Search

For high-power and high-voltage applications, silicon is by far the dominant semiconductor material. However, silicon has many limitations, e.g. a relatively low thermal conductivity, electric breakdown occurs at relatively low fields and the bandgap is 1.1 eV which effectively limits operation to temperatures below 175 deg.n C. Wide-bandgap materials, such as silicon carbide (SiC), gallium nitride (GaN) and diamond offer the potential to overcome both the temperature and power handling limitations of silicon. Diamond is the most extreme in this class of materials. By the fundamental material properties alone, diamond offers the largest benefits as a semiconductor material for power electronic applications. On the other hand, diamond has a problem with a large carrier activation energy of available dopants which necessitates specialised device concepts to allow room temperature (RT) operation. In addition, the role of common defects on the charge transport properties of diamond is poorly understood. Notwithstanding this, many proof-of-principle two-terminal and three-terminal devices have been made and tested. Two-terminal electronic diamond devices described in the literature include: p-n diodes, p-i-n diodes, various types of radiation detectors, Schottky diodes and photoconductive or electron beam triggered switches. Three terminal devices include e.g. MISFETs and JFETs. However, the development of diamondmore » devices poses great challenges for the future. A particularly interesting way to overcome the doping problem, for which there has been some recent progress, is to make so-called delta doped (or pulse-doped) devices. Such devices utilise very thin ({approx}1 nm) doped layers in order to achieve high RT activation.« less

Epitaxial formation of SiC nanocrystals has been investigated on single crystal silicon surfaces. A simple and cheap method using reactive annealing in CO has been developed and patented by our group (BME AFT and MTA MFA). By this technique epitaxial 3C‐SiC nanocrystals can be grown at the Si side of a SiO2/Si interface without void formation at the SiC/Si interface. CO diffusion and SiC nanocrystal formation on different silicon based systems (SiO2/Si, Si3N4/3Si and SiO2/LPCVD poly‐Si) after CO treatment at 105 Pa at elevated temperatures (T>1000° C) will be presented. By optimizing the annealing time a thin continuous nanocrystalline SiC layer has been formed. Applying a patterned Si3N4 capping layer as a barrier layer against CO diffusion, SiC nanocrystal formation at the Si3N4/Si interface is inhibited. We will present the selective growth of SiC nanocrystals using the before mentioned technique.

Raman microscopy has been used to study the carrier concentration and mobility in n‐doped 3C‐SiC epilayers grown on different silicon substrates, namely (100) Si and (111) Si on axis and off‐axis towards the [110] direction. By analyzing the longitudinal optical phonon‐plasmon coupled mode (LOPC), we were able to estimate the 3C‐SiC electron bulk mobility (μ) within the range between 5 and 500 cm2/Vs. The carrier concentration (n) was ranging from 2×1016 to 6×1018 cm−3. The observed trend shows a reduction in the electron mobility as the carrier concentration increases for films grown on any substrate considered, accordingly to the existent theory. For equal values of doping concentration, 3C‐SiC epitaxial films grown on (100) Si substrates show a higher mobility than films grown on (111) Si counterparts. This could be ascribed to a higher defect density in (111) Si samples. A deeper characterization by performing Raman maps shows a broadening and a splitting of the 3C‐SiC transverse optical (TO) peaks fo...

Residual stress and carrier lifetime variation have been measured in free‐standing n‐type 3C‐SiC wafer grown on undulated Si substrate. We identify extended regions of residual stress that lie parallel to epilayer surfaces. The opposite polarity of stress is identified toward the interface and toward the top surface. Integrated carrier lifetime has been determined by random defect density distribution which is enhanced in the areas of double‐positioning boundary defects. It is shown that carrier lifetimes are severely reduced by the presence of residual stress towards epilayer surfaces. In this way lifetime depth‐distribution can be mistakenly attributed to enhanced surface recombination.

The electrical properties of SiO2/3C‐SiC structures fabricated by rapid thermal processing (RTP) using oxynitridation and hydrogenation processes have been examined. It is shown that the growth of the SiO2 films in a RTP chamber is one order of magnitude faster than in a conventional furnace. As well as being fast, this process leads to oxide films with quality comparable or even better than the one grown in classical furnaces. It is revealed that with respect to oxides grown in N2O gas, without any pre‐annealing step, hydrogenation by H2 surface pre‐treatment reduces the density of interface traps and effective oxide charge in SiO2 layers. The improvement in the quality of the SiO2/3C‐SiC interface by H2 pre‐oxidation annealing is discussed.

