penetrating X-rays would overcome these severe setbacks. The single photon counting chip MEDIPIX2 http://www.cern.ch/medipix exhibits exactly this feature. First simulations and measurements have been carried out using a dental X-ray source. As a demonstrator a real tooth has been used with different cavities and filling materials. Simulations showed in general larger improvements as compared to measurements regarding SNR and contrast: A beneficial factor of 4% wrt SNR and 25% for contrast, measurements showed factors of 2.5 and up to 10%, respectively. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: We present the results of photoluminescence (PL) study of CdTe:Ge substrates, which form part of Hg1-xCdxI2/ CdTe heterostructures grown by low-temperature (< 250degreesC) vapor phase epitaxy (VPE), using a combination of several PL techniques such as spectroscopy, cross-section mapping and profiling. It is demonstrated that the VPE growth causes a simultaneous and interactive transformation of different defect subsystems like native defects, residual impurities and dopants all over CdTe substrate bulk. The dominant processes were found to be the formation of iodine compensating complexes associated with cadmium vacancies and the gettering of defects at layer/substrate interface. (C) 2003 Elsevier B.V. All rights reserved.
Abstract: Vapour pressure measurement in the Cd-Zn-Te system is reported for T = 700-1300 K and P less than or equal to 760 mm Hg. From the experimental data for x = 0.05, 0.10, 0.15. 0.25. 0.5. 0.75, 0.80. 0.90 and 1.0, complete P-T projection of the phase diagram for the quasi-binary Cd1-xZnxTe system has been constructed. Transformation of the P-T projection was traced for the whole range of the Cd1-xZnxTe solid solution as a function of the ZnTe concentration. From detailed Studies of sections x = 0.05, 0.1, 0.15 and 1.0. maximum non-stoichiometry as a function of temperature was determined both for Te and metal solubility. For ZnTe the solidus volume is on the Te side of the stoichiometric plane (50 at.% Te). Zn-boundary of ZnTe solidus is 50.004-50.005 at.% Te in composition at 826-1153 K. Formation of the Cd1-xZnxTe solid solution leads to extension of the homogeneity range, especially at high temperatures. Increase of ZnTe content in the solid solution results in a shift of the solidus toward Te, so that already for x = 0.15 the solidus does not contain the stoichiometric plane. (C) 2003 Elsevier B.V. All rights reserved.
Abstract: The growth of high resistivity CdTe and (Cd,Zn)Te is successfully performed with different kinds of doping. In the literature intentionally undoped as well as doped crystals are presented with resistivities up to 10(10) Omegacm. In this paper we review the growth of high resistivity CdTe and (Cd,Zn)Te. The mechanism of compensation is discussed regarding the different dopants and deep levels, which seem to be responsible for the high resistivity. A common compensation model explains the high resistivity by deep levels. The doping and the influence on the compensation mechanism is compared for several elements like tin, germanium and chlorine. The material properties and the crystal quality of undoped and doped CdTe as well as (Cd,Zn)Te are shown.
Abstract: CdTe crystals have been grown by the Vertical Bridgman method with diameters of 25, 45 and 75 mm. Improvements of the crystallinity could be achieved by reduction of twins and growth of large single crystalline grains up to 40 x 40mm(2). CdTe:Ge has been grown with resistivities up to 2 x 10(10) Omega cm. The samples showed a mobility-lifetime product of 5.7 x 10(-5) for holes. (C) 2003 Elsevier Science B.V. All rights reserved.
