Nuova ricerca

FILIPPO NAVA

OSPITE CON ACCESSO AL SERVIZIO VPN
Dipartimento di Scienze Fisiche, Informatiche e Matematiche sede ex-Fisica

Home | Didattica |


Pubblicazioni

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

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
abstract

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
abstract

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 - Effect of heavy proton and neutron irradiations on epitaxial 4H-SiC Schottky diodes [Articolo su rivista]
S., Sciortino; F., Hartjes; S., Lagomarsino; Nava, Filippo; M., Brianzi; V., Cindro; C., Lanzieri; M., Moll; P., Vanni
abstract

In this work we report electrical characterizations on heavily irradiated epitaxial H-4-SiC Schottky diodes. Even after an irradiation at a fluence of 1.4 x 10(16) p/cm(2) and 7 x 10(15) n/cm(2), we found the diodes still able to detect alpha and beta particles with a charge collection efficiency (CCE) ranging from 25 to 30% after proton irradiation and about 18% after neutron irradiation, at the highest reverse bias applied. This corresponds to a charge collection distance (CCD) of 7 mu m after the proton irradiation and 5 mu m after the neutron irradiation. As the irradiation level approaches the range similar to 10(15)/cm(2), the material behaves as intrinsic due to a very high compensation effect.

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.
abstract

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
abstract

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 - Deep levels in silicon carbide Schottky diodes [Articolo su rivista]
A., Castaldini; A., Cavallini; L., Polenta; Nava, Filippo; Canali, Claudio; C., Lanzieri
abstract

Native or process-induced defective states may significantly affect the transport properties of silicon carbide devices. For this reason, it is of major importance to detect them and, when possible, to identify their origin. This contribution deals with the deep levels detected by deep level transient spectroscopy analyses in silicon carbide Schottky detectors. Current-voltage and capacitance-voltage characteristics have also been studied to investigate Schottky barrier properties and diode quality. On the basis of the comparison with literature data, some of the deep levels found can be attributed to impurities introduced during growth. (C) 2002 Elsevier Science B.V. All rights reserved.

2002 - Double-junction effect in proton-irradiated silicon diodes [Articolo su rivista]
A., Castaldini; A., Cavallini; L., Polenta; Canali, Claudio; Nava, Filippo
abstract

This article concerns the existence of a double-junction effect in proton-irradiated silicon p(+)-nu-n(+) (pin) diodes demonstrated by surface potential and optical-beam-induced current (OBIC) investigations. By increasing the diode biasing, the junctions existing at both ends in the irradiated devices move towards each other up to join, hence, causing a full depletion of the diodes. Due to the inhomogeneity of the electric field, however, the charge-carrier collection is strongly dependent on the position. The extent of the diode depletion layers at both ends and the shape of the electric field within the p(+)-nu-n(+) diodes are determined as a function of the bias applied. Three-dimensional photocurrent maps obtained by OBIC profiles allow for imaging the double junction. The deep level analysis evidences the presence of irradiation-induced defects, which are responsible for the double-junction effect.

2002 - Electric field distribution in irradiated silicon detectors [Articolo su rivista]
A., Castaldini; A., Cavallini; L., Polenta; Nava, Filippo; Canali, Claudio
abstract

Particle irradiation causes dramatic changes in bulk properties of p (+)-n-n(+) silicon structures operating as particle detectors. Several attempts to model and justify such variations have been proposed in the last few years. The main unsolved problem remains in the determination of the electric field and depletion layer distributions as key-parameters to estimate the collection efficiency of the detector. By using optical beam induced current (OBIC) and surface potential (SP) measurements we determined the behavior of the electric field and confirmed the existence of a double-junction structure appearing after irradiation. (C) 2002 Elsevier Science B.V. All rights reserved.

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
abstract

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 - Characterisation of silicon carbide detectors response to electron and photon irradiation [Articolo su rivista]
M., Bruzzi; Nava, Filippo; S., Russo; S., Sciortino; P., Vanni
abstract

A preliminary study of the response of SiC devices to 22 MeV electrons and 6 MV photon beams from a linear accelerator is presented in view to assess the feasibility of SiC-based dosimeters. The devices used are 4H-SiC epitaxial n-type layers deposited onto a 4H-SiC n(+)-type substrate wafer doped with nitrogen. Schottky contacts have been formed by gold deposition on the epitaxial layer. The released charge has been observed to increase linearly with the electron dose up to 10 Gy. A linear dependence of the current response of the devices has been also observed as a function of the photon dose-rate in the 2-7 Gy/min range. A preliminary study of the photoconductive response to UV irradiation of semi-insulating 6H-SiC substrates is also reported on samples, with a bulk resistivity of approximate to 10(11) Omega cm, produced with a modified Lely technique.

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
abstract

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.

