In this work, we demonstrate a fully integrated three-axis Hall magnetic sensor by exploitingmicrofabrication technologies applied to a GaAs-based heterostructure. This allows us toobtain, by the same process, three mutually orthogonal sensors: an in-plane Hall sensor andtwo out-of-plane Hall sensors. The micromachined devices consist of a two-dimensionalelectron gas AlGaAs/InGaAs/GaAs multilayer which represents the sensing structure, grownon the top of an InGaAs/GaAs strained bilayer. After the release from the substrate, thestrained bilayer acts as a hinge for the multilayered structure allowing the out-of-planeself-positioning of devices. Both the in-plane and out-of-plane Hall sensors show a linearresponse versus the magnetic field with a sensitivity for current-biased devices higher than1000 V A-1 T-1, corresponding to an absolute sensitivity more than 0.05 V T-1 at 50 ?A.Moreover, Hall voltage measurements, as a function of the mechanical angle for both in-planeand out-of-plane sensors, demonstrate the potential of such a device for measurements of thethree vector components of a magnetic field.

We report on the first dielectric investigation of high-k yttrium copper titanate thin films, which were demonstrated to be very promising for nanoelectronics applications. The dielectric constant of these films is found to vary from 100 down to 24 (at 100 kHz) as a function of deposition conditions, namely oxygen pressure and film thickness. The physical origin of such variation was investigated in the framework of universal dielectric response and Cole-Cole relations and by means of voltage dependence studies of the dielectric constant. Surface-related effects and charge hopping polarization processes, strictly dependent on the film microstructure, are suggested to be mainly responsible for the observed dielectric response. In particular, the bulky behaviour of thick films deposited at lower oxygen pressure evolves towards a more complex and electrically heterogeneous structure when either the thickness decreases down to 50 nm or the films are grown under high oxygen pressure.

A detailed study of the electrical properties of planar AlGaN/GaN Schottky diodes is presented, the focus being on the role of the two dimensional electron gas (2DEG) depletion and the diodes non-idealities in different voltage regimes. The 2DEG depletion behavior is inferred from the analysis of capacitance and current measurements with transition from vertical to lateral diode operation occurring at Vpinch-off =4V. In particular, the sub-micrometer depletion width, laterally extending from the edge of the Schottky contact under high reverse voltages, is evaluated on the basis of a simple fringe capacitance model. Current transport mechanisms are discussed, investigating the interrelation between 2DEG, Poole-Frenkel effect, and defects. With regard to defects, the role of dislocations in the AlGaN/GaN diode non-idealities, usually interpreted in terms of Schottky barrier inhomogeneities, is critically addressed. Photocurrent spatial mapping under high reverse voltage points out the not uniform electric field distribution around the Schottky contact and highlights the presence of local photo-conductive paths, likely associated with the dislocations near the edge of the Schottky contact. Published by AIP Publishing.

Reverse-bias stress testing has been applied to a large set of more than fifty AlGaN/GaN high electron mobility transistors, which were fabricated using the same process but with different values of the AlN mole fraction and the AlGaN barrier-layer thickness, as well as different substrates (SiC, sapphire). Two sets of devices having different defect types and densities, related to the different growth conditions and the choice of nucleation layer, were also compared. When subjected to gate-drain (or gate-to-drain and source short-circuited) reverse-bias testing, all devices presented the same timedependent failure mode, consisting of a significant increase in the gate leakage current. This failure mechanism occurred abruptly during step-stress experiments when a certain negative gate voltage, or "critical voltage", was exceeded, or, during constant voltage tests, at a certain time, defined as "time to breakdown". Electroluminescence (EL) microscopy was systematically used to identify localized damaged areas that induced an increase of gate reverse current. This current increase was correlated with the increase of EL intensity, and significant EL emission during tests occurred only when the critical voltage was exceeded. Focusedion- beam milling produced cross-sectional samples suitable for electron microscopy observation at the sites of failure points previously identified by EL microscopy. In high-defectivity devices, V-defects were identified that were associated with initially high gate leakage current and corresponding to EL spots already present in untreated devices. Conversely, identification of defects induced by reverse-bias testing proved to be extremely difficult, and only nanometer-size cracks or defect chains, extending vertically from the gate edges through the AlGaN/GaN heterojunction, were found. No signs of metal/semiconductor interdiffusion or extended defective areas were visible. The weak dependence on AlGaN properties, the strong process depend- nce, the time dependence, and the features of the localized damage identified by EL and electron microscopy, suggest a multi-step failure mechanism initiated by a process-induced weakness of the gate Schottky junction, which enhances current injection into pre-existing defects. As a result, further defects are generated or activated, eventually resulting in a percolation conductive path and permanent damage. A low impedance path between the device gate and the channel is formed, increasing gate leakage current and possibly resulting in device burn-out.