Abstract: At the new synchrotron light source ANKA (Angstrom Karlsruhe, Germany) one beamline is optimised for white beam synchrotron topography. First topographs taken at various geometries and materials demonstrate the easy operation and the high resolution. Details about the experimental set-up are given. Large area and section topography in transmission allow a quantitative analysis of the type of dislocations and the dislocation density up to 10(6) cm(-2) which is shown for GaSb:Te and InP:S, respectively. For highly absorbing materials like CdTe the back reflection geometry is adequate to analyse dislocation networks, twins and small angle grain boundaries. The grazing incidence method is used for the characterisation of strain and defects as a function of depth by varying the tilt of the sample which is helpful for processed devices like CdTe strip detectors for X-rays. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: Thermodynamic conditions for post-growth annealing to prepare near stoichiometric semi-insulating CdTe are studied for undoped, and Sn and Ge-doped single crystals. The main aim of the annealing procedure was to obtain high resistive material with the minimal concentration of native and foreign defects. The high temperature (200 - 1000 degreesC) in-situ conductivity sigma and Hall effect measurements were used to control the native defect density and to determine the Cd pressure p(Cd) at which shallow defects are compensated. It is shown that contrary to the undoped samples in which the change of the type of conductivity by variations of p(Cd) is easy, the Sn- and Ge-doped samples exhibit a much more stable behavior due to the Sn (Ge) self-compensation. It was found that: the temperatures near 500 degreesC is optimum for the real-time annealing of bulk samples, the chemical diffusion is sufficiently fast at these temperatures while the uncontrolled change of defect density distribution during the subsequent cooling process is minimized. The time-dependent charge measurement technique was used to characterize the room-temperature specific resistivity distribution in the as-grown crystals, which indirectly controls the dynamic of solidification process. This allows us to consider the specific resistivity distribution along the growth direction of each crystal in terms of superposition of segregation and self-compensation phenomena.
Abstract: The heat capacity of ZnTe was measured in an adiabatic calorimeter in the temperature range from 15 K to 330 K. The following standard thermodynamic properties of ZnTe were derived: C-p,m(degrees) (298.15 K) = (49.52 +/- 0.10) J . K-1 . mol(-1); S(m)degrees(298.15 K) = (81.94 +/- 0.17) J . K-1 . mol(-1); H(m)degrees(298.15K) -H(m)degrees(0) = (10.98 +/- 0.02) kJ . mol(-1); and Phi(m)degrees (298.15K) = (45.12 +/- 0.10) J . K-1 . mol(-1). The Debye temperature, calculated from In the massic heat capacity, is Theta(D) = 252 K. (C) 2002 Elsevier Science Ltd. All rights reserved.
Abstract: The heat capacity of ZnTe was measured in an adiabatic calorimeter in the temperature range from 15 to 330K and by DSC at T=290-925K. The following standard thermo-d,ynamic properties of ZnTe were derived: C-p(0) (298.15 K) = (49.52 +/- 0.10) J K-1 mol(-1); S-0(298.15 K) = (81,94 +/- 0.17) J K-1 mol(-1); H-0(298.15 K) - H-0(0 K) = (10.98 +/- 0.02) kJ mol(-1;) Phi(n)(298.15 K) (45.12 +/- 0.10) J K-1 mol(-1). Temperature dependences of the heat capacity and the Gibbs free energy function are derived for the temperature range up to 1500 K. The Debye temperature calculated from the specific heat data is Theta(D) = 252 K.
Abstract: Quasi-chemical formalism is used to evaluate high temperature (600 degreesC-1000 degreesC) in situ conductivity and Hall effect measurements and simultaneously tellurium atom fraction in CdTe along the three-phase curve. We show that the electric properties can be described only by two native defects-cadmium interstitial as the divalent donor and cadmium vacancy as the divalent acceptor. Close to Te saturation, another native defect must be involved in the model to allow the deviation from the stoichiometry irrespective of the low density of electrically charged defects. Deep divalent donor Te-Cd having both levels near or below the midgap best describes all the high-temperature experimental data.
Abstract: P-T-X data are the thermodynamic basis for modeling the crystal growth of materials with controlled stoichiometry. In this paper results of the first direct experimental measurement of the total vapor pressure are reported for three-phase equilibria SLV (solid-liquid-vapor) and VLS (vapor-liquid-solid) in Zn-Te, and the P-T projection of the P-T-X phase diagram is constructed. Geometrical analysis of the phase equilibrium showed that three congruent processes are observed in the Zn-Te system: congruent melting (S = L), sublimation (S = V), and vaporization (L = V). All three congruent curves are tangent to SLV. Consequently, all three congruent points are on the Te-side of the maximum melting point of ZnTe. The vapor pressure scanning method was applied to determine the maximum non-stoichiometry as a function of the temperature. The solidus volume of ZnTe was shown to be on the Te-side of the stoichiometric plane.