2001 - Epitaxial silicon carbide for X-ray detection [Articolo su rivista]
G., Bertuccio; R., Casiraghi; Nava, Filippo
abstract

We present the first experimental results of X-ray detection and spectroscopy by means of Schottky junctions on epitaxial silicon carbide (SLC). The devices have a junction area of 3 mm(2) on an n-type 4H-SiC layer 30 mum thick with a dopant concentration of 1.8 X 10(15)cm(-3) At 300K, the reverse current density of the best device varies between 2 pA/cm(2) and 18 pA/cm(2) as the mean electric field is increased from 40 kY/cm up to 170 kV/cm. The devices have been tested with X and gamma raysfrom Am-241; the best measured energy resolution is 2.7 keV FWHM at room temperature.

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
abstract

2000 - Influence of substrate on the performance of semi-insulating GaAs detectors [Articolo su rivista]
R., Baldini; P., Vanni; Nava, Filippo; Canali, Claudio; C., Lanzieri
abstract

A study of the carrier transport mechanism, the charge collection efficiency and the energy resolution has been carried out on semi-insulating GaAs X-ray detectors realised on substrates with concentrations of acceptor dopants N-a, varying from 10(14) to 10(17) cm(-3). The electron collection efficiency (ECE) and the reverse current were found to decrease with increasing N-a, while the resistivity of the material was found to increase. At room temperature, the best collection efficiency (95%) and the best energy resolution (13.7 keV FWHM) for 59.5 keV X-rays of the Am-241 source, have been achieved with the less doped detectors (N-1 - 10(14) cm(-3)). The concentrations of ionised EL2(+), determined by optical measurements in IR regions, was shown to increase with N-a and to be quasi-inversely proportional to the ECE values. This behaviour strongly supports the hypothesis that the EL2 defects play a main role in the compensation of the material and in the limitation of the detection properties. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

1999 - Epitaxial silicon carbide charge particle detectors [Articolo su rivista]
Nava, Filippo; P., Vanni; C., Lanzieri; Canali, Claudio
abstract

The radiation detection properties of Schottky detectors made on epitaxial layers of 4H silicon carbide were evaluated. Exposure to 5.48 MeV alpha particles from a Am-241 source in vacumn led to robust signals from the detectors. The collection of the charge carriers was found to increase linearly with the square root of the applied reverse bias. (C) 1999 Elsevier Science B.V. All rights reserved.

1999 - Evidence for plasma effect on charge collection efficiency in proton irradiated GaAs detectors [Articolo su rivista]
Nava, Filippo; P., Vanni; Canali, Claudio; E., Vittone; P., Polesello; U., Biggeri; C., Leroy
abstract

The radiation damage in 100 mu m thick Schottky diodes made on semi-insulating undoped GaAs materials, were studied using alpha-, beta-, proton- and gamma-spectroscopy as well as I-V measurements. The results have been analysed within the framework of the Hecht model to investigate the influence of the plasma produced by short-range strongly ionising particles on the detector performance after 24 GeV proton irradiation. It has been found that with the mean free drift lengths for electrons and holes determined from alpha-spectra in overdepleted detectors, the charge collection efficiency for beta-particles, cce(beta), is well predicted in the unirradiated detectors, while in the most irradiated ones, the cce(beta) is underestimated by more than 40%. The observed disagreement can be explained by assuming that the charge carrier recombination in the plasma region of such detectors, becomes significant. (C) 1999 Elsevier Science B.V. All rights reserved.

1999 - Integration of front-end electronics with GaAs pixel detectors: Experimental and feasibility analysis [Articolo su rivista]
G., Bertuccio; Canali, Claudio; G., De Geronimo; A., Longoni; Nava, Filippo; C., Lanzieri
abstract

This work aims to study the feasibility of the integration, on the same chip, of GaAs pixel detectors and front-end electronics employing GaAs metal semiconductor FET's (MESFET's) or high electron mobility transistors (HEMT's), The interest of fully integrated GaAs systems lies in X and gamma-ray spectroscopy and imaging for scientific, industrial, and medical applications, The system design criteria and the prediction of the performance have been derived on the basis of recent experimental results on semi-insulating GaAs pixel detectors. Measurements of the relevant parameters of GaAs FET's suitable for the stringent requirements of a spectroscopy-grade front-end amplifier are analyzed. It is shown that an optimized GaAs integrated system can reach an electronic noise level below 100 electrons rms (<1 keV FWHM) even at room temperature. Some open problems regarding the detector-electronics integration are highlighted and discussed.

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
abstract

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.

