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DOCENTE A CONTRATTO presso: Dipartimento di Ingegneria "Enzo Ferrari"

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2008 - On the UV responsivity of neutron irradiated 4H-SiC [Articolo su rivista]
Anna, Cavallini; Antonio, Castaldini; Nava, Filippo

We report on UV responsivity of 4H-SiC photodiodes irradiated by 1 MeV neutrons.Current-voltage characteristics, photoresponse spectra, and responsivity were obtained with lightwavelength from 200 to 450 nm. Photoresponse results slightly affected by irradiation up to thethreshold fluence critical=81014 cm−2. At fluences critical the rejection rate is in the order of103 in the range of 200–320 nm while it is less than 102 at about 320 nm. The abrupt increase inmidgap traps induced by irradiation at critical, observed by photoinduced current transientspectroscopy, proves carrier generation/trapping to be the controlling mechanism for theresponsivity. © 2008 American Institute of Physics.

2008 - Silicon Carbide and its use as a radiation detector material [Articolo su rivista]
Nava, Filippo; G., Bertuccio; A., Cavallini; E., Vittone

We present a comprehensive review of the material properties of the epitaxial 4H silicon carbide polytype. Particular emphasis is placed on those aspects of this material related to high temperature and harsh environment ionizing radiation detector operations.A review of the material characterization methods and electrical contacting issues and how these are related to detector performances is presented.The more recent best data of charge transport across the Schottky barrier and how these are related to spectrometer performance are presented.

2006 - Radiation detection properties of 4H-SiC Schottky diodes irradiated up to 10(16) n/cm(2) by 1 MeV neutrons [Articolo su rivista]
Nava, Filippo; A., Castaldini; A., Cavallini; P., Errani; V., Cindro

We report the results of an experimental study on the radiation hardness of 4H-SiC diodes used as alpha-particle detectors with 1 MeV neutrons up to a fluence of 8 x 10(15) n/cm(2). As the irradiation level approaches the range 10(15) n/cm(2), the material behaves as intrinsic due to a very high compensation effect and the diodes are still able to detect with a reasonable good Charge Collection Efficiency (CCE = 80%).For fluences > 10(15) n/cm(2) CCE decreases monotonically to approximate to 20 % at the highest fluence. Heavily irradiated SiC diodes have been studied, by means of Photo Induced Current Transient

2005 - Silicon carbide for alpha, beta, ion and soft X-ray high performance detectors [Relazione in Atti di Convegno]
Bertuccio, G; Binetti, S; Caccia, S; Casiraghi, R; Castaldini, A; Cavallini, A; Lanzieri, C; Le Donne, A; Nava, Filippo; Pizzini, S; Rigutti, L; Verzellesi, Giovanni; Vittone, E.

High performance SiC detectors for ionising radiation have been designed, manufactured and tested. Schottky junctions on low-doped epitaxial 4H-SiC with leakage current densities of few pA/cm(2) at room temperature has been realised at this purpose. The epitaxial layer has been characterised at different dose of radiations in order to investigate the SiC radiation hardness. The response of the detectors to alpha and beta particle and to soft X-ray have been measured. High energy resolution and full charge collection efficiency have been successfully demonstrated.

2003 - Radiation tolerance of epitaxial silicon carbide detectors for electrons, protons and gamma-rays [Articolo su rivista]
Nava, Filippo; E., Vittone; P., Vanni; Verzellesi, Giovanni; Pg, Fuochi; C., Lanzieri; M., Glaser

Particle detectors were made using semiconductor epitaxial 4H-SiC as the detection medium. The investigated detectors are formed by Schottky contact (Au) on the epitaxial layer and an ohmic contact on the back side of 4H-SiC substrates with different micropipe densities from CREE. For radiation hardness studies, the detectors have been irradiated with protons (24 GeV/c) at a fluence of about 10e14 cm^(-2) and with electrons (8.2 MeV) and gamma-rays (Co-60 source) at doses ranging from 0 to 40 Mrad. We present experimental data on the charge collection properties by using 5.48, 4.14 and 2.00 MeV alpha-particles impinging on the Schottky contact. Hundred percent charge collection efficiency (CCE) is demonstrated for reverse voltages higher than the one needed to have a depletion region equal to the alpha-particle projected range, even after the irradiation at the highest dose. By comparing measured CCE values with the outcomes of drift-diffusion simulations, values are inferred for the hole lifetime, within the neutral region of the charge carrier generation layer. The hole lifetime was found to decrease with increasing radiation levels, ranging from 300 ns in non-irradiated detectors to 3 ns in the most irradiated ones. The diffusion contribution of the minority charge carriers to CCE is pointed out.

