M. Jakšić

IBIC investigations on CVD diamond

A 3 MeV proton microbeam has been used for the first time both to investigate the transport properties and to probe the electrical field in a CVD diamond sample of detector grade. Qualitative results concerning the spatial distribution of charge collection efficiency and, consequently, of the collection length are obtained. Collection length seems to be of the order of several tens of microns only in small regions of the same dimensions, characterized probably by a good crystalline behaviour. A strong polarization due to the creation of a local internal electric field and ascribed to the space-charge accumulated in the grain boundaries, makes the measurements very difficult and reduces the statistical validity of the data. However, imaging of physical quantities like collection length is concluded to be a viable technique and it will constitute a useful and powerful method of investigating homogeneity of nuclear detectors and of other similar devices.

Micromachining of silicon with a proton microbeam

Abstract In the recent years the fabrication of sensors and actuator devices on a microscopic scale and their integration with electronic devices and micro-electromechanical systems (MEMS) has become an area of considerable commercial and technological interest, with huge development potentialities. High energy ion microbeam is a suitable tool for such purposes. In this paper we present an alternative way to exploit the lithographic properties of micro ion beams based on the selective damage of silicon to produce porous silicon microstructures. We used a 2 MeV proton microbeam to irradiate definite areas of silicon samples in order to produce damaged layers localised at the end of the proton trajectories. By performing an electrochemical etching in a suitable HF solution, a porous silicon pattern, complementary to the irradiated one, is always formed. The main effect of the damage on the porous silicon formation is to reduce the velocity of formation. To interpret this, such dead layers can be seen to be more or less opaque to the migration of free holes. Consequently the patterned region can be more or less revealed according to the formation time. The procedure allows for the production of microstructures of porous silicon whose unique properties are of great interest for applications. Preliminary results obtained on silicon samples, with different doping levels (p+, p, n+) and irradiating regions with different areas (from 200×200 μm 2 to 25×25 μm 2 ) are presented in order to evaluate the most suitable range of exposure and aspect-ratio of the microstructures.

The 2000 IAEA intercomparison of PIXE spectrum analysis software

Seven programs for analysis of PIXE spectra were compared using the 2000 IAEA test spectra, i.e. Geopixe, Gupix, Pixan, Pixeklm, Sapix, Winaxil and Witshex. A systematic statistical study of the analysis results was performed based on z-scores. The results indicate that most of the programs perform reasonably well with respect to peak areas. Except for a very rare exception, the statistical analysis shows that the participants generally reported smaller uncertainties than would have been expected from the reference uncertainty values. The results show that all the participants reported a number of statistically significant “false hits” and “misses” in their reports. All this indicates that programs in general still leave room for further improvements.

Data acquisition and scan control system for nuclear microprobe and other multiparameter experiments

Abstract A multipurpose data acquisition and beam control system was developed for use on a personal computer (PC) in an MS Windows graphical environment. Data acquisition is performed via a FAST MPA/PC adapter card and MPA/LBB large bus-box by a host computer. Beam and sample positioning are controlled from another computer connected to the host. Applications in the nuclear microprobe and experimental nuclear physics are presented.

Direct fabrication of three-dimensional buried conductive channels in single crystal diamond with ion microbeam induced graphitization

Abstract We report on a novel method for the fabrication of three-dimensional buried graphitic micropaths in single crystal diamond with the employment of focused MeV ions. The use of implantation masks with graded thickness at the sub-micrometer scale allows the formation of conductive channels which are embedded in the insulating matrix at controllable depths. In particular, the modulation of the channels depth at their endpoints allows the surface contacting of the channel terminations with no need of further fabrication stages. In the present work we describe the sample masking, which includes the deposition of semi-spherical gold contacts on the sample surface, followed by MeV ion implantation. Because of the significant difference between the densities of pristine and amorphous or graphitized diamond, the formation of buried channels has a relevant mechanical effect on the diamond structure, causing localized surface swelling, which has been measured both with interferometric profilometry and atomic force microscopy. The electrical properties of the buried channels are then measured with a two point probe station: clear evidence is given that only the terminal points of the channels are electrically connected with the surface, while the rest of the channels extends below the surface. IV measurements are employed also to qualitatively investigate the electrical properties of the channels as a function of implantation fluence and annealing.

