Roberto Vittorio Murri

Magnetic and electronic transport percolation in epitaxial Ge 1 – x Mn x films

Electronic transport and magnetic properties of Ge{sub 1-x}Mn{sub x}/Ge(100) films are investigated as a function of Mn dilution. Depending on x, characteristic temperatures separate different regimes in both properties. Resistivity exhibits an insulatorlike behavior in the whole temperature range and, below about 80 K, two distinct activation energies are observed. At a higher temperature value, T{sub R}, resistivity experiences a sudden reduction. The Hall coefficient shows a strong contribution from the anomalous Hall effect and, at T{sub R}, a sign inversion, from positive to negative, is recorded. The magnetic properties, inferred from magneto-optical Kerr effect, evidence a progressive decrease of the ferromagnetic long range order as the temperature is raised, with a Curie temperature T{sub C} not far from T{sub R}. The transport and magnetic results are qualitatively consistent with a percolation mechanism due to bound magnetic polarons in a GeMn diluted magnetic semiconductor, with localized holes [A. Kaminski and S. Das Sarma, Phys. Rev. B 68, 235210 (2003)].

Characterization of porous Al2O3SiO2/Si sensor for low and medium humidity ranges

Abstract We present the preliminary a.c. electrical characterization of a humidity sensor based on a thin Al 2 O 3 porous layer grown on an SiO 2 /Si substrate. The sensor is prepared by sputter deposition of a thin film of Al, followed by its anodic oxidation in a sulphuric acid solution. A gold electrode is deposited on the substrate tilted at a grazing angle with respect to the substrate plane. The electrical a.c. measurements are made in the range 100 Hz-15 MHz and in a small chamber where either the relative humidity or the temperature could be easily changed. The sensor response is not influenced by interfering gases like CO, CO 2 , NO 2 , CH 4 , C 2 H 6 and H 2 . The experimental results of the impedance spectroscopy are compared with the frequency responses of different equivalent circuits of the sensor.

MR Imaging and Osteoporosis: Fractal Lacunarity Analysis of Trabecular Bone

We develop a method of magnetic resonance (MR) image analysis able to provide parameter(s) sensitive to bone microarchitecture changes in aging, and to osteoporosis onset and progression. The method has been built taking into account fractal properties of many anatomic and physiologic structures. Fractal lacunarity analysis has been used to determine relevant parameter(s) to differentiate among three types of trabecular bone structure (healthy young, healthy perimenopausal, and osteoporotic patients) from lumbar vertebra MR images. In particular, we propose to approximate the lacunarity function by a hyperbola model function that depends on three coefficients, alpha,beta, and gamma, and to compute these coefficients as the solution of a least squares problem. This triplet of coefficients provides a model function that better represents the variation of mass density of pixels in the image considered. Clinical application of this preliminary version of our method suggests that one of the three coefficients, beta, may represent a standard for the evaluation of trabecular bone architecture and a potentially useful parametric index for the early diagnosis of osteoporosis

Urbach tail in amorphous gallium arsenide films

Measurements using photothermal deflection spectroscopy (PDS) have been performed on samples of amorphous gallium arsenide deposited by rf sputtering of monocrystalline GaAs targets with and without hydrogen. The trend of the Urbach energy, E 0 , and Tauc gap, E g , as a function of some of the deposition parameters is discussed. In particular, the behavior of E 0 shows a decrease as a function of the quantity ( W / p ) 1/2 , where W is the discharge power and p the pressure of the deposition chamber. E g increases with the hydrogen pressure and tends to saturate when p H 2 is greater than 0.1 Pa. E 0 shows exactly the opposite trend.

Analysis of the X-ray Work Flow in Two Diagnostic Imaging Departments With and Without a RIS/PACS System

A traditional radiological workflow is compared with one based on radiology information system/picture archiving and communication system (RIS/PACS). X-ray workflow process was considered in both radiology departments. First, the study identified the main phases of the research work as follows: Process Analysis, Data Collection and Elaboration, Interpretation. Afterwards, the main steps of the whole image acquisition process were defined, and each step was divided into a number of elementary operations. Then, the time required to complete each of these was measured. Data collected were elaborated and synthesized to obtain time frequency distributions for each step and evaluation of the total time for the whole working flow. Statistical elaboration of the collected data shows that x-ray working time decreases, between 35% and 57%, when RIS/PACS is used. Detailed analysis of the whole working process allows identification of possible critical points to improve the image acquisition process.