2002 - Investigation on the charge collection properties of a 4H-SiC Schottky diode detector [Articolo su rivista]
Verzellesi, Giovanni; Vanni, Paolo; Nava, Filippo; Canali, Claudio

We present experimental and theoretical data on the charge collection properties of a 4H-SiC epitaxial Schottky diode exposed to 5.48- and 2.00-MeV alpha particles. Hundred percent Charge Collection Efficiency (CCE) is, in particular, demonstrated for the 2.00-MeV alpha particles at reverse voltages higher than 40 V. By comparing measured CCE values with the outcomes of drift-diffusion simulations, a value of 500 ns is inferred for the hole lifetime within the lowly doped, active layer of virgin samples. The contributions of diffusion and funneling-assisted drift to CCE at low reverse voltages are pointed out.

2001 - Charge particle detection properties of epitaxial 4H-SiC Schottky diodes [Relazione in Atti di Convegno]
Nava, Filippo; P., Vanni; Verzellesi, Giovanni; A., Castaldini; A., Cavallini; L., Polenta; R., Nipoti; C., Donolato

This work presents measurements of the charge-collection properties of 4H-SiC Schottky diodes under alpha radiation and investigates the influence of native and alpha induced defects on the detector performance. The contribution of the diffusion of minority carriers to the charge collection efficiency is pointed out. Vaues of 500 ns and 95 us are inferred for the hole and electron lifetime respectively.

2000 - Charge particle detection properties of epitaxial 4H-SiC Schottky diodes [Abstract in Atti di Convegno]
Verzellesi, Giovanni; Vanni, Paolo; Nava, Filippo; E., Vittone; C., Manfredotti; A., LO GIUDICE; A., Castaldini; A., Cavallini; L., Polenta; R., Nipoti; C., Donolato

1999 - Polymerization and Characterization of 4,4’-bis(alkylsulfanyl)-2,2’-bithiophenes [Articolo su rivista]
Iarossi, Dario; Mucci, Adele; Schenetti, Luisa; Seeber, Renato; F., Goldoni; Affronte, Marco; Nava, Filippo

Regioregular polymers poly[4,4'-bis(butylsulfanyl)-2,2'-bithiophene] (P1)and poly[4,4'-bis(octylsulfanyl)-2,2'-bithiophene] (P2) were prepared from 4,4'-bis(butylsulfanyl)- and 4,4'-bis(octylsulfanyl)-2,2'-bithiophene, by oxidative polymerization with FeCl3, and characterized by 1-H, and 13-C NMR, FT-IR, ad UV-vis spectroscopies, atomic force microscopy (AFM) , electrical conductivity, and cyclic voltammetry measurements. They exhibit weight-average molecular weights of 8 and 70 kDa, respectively, and are both readily soluble in CHCl3, CH2Cl2, CS2, toluene, and THF. P1 has a lamellar structure, and P2 is a flexible and compact film that can be easily processed and manipulated. In the neutral state, they show UV-vis absorption maxima at about 470 nm in CHCL3 solution and exhibit marked solvatochromism and thermochromism. Furthermore, thay can be cast from orange-red solution to form a violet film. The p-doping potentials have been determined, and the possibility of electrogenerating corresponding polymers has been checked. First comparative characterizations of the electrogenerated polymers bave been carried out.

1993 - Electrical and optical properties of silicide single crystals and thin films [Articolo su rivista]
Nava, Filippo; K. N., Tu; O., Thomas; J. P., Senateur; R., Madar; A., Borghesi; G., Guizzetti; U., Gottlieb; O., Laborde; Bisi, Olmes

