A. Valentini

Ion‐beam sputtering deposition of fluoropolymer thin films

It is shown that it is possible to deposit thin films with various CFx composition (1.26≤x≤1.83) by ion‐beam sputtering. These materials with ‘‘teflon‐like’’ composition have been deposited at room temperature by Ar ion‐beam sputtering of a teflon target; the film chemical composition has been determined by electron spectroscopy for chemical analysis. The fluorine‐to‐carbon ratio of the films, as well as their crosslinking degree, is shown to depend on the energy of the ions impinging on the target.

Optical spectra and photoluminescence of C60 thin films

Abstract We report on transmission and reflectance spectra of fullerite C60 thin films deposited on quartz substrate. An Urbach tail of states is present below the absorption edge due to the disordered structure of the films. Photoluminescence (PL) spectra have been measured from 10 to 300 K. Recombination of self-trapped excitons and their phonon replicas are present in the PL spectra. Temperature dependence of the integrated intensity of each emission line is also reported.

Detection of combustible gases by means of a ZnO-on-Si surface acoustic wave (SAW) delay line

Abstract ZnO-on-Si surface acoustic wave (SAW) delay lines have been examined as detectors of some combustible gases (CO, NO, H2), reactive gas such as O2 and other gases (CH4, N2, Ar). Measurements have been performed at temperatures of 20–160 °C, operation frequencies of 100–200 MHz and atmospheric pressure. The ZnO thin films have been prepared by reactive r.f. diode sputtering in an O2/Ar atmosphere. The piezoelectric ZnO films deposited on the acoustic propagation path ensure both electromechanical transduction and gas detection. The relative changes of SAW phase velocity (acoustic response) upon absorption and desorption of a gas have been measured and compared with the change of electrical resistivity (electrical response) measured on the sensor in the same conditions. The effect of preliminary NO exposure on the acoustic responses for other gases is demonstrated. The time dependence of the acoustic response to NO is shown to be remarkably different from both its electrical response and from the acoustic responses to all other test gases. Therefore the shape of the time dependence can be used as a qualitative parameter for selective identification of NO. The relatively small interaction of Ar and N2 with ZnO film makes them good carriers for other gases. Concerning the acoustic response, two possible processes of gas-molecule sorption by the ZnO film are briefly discussed.

The stability of zinc oxide electrodes fabricated by dual ion beam sputtering

The stability of the electrical and optical properties of dual ion beam sputtered zinc oxide films, with resistivities of 10−3 Ω cm, were investigated. ZnO films were deposited at room temperature by argon ion beam sputtering of a zinc oxide target. Argon or hydrogen/argon mixtures of positive ions were irradiated by the second source on the growing film. After heat treatment in air, no significant changes in the electrical and optical properties are observed for films irradiated by hydrogen/argon ions. ZnO films prepared by this method would be useful in the production of transparent and conductive electrodes for practical use at high temperature.

Organic multilayers as distributed Bragg reflectors

We report on a distributed Bragg reflector (DBR) operating in the IR spectral region, based on a fluorocarbon polymer (CFx) multilayer. In this organic structure, the periodic variation of the refractive index is obtained by embedding gold clusters in the CFx, resulting in a periodic CFx/CFx(Au) layered structure with a refractive index discontinuity in the infrared spectral region of about Δn∼0.4. We have fabricated a quarter-wavelength stack consisting of 4 CFx/CFx(Au) pairs, with a nominal Au concentration equal to 17%. The DBR shows a reflectance peak of about 70% at 1.7 μm and a full width at half maximum of about 0.4 μm.

Evaluation of semi-insulating InP crystals for nuclear radiation

Abstract We present a preliminary study of the performance of semi-insulating indium phosphide (InP) crystals and its possible use in nuclear radiation detection at room temperature. Devices having an active area of 12 mm 2 showed a 5% resolution (FWHM) for 5.48 MeV alpha particles. Analyses of photo spectra indicated that the trapping of holes was primarily responsible for degrading the alpha response.

Design of an integrated low-noise read-out system for DNA capacitive sensors

This paper presents an electronic system for a fast DNA label-less detection. The sensitivity of the capacitive sensor in use is improved by depositing an insulating self-assembled monolayer (SAM) over the golden electrodes. The capacitance shift due to the hybridization effect is monitored by means of a charge-sensitive amplifier and digitalized by means of a comparator and a counter. The read-out solution demonstrates the ability to identify a 0.01% variation on the capacitive value of the sensor. Results from measurements with the optimized sensor show the reliability of the electronics. The investigated solution is suitable for monolithic systems or for a micro-fabricated array of sensors. An example of the integrated front-end is described and performances and noise evaluation are reported here.

Correlation between surface chemical composition and vapor sensing properties of gold-fluorocarbon nanocomposites

Abstract Chemical and morphological characterization of gold-fluoropolymer (CF x (Au)) nanocomposites, deposited by ion beam co-sputtering, has been carried out in order to establish a correlation between the morphological/chemical structure of the thin film and its performance as vapor sensor. Results derived from the analytical characterization allow to reach a deeper understanding of the swelling phenomena involved in the sensing processes.

Oxygen gas-sensing characteristics for ZnO(Li) sputtered thin films

Abstract The oxygen gas-sensing characteristics of ZnO sputtered thin films, undoped or Li doped, versus their working temperatures are reported in this paper. The Li content and the O/Zn ratio are obtained by measurements of nuclear reaction analysis (NRA) and Rutherford back-scattering (RBS), respectively. The structural properties of ZnO and ZnO(Li) films are studied by means of X-ray diffraction (XRD); the preferential orientation of the thin films is found to be (002). We have measured the variation of the electrical conductance for O2 concentrations between 100 and 2 × 105 ppm by means of absorption isotherms at temperatures varying from 400 to 550 °C. The oxygen sensitivity and the response times for these ZnO thin films are reported here; the possible interaction mechanisms of gaseous O2 with the bulk of pure or Li-doped films will also be given.

ZnO/CdTe heterojunctions prepared by r.f. sputtering

Abstract ZnO/CdTe heterojunctions were prepared by r.f. sputtering of ZnO films onto p-type CdTe single crystals. The ZnO deposition was carried out using a ZnO target and an Ar-H2 mixture as the sputtering gas. The ZnO films obtained under these conditions show a very good optical transmission (about 90%) between 0.4 and 0.8 μm and a low resistivity. The electrical properties of the ZnO/CdTe heterojunctions were studied by means of current-voltage (I-V) and capacitance-voltage (C-V) measurements carried out at different temperatures. The dark I-V characteristics show that a multistep tunnelling mechanism controls the transport of charge carriers across the junction. The presence of interface states was shown to play an important role in the electrical properties of the junction. Tentative measurements of the solar energy conversion efficiency gave a value of about 3.5% without any attempt at optimization of the cell parameters.