Pb thin films were prepared by pulsed laser deposition on a Si (100) substrate at different growth temperatures to investigate their morphology and structure. The morphological analysis of the thin metal films showed the formation of spherical submicrometer grains whose average size decreased with temperature. X-ray diffraction measurements confirmed that growth temperature influences the Pb polycrystalline film structure. A preferred orientation of Pb (111) normal to the substrate was achieved at 30° C and became increasingly pronounced along the Pb (200) plane as the substrate temperature increased. These thin films could be used to synthesize innovative materials, such as metallic photocathodes, with improved photoemission performances. © 2014 American Vacuum Society.

We report on effective prevention of GaAs corrosion in a cell culture liquid environment by means of polymerized (3-mercaptopropyl)-trimethoxysilane thin film coatings. Aging in physiological solution kept at 37 °C revealed no significant oxidation after 2 weeks, which is the typical period of incubation of a neuron cells culture. The method was also applied to High Electron Mobility Transistors (HEMT) arrays with unmetallized gate regions, in view of their application as neural signal transducers. Significant reduction of the degradation of the HEMT behavior was obtained, as compared to uncoated HEMTs, with good channel modulation efficiency still after 30 days aging

In this work two quantum dot (QD) solar cell structures have been proposed and compared as potential solutions for the realization of the Intermediate Band Solar Cell concept: the well known dot/barrier material system InAs / GaAs and an engineered InAlGaAs/AlGaAs combination. The Al-based structures have been obtained by a suitably developed growth procedure with the aim of increasing island density and engineering the absorption spectrum and the energy band profile in the near infrared region. Along with tunability of the confined electron energy levels, the proposed Al-based structures exhibit transport features, such as reduced edge recombination losses and lower reverse saturation current density with respect to the InAs/GaAs QD system, which can be useful for enhancing device performances. © 2014 AEIT.

The nanoscaling of metamaterial structures represents a technological challenge toward their application in the optical frequency range. In this work we demonstrate tailored chiro-optical effects in plasmonic nanohelices, by a fabrication process providing a nanometer scale control on geometrical features, that leads to a fine tuning of operation band even in the visible range. Helicoidal 3D nanostructures have been prototyped by a bottom-up approach based on focused ion and electron beam induced deposition, investigating resolution limits, growth control and 3D proximity effects as a function of the interactions between writing beam and deposition environment. The fabricated arrays show chiro-optical properties at the optical frequencies and extremely high operation bandwidth tailoring dependent on the dimensional features of these 3D nanostructures: with the focused ion beam we obtained a broadband polarization selection of about 600 nm and maximum dissymmetry factor up to 40% in the near-infrared region, while with the reduced dimensions obtained by the focused electron beam a highly selective dichroic band shifted toward shorter wavelengths is obtained, with a maximum dissymmetry factor up to 26% in the visible range. A detailed finite difference time domain model highlighted the role of geometrical and compositional parameters on the optical response of fabricated nanohelices, in good agreement with experimental results.

