Actually MEMS technology allows to fabricate free standing and bended cantilevers by acting on stress/strain properties and thicknesses of materials. In particular, by means of MEMS technology it is possible to realize ring or spiral layouts with piezoelectric materials. The mechanical movement due to the piezoelectric resonance can be used in order to modulate a signal travelling in the MEMS and radiating in the free space as happens in antennas. In this work we provide an accurate study regarding the design approach of piezoelectric aluminium nitride (AlN) ring antenna. The study is developed by means of a tailored 3D FEM tool which allows to analyze the piezoelectric resonances and to design the ring micro-antenna in the THz range. Finally we provide the technology and we measure the piezoelectric resonances of ring antennas.

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.

This paper presents a new AlN-based MEMS devices suitable for vibrational energy harvesting applications. Due to their particular shape and unlike traditional cantilever which efficiently harvest energy only if subjected to stimulus in the proper direction, the proposed devices have 3D generation capabilities solving the problem of device orientation and placement in real applications. Thanks to their particular shape, the realized devices present more than one fundamental resonance frequencies in a range comprised between 500 Hz and 1.5 kHz, with a voltage generation higher than 300?V and an output power up to 0.4 pW for single MEMS device.

In this work the micro-fabrication of flexible MicroElectroMechanical transducers based on the piezoelectric effect is reported. We developed the technological protocol to realize a piezoelectric transducer composed by a Molybdenum (Mo) top electrode, the Aluminum Nitride active layer and a Mo bottom electrode on a polymeric tape. The process starts from the DC sputtering deposition of the Molybdenum layer at room temperature on Kapton HN. The Molybdenum is chosen not only for its electrical properties but also because it enhances the crystal orientation of AlN. The next step is the deposition of AlN that occurs at high temperature, around 250 degrees C. Temperature and physical sputtering enhanced by applying a DC bias on the substrates are two important parameters to improve the crystal orientation of the film. These extreme growth conditions guarantee a very good crystal structure without damaging the Kapton substrate. Then a final Mo layer is sputtered at room temperature. SU8-25 thick photo resist is used to define the top electrode and the AlN layer, and in a second mask step the Mo bottom electrode. The developer, the PG remover and SU8 negative resist itself have shown a chemical compatibility with Kapton HN. We measured the piezoelectric response on a capacitor test structure: through the Dynamic Mechanical Analyzer we applied controlled forces, and at the same time, by an LCR meter we performed measurements of the capacitance. (C) 2011 Elsevier B.V. All rights reserved.

In this paper, we analyze the birefringence effect in circular photonic crystals (CphCs). The studied CphCs are dielectric rings (DRs) and photonic crystals with cylindrical air holes arranged in circular patterns. The dielectric concentric circular patterns admit two preferred propagation directions defined by an extraordinary and an ordinary refractive index, representing two electric field polarizations. These electric fields are diffracted inside the crystal or are localized in a central microcavity region. We prove the induced artificial anisotropy in DRs through the geometrical equivalence with the corresponding thin-film multilayer structure. The equivalence is obtained through the geometrical synthesis of the wavefront propagation inside the artificial anisotropic structure. As applications, we analyze a Si/SiO2 DR Bragg reflector and a GaAs CphC microcavity resonator. The Bragg theory is validated by numerical time-domain approaches that are well suited to solve scattering problems. The microcavity resonance analysis and the Q-factor evaluation are performed by the finite-element method modeling.

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.

In this work, we analyze the pressure sensing of a thin film molybdenum/aluminumnitride/molybdenum (Mo/AlN/Mo) microwave/RF MEMS filter fabricated by a simple technology. After an experimental characterization in a frequency range between 1 and 36 GHz, we focused on the piezoelectric effect due to the stress properties of the piezoelectric AlN layer by applying forces by means of weights. Variations in the bandpass region of the microwave/RF filter are observed by proving high sensitivity also for low applied weights. We check by a properly designed three-dimensional (3D) finite-element method (FEM) tool the pressure-sensing property of the proposed device. Finally, we analyze the bad gap property of a chip with central defect around 40 GHz. © Copyright 2011 Cambridge University Press and the European Microwave Association.

