C. Paolini

Theory of ion beam induced charge collection in detectors based on the extended Shockley–Ramo theorem

Abstract An analysis of the charge collection process induced by focused MeV ion beams in semiconductor devices is presented. It is based on the extended Shockley–Ramo theorem that provides a rigorous mathematical tool for the calculation of the induced charge and current under the assumption of a quasi-steady-state operation of the semiconductor device. A complete description of the theory and underlying assumption is given as well as a simple application of the method aimed to evaluate the main transport properties of fully depleted semiconductors from the analysis of frontal and lateral ion beam induced charge collection (IBICC) measurements.

Ionoluminescence in CVD diamond and in cubic boron nitride

Abstract Using the new ion beam-induced luminescence (IBIL) apparatus in National Legnaro Laboratories, Italy, a series of measurements concerning both wide-area luminescence spectra and monochromatic luminescence maps with a space resolution of a few μm has been carried out on several CVD diamond and c-BN samples. Protons of 2 MeV with a penetration depth of approximately 25 μm have been used in order to investigate the materials in the bulk. These measurements have been correlated with particle-induced X-ray emission (PIXE) and EPR data. The measurements have been performed at increasing proton doses in order to also investigate the radiation hardness of luminescence peaks. The results indicate that ionoluminescence of CVD diamond is dominated by three bands at approximately 2, 2.4 and 2.9 eV, with the intermediate band being very radiation-hard, and the other two radiation-weak. The band at 2 eV is correlated with N content, and is particularly high in samples with poor electronic properties. IBIL in c-BN is also dominated by three bands, one at approximately 2 eV, and the other two at higher energies with respect to CVD diamond. All these three bands seem to be relatively radiation-hard with respect to CVD diamond, and to be related to defects induced by doping.

Effects of light on the primed' state of CVD diamond nuclear detectors

Abstract Diamond radiation detectors produced by Chemical Vapour Deposition (CVD) have been extensively studied as nuclear detectors both for the application as tracking detectors in high energy physics experiments and for applications in nuclear industry domain. A meaningful measurement to characterise the performance of such devices is the evaluation of the charge collection distance (CCD), i.e. the mean distance the charge carriers travel before being trapped. It is well known that CCD increases with the absorbed dose (priming or pumping process). The priming of diamond is usually explained by the saturation of active traps in the diamond bulk, which are filled by charge carriers generated by the ionisation. However, such a primed state is metastable, i.e. the exposure to light can de-pumps diamond to its initial state. The analysis of ‘pumping’ and ‘de-pumping’ processes is then useful to explain trapping mechanisms and polarisation effects, which limit the performances of diamond detectors. This paper deals with an investigation of the ‘pumping’ process on detector grade CVD diamond samples exposed to some doses of irradiation (X-rays or beta particles). Photoconductivity measurements carried out during monochromatic illumination as well as optical bleaching spectra of thermoluminescence glow curves highlight the passivation effects of some trap levels due to irradiation. A possible interpretation of these effects is presented and discussed, taking into account charge collection efficiency measurements carried out using Am-241 alpha particles.

Growth, contacting and ageing of superconducting Bi-2212 whiskers

We report the growth of highly oriented microscopic whisker-like crystals in the Bi–Sr–Ca–Cu–O system by means of glassy precursors. The dependence of the growth on the stoichiometric composition and on the temperature and duration of the annealing process has been studied. Chemical impurities have been investigated from the point of view of the morphology, the elemental composition and the crystal structure, identifying the presence of CuAl2O4, Sr1.2Bi0.8O3 and Al. Electrical contacts have been fabricated by means of thermal evaporation and diffusion. Their sizes have been carefully measured, achieving contact resistivity in the range of 0.2–3.7 × 10−6 Ω cm2. A very slow degrading of the contacts and the crystals on a time scale of a few years has been detected. This is associated with a decrease in Tc, which is probably due to oxygen release from the material.

CVD diamond microdosimeters

Abstract CVD diamond detectors have been evaluated for use as X-ray dosimeters. The response both at radiological diagnostic and at therapeutic energies of one “detector grade” commercially available parallelepiped CVD sample has been measured and compared with the performances of a “needle shaped” CVD diamond detector grown by the hot-filament CVD technique on a tungsten tip. All the detectors were demonstrated to be very suitable for bio-medical application: they are linear in dose rate, stable and relatively fast, with a signal-to-noise ratio higher than 1000.