Aiming at the growth of diamond nanowires on carbon nanotubes we have studied carbon nanotubes for treatment in pure hydrogen in Hot‐Filament assisted chemical vapour deposition reactor (HFCVD). Purified multiwalled carbon nanotubes were dispersed onto silicon substrates, and annealed in pure hydrogen in HFCVD reactor. Important parameters were substrate temperature (650 to 870° C), nanotube density, annealing time (1 to 4 hours), hot‐filament temperature (HFVCD, 1900 to 2350° C), and gas pressure (30 to 70 Torr). No catalyst was used, and no methane was introduced in the chamber. Annealed samples were studied by Field Emission SEM and EDX analysis. Results show that for a filament temperature of 2000 to 2350° C, carbon nanotubes are rather stable in atomic hydrogen environment. For higher filament temperatures (>2350° C) nanotubes are converted into microcrystalline graphite, and for lower temperatures (1900–1950° C) nanotubes are etched away. In the conditions in which nanotubes are stable, growth of na...

The present work demonstrates the applicability of FTIR‐ellipsometry to determine vibrational properties and stress distributions in thin and ultra thin 3C‐SiC heteroepitaxial layers grown on Si(111) and Si(100) substrates. Additionally, they are compared to the properties of hexagonal SiC layers formed on Si(111) are analyzed.

International audience; The stability of (001)‐oriented 3C silicon carbide crystals is studied by a method coupling high resolution x‐ray diffraction and numerical simulations. The analysis of the diffuse scattering intensity distribution along selected directions in reciprocal space allows us to obtain qualitative and quantitative informations regarding the 3C‐6H transition. Our latest results concerning the influence of the initial crystal quality (presence of defects) and of annealing time on the 3C‐6H transition are presented in this article.

In this work we report on membranes made of nanocrystalline diamond (NCD) and AlN for the use in tunable micro‐optics. For the growth of the AlN and NCD thin films, magnetron sputtering and chemical vapor deposition techniques have been used, respectively. A chemical‐mechanical polishing process of NCD layers has been introduced, which is crucial for the growth of c‐oriented, fiber textured AlN films. AlN layers deposited on as grown and polished nanocrystalline diamond along with free standing membranes have been compared by studying microstructure, surface morphology, piezoelectrical response as well as optical properties.

The growth of cubic group III‐nitrides is a direct way to eliminate polarization effects, which inherently limit the fabrication of normally‐off hetero‐junction field‐effect transistors (HFETs) in GaN technology. HFET structures were fabricated of non‐polar cubic AlGaN/GaN hetero layers grown by plasma assisted molecular beam epitaxy on free standing 3C‐SiC (001). The electrical insulation of 3C‐SiC was realized by Ar+ implantation before c‐AlGaN/GaN growth. HFETs with normally‐off and normally‐on characteristics were fabricated of cubic AlGaN/GaN. Capacitance‐voltage characteristics of the gate contact were performed to detect the electron channel at the c‐AlGaN/GaN hetero‐interface.

Nowadays crystalline Silicon Carbide (SiC) thin films are promising coatings for high‐temperature, high‐power and high‐frequency electronic devices as well as for optical (for Infrared and Extreme Ultraviolet, EUV) and optoelectronic applications because of its physical and electrical properties. We have deposited Silicon Carbides thin film for by means of Pulsed Laser Deposition (PLD) in off‐axis geometry to obtain crystalline 3C‐SiC using two different target such as hot‐pressed SiC and crystalline 3C‐SiC on Silicon (100) and Silicon (111) substrates. At high temperature, and in the same deposition condition such as target‐substrate distance, laser fluence, repetition rate, the resulted films are quite different resulting dependent from the target and the substrates, being polycrystalline with the hot‐pressed target and oriented nanocrystalline when using the 3C‐SiC crystalline target. The films have been characterized by means of Atomic Force microscopy (AFM), Scanning Electron Microscopy (SEM), Infrared Transmittance and EUV reflectance.