1998 - Analysis of uniformity of as prepared and irradiated SI GaAs radiation detectors [Articolo su rivista]
Nava, Filippo; P., Vanni; C., Canali; G., Apostolo; C., Manfredotti; P., Polesello; E., Vittone
abstract

SI (semi-insulating) LEC (Liquid Encapsulated Czochralsky) GaAs (gallium arsenide) Schottky barrier detectors have been irradiated with high energy protons (24 GeV/c, fluence up to 16.45x10(13) p/cm2). The detectors have been characterised in terms of I/V curves, charge collection efficiency (cce) for incident 5.48 MeV alpha-, 2 MeV proton and minimum ionizing beta-particles and of cce maps by microprobe technique IBIC (Ion Beam Induced Charge). At the highest fluence a significant degradation of the electron and hole collection efficiencies and a remarkable improvement of the FWHM energy resolution have been measured with alpha- and proton particles. Furthermore the reduction in the cce is greater than the one measured with beta-particles and the energy resolution worsens with increasing the applied bias, V-a, above the voltage V-d necessary to extend the electric field all the way to the ohmic contact. On the contrary, in the unirradiated detectors the charge collection efficiencies with alpha-, beta- and proton particles are quite similar and the energy resolution improves with increasing V-a > V-d. IBIC spectra and IBIC space maps obtained by scanning a focused (8 mu m(2)) 2 MeV proton microbeam on front (Schottky) and back (ohmic) contacts, support the observed electric field dependence of the energy resolution both in unirradiated and most irradiated detectors. The results obtained let us explain the effect of the electric field strength and the plasma on the collection of the charge carriers and the FWHM energy resolution.

1997 - Electric-field behavior and charge-density distribution in semi-insulating gallium arsenide Schottky diodes [Articolo su rivista]
Castaldini, A; Cavallini, A; Polenta, L; Canali, C; Delpapa, C; Nava, Filippo
abstract

The behavior of the electric field and the charge-density distribution in semi-insulating gallium arsenide Schottky diodes have been analyzed by optical-beam-induced current and surface potential;measurements. The electric field exhibits three different regions across the detector, the characteristics of which depend on the reverse applied voltage. Furthermore, a positive box-shaped space charge region exists, separated from the Schottky barrier by a neutral space-charge region, and widens and moves towards the Ohmic contact at increasing the reverse bias voltage. This study adds substantial information to the knowledge of the space-charge distribution in semi-insulating Schottky diodes, discriminates between the existing models on the electric field, and provides essential information to understand nuclear detector performance.

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
abstract

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
abstract

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.

1992 - THEORY AND EXPERIMENT ON THE OPTICAL-PROPERTIES OF CRSI2 [Articolo su rivista]
Bellani, V; Guizzetti, G; Marabelli, F; Piaggi, A; Borghesi, A; Nava, Filippo; Antonov, Vn; Antonov, Vn; Jepsen, O; Andersen, Ok; Nemoshkalenko, Vv
abstract

The optical properties of CrSi2, both in polycrystalline and single-crystal form, were investigated between 0.01 and 5 eV. The dielectric functions were determined by different methods: Kramers-Kronig transformations of the near-normal reflectivity over the whole spectral range; direct measurement by spectroscopic ellipsometry from 1.4 to 5 eV; numerical inversion of the reflectance from two films with different thickness. The main difference between thin-film and single-crystal data is the presence, in the latter, of a strong free-carrier response, preventing the determination of the intrinsic absorption edge (interband optical gap). Moreover, the optical properties of CrSi2 were calculated within the local-density approximation using the semirelativistic linear-muffin-tin-orbital method. The band structure, the l-projected densities of states, the complex dielectric function, and the optical reflectivity were obtained in the energy range from 0 to 5 eV. The theoretical calculations are compared with the experimental data.

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

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.

1991 - OPTICAL STUDY OF NIOBIUM DISILICIDE POLYCRYSTALLINE FILMS [Articolo su rivista]
Amiotti, M; Borghesi, A; Marabelli, F; Guizzetti, G; Nava, Filippo
abstract

NbSi2 polycrystalline films, coevaporated and thermally annealed, were subjected to chemical and structural characterization, and then studied by reflectance from 0.06 to 6 eV and ellipsometry from 1.4 to 5 eV. The dielectric functions, obtained from Kramers-Kronig analysis and directly from ellipsometry, are also presented. Low-frequency free-carrier response is discussed in terms of the Drude model; the high-frequency interband structures are interpreted on the basis of the calculated density of states and photoemission results. A comparison is made with the optical properties of isoelectronic VSi2 and TaSi2 Polycrystalline films.

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

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
abstract

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
abstract

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
abstract

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
abstract

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
abstract

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
abstract

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

1981 - Diffusion coefficient of electrons in silicon [Articolo su rivista]
Brunetti, R.; Jacoboni, C.; Nava, F.; Reggiani, L.; Bosman, G.; Zijlstra, R. J. J.
abstract

This paper reports an experimental and theoretical analysis of the diffusivity of electrons in Si as function of temperature, field strength, and field direction. Results for the longitudinal diffusion coefficient have been obtained experimentally for fields applied along 〈111〉 and 〈100〉 directions with time-of-flight and noise measurements. Calculations have been performed with the Monte Carlo procedure. The theoretical analysis, which includes an extensive discussion of the intervalley diffusion process, has yielded a revised version of the silicon model which correctly interprets both the new diffusion data and other well-established electron transport properties. The revision of the model is mainly concerned with the relative weights of f and g intervalley scattering mechanisms. In fact the interpretation of the anisotropy of the diffusion allows separate estimates of the two types of scattering through their different effects on the intervalley diffusion which comes about when electrons have different drift velocities in different valleys.