Energy harvesting at low frequency is a challenge for microelectromechanical systems. In this workwe present a piezoelectric vibration energy harvester based on freestanding molybdenum Mo andaluminum nitride AlN ring-microelectromechanical-system RMEMS resonators. Thefreestanding ring layout has high energy efficiency due to the additional torsional modes which areabsent in planar cantilevers systems. The realized RMEMS prototypes show very low resonancefrequencies without adding proof masses, providing the record high power density of30.20 W mm-3 at 64 Hz with an acceleration of 2g. The power density refers to the volume of thevibrating RMEMS layout.

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.

This paper presents a self-rolled metalized InGaAs/GaAs bi-layer suitable to be used for magnetic field mapping applications. Numerical and preliminary experimental results will be presented demonstrating that the proposed micro-tube structure is an optimum candidate to be used as THz collector of electromagnetic energy.

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.

Herein we describe the realization of nanowalled polymeric microtubes through a novel andversatile approach combining the layer-by-layer (LbL) deposition technique, the self-rolling ofhybrid polymer/semiconductor microtubes and the subsequent removal of the semiconductortemplate. The realized channels were characterized in detail using scanning electron and atomicforce microscopes. Additionally, we report on the incorporation of a dye molecule within thenanowalls of such microtubes, demonstrating a distribution of the fluorescence signalthroughout the whole channel volume. This approach offers the possibility to tailor theproperties of micro/nanotubes in terms of size, wall thickness and composition, thus enablingtheir employment for several applications.

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

We report on the fabrication of an ultra-compact optical filter based on photonic crystal free-standing membranes in bi-layer configuration. The basic heterostructure consists of two 376 nm-thick GaAs-membranes sandwiched between air on a GaAs substrate. The air gap between the two membranes is 520 nm thick. The normal-incidence reflectance measurements and the numerical simulation of reflection spectra show a high sensitivity to the holes diameter

We demonstrate an ultracompact optical filter based on two coupled high-index contrast GaAs photonic crystal (PhC) membranes. The PhC membranes consist of a square lattice of air holes and behave as a FabryPerot cavity whose reflectivity and transmissivity depend on the air gap between the two membranes. The normal-incidence reflectance measurements and the numerical simulation of reflection spectra show a high sensitivity to the geometrical parameters, such as the distance between the slabs, whose control would make the device suitable for a new class of tunable optical filters.

In this paper it is reported a novel approach for the fabrication of polymeric microtubes based on the combination of semiconductor strain released thin films and Layer-by-Layer (LbL) deposition technique. The structure consisting of a LbL self-assembled polylectrolytes (PEs) film deposited onto a strained GaAs/InGaAs bilayer, was properly patterned and structured to enable the self rolling of an array of channels of different lengths. Then, the semiconductor film, acting as a sacrificial template, was selectively etched to obtain polymer microtubes. The so-realized polymeric channels were characterized in detail using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Additionally, such microtubes were analyzed by confocal microscopy to prove the successful incorporation of a dye molecule within the polymeric nanowalls.

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 report on the fabrication and characterization of stress-driven aluminum nitride (AlN) cantilevers to be applied as flow sensor for fish lateral line system. The fabricated structures exploit a multilayered cantilever AlN/molybdenum (Mo) and a Nichrome 80/20 alloy as piezoresistor. Cantilever arrays are realized by using conventional micromachining techniques involving optical lithography and etching processes. The fabrication of the piezoresistive cantilevers is reported and the operation of the cantilever as flow sensor has been investigated by electrical measurement under nitrogen flowing condition showing a sensitivity to directionality and to low value applied forces.

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.