We report on carbon doping of cubic GaN by CBr4 using plasma‐assisted molecular beam epitaxy on 3C‐SiC (001) substrates. The samples consist of a 70 nm thick GaN buffer followed by a 550 nm thick GaN:C layer. Carbon doping is realized with a home‐made carbon tetrabromide sublimation source. The CBr4 beam equivalent pressure was established by a needle valve and was varied between 2×10−9 mbar and 6×10−6 mbar. The growth was controlled by in‐situ reflection high energy electron diffraction. The incorporated carbon concentration is obtained from secondary ion mass spectroscopy. Capacitance voltage characteristics were measured using metal‐insulator‐semiconductor structures. Capacitance voltage measurements on nominally undoped cubic GaN showed n‐type conductivity with ND‐NA = 1×1017 cm−3. With increasing CBr4 flux the conductivity type changes to p‐type and for the highest CBr4 flux NA‐ND = 4⋅5×1018 cm−3 was obtained.

In this article, using kinetic Monte Carlo simulations, we study two morphological instabilities, namely step bunching and step meandering, that are known to occur during the epitaxial growth of Silicon Carbide on misoriented substrates. Specifically, we analyze the simulated final, i.e. after the growth process, step roughness (related to the step meandering instability) as function of the growth and temperature during growth of cubic SiC polytype for substrates with miscut towards the [110] and [112] directions and compare these results with recent experimental findings. We also study the simulated final terrace width distribution (related to the step bunching instability) as function of the substrate polytype finding that the cubic polytype does not suffer of neither the geometrical nor the energetic surface instability. For this reason the cubic polytype, showing a better planarity, could be a good candidate for subsequent metallization or oxidation processes. Finally, we show that, on the basis of the simulated morphologies of the hexagonal polytypes with miscut towards the [1–100] direction it is possible to indicate solutions to increase the quality of the heteroepitaxial growth of 3C on 6H.

Thin SiO2 layers were thermally grown onto cubic silicon carbide (3C‐SiC) heteroepitaxial layers of different surface roughness and with different types of near‐surface epitaxial defects. Localized dielectric breakdown (BD) was studied by electrically stressing the system using Conductive Atomic Force Microscopy (C‐AFM), which constitutes a means to directly and simultaneously observe localized dielectric failure as a function of stress time and surface morphology with nanoscale lateral resolution. The BD kinetics was evaluated by fitting the experimental failure ratios as a function of stress time to the failure probability described by Weibull statistics, in turn allowing to distinguish between defect‐induced (extrinsic) and intrinsic dielectric BD events. The results give useful information about how morphological features at the 3C‐SiC surface influence the BD generation in thermally grown oxides on this polytype.

Discrete series of lines have been observed for many years in N‐Al DAP (Donor Acceptor Pair) spectra in 3C‐SiC. Unfortunately, up to now, there has been no quantitative analysis for the splitting of lines in a given shell. This is done in this work for N‐Al DAP spectra in 3C‐SiC. The samples were non‐intentionally doped 3C‐SiC layers grown by CVD on a VLS seeding layer grown on a 6H‐SiC substrate. From low temperature photoluminescence measurements, strong N‐Al DAP emission bands were observed and, on the high energy side of the zero‐phonon line, we could resolve a series of discrete lines coming from close pairs. Comparing with literature data, we show that the splitting energy for a given shell is constant and, to explain this shell substructure, we consider the non equivalent sets of sites for a given shell. Results are discussed in terms of the ion‐ion interaction containing third and forth multipole terms.

Epitaxial 3C‐SiC layers have been grown on 6H‐SiC substrate via sublimation growth in a RF induction heated reactor. The growth process was carried out at a source temperature 2000° C under vacuum condition at temperature gradient in the range of 5–8° C/mm. The morphological investigation by optical microscope with Nomarski interference contrast in transmission mode reveals 100% inclusion free 3C‐SiC for the layer grown at temperature gradient of 8° C/mm. 2Theta‐omega scan shows 0006 and 111 peaks coming from the substrate and the layer, respectively. The higher intensity of 3C‐SiC 111 peak for the layer grown at temperature gradient of 8° C/mm is due to continuous nature of 3C‐SiC domains compared to that grown at lower temperature gradient. The influence of the temperature gradient on the morphology of layers investigated by Atomic Force Microscopy (AFM) is discussed.

In this paper, the evolution of the electrical characteristics of Pt Schottky contacts to cubic silicon carbide (3C‐SiC) upon annealing was investigated. The Pt/3C‐SiC system was structurally and electrically characterized upon annealing in the temperature range 500° C–900° C. X‐ray diffraction (XRD) showed the formation of platinum silicide (Pt2Si) starting at an annealing temperature of 500° C. After metal preparation, the current‐voltage (I–V) characteristics of Pt/3C‐SiC diodes exhibited a lower leakage current and a higher Schottky barrier height with respect to the previously studied Au/3C‐SiC system [1]. A further improvement was observed after annealing at 500° C, and attributed to the consumption of a SiC surface‐layer by the reaction, thus reducing the influence of the original interface states. On the other hand, annealing at higher temperatures (700° C and 900° C) caused a degradation of the contact properties, which can be related to the evolution of carbon clusters released during reaction.