Electrical transport and optical properties of transition-metal silicides are reviewed. They are integrated with thermal properties of single-crystal silicides. Most of these compounds behave as metals while some of them behave as semiconductors. The former show an increasing electrical resistivity ρ with increasing temperature. Several of them show a non-classical deviation of ρ(T) from linearity in the high-temperature limit. This deviation, related to intrinsic properties of the compound, can be affected both in sign and in amount by the presence of foreign atoms (impurities) and structural defects. Moreover, defects dominate the electrical transport at low temperatures both in metallic and semiconducting compounds. Therefore, the interpretation of the electrical properties measured as a function of temperature may give a non-realistic description of silicide intrinsic properties. Since also other physical properties, like thermal and optical ones, can be strongly affected by impurities and defects, results about single-crystal silicides will be first illustrated. Single-crystal preparation and structural characterization are described in detail, with emphasis on crystalline quality in terms of residual resistivity ratio. The electrical quantities, resistivity and magnetoresistance, are measured as a function of temperature and along the main crystallographic directions. The effect of impurities and defects on the transport properties is then evaluated by examining the electrical transport of polycrystalline thin-film silicides. The different contributions to the total resistivity are measured by changing: (i) film stoichiometry, (ii) impurity concentration, (iii) texture growth and (iv) film thickness. Hall-coefficient measurements are briefly discussed with the main purpose to evidence that great caution is necessary when deducing mobility and charge-carrier density values from these data. The theoretical models currently used to interpret the low- and high-temperature resistivity behavior of the metallic silicides are presented and used to fit the experimental resistivity curves. The results of these studies reveal that in several cases there are well-defined temperature ranges in which a specific electron—phonon scattering mechanism dominates. This allows a more detailed study of the microscopic processes. The optical functions from the far-infrared to the vacuum ultraviolet, derived from Kramers—Krönig analysis of reflectance spectra or directly measured by spectroscopic ellipsometry, are presented and discussed for some significant metallic disilicides, both single crystals and polycrystalline films. Different physical phenomena are distinguished in the spectra: intraband transitions at the lowest photon energies, interband transitions at higher energies, and collective oscillations. In particular, the free-carrier response derived from this analysis is compared with the transport results. The interpretation of the experimental spectra is based on the calculated electronic structures or optical functions. Moreover, it is shown how the optical studies contribute to assess definitively the semiconducting character of some disilicides. Specific-heat measurements on single crystals between 0.1 and 8 K are reported. The Debye temperature and the density of electronics states at the Fermi surface are deduced from the lattice and electronic contributions, respectively. Some silicides have been found superconductors with small electron—phonon coupling constants. Emphasis is given to the comparison between the properties deduced from these studies and those obtained from the analysis of electrical transport data. The final part of this review is devoted to the calculation of some microscopic physical quantities, as for example the electron mean free path, the charge-carrier density, the Fermi velocity. The parameters of the best fit to the experimental resistivity curves, the f

1992 - Processing high-quality silicon for microstrip detectors [Articolo su rivista]
Nava, Filippo; Ottaviani, Giampiero; Tonini, Rita; Frabboni, Stefano; A., Alberigi Quaranta; P., Cantoni; P. L., Frabetti; L., Stagni; G., Queirolo; P. F., Manfredi

An investigation has been made into the behaviour of high-purity silicon (HP-Si) during the fabrication of microstrip detectors. The resistivity of the silicon used is 3 k-OMEGA cm. The investigation is centred on standard bipolar processes based on ion implantation. It is found that, comparing the processes used, the best diode characteristics are achieved when a heat treatment at 600-degrees-C is used after the ion-implantation step, whereas the worst results from an implantation and a 900-degrees-C heat treatment. Thus it is shown that if integration of the electronic circuitry and the detector on a single chip is required, then the high-temperature heat treatments must be done before the ion-implantation step needed for detector fabrication.

1991 - Far-infrared spectroscopy of thermally annealed tungsten silicide films [Articolo su rivista]
M., Amiotti; A., Borghesi; G., Guizzetti; Nava, Filippo; Santoro, Giorgio

The far-infrared transmittance spectrum of tungsten silicide has been observed for the first time. WSi2 polycrystalline films were prepared by coevaporation and chemical-vapour deposition on silicon wafers, and subsequently thermally annealed at different temperatures. The observed structures are interpreted, on the basis of the symmetry properties of the crystal, such as infrared-active vibrational modes. Moreover, the marked lineshape dependence on annealing temperature enables this technique to analyse the formation of the solid silicide phases.

1990 - Far Infrared Vibrational Spectroscopy in CrSi_2 [Articolo su rivista]
A., Borghesi; A., Piaggi; Franchini, Anna; G., Guizzetti; Nava, Filippo; Santoro, Giorgio

We present and discuss reflectance and trasmittance measurements on CrSi, polycrystalline films in the (100 + 700) cm-' wavenumber range. The spectra show that the compound has a nonmetallic behaviour. The strong and sharp structures observed are interpreted on the basis of the symmetry properties of the normal modes of CrSie, and their strength is attributed to a dynamical charge. Moreover, preliminary lattice dynamicscalculations allow to explain the relative intensity of the infrared-active modes.

1990 - Optical and vibrational properties of Cr and Fe disilicides [Relazione in Atti di Convegno]
A., Borghesi; A., Piaggi; A., Stella; G., Guizzetti; Nava, Filippo; Santoro, Giorgio

We present and discuss reflectance and trasmittance measurements on CrSi_2 an FeSi_2 polycristalline films, The strong and sharp structures observed in the 100-700 cm^-1 wavenumber range are interpreted basin on normal modes symmetry properties. We also examine the optical response in the energy region around the band-gap for CrSi_2 and determine the dispersion of its complex refractive index.

1987 - Electrical and optical properties of near-noble silicides [Articolo su rivista]
Bisi, Olmes; M. G., Betti; Nava, Filippo; A., Borghesi; G., Guizzetti; L., Nosenzo; A., Piaggi

The electrical and optical properties of near-noble silicides have been studied by performing a joint theoretical and experimental analysis of the most stable palladium-silicon compound, i.e. Pd2Si.