The polarization scan of both pumps in a noncollinear second-harmonic experiment is shown to be a powerful tool for identifying the different components of plane misalignment angles in a nonlinear crystal. Here, we report an optical axis misalignment as small as 1 degrees, in a 380 mu m thick 4H-SiC sample, by means of 130 fs pulsed laser of 830 nm wavelength. The optical axis misalignment is confirmed by the x-ray structural analysis. (C) 2013 Optical Society of America

In this work, we report on the competition between two-step two photon absorption, carrier recombination,and escape in the photocurrent generation mechanisms of high quality InAs/GaAs quantumdot intermediate band solar cells. In particular, the different role of holes and electrons ishighlighted. Experiments of external quantum efficiency dependent on temperature and electricalor optical bias (two-step two photon absorption) highlight a relative increase as high as 38% at10K under infrared excitation. We interpret these results on the base of charge separation by phononassisted tunneling of holes from quantum dots. We propose the charge separation as an effectivemechanism which, reducing the recombination rate and competing with the other escapeprocesses, enhances the infrared absorption contribution. Meanwhile, this model explains why thermalescape is found to predominate over two-step two photon absorption starting from 200 K,whereas it was expected to prevail at lower temperatures (70 K), solely on the basis of the relativelylow electron barrier height in such a system. VC 2016 AIP Publishing LLC.

In this work, we experimentally investigate the chiro-optical properties of 3D metallic helical systems at optical frequencies. Both single and triple-nanowire geometries have been studied. In particular, we found that in single-helical nanostructures, the enhancement of chiro-optical effects achievable by geometrical design is limited, especially with respect to the operation wavelength and the circular polarization conversion purity. Conversely, in the triple-helical nanowire configuration, the dominant interaction is the coupling among the intertwined coaxial helices which is driven by a symmetric spatial arrangement. Consequently, a general improvement in the g-factor, extinction ratio and signal-to-noise-ratio is achieved in a broad spectral range. Moreover, while in single-helical nanowires a mixed linear and circular birefringence results in an optical activity strongly dependent on the sample orientation and wavelength, in the triple-helical nanowire configuration, the obtained purely circular birefringence leads to a large optical activity up to 8°, independent of the sample angle, and extending in a broad band of 500 nm in the visible range. These results demonstrate a strong correlation between the configurational internal interactions and the chiral feature designation, which can be effectively exploited for nanoscale chiral device engineering.

Fabrication and characterization of chiral metallic nanospirals for application as metamaterials in the visible and near infrared range are described. The structures consist of platinum helicoidal three-dimensional nanostructures realized by focused ion beam induced-deposition, where the interaction with incident light can be controlled as a function of light circular polarization state and spectral region, showing a circular dichroism across a wide range of optical wavelengths. An accurate size control and nanometer resolution on the fabrication of the chiral structures are achieved by exploring substrate surface charge effects on substrates with different electrical properties and by studying and implementing an accurate scanning procedure for the nanostructure growth that allows compensation of the proximity and charge effects. Optical measurements carried out on the nanospiral arrays using a high spatial resolution setup show a transmittance difference of the right- and left-circular polarized light near to 40%.

Three dimensional helical chiral metamaterials resulted in effective manipulation of circularly polarized light in the visible infrared for advanced nanophotonics. Their potentialities are severely limited by the lack of full rotational symmetry preventing broadband operation, high signal-to-noise ratio and inducing high optical activity sensitivity to structure orientation. Complex intertwined three dimensional structures such as multiple-helical nanowires could overcome these limitations, allowing the achievement of several chiro-optical effects combining chirality and isotropy. Here we report three dimensional triple-helical nanowires, engineered by the innovative tomographic rotatory growth, on the basis of focused ion beam-induced deposition. These three dimensional nanostructures show up to 37% of circular dichroism in a broad range (500-1,000 nm), with a high signal-to-noise ratio (up to 24 dB). Optical activity of up to 8 degrees only due to the circular birefringence is also shown, tracing the way towards chiral photonic devices that can be integrated in optical nanocircuits to modulate the visible light polarization.