Single‐phase rock‐salt MgZnO films with high Zn content were successfully fabricated on the templates of MgO (111)/α‐sapphire (0001) by radio‐frequency plasma assisted molecular beam epitaxy. The influence of growth temperature on epitaxy of MgZnO alloy films was investigated by the combined studies of crystal structures, compositions, and optical properties. It is found that the incorporation of Zn atoms into the rock‐salt MgZnO films is greatly enhanced at low temperature, confirmed by in‐situ reflection high‐energy electron diffraction observations and ex‐situ X‐ray diffraction characterization. Zn fraction in the single‐phase rock‐salt Mg0.53Zn0.47O film was determined by Rutherford backscattering spectrometry. Optical properties of the films were investigated by transmittance spectroscopy and reflectance spectroscopy, both of which demonstrate the solar‐blind band gap and its dependence on Zn content.

Heavily Boron‐doped MPCVD‐grown (Microwave Plasma Chemical Vapour Deposition) polycrystalline diamond layers were observed to have a superconducting character depending exclusively on the growth time, i.e. grown under identical experimental conditions. To elucidate such behavior, morphology aspects are investigated using Transmission Electron Microscopy (TEM). Diffraction contrasts and High Angle Annular Dark Field (HAADF) observations show that grains are thicker and boron enrichment occur at grain boundary in the superconducting samples. Interfacial effects are the main difference between the two samples, in the non‐superconducting ones descohesion effect is observed. In both samples boron concentration is nearly constant except in isolated grains and twins, where their crystal orientation probably influences boron incorporation during growth. In summary, as grain configuration changes along the growth, a threshold thickness should be over passed to allow an optimum transport between the diamond grains.

Using optical techniques, we analyzed an impact of non‐intentional Al mpurity and twin boundaries to photoelectrical properties of sublimation‐grown 3C heterostructures. Differential transmission techniques revealed Al related contribution to probe beam absorption with cross‐section σAl = (1.8±0.5)×10−17 cm2 at 1064 nm, being four times stronger that the free‐carrier absorption cross‐section at given wavelength. Temperature dependent carrier recombination rates provided trap activation energy of 170 and 210 meV in two samples with different Al concentration. Saturation of probe beam absorption with excitation allowed determination of electrically active Al concentration, not gettered at grain boundaries. Increase of room‐temperature mobility with injection in the highly defective layer and the corresponding lifetime decrease pointed out contribution of point and structural defects to carrier scattering.

1‐D and 2‐D detector arrays have been realized on CVD‐diamond. The relatively high resistivity of diamond in the dark allowed the fabrication of photoconductive “sandwich” strip (1D) or pixel (2D) detectors: a semitransparent light‐receiving back‐side contact was used for detector biasing. Cross‐talk between pixels was limited by using intermediate guard contacts connected at the same ground potential of the pixels. Each pixel photocurrent was conditioned by a read‐out electronics composed by a high sensitive integrator and a Σ‐Δ ADC converter. The overall 500 μs conversion time allowed a data acquisition rate up to 2 kSPS. The measured fast photoresponse of the samples in the ns time regime suggests to use the proposed devices for fine tuning feedback of high‐power pulsed‐laser cavities, whereas solar‐blindness guarantees high performance in UV beam diagnostics also under high intensity background illumination. Offering unique properties in terms of thermal conductivity and visible‐light transparency, diamond represents one of the most suitable candidate for the detection of high‐power UV laser emission. The technology of laser beam profiling is evolving with the increase of excimer lasers applications that span from laser‐cutting to VLSI and MEMS technologies. Indeed, to improve emission performances, fine tuning of the laser cavity is required. In such a view, the development of a beam‐profiler, able to work in real‐time between each laser pulse, is mandatory.

The value of the full width at half maximum of the rocking curve of symmetric 3C SiC reflection, often considered as the “quality indicator” of the epitaxial film, is in fact a convolution of several contributions, among which the curvature related part may play a dominant role. Thus, a precise determination of the curvature related broadening is necessary to extract the information on the film quality from the rocking curve. In this paper we demonstrate experimentally the coexistence of two independent curvature related broadening effects. We also propose an analytical model that describes quantitatively both effects.