Electrical transport properties of microcrystalline silicon grown by plasma enhanced chemical vapor deposition

The dark conductivity and Hall mobility of hydrogenated silicon films deposited varying the silane concentration f=SiH4∕(SiH4+H2) in a conventional plasma enhanced chemical vapor deposition system have been investigated as a function of temperature, taking into account their structural properties. The electrical properties have been studied in terms of a structural two-phase model. A clear transition from the electrical transport governed by a crystalline phase, in the range 1%⩽f⩽3%, to that controlled by an amorphous phase, for f>3%, has been evidenced. Some metastable effects of the dark conductivity have been noticed.

Growth and magnetic properties of MnGe films for spintronic application.

Morphological and magnetic properties of MnxGe1−x films have been investigated by scanning tunneling microscopy (STM) and magneto-optical Kerr effect (MOKE) measurements, respectively. Several MnxGe1−x alloys were grown by molecular beam epitaxy (MBE) on Ge(1 0 0) substrates, varying the growth temperature and alloy composition (x). STM analysis demonstrated island morphology with islands having a mean dimension ranging from about 100 to 130 nm, depending on the substrate temperature and Mn content in the film. Growth conditions also influence the island distribution. MOKE analysis, carried out on all the MnxGe1−x alloys, showed only a negligible hysteresis effect in the investigated temperature range from about 12 to 300 K. At low temperatures (below 70–110 K, depending on the sample), the MOKE signal tends to saturate at a magnetic field intensity less than about 0.5 T, indicating a superparamagnetic behavior. On the contrary, above that temperature the films do not show a magnetic character. The features of the MOKE curves depend on the growth parameters.

Structural characterization of unhydrogenated amorphous GaAs.

Abstract Films of unhydrogenated amorphous gallium arsenide have been deposited by reactive rf sputtering. Substrate temperature or deposition power or argon pressure were allowed to change, in different deposition runs, to obtain groups of specimens deposited with only one variable parameter. Transmission electron microscopy (TEM) and transmission high energy electron diffraction (THEED) measurements have been made in order to determine the structure and the morphology of the films. The THEED patterns show that the structure has a short range order and a weak dependence on deposition conditions. The TEM micrographs show that the morphology of the films is strongly affected by the deposition conditions.

Medical imaging and osteoporosis: fractal's lacunarity analysis of trabecular bone in MR images

The aim of this study was to develop a method of MR image analysis able to provide parameter(s) sensitive to bone microarchitecture changes in aging and osteoporosis onset and progression. The method has been built taking into account fractal properties of many anatomic and physiologic structures. Fractal lacunarity analysis has been used to determine relevant parameter(s) to differentiate among three types of trabecular bone structure (healthy young, healthy perimenopaused, and osteoporotic patients) from lumbar vertebra MR images. In particular, we propose to approximate the lacunarity function by a hyperbola model function, that depends on three different coefficients, /spl alpha/, /spl beta/, /spl gamma/, and to compute these coefficients as the solution of a least squares problem. This term of coefficients provides the model function that better represents the variation of mass density of pixels in the image considered. Clinical application of this preliminary version of our method suggests that one of the three coefficients, namely /spl beta/, may represent a standard for an evaluation of trabecular bone architecture and a potential useful parametric index in early diagnosis of osteoporosis.

An Al2O3 sensor for low humidity content: Characterization by impedance spectroscopy.

Abstract A sensor able to measure very low humidity concentrations and with possible applications for geophysical research has been developed using Al 2 O 3 porous thin films. The sensor is obtained by anodization in sulphuric acid of an Al thin film deposited by d.c. sputtering. The electrical properties are evaluated using impedance spectroscopy for different contents of water vapour in the range 20 to 200 ppmv and for temperatures from −60 to +40 °C. The response of the sensor changes according to the frequency of the applied voltage. These results allow us to formulate a hypothesis on the mechanism of water absorption.