Abstract: Processes of growth of semi-insulating Cd1-xZnxTe:Cl crystals (x = 0.0002 and 0.1) of n-type conductivity are investigated. From the grown crystals detectors for X-ray computer tomography with small value of photocurrent memory (afterglow) (0.1-0.3%) are obtained.
Abstract: A complex investigation of defect structure of high-resistivity Ge-doped CdTe by a number of optical, photoelectrical and electrical methods was performed. It was found that material properties are strongly influenced by the presence of centers of strong recombination (S-centers) and photosensitivity (R-centers). A model of energy levels dominating the recombination processes in the material was elaborated, where the role of Ge, Fe and Cu related as well as native defects (Cd vacancy) is discussed. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: High-temperature heat capacities were measured using the Setarain DSC 111 in the temperature range 290-925 K. Experimental data were smoothed jointly with earlier determined low-temperature heat capacities to calculate thermodynamic properties in temperature range 15-925 K. Extrapolation of heat capacity curve to 1500 K allows to calculate Gibbs free energy at high temperatures. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: High precision concerning the dose calibration of X-ray sources is required when counting and integrating methods are compared. The dose calibration for a dental X-ray tube was executed with special dose calibration equipment (dosimeter) as function of exposure time and rate. Results were compared with a benchmark spectrum and agree within +/- 1.5%. Dead time investigations with the Medipix1 photon-counting chip (PCC) have been performed by rate variations. Two different types of dead time, paralysable and non-paralysable will be discussed. The dead time depends on settings of the front-end electronics and is a function of signal height, which might lead to systematic defects of systems. Dead time losses in excess of 30% have been found for the PCC at 200 kHz absorbed photons per pixel. (C) 2002 Published by Elsevier Science B.V.
Abstract: We report the possibility to grow semi-insulating (Cd,Zn)Te:Se crystals by the modified Markov method (MMM) from the vapor phase. When oriented Cd(Te,Se), seed material was used for. the growth, lattice matching and doping with Se resulted in an increase of resistivity up to several units of 10(9) Omega cm., A homogenous Zn distribution was also observed, The content of Se and Zn in the seed and grown crystals was determined by photoluminescence and energy dispersive analysis by x ray measurements. Resistivity and Zn concentration maps show a good spatial correlation in which the highest values, of resistivity correspond. to the areas of the highest Zn concentration. The main part of the crystals demonstrate a low extinction coefficient in the IR spectral region, except for the Zn inhomogeneity region. (C) 2002 American Institute of Physics. [DOI: 10.1063/1.1533124].
Abstract: Morphology and analysis of composition of inclusions were done by secondary electron microscopy and spatially resolved energy-dispersive analysis of x-ray on semiintrinsic CdTc:Cl and CdTe:Zn:Cl crystals grown from the vapor phase by the modified Markov technique and on undoped CdTc crystals grown from the melt by the Bridgman method. In CdTe:Cl and CdTe:Zn:Cl crystals nonstoichiometric inclusions of about 10-20 mum were found, which contain high concentrations of Cl and Na impurities. The Cl is concentrated in small precipitates of 1-2 mum inside these inclusions. After short-time low-temperature annealing (600 degreesC), the inclusions mostly disappeared.