We have studied the growth of zinc‐blende GaN and AlGaN layers, structures and bulk crystals by molecular beam epitaxy (MBE). We have developed a process for growth by MBE of free‐standing cubic GaN layers. Undoped thick cubic GaN films were grown on semi‐insulating GaAs (001) substrates by a modified plasma‐assisted molecular beam epitaxy (PA‐MBE) method and were removed from the GaAs substrate after the growth. The resulting free‐standing GaN wafers with thicknesses in the 30–100 μm range may be used as substrates for further epitaxy of cubic GaN‐based structures and devices. We have developed procedures to cleave the wafers into 10×10 mm2 square substrates and to polish them to produce epi‐ready surfaces. The first GaN/InGaN LEDs on our zinc‐blende GaN substrates have been demonstrated by our collaborators at Sharp Laboratories of Europe.

Nous montrons dans ce travail que l’hydrogene, qui est un element leger diffusant rapidement dans le silicium, passive non seulement les liaisons pendantes et les defauts cristallographiques inter-ou intra-grains dans les materiaux polycristallins, mais egalement les impuretes chimiques qui sont responsables en tant que centres de recombinaison de la degradation des proprietes de transport du materiau.

There are two different domains of using silicon semiconductors as gamma-ray dosimeters which are widely separated if one considers the dose-rate sensitivity levels which can be reached, namely the low-dose-rate region which is that of the personal monitoring and the high-level one corresponding mainly to area survey or catastrophic flash accidents (for example). Such sensitivities depend strongly on the properties of the detector (surface, carrier lifetime) as well as on the working conditions (applied bias, integration time) and on detector mounting (shielding). All these points are discussed here with the help of the theoretical sensitivity curves we calculated for a 7.6 mm 2 n-type silicon detector mounted behind Sn and Al γ-ray moderators. These calculations are done for the 662 keV 137 Cs γ-rays which are used in our studies mostly as the normalization point of all of our graphs.

This work presents direct evidence for a correlation between rapid thermal process-induced recombination centers and co-implanted metallic impurities in ion implanted silicon. Experimental evidence includes the dose dependence of the minority carrier diffusion length measured by the SPV technique, SIMS and RBS analysis of high-dose implantations which show the presence of heavy metals, the dependence of the final diffusion lengths on the mass of the implanted ions, as well as the successful modification of an implantation equipment.

The European Materials Society — now 20 years old — offers a focus on functional materials and a willingness to engage in policy issues. The success of present and future activities depends on its ability to remain flexible.

To date polarimetry in astrophysics in the energy domain from hard X-rays up to soft gamma-rays has not been pursued due to the difficulties involved in obtaining sufficient sensitivity. Indeed for those few instruments which are capable of performing this type of measurement, polarimetry itself plays a secondary role in the mission schedule, as the efficiencies and polarimetric Q factors are relatively limited. In order to perform efficient polarimetric measurements for hard X and soft gamma-ray sources, with an instrument of relatively robust and simple design, a CdTe based telescope (CIPHER: Coded Imager and Polarimeter for High Energy Radiation) is under study. This instrument is based on a thick (10 mm) CdTe position sensitive spectrometer comprising four modules of 32×32 individual pixels, each with a surface area of 2×2 mm 2 . The polarimetric performance and design optimisation of the CIPHER detection surface have been studied by use of a Monte Carlo code based on GEANT4 modules. Simulations show that we can achieve a Q factor better than 0.5 and Compton double event efficiency better than 11% between 100 keV and 1 MeV. Herein we will present and discuss the general problems that affect polarimetric measurements in space such as the inclination of the source with respect to the telescope optical axis and space background radiation. Q factor calculations for several beam inclinations as well as for space background together with simulated astronomical sources will be presented and discussed.