Time-resolved ion beam-induced charge collection measurement of minority carrier lifetime in semiconductor power devices by using Gunn's theorem

Abstract Ion microbeam techniques like ion beam-induced charge collection (IBICC) are very powerful methods in order to investigate and to map the transport properties in different technologically important semiconductors and in particular in materials proposed for nuclear detection. Time-resolved ion beam-induced charge collection (TRIBICC) represents a further improvement with respect to more traditional IBICC, since it can supply not only the charge collection efficiency (and through it data on mobility and trapping time of carriers in drift regions) but also the time behaviours of the charge collection. For long collection times, this means to gather information, also about diffusion lengths and lifetimes of carriers in the diffusion regions, which are always present in undepleted electronic devices, in particular power devices, and which are of paramount importance as inputs for simulation codes. By TRIBICC, in fact, some difficulties could be avoided in analysis of data collected in cases when lifetimes and shaping times of electronic chain are similar, and the sensitivity of the method is worse. In order to suitably analyse TRIBICC data, a theoretical model should be available: in general, Ramos theorem is used, but its validity in cases when space charge is present is questionable. A more general and powerful method is presented in this work by using Gunns theorem and a particular formulation of the generation function in order to solve the adjoint of the continuity equation in the time-dependent case. An application of this method to a commercial power device is presented and discussed.

Imaging of charge collection properties of a CVD diamond detector using X-ray-induced current microscopy

Abstract A 5.5-keV X-ray microprobe was used at the European Synchrotron Radiation Facility in Grenoble (France) to image the transport properties of a ‘detector grade’ polished CVD diamond sample. Qualitative results concerning the spatial distribution of the X-ray-induced current were obtained with a submicrometer spatial resolution. Particular attention has been paid to the dependence of the photoresponse on bias polarity. Interpretation of the photocurrent maps has been drawn on the basis of previous results obtained using the ion beam-induced charge collection (IBIC) technique. Priming effects and polarisation effects are reported and discussed.

CARBON INFLUENCE IN THE SYNTHESIS OF MgB2 BY A MICROWAVE METHOD

We describe a method to produce MgB2 bulk via a microwave processing, which has not been reported yet for this material. We used two experimental arrangements, which were different in the form of the graphite thermal activator responsible for the microwave absorption. The reaction products have been analyzed from the morphological, structural and electronic point of view. The results show that the critical temperature is decreased by about 4 K when the graphite can diffuse inside the reaction cell and is incorporated in the samples, even if no evidence of carbon substitution in the MgB2 lattice is given by the lattice constants. We point out that excellent conductivity features are achieved for nearly carbon-free samples.

Micro-IL and micro-PIXE studies of rich diamond meteorites at Legnaro nuclear microprobe

Abstract A combination of micro-ionoluminescence (micro-IL) and micro-PIXE was used to characterize diamond grains inside a type of meteorites known as ureilites. Ureilites are a group of achondrites unique in containing relatively large amounts of carbon occurring as diamond, graphite or lonsdaleite. A shock origin for ureilitic diamonds has been widely accepted though an exact knowledge of the conditions during high-pressure graphite conversion is not yet achieved. Micro-IL is a very powerful technique for material investigation and particularly for diamond analysis. Using this technique we were able to identify the occurrence of the diamond phase inside carbon meteoritic inclusions and to perform micro-PIXE analysis on single diamond grains. In fact, IL in low nitrogen content diamonds is dominated by A-band emission (centered at about 2.9 eV) and so, considering only IL monochromatic map at such a spectral band, it was possible to identify them. By making measurements directly on the meteorites, contamination during chemical extraction processes was avoided and it was possible to study not only the diamond phase, but also its distribution inside carbon inclusions.

Blue light sensitization of CVD diamond detectors

Abstract CVD diamond needs to be ‘primed’ or ‘pumped’ by X-rays or by high energy electrons in order to be qualified for nuclear detection. In this ‘pumped’ state, CVD diamond detector performances are reported to be more reproducible, homogeneous and averagely better. The pumping effect is usually attributed to filling of (hole) traps, which are responsible for the charge collection efficiency of the detector. In this work, the possibility of filling traps selectively by light has been systematically investigated, in order to find the kind of ‘background light’ in which the detectors could preserve their performances indefinitely in time. Irradiation by the blue light before or/and during the detection of nuclear particles seems to avoiding the large doses needed for pumping. Other wavelengths seem to be totally inefficient from this point of view. It turns out that electrons are responsible for charge transport during blue light priming, while holes give the largest contribution during X-ray priming. The results will be presented as maps of charge collection efficiency in CVD diamond detectors as obtained by IBIC (Ion Beam Induced Charge) carried out by using a proton microbeam.