Marco Stefancich

Model for Schottky barrier and surface states in nanostructured n-type semiconductors

A semiclassical model for Schottky contacts to be applied to nanosized polycrystalline n-type semiconductors was developed. To this purpose we determined the density of surface states as a function of the mean grain radius, which establishes the Schottky barrier height. The intergranular potential shape was investigated in depletion approximation under spherical geometry and a critical revision of this method was proposed. The model was then extended to also include nanostructured materials, which could not be considered in the previous approach. Thus we were able to explain the flattening of the band bending and the decrease in the surface state density, which are experimentally observed when the granulometry is very fine.

Experimental study for the feasibility of a crystalline undulator

We present an idea for creation of a crystalline undulator and report its first realization. One face of a silicon crystal was given periodic microscratches (grooves) by means of a diamond blade. The x-ray tests of the crystal deformation due to a given periodic pattern of surface scratches have shown that a sinusoidal-like shape is observed on both the scratched surface and the opposite (unscratched) face of the crystal; that is, a periodic sinusoidal-like deformation goes through the bulk of the crystal. This opens up the possibility for experiments with high-energy particles channeled in a crystalline undulator, a novel compact source of radiation.

Thermal conductance imaging of graphene contacts

Suspended graphene has the highest measured thermal conductivity of any material at room temperature. However, when graphene is supported by a substrate or encased between two materials, basal-plane heat transfer is suppressed by phonon interactions at the interfaces. We have used frequency domain thermoreflectance to create thermal conductance maps of graphene contacts, obtaining simultaneous measurements of the basal-plane thermal conductivity and cross-plane thermal boundary conductance for 1–7 graphitic layers encased between titanium and silicon dioxide. We find that the basal-plane thermal conductivity is similar to that of graphene supported on silicon dioxide. Our results have implications for heat transfer in two-dimensional material systems, and are relevant for applications such as graphene transistors and other nanoelectronic devices.

Crystal deflector for highly efficient channeling extraction of a proton beam from accelerators

The design and manufacturing details of a new crystal deflector for proton beams are reported. The technique allows one to manufacture a very short deflector along the beam direction (2 mm). Thanks to that, multiple encounters of circulating particles with the crystal are possible with a reduced probability of multiple scattering and nuclear interactions per encounter. Thus, drastic increase in efficiency for particle extraction out of the accelerator was attained (85%) on a 70 GeV proton beam. We show the characteristics of the crystal deflector and the technology behind it.

Optical efficiency of solar concentrators by a reverse optical path method

A method for the optical characterization of a solar concentrator, based on the reverse illumination by a Lambertian source and measurement of intensity of light projected on a far screen, has been developed. It is shown that the projected light intensity is simply correlated to the angle-resolved efficiency of a concentrator, conventionally obtained by a direct illumination procedure. The method has been applied by simulating simple reflective nonimaging and Fresnel lens concentrators.

Monitoring of concentrated radiation beam for photovoltaic and thermal solar energy conversion applications

Methods for evaluating the light intensity distribution on receivers of concentrated solar radiation systems are described. They are based on the use of Lambertian diffusers in place of the illuminated receiver and on the acquisition of the scattered light, in reflection or transmission mode, by a CCD camera. The spatial distribution of intensity radiation is then numerically derived from the recorded images via a proprietary code. The details of the method are presented and a short survey of the main applications of the method in the photovoltaic and thermal solar energy conversion field is proposed. Methods for investigating the Lambertian character of commercial diffusers are also discussed.

Low-power thick-film gas sensor obtained by a combination of screen printing and micromachining techniques

Abstract A novel prototype of low-power thick-film gas sensor deposited by screen-printing onto a micromachined hotplate is presented. The micro-heater is designed to maintain a film temperature of 400°C with less than 30 mW of input power. The fabrication process involves a combination of standard, VLSI-compatible, micromachining procedures and computer-aided screen-printing. A dielectric membrane of Si 3 N 4 and SiO 2 has been obtained with an embedded poly-Si resistor acting as a heating element. The bonding pad and contacts have been realised by a Ti/TiN/Cr/Au structure and the sensing film has been deposited by a screen-printing technique. Here follows a characterisation of a device, based on SnO 2 sensing film, at working conditions together with the response curve for CH 4 and NO 2 . We will also address some important improvements to the micro-hotplate structure, which leads to an increased flexibility of the device.

Characterization of CPC solar concentrators by a laser method

In this paper we present a method of optical characterization of solar concentrators based on the use of a laser beam. The method, even though constrained by lengthy measurements, gives nevertheless interesting information on local mirror surface defects or manufacturing defects, like internal wall shape inaccuracies. It was applied to 3D-CPC-like concentrators and the measurements were supported by optical simulations with commercial codes. The method, simple to apply, requires just a laser to scan the CPC input aperture following a matrix-like path, at a controlled orientation of the beam. Maps of optical efficiency as function of the laser beam incidence angle are obtained by matching the CPC exit aperture with a photodetector with an efficient light trapping. The integration of each map gives the CPC efficiency resolved in angle of incidence, so curves of optical transmission (efficiency) as function of incidence angle can be drawn and the acceptance angle measured. The analysis of the single maps allows to obtain interesting information on light collection by the different regions of CPC input area. It reveals, moreover, how the efficiency of light collection depends on several factors like surface reflectivity, number of reflections of the single beam, local angle of incidence, local surface defects, and so on. By comparing the theoretical analysis with the experimental results, it is possible to emphasize the effects directly related to manufacturing defects.

Effective AFM cantilever tip size: methods for in-situ determination

In atomic force microscopy (AFM) investigations, knowledge of the cantilever tip radius R is essential for the quantitative interpretation of experimental observables. Here we propose two techniques to rapidly quantify in-situ the effective tip radius of AFM probes. The first method is based on the strong dependency of the minimum value of the free amplitude required to observe a sharp transition from attractive to repulsive force regimes on the AFM probe radius. Specifically, the sharper the tip, the smaller the value of free amplitude required to observe such a transition. The key trait of the second method is to treat the tip?sample system as a capacitor. Provided with an analytical model that takes into account the geometry of the tip?sample?s capacitance, one can quantify the effective size of the tip apex fitting the experimental capacitance versus distance curve. Flowchart-like algorithms, easily implementable on any hardware, are provided for both methods, giving a guideline to AFM practitioners. The methods? robustness is assessed over a wide range of probes of different tip radii R (i.e. 4?<?R?<?50?nm) and geometries. Results obtained from both methods are compared with the nominal values given by manufacturers and verified by acquiring scanning electron microscopy images. Our observations show that while both methods are reliable and robust over the range of tip sizes tested, the critical amplitude method is more accurate for relatively sharp tips (4?nm?<?R?<?10?nm).

Crystal undulator as a novel compact source of radiation

A crystalline undulator (CU) with periodically deformed crystallographic planes is capable of deflecting charged particles with the same strength as an equivalent magnetic field of 1000 T and could provide quite a short period L in the sub-millimeter range. We present an idea for creation of a CU and report its first realization. One face of a silicon crystal was given periodic micro-scratches (grooves), with a period of 1 mm, by means of a diamond blade. The X-ray tests of the crystal deformation have shown that a sinusoidal-like shape of crystalline planes goes through the bulk of the crystal. This opens up the possibility for experiments with high-energy particles channeled in CU, a novel compact source of radiation. The first experiment on photon emission in CU has been started at LNF with 800 MeV positrons aiming to produce 50 keV undulator photons.