The polarisation of astrophysical source emissions in the energy range from a few tens of keV up to the MeV region is an almost unexplored field of high energy astrophysics. In order to improve the capabilities of performing polarimetric studies of hard X and soft gamma ray sources through Compton polarimetry, a CdTe based telescope (CIPHER: Coded Imager and Polarimeter for High Energy Radiation) is under study. This instrument is based on a thick (10 mm) CdTe position sensitive spectrometer made of four modules of 32x32 individual pixels, each with a surface area of 2x2 mm2, corresponding to about 160 cm2 active detection area. This detector, due to its intrinsic geometry, could allow efficient polarimetric measurements to be made between 100 keV and 1 MeV. In order to predict the polarimetric performance and to optimise the design and concept of the CIPHER detection plane, a Monte Carlo code based on GEANT4 library modules was developed to simulate the detector behaviour under a polarised photon flux. The Compton double event efficiency, as well bi-dimensional double event distribution maps and the corresponding polarimetric modulation factor will be presented and discussed. Modulation (Q) factors better than 0.30 and double event total efficiencies over 10 % for an energy range from 100 keV to 1 MeV have been obtained.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The design and preliminary test results of a semiconductor silicon dosimeter are presented in this article. Use of this dosimeter is foreseen for real-time skin dose control in interventional radiology. The strong energy dependence of this kind of radiation detector is well overcome by filtering the silicon diode. Here, the optimal filter features have been calculated by numerical Monte Carlo simulations. A prototype has been built and tested in a radiological facility. The first experimental results show a good match between the filtered semiconductor diode response and an ionization chamber response, within 2% fluctuation in a 2.2 to 4.1 mm Al half-value layer (HVL) energy range. Moreover, the semiconductor sensor response is linear from 0.02 Gy/min to at least 6.5 Gy/min, covering the whole dose rate range found in interventional radiology. The results show that a semiconductor dosimeter could be used to monitor skin dose during the majority of procedures using x-rays below 150 keV. The use of this device may assist in avoiding radiation-induced skin injuries and lower radiation levels during interventional procedures.

Even though it is recognized that the study of polarization from cosmic high-energy sources can give very important information about the nature of the emission mechanism, to date very few measurements have been attempted. For several years we have proposed the use of a thick CdTe array as a position sensitive spectrometer for hard X- and soft gamma-ray astronomy, a design which is also efficient for use as a polarimeter at energies above approximately 100 keV. Herein we describe the preliminary results of our study of a polarimeter based on 4096 CdTe microcrystals that we would like to develop for a high altitude balloon experiment. We present the telescope concept with a description of each subsystem together with some results on activities devoted to the optimization of the CdTe detector units' response. Furthermore we give an evaluation of the telescope performance in terms of achievable spectroscopic and polarimetric performance. In particular we will show the results of Monte Carlo simulations developed to evaluate the efficiency of our detector as a hard X ray polarimeter.

Thin CdTe detectors (3 X 5 mm2 electrode area and 0.5 mm thick), mounted in back-to-back configuration with common anode, have been characterized, aiming at determining the actual detection region with good spectroscopy performance. This geometry allows one to use devices with 1.0 mm useful absorbing thickness when irradiated in the classical Planar Parallel Field (PPF) configuration, and a large active area (3 X 1 mm2) and high absorbing thickness (5 mm) when irradiated in the Planar Transverse Field (PTF) configuration, while the charge collection takes place on maximum distances of 0.5 mm. These dimensions have been chosen on the basis of previous studies performed on various detectors irradiated in the PTF configuration which have shown that the 'best spectroscopy region' width assumes a constant value of about 0.4 mm in the thickness range 1.0 - 2.5 mm, while it is largely reduced when using smaller electrode area (2 X 2 mm2) 0.5 mm thick devices. The tests were performed by using a well collimated (0.1 X 1 mm2, 20 mm thick collimator) photon beam in the 10 - 300 keV energy range. The results concerning the spectroscopic behavior of both the coupled detectors and of single detectors (for comparison) have shown that the edge effects observed in smaller detectors with the same configurations are significantly reduced, giving better energy resolutions at Ex equals 122 keV, but no wider good spectroscopy regions; at low energies the effects of larger electrodes (increased electric capacitance and leakage current) result in a higher noise and increased detection threshold.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In this paper, we will give first the description of a versatile material analyzer for x-ray crystallographic and compositional analyses. We will describe how we calculate and elaborate glass capillaries; we will describe some important parameters of the total reflection of x-rays: focusing, transparency, gain and divergence. Our first results concern a compositional study of CdZnTe crystals with a n x-ray beam diameter of 10 micrometers and we will show the variation of Zn on the surface of such a crystal. Another possibility of this device is x-ray microtopography of a film or of a wafer. We show, on one hand the way to find in classical centers by Laue micropatterns on the other hand we could establish a Bragg reflection pattern like those obtained in classical x-ray surface topography. In some heat treatments we can see so-called texture effects. We have also on our device the possibility to see microtexture effects in a way which can be compared to the Seeman-Bohlin experiment. The compositional experiments were also tested by SEM in order to see how these two devices complete each other. We can notice in some studies the advantages of x-rays because photons are less destructive than electrons.