1986 - Electrical and structural characterization of Nb - Si thin film alloys [Articolo su rivista]
Nava, Filippo; P., Psaras; H., Takai; K. N., Tu; Valeri, Sergio; Bisi, Olmes

The structural and electrical properties of a Nb-Si thin alloy film as a function of temperature have been studied by Auger electron spectrometry, Rutherford backscattering spectroscopy, transmission electron microscopies, and in situ electrical resistivity and Hall coefficient measurements. The NbSi2,8 films were deposited by double electron-gun coevaporation onto oxidized silicon. For electrical measurements samples of a van der Pauw pattern were made through metallic masks. In the as-deposited state the coevaporated alloy film was amorphous. Upon annealing a precipitous drop in resistivity near 270°C has been determined to be the amorphous to crystalline phase transformation. The kinetics of the transformation has been determined by isothermal heat treatment over the temperature range of 224°C to 252°C. An apparent activation energy of 1.90 eV has been measured. The nucleation and growth kinetics in the crystallization process show a change in the power of time dependence from 5.5 to 2.4. The microstructures of films at various states of annealing have been correlated to the resistivity change. The crystalline NbSi2 shows an anomalous metallic behavior. The resistivity (p) versus temperature curve has a large negative deviation from linearity (dfl) and it approaches a saturation value (psat) as temperature increases. The resistivity data are fitted by two empirical expressions put forth to explain the resistivity behavior in A15 superconductors at low and high temperatures. One is based on the idea that ideal resistivity must approach some limiting value in the regime where the mean free path becomes comparable to the interatomic spacing and the other is based on a selective electron-phonon assisted scattering. In spite of the wide temperature range of analysis, it is not possible to choose one of them due to the fact that the best fit in both cases is nearly the same. The Hall coefficient (RH) changes sign from negative above ~250°K to positive below ~ 250°K.

1986 - Electrical characterization of alloy thin films of VSi2 and V3Si [Articolo su rivista]
Nava, Filippo; Bisi, Olmes; P., Psaras; H., Takai; K. N., Tu

In situ resistivity measurement was used to study the crystallization and the electrical conduction processes of V3Si and VSi2 thin films as a function of temperature. The films were deposited by dual-electron-beam coevaporation through metallic masks in a van der Pauw configuration onto oxidized silicon substrates. In the as-deposited state the alloy thin films were amorphous. For the electrical characterization of VSi2, silicon-rich specimens with a V:Si atomic ratio of 1:3 were used.The kinetics of crystallization are well described by an equation of the Johnson-Mehl-Avrami type.Measurements of the electrical resistivity varrho over a wide temperature range (2–1100 K) on samples preheated at a high temperature exhibit a similar behavior for both the vanadium silicides, i.e. varrho rises less rapidly with temperature T than the Bloch-Grüneisen theory predicts and seems to approach a saturation value at the higher temperatures. An empirical formula is used to obtain a best fit of the resistivity curves. Polycrystalline V3Si thin film became superconductive at 15 K with a residual resistivity ratio of 10.6.

1986 - Electrical transport properties of V3Si, V5Si3 and VSi2 thin films [Articolo su rivista]
NAVA, Filippo; BISI, Olmes; K. N., TU

Resistivity measurements in a wide temperature range (2–1100 K) have been performed on thin films of V3Si, V5Si3, and VSi2 formed on an inert substrate. An anomalous resistivity behavior has been observed in these metallic compounds: The resistivity deviates from linearity and approaches a saturation value at the higher temperatures. The resistivity data can be fitted quite well to a phenomenological expression based on the idea that a limiting resistivity is reached when the electron mean free path is of the order of the interatomic spacing. The electron mean free paths, which have been computed from the experimental data, lend support to the above idea. The saturation phenomenon in V3Si and V5Si3 compounds is characterized by a limiting resistivity of the same magnitude as observed in several A15 materials and in the Mooij correlation, yet in VSi2 the resistivity saturates to a much higher value. The V3Si is a superconductor with a transition temperature around 15 K and a residual resistivity ratio of 10.6. On the other hand, V5Si3 and VSi2 thin compound films do not show superconductivity state down to 2 K. The temperature dependence of the Hall coefficient gives evidence of a complex and different electronic structure of the three compounds.

1981 - Diffusion coeffcient of electrons in Si [Articolo su rivista]
Brunetti, Rossella; Jacoboni, Carlo; Nava, Filippo; L., Reggiani; G., Bosman; R. J. J., Zijlstra

An experimental and theoretical analysis of the diffusivity of electrons in si is presented.