Abstract: Knudsen cell mass spectrometry has been applied to study the quasi-binary system CdTe-ZnTe at 900 K. Composition of the vapors, vapor pressures, activities, and free energy of mixing were determined. Activities of CdTe were shown to have negative deviation from ideal behavior, whereas those of ZnTe exhibited positive deviation. Free energy of mixing in CdTe-ZnTe is more negative than that of the ideal solution. Pressure-composition P-X section of the Pressure-temperature-composition P-T-X phase diagram was constructed. and relationship between the composition of the crystal, vapor pressure and composition of the vapor was determined in the whole concentration range. At 900K. CdTe-ZnTe was found to be a continuous solid solution with no phase separation and without congruent sublimation. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: CdTe:Zn:V crystals grown by the seeded Bridgman method in microgravity conditions during the STS95-Spacelab-AGHF-1 mission and in the ground laboratory (1-g) were analyzed and compared. The results obtained clearly show that the structural quality of the space crystal is better. Density of inclusions, concentration of dislocations, and presence of stresses are lower in the microgravity-grown (mu-g) crystal. The 1-g crystal contains twins and grains from the beginning of the growth process, that is, from the near-seed region. In general, the concentration of inclusions and amount of segregated impurities on the 1-g crystal are larger than in the mu-g crystal. X-ray rocking curves and low-temperature photoluminescence spectra demonstrate the relatively high quality of both crystals on a microscale at the beginning of the. growth and show that the 1-g conditions were worse at the end. The results of this investigation demonstrate a positive role of contactless growth and mu-g conditions in the melt in suppressing the creation of inclusions and dislocations.
Abstract: Synchrotron topography characterization of vapor grown ZnTe crystals showed that the available data in this system are not sufficient to control the growth parameters. First P-T-X phase equilibrium studies confirm that the stoichiometric plane X=50 at.% Te is completely outside of the single-phase volume of ZnTe. The Te boundary exhibits retrograde solubility. The maximum Te non-stoichiometry of 0.0186 at.% was found at T = 1344 K. (C) 2001 Published by Elsevier Science Ltd.
Abstract: Characterization studies of Cd1-xZnxTe (0 <x <0,24) crystals, CZT, grown by high pressure vertical Bridgman (HPVB), low pressure (LPB) vertical modified Bridgman (VB), horizontal modified Bridgman (HB), and physical vapor deposition (PVD) methods were performed, For selected melt-grown ingots, the liquid/solid segregation coefficients of some of the impurities were established. For most of the crystals, the surface and the bulk crystallinity were determined using triple and double axis X-ray diffraction techniques (TAD and DAD XRD). X-ray topography maps were also used to study macroscopic defects, The difference in properties of CZT grown by these methods are discussed. (C) 2001 Published by Elsevier Science B.V.
Abstract: The experiment STS95-Spacehab-AGHF1 has been devoted to the study of the dewetting phenomenon applied to the growth of CdTe:Zn:V. Two samples have been introduced in quartz ampoules and solidified directionally. However, one sample has been grown at a very high rate, due to untimely furnace shut-off. After the analysis of the experimental data, the growth rate and thermal gradient have been estimated. Metrology of the samples, surface roughness and microscopic analysis show that both space samples presented similar partial dewetting, with the conclusion that the growth rate is not involved in the phenomenon. The ground reference sample has shown st closer contact and moulding to the crucible wall. A special design of one ampoule prevented the establishment of any pressure difference between the hot and cold parts of one space sample, then demonstrating that such differences are not necessarily involved when dewetting occurs. The structural analysis is still under progress. The sample grown at high rate shows twins from the very beginning. The ground reference sample is polycrystalline from the beginning of the solidification. The space sample grown under nominal conditions is a single crystal on the first 11 mm, demonstrating that dewetting, even partial, is beneficial to the crystal structural quality. (C) 2001 Elsevier Science Ltd. All rights reserved.
Abstract: Semi-insulating CdTe single crystals doped with Ga were grown from the vapour phase by the modified Markov technique MMT. The study of the resistivity map in the cross-sections cut along the growth direction has been performed. The compensation phenomenon is analysed in the framework of the three levels Fermi-statistic model. It is shown that a semi-insulation behaviour throughout the ingot is due to the compensation of shallow impurities by the deep level. From the low-temperature photoluminescence spectra it was concluded that shallow donors (Gac) are partly compensated by (Ga-Cd-V-Cd)(-) and (Ga-Cd-Cd-Tc) complexes and by residual acceptors (Na-Cd, Cu-Cd). The microscopic structure of (Ga-Cd-Cd-Tc)) complex is proposed based on the value of its local phonon mode and the growth conditions. A native defect like Te-Cd which has a deep level near the middle-band-gap is suppose to dive a stable compensation and a tolerance for variation in shallow impurity concentrations.
