Nicola Pinto

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.

Structural, electrical, electronic and optical properties of melanin films

We present thick, uniform and rather flat melanin films obtained using spray deposition. The morphology of the films was investigated using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Temperature-dependent electrical resistance of melanin thin films evidenced a semiconductor-like character and a hysteretic behavior linked to an irreversible process of water molecule desorption from the melanin film. X-ray Photoelectron Spectroscopy (XPS) was carried out to analyze the role of the functional groups in the primary and secondary structure of the macromolecule, showing that the contribution of the 5,6-dihydroxyindole-2-carboxylic acid (DHICA) subunit to the molecule is about 35%. Comparison of the optical absorption of the thick (800nm) and thin (80nm) films showed a spectral change when the thickness increases. From in vacuum photoconductivity (PC) measured at controlled temperatures, we suggest that the melanin films exhibit a possible charge transport mechanism by means of delocalized

Urbach tail in amorphous gallium arsenide films

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Morphological and structural evolutions of diluted Ge1−xMnx epitaxial films

states along the stacked planar secondary structure.

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

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.

Growth and magnetic properties of MnGe films for spintronic application.

We investigate the structural and morphological evolutions of Ge1−xMnx films, grown by molecular beam epitaxy on Ge(100), as a function of Mn nominal concentration (x). We show that in our experimental growth conditions (growth temperature TG∼160°C), Mn atoms incorporated in the matrix increases with x up to a concentration m∼0.03. Magnetic properties of the samples are mainly related to Ge3Mn5 cluster phase, while transport properties are connected to Ge:Mn matrix.

Structural characterization of unhydrogenated amorphous GaAs.

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.

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

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.

Photoconductivity of 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.