Tailoring of electronic and optical properties of self-assembled InAs quantum dots (QDs) is a critical limit for the design of several QD-based optoelectronic devices operating in the telecom frequency range. We describe how fine control of the strain-induced surface kinetics during the growth of vertically stacked multiple layers of QDs allows for the engineering of their self-organization process. Most noticeably, this study shows that the underlying strain field induced along a QD stack can be modulated and controlled by time-dependent intermixing and segregation effects occurring after capping with a GaAs spacer. This leads to a drastic increase of the TM/TE polarization ratio of emitted light, not accessible from conventional growth parameters. Our detailed experimental measurements, supported by comprehensive multi-million atom simulations of strain, electronic and optical properties, provide in-depth analysis of the grown QD samples allowing us to give a clear picture of the atomic scale phenomena affecting the proposed growth dynamics and consequent QD polarization response. © 2014 IOP Publishing Ltd.

A model for realistic InAs quantum dot composition profile is proposed and analyzed, consisting of a double region scheme with an In-rich internal core and an In-poor external shell, in order to mimic the atomic scale phenomena such as In-Ga intermixing and In segregation during the growth and overgrowth with GaAs. The parameters of the proposed model are derived by reproducing the experimentally measured polarization data. Further understanding is developed by analyzing the strain fields which suggests that the two-composition model indeed results in lower strain energies than the commonly applied uniform composition model.

We propose the realization of a compact fully-passive biotelemetry tag composed of a high-electron mobility transistor (HEMT) connected to a wireless link. The Gallium Arsenide based gateless HEMT serves both as the environmental sensing element and as the amplitude modulator of the carrier signal received by the antenna. A prototype demonstrator operating in the MHz range has been developed: it consists of an array of transistors with different gate geometries and two spiral loop resonators implementing the wireless link. More specifically, one resonator (Tag-resonator) is connected to the array of transistors, while the other one (Reader-resonator) is connected to a power generator/reader device; the wireless link uses the magnetic coupling between the two resonators. Experimental results demonstrate that the reader-resonator exhibits an intensity modulation of the resonance dip depending on the voltage applied to the HEMT gate. These results will be used as a guideline for the realization of biocompatible sub-millimeter tags operating in the Gigahertz frequency range.b © 2013 Elsevier B.V. All rights reserved.

The present invention concerns an integrated triaxial magnetic sensor device 100 apt to detect a magnetic field comprising: a substrate 120 having a surface defining a reference plane (x,y); a first sensor unit 101 arranged on a first main surface 121 of the substrate in a first plane substantially parallel to the reference plane; a second sensor unit 102 arranged on a second plane, and a third sensor unit 103 arranged on a third plane, the second and third planes being not parallel to the reference plane. The device further comprises: a first cantilever structure 115 raised with respect to the reference plane by a first elevation angle and having a second main surface 125 arranged along the second plane, the first cantilever structure including the second sensor unit 102, and a second cantilever structure 116 raised with respect to the reference plane by a second elevation angle and having a third main surface 126 arranged along the third plane, the second cantilever structure including the third sensor unit 103, in which the first and the second cantilever structure are structurally connected to the substrate 120 through a respective first and second hinge structure 113, 114 curved with respect to the reference plane and bearing the respective cantilever structure while maintaining it raised with respect to the reference plane.

Optical logic gate having a second-harmonic generator element that receives a first and a second optical input signal respectively having a first and a second angular frequency and respectively having a first and a second polarization, and which provides a second-harmonic optical signal having a third angular frequency and a third polarization. The third angular frequency is equal to the sum of the first and the second angular frequency. The third polarization is a function of the first and the second polarization. The second-harmonic generator element includes a second-harmonic generator layer in a material having a non-null second-order optical tensor.