Abstract: This paper presents data on experimentally and theoretically deduced total energies and energy level positions in the band-gap for eight elementary defects in CdTe. Based on these data, the total energy dependence on the Fermi-level position in the band-gap is graphically analysed. Two types of defect reactions, which are responsible for cadmium vacancy (V-Cd) creation and transformation, are discussed. It is shown that the most probable candidate for the native deep-level donor is a tellurium antisite. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: The growth of CdTe from the vapour phase offers several improvements in crystal quality and homogeneity. CdTe and (Cd, Zn)Te were grown by the modified Markov technique. The transport properties and the detector performance are given and compared to melt grown material. (C) 1999 Elsevier Science B.V. All rights reserved.
Abstract: CdTe and (Cd,Zn)Te crystals were compared by their material properties. The compensation mechanism of(Cd,Zn)Te and the influence of deep levels are compared to CdTe. The detector grade of(Cd,Zn)Te is obtained with a peak-to-valley ratio of 3.5 for a Co-57 spectrum. The homogeneity of electrical properties is discussed by photoluminescence and TDCM mappings. Recent applications of(Cd,Zn)Te detectors are presented. (C) 1999 Elsevier Science B.V. All rights reserved.
Abstract: The traditional compensation model to explain the high resistivity properties of CdTe is based on the presence of a deep acceptor level of the cadmium vacancy in the middle of the band gap. A new compensation model based on a deep intrinsic donor level is presented. The compensation model is used together with an appropriate segregation model to calculate axial distributions of resistivity which are compared with spatially resolved resistivity measurements. The Te-antisite defect is discussed as a possible origin cause of this intrinsic defect, which is also supported by theoretical calculations. (C) 1998 American Institute of Physics. [S0021-8979(98)02224-5].
Abstract: Gallium Arsenide layers grown using low pressure Vapour-Phase Epitaxy (LP-VPE) were studied with CV, Hall-measurements and Photoluminescence. The results have been analysed for the general investigation of the influence of material properties on particle detector performance for X-ray applications. This p-type material exhibits a free carrier concentration of 1.3 x 10(11) cm(-3) at room temperature and was compensated by the presence of shallow donors and deep accepters. Because of this, the detector performance was restricted by the space-charge density of the order of 10(14)cm(-3). (C) 1998 Elsevier Science B.V. All rights reserved.
Abstract: V and Ga doped CdTe single crystals with one inch diameter were grown without wall contact in a semi-closed vapour phase system (modified Markov method). By vapour transport modelling, we demonstrate that volatile impurities and excess species are enabled to condense in a heal sink connected to the growth chamber. Vapour composition and component fluxes are controlled by the temperature profile, in particular by the sink temperature. The grown crystals exhibit pronounced {111}, {110} and {100} facetting. The influence of the deep heat sink on interface stability is discussed in terms of growth morphology and formation of inclusions. Piezobirefringence measurements indicate nearly stress-free growth. The resistivities of the grown crystals are up to 3 x 10(9) Omega.cm.
Abstract: Vanadium and gallium doped CdTe crystals were grown from the vapour phase (modified Markov method) and (Cd0.9Zn0.1)Te:V from the melt (vertical Bridgman). The crystals were characterized by photoinduced current transient spectroscopy (PICTS), photoluminescence (PL) and time dependent charge measurements (TDCM). Transitions from different charge states (V2+/V3+) Of the vanadium donor have been observed in the V-doped crystals by PICTS. A shallow donor level (dE = 0.068 eV) and the Ga A-center have been identified by PICTS and PL measurements in CdTe:Ga. In case of V-doping high resistivity is achieved all over the crystal while Ga-doping results in a high resistivity region only in the middle of the crystal. Calculation of the resistivity by means of a compensation model shows that for both dopants an additional not observed deep donor has to be assumed in order to describe the resistivity distributions.