An analysis of the variation in size of the best spectroscopy region as a function of the inter-electrode distance for planar CdTe detectors is reported. Four detectors with inter-electrode distance ranging between 1.0 and 2.5 mm, in steps of 0.5 mm, were investigated. The measurements were carried out by scanning the detectors with a narrow beam of 122 keV photons, obtained by using a 20 mm thick tungsten collimator having a 0.2 X 2 mm 2 collimating channel. The detectors were irradiated in the configuration with the electric field perpendicular to the incoming radiation, Planar Transverse Field. A 'best charge collection region' has been identified, close to the cathode and approximately 0.4 mm wide, independent of the detector thickness in the investigated range, in which the photopeak amplitude and the energy resolution assume the best and sufficiently constant values. When going outside this region, both these parameters become quickly worse, with a trend well described by the Hecht's relationship. These feature appear essentially in the energy range 30 x < 250 keV, a very important one for space astrophysical applications, as well as in medical radiology. At higher energies, changes in the prevalent photon interaction mechanism require use of alternative methods to improve the spectroscopic performance. An appropriate selection of the width of the irradiated region can give the possibility to match the requirements of particular applications.

A dedicated to nuclear cardiology mobile camera based on a detection matrix 15 X 15 cm of 2304 CdTe detector elements 2.83 X 2.83X 2 mm has been developed by an European Community support including Academic and Industrial Centers. The decision was taken to use CdTe since the maker (Eurorad-F) has a great experience in order to produce high grade, good homogeneity, stability and transport properties of hole twice greater than CdZnTe.

In recent years silicon detectors have nearly replaced Geiger-Muller tubes in personal dosemeters for gamma and X- rays. Several tentatives have been undertaken to extend these dosemeters to fast neutrons, but problems arose since in the generally present mixed fields it appeared difficult to separate the effect of both kind of radiations. In this paper, a method for fast neutron monitoring is investigated. It results from a computer simulation analysis of fast neutrons and gamma-ray interactions in silicon detectors. For neutron energies between 0.75 and 15 MeV, it proposes a real time personal dosemeter, with a response accuracy better than 30% in a mixed neutron and gamma-ray field.

The electron-hole pair production energy (∼ 4.43 eV) in cadmium telluride semiconductor based detector would allow, in principle, to achieve spectroscopic performance close to germanium. In fact the material impurities, crystal lattice defects and low charge mobility, mainly for holes, can severely degrade the spectroscopic properties through charge trapping/detrapping and recombination phenomena, limiting the usable distance between the electrodes. The charge collection efficiency depends, following the Hecht relation, on the distance between the charge production location and the collecting cathode; we have studied the dependence of some CdTe spectroscopic parameters (energy resolution, photo-peak gain and efficiency) from this distance. A set of experimental tests were performed using a well collimated photon beam. The beam spot, obtained from a 57Co radioactive source through a thick (20 mm) tungsten collimator having a 0.2×2 mm window, was positioned between the two electrodes of a CdTe crystal of size 3×5×2 mm3, with the electrodes deposited on the 3×5 mm2 sides.The detector was irradiated on the face having the area of 2×3 mm2, in the arrangement with the collecting field orthogonal to the incoming radiation (PTF configuration), and the largest side of the spot parallel to the electrode plane. Using a micro-positioning system a fine scanning of the interelectrodic region was performed.In order to evaluate the detector behavior in a wider energy range, further tests using energies between 14 keV to 662 keV have been performed.The results of these measurements are presented and discussed in order to emphasize a method to improve CdTe detector spectroscopic capabilities and to understand the detection behavior

In recent years the performance of room-temperature semiconductor detectors such as CdTe are improved and they are now suitable candidates for several applications. However, some key parameters that can severely affect such perfomances have not been measured yet. Thus we have studied the damaging of a set of CdTe detectors irradiated in a 60Co gamma-cell in a wide range of doses and dose-rates. A full characterization of the performance of irradiated detectors has been obtained by means of spectroscopic, electrostatic, photo-induced current transient spectroscopy and photo-deep level transient spectroscopy measurements to quote the energy resolution, the leakage current, the activation energy and capture cross-section of deep level defects, respectively.