Energy threshold for the creation of nanodots on SrTiO3 by swift heavy ions

We present experimental and theoretical data on the threshold behaviour of nanodot creation with swift heavy ions. A model calculation based on a two-temperature model that takes into account the spatially resolved electron density gives a threshold of 12 keV nm−1 below which the energy density at the end of the track is no longer high enough to melt the material. In the corresponding experiments, we irradiated SrTiO3 surfaces under grazing incidence with swift heavy ions. The resulting chains of nanodots were analysed by atomic force microscopy (AFM). In addition, some samples of SrTiO3 irradiated under normal incidence were analysed by transmission electron microscopy (TEM). Both experiments showed two thresholds, which were connected with the appearance of tracks and the creation of fully developed tracks. The threshold values were similar for surface and bulk tracks, suggesting that the same processes occur at both glancing and normal incidence. 5 Author to whom any correspondence should be addressed.

Investigation on the electric field profile in CdTe by ion beam induced current

Abstract The charge collection efficiency profile along the thickness of a CdTe detector was measured for first time by using a proton microbeam and a polished cross-section of the devices. Different samples were investigated and tests were carried out along different lines from cathode to anode, with different biases and polarities, and with different shaping times. The effects of all these parameters are evident in the experimental curves. By fitting these curves, the behaviour of the drift length for electrons and holes is obtained, if a constant ratio between them is assumed. If the mobilities and trapping times are uniform, the electric field profile is obtained. The profile has a minimum at the anode and a maximum towards the cathode, at least for large bias voltage.

Trace element characterization of coal fly ash particles

Abstract XRF, PIXE, RBS and STIM were used for a study of trace element distributions in Greek lignites and their ashes. Samples of fly ash collected from electrostatic precipitators have been separated into seven fractional sizes ranging from 300 μm to less than 25 μm. Trace element concentrations show variation as function of grain size. In addition, the results of analyses of 50 individual fly ash particles are presented.

New developments in IBIC for the study of charge transport properties of radiation detector materials

Abstract A study of charge transport properties in various radiation detector materials (Si, Si(Li), CdTe, GaAs, CVD and natural diamond) has become in recent years one of the major applications of the nuclear microprobe technique IBIC. In order to extend the capabilities of IBIC, further developments of the technique were considered. By changing the projectile type and energy, different sample layers in the range between one and several hundreds of microns can be imaged. A trigger signal from simultaneously detected secondary electrons or a transmitted ion enables time-resolved IBIC imaging to be carried out. Furthermore, simultaneous STIM imaging showing areal density distributions, can be essential for the interpretation of IBIC images.

Probability of divacancy trap production in silicon diodes exposed to focused ion beam irradiation

We present ion beam induced charge (IBIC) measurements of the critical displacement damage dose Dd values and modeling of the probability of divacancy trap production in p+−n−n+ silicon diodes exposed to megaelectron volt energy ion beam irradiation. The normalized induced charge (Q0/Q) measured by He ion probe in tested silicon diodes irradiated by focused He, Li, O, and Cl ion beams with energies of about 0.3 MeV/u increases linearly with Dd according to the modified radiation damage function and nonionizing energy loss (NIEL) theory. A simple IBIC model based on Gunn theorem showed clear dependence of the induced charge Q and corresponding equivalent damage factor Ked value on both a depth profile of charge created by ionizing particle (probe) and a depth distribution of stable defects created from primary defects produced by damaging ions. The average probability of the divacancy production (defined as the ratio of the final electrical active defect quantity and primary ion induced vacancy quantity fo...