Abstract: Schottky diodes made of semi-insulating (SI) gallium arsenide and bulk material received from various manufactures were studied by using magnetic circular dichroism of absorption (MCDA), near-infrared absorption (NIR), CV- and Hall-measurements, The results have been analysed to investigate the influence of the EL2/EL2(+)-concentration to the detector properties, The MCDA measurements of these different materials showed a variation of the EL2(+)-concentration between 3.0 and 6.5 x 10(15) cm(-3), A likely dependence of the charge collection efficiency (CCE) for alpha particles on the EL2(+)-concentration was observed. Also an influence of the EL2(+)-concentration on the space charge density measured by CV and the leakage current density can be seen, No correlation between the room temperature mobility and the EL2(+)-concentration was found. We conclude that the EL2(+)-concentration and the position of the Fermi level have a influence on the detector performance.
Abstract: The influence of growth conditions on radiation detector performance was studied by examining Cd(Te,Se):Cl crystals. They were grown by the travelling heater method under microgravity conditions during the PHOTON 8 mission. An additional forced convection was produced by a rotating magnetic field (400 Hz) of 2 mT. The influence on crystal quality was analyzed by several characterization methods such as time dependent charge measurement, photo-induced current transient spectroscopy and admittance spectroscopy. The charge collection efficiency of detectors was increased from 10% without magnetic field up to 66% by the influence of the rotating magnetic field. The improved detector performance is explained by the increased resistivity and a reduced number of deep levels.
Abstract: CdTe:Cl crystals were grown from the liquid and from the vapour phase under microgravity( mu g) conditions on board the unmanned EURECA I mission. The resistivity distribution of the grown crystals was measured by time dependent charge measurement (TDCM). Photo induced current transient spectroscopy (PICTS) was used to investigate the deep level properties. The axial resistivity distributions of the crystals grown in space differ significantly from Ig reference crystals. In the case of vapour growth, these differences can be explained by an additional laminar flow under Ig conditions. Supercooling has to be considered in growth from a Te zone under mu g.
Abstract: CdTe0.9Se0.1:Cl is a detector grade material for gamma and X-rays. Its high resistivity and the high mobility lifetime product yield a high charge collection efficiency of 90 percent. CdTe0.9Se0.1:Cl was used for the first time to built up a strip detector. The detector performance was investigated by a Co-57 source. The signal behaviour, charge collection efficiency and coupling effects were analyzed for different strips. The comparison between the signal amplitude of all strips showed a good homogeneous response for the device. For a single strip a charge collection efficiency of more than 40 percent was obtained.
Abstract: According to Ramo's theorem the charge collection efficiency of a particle detector is mainly influenced by the field distribution between the contacts of a Schottky diode. In semi-insulating GaAs material a space charge layer is formed due to deep levels needed for the compensation of accepters. In this paper the deep levels and their influence on the distribution of the electric field is studied experimentally by different methods of electrical characterization. It is found that the electrical active concentration of the midgap donor of similar to 10(15) cm(-3) at an energy of 0.67 eV below the conduction band is only about one tenth of its total concentration of similar to 10(16) cm(-3) as measured by infrared absorption. The Schottky barrier leakage current is found to be responsible for the variation of the electrically active deep centers and it therefore influences the charge collection efficiency (c.c.e.). The c.c.e, turns out to be inversely proportional to the active concentration of deep centers. These results are supported by our modelling of the c.c.e.: Using the transport and the Poisson equation the electrical field distribution can be calculated through the coupling of the quasi-Fermi levels and the compensation mechanism. The model calculations of charge collection efficiencies for both alpha particles and protons are confirmed by the experimental results. The work is performed within the framework of the RD8 project.
Abstract: CdTe:Cl crystals were grown from the vapour phase by the sublimation travelling heater method (STHM) in space on board of the unmanned EURECA I mission. They were characterized by time dependent charge measurement (TDCM) and photoinduced current transient spectroscopy (PICTS). Axial cuts of the space grown crystals investigated by TDCM reveal an axial symmetric resistivity distribution along the growth direction, which can be explained by segregation effects of chlorine.