Detecteurs de rayons X a semi-conducteurs. Evolutions recentes Les progres recents dans la detection des rayons X a l'aide de semi-conducteurs sont passes en revue. Le developpement des materiaux a large bande interdite d'une part, l'utilisation des techniques micro-electroniques sur le silicium d'autre part, permettent d'envisager la spectrometrie des rayons X a temperature ambiante, avec des performances proches de celles obtenues a l'aide de detecteurs refroidis. Les possibilites offertes par de nouveaux dispositifs semi-conducteurs dans le domaine de l'imagerie X sont quant a elles en pleine evolution.

This chapter focuses on those methods suitable for nuclear detection that constitute the most promising application to date. For this purpose, one needs a reasonable mobility lifetime product for electrons and holes, high resistivity, a low number of grain boundaries and a low concentration of trapping centers. At the same time, production rate and reproducibility are of interest. The quality of the material and its electrical characteristics are dependent on structural defects and chemical dopants: vacancies, interstitials, loops, and associations. The behavior of these defects is determined by composition, temperature and pressure, which are the main phase diagram parameters. The chapter describes the thermodynamic properties of the cadmium-tellurium (CdTe) system (phase diagram and equilibrium relationship between components) that summarizes the various combinations of composition, temperature and partial pressure under which the three phases-solid, liquid, and gas-coexist in equilibrium.

Among the large family of binary compounds, only a few can potentially be used as nuclear radiation detector material. This chapter shows a few selected semiconductors with their selection criteria. The chapter discusses the cadmium-tellurium (CdTe) nuclear detectors and its applications. When the CdTe material is high resistivity, CdTe nuclear detectors can be of the homojunction type with two similar contacts, ohmic or not. CdTe detectors have not yet been employed in the large number of applications that they can, in principle, address. Several categories of the various applications can be considered: chronological, detector type, or subject of interest. The last classification, mixing with some of the others, seems to be the more appropriate. Both scientific and industrial fields offer applications to CdTe spectrometers when room temperature operation, small size, low voltage requirements, and reasonable resolution are needed.

Inspection and investigation is the only way to select material with desired properties for conventional semiconductor materials. This demonstrates the importance of characterization of cadmium-tellurium (CdTe) material. The choice of characterization methods and their combination determines the success or failure of device applications. This chapter summarizes of the important measurement methods and properties for nuclear detector applications. Conventional analysis methods for semiconductors generally fail in the case of high resistivity CdTe, due to the difficulty in obtaining ohmic contacts and due to the thermal instability even at rather low temperature. The greatest part of the analysis can and must be done through detector operation itself. The chapter provides a considerable amount of characterization data, results and experiments concerning II-VI compounds crystals and especially CdTe material. The review of methods and characteristics is restricted to subjects of interest to people concerned with nuclear detectors.

Two electric field profile analysis methods have been considered for evaluating the internal field distribution in cadmium telluride nuclear radiation detectors. A theoretical model is given, followed by two experimental measurements, using the induced radiation current pulse profile and the optical rotation by the Pockels effect of polarized IR light. Finally, theoretical and experimental results are discussed.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This paper reviews the progress achieved in recent years in the field of cadmium telluride X and gamma-ray detectors. Both material and structure of detectors are considered as well as recent progress in pulse handling.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Cadmium telluride (CdTe) constitutes an unique material for X-ray imaging, due to its outstanding properties: room temperature operation, high atomic numbers giving high photoelectric absorption properties, long term stability under irradiation. This paper deals with a procedure we have developed for the use of CdTe detectors in imaging purposes. For demonstration purpose, a 64 linear array has been realized, all detectors being biased at the same voltage. Each pixel is equivalent to a capacitance and a resistance in parallel. The loaded capacitance is disconnected just before irradiation by the X-rays by means of a MOS switching transistor. The detector's equivalent capacitance discharges under irradiation trough the parallel resistance by an amount directly proportional to the charges deposited in the detector, i.e. to the received radiation. In order to measure the corresponding charges, the capacitances of all pixels are successfully loaded for a short time, by switching on a shift register. Each charging current generates between the terminals of a common resistor, a voltage pulse proportional to the beam absorbed. This approach offers several advantages, in particular, to correct the dispersion introduced by differences in detectors, as well as by electronic circuits. Furthermore, it facilitates the use of a dummy line in order to reduce the dark current and parasitic commutation signals. Images of phantoms exposed to the X-ray beam have been visualized with success by means of this system.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The IR absorption spectrum of KI etched HgI2 single crystals shows a great number of bands in the range 400 - 4000 cm-1, indicating the formation of a complex. This spectrum presents similarities and differences with that of KHgI3.H2O. The index of refraction and the relative evolution of the complex layer thickness have been determined optically.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.