Abstract: The vapour growth of CdTe:V and CdTe:Ti by the sublimation travelling heater method (STHM) is reported for the first time. Characterization of the crystals has been performed by time dependent charge measurement (TDCM) and photoinduced current transient spectroscopy (PICTS). Axial cuts of CdTe:V crystals showed a very homogeneous and high resistivity distribution in the range of 10(10) Omega.cm. The PICTS spectra depict three levels which can be attributed to transition metal doping. The expected deep donor level can be identified clearly to be at E(v) + 0.95 and 0.94 eV in CdTe:V and CdTe:Ti, respectively.
Abstract: Several CdTe crystals doped with vanadium (V) or titanium (Ti) were grown by a vertical Bridgman technique and the Sublimation Travelling Heater Method (STHM). Spatially resolved resistivity measurements were performed using the Time Dependent Charge Measurement (TDCM) method. It is shown that V doping leads to a more homogeneous resistivity distribution in contrast to Ti. Photoinduced Current Transient Spectroscopy (PICTS) has been used to characterize the deep levels. Up to three additional levels are introduced through the doping with transition metals.
Abstract: Semi-insulting CdTe bulk crystals were grown from the vapour phase in both closed and semi-open arrangements. The results of the growth experiments are discussed in terms of various electrical and optical characterization methods. Van der Pauw measurements and time dependent charge measurements (TDCM) were used to determine the resistivity. Deep level defects were investigated by means of photoinduced current transient spectroscopy (PICTS). For one of the most important fields of application, detector spectra of the vapour phase material are measured and discussed.
Abstract: Semi-insulating titanium and vanadium doped cadmium telluride crystals were grown from the vapour phase. We show results of the electrical characterisation obtained by Hall measurements, photoinduced current transient spectroscopy (PICTS), admittance spectroscopy and time-of-flight measurements. The crystals with a resistivity of about 10(9) Omega.cm showed a deep level, which was identified for CdTe:V at - 0.77 eV and a similar value at - 0.66 eV for CdTe:Ti, both with reference to the conduction band. The mu tau products are almost the same for both materials in the range of 10(-5) cm(2)/V. A compensation model, which was developed for semi-insulating GaAs, is adapted to semi-insulating vanadium- and titanium-doped cadmium telluride. We show that the data from the compensation model correspond to the measured properties.
Abstract: CdTe and CdTe0.9Se0.1 crystals were grown by the travelling heater method under different material transport regimes. The different transport regimes in the liquid zone were either caused by diffusion (and residual convection), or by 1 g convection, or by forced convection. The forced convection was produced by a B = 2 mT rotating magnetic field (400 Hz) under mug conditions. The equations for describing the model of the rotating magnetic field are derived, showing that the magnetic force term in the Navier-Stokes equation can be calculated separately in the case of ug conditions and small magnetic field. The rotating magnetic field generates a stable steady flow in the solution zone, which improves the radial and axial distribution of mutau products.
Abstract: CdTe is one of the most encouraging semiconductor materials in the field of room temperature gamma- and X-ray spectroscopy. To improve the detector properties, the temary systems (Cd,Zn)Te and Cd(Te,Se), and CdTe were grown by vertical Bridgman technique. To achieve low noise detectors, the resistivity of all materials was increased by chlorine doping. The crystals were characterized by electrical (Hall measurements, photoinduced current transient spectroscopy) methods. The numbers of deep levels influencing the resistivity were reduced by introducing Se into the CdTe system. A common deep level close to the middle of the bandgap has been identified, responsible for the compensation effect in all three systems. In addition high resistivity and n-type conductivity were achieved in CdTeSe materials for the first time. Detectors were tested by irradiation with a particles and low gamma-rays. Best charge collection efficiency (CCE) of 91% was recorded for CdTe0.9Se0.1 for both radiation types and smallest FWHM of about 10% at 60 keV.
