We report on a variable capacitor that is formed between Schottky contacts and the two dimensional electron gas (2DEG) in a planar metal-semiconductor-metal structure. Device capacitance at low bias is twice the series capacitance of anode and cathode, enhancing to a maximum value, Cmax, at a threshold voltage, before reaching a minimum, Cmin, lower than the geometric capacitance of the coplanar contacts, thus resulting in ultra high Cmax=Cmin tuning ratio. Sensitivity, the normalized change of capacitance with voltage, is also very large. The dense reservoir of the 2DEG charge maintained between contacts is shown to be responsible for this remarkable performance.

Capacitance-Voltage characteristics of a metal-semiconductor-metal structure with an embedded two-dimensional electron system are reported. The device functions based on a quantum capacitance, which is activated by local illumination. The giant sensitivity to light and voltage makes this device a candidate for capacitive-based photodetection. © OSA 2013.

A metal-semiconductor-metal capacitor with embedded two-dimensional charge is designed and fabricated. Capacitance-Voltage characteristics exhibit switchability with a large voltage sensitivity. Maximum and minimum capacitances outperform previous predictions with potential applicability in RFICs and VLSI for reducing the cross-talk among transmission lines and achievement of higher integrations. The device can replace bulky conductors with its negative capacitance feature. The large light sensitivity in the C-V makes this capacitor an ideal candidate for monolithic microwave-photonic integrated circuits.

In this work we develop surface-micromachinedRF MEMS switches in III-V technology making use ofmaterials which can be alternative to the ones commonly used.In this way, some technological constraints concerning RFMEMS reliability can be overcome. Specifically, we evaluatethe potential of tantalum nitride (TaN) and tantalumpentoxide (Ta2O5) to be used for the switches actuation padsand dielectric layers, respectively. To this scope, acompositional, structural and electrical characterization ofTaN and Ta2O5 films as a function of the depositionparameters (temperature, sputtering mixture composition, andfilm thickness) is performed. Both shunt and series switchesare prepared and show good switching capabilities by apreliminary analysis. The complete device characterization isin progress and will be presented.

Moderation of internal quantum mechanical energies, such as exchange energy of an unconventional contact, comprised of a system of 2-D charge carriers, improves performance merits of variable capacitors, varactors, mainly in tuning ratio (TR), and sensitivity, S. Energy transfer from the unconventional contact to the dielectric increases the energy density and enhances the capacitance of the varactor. Here, we analyze the performance of an unconventional varactor based on a planar metal-semiconductor-metal (MSM) structure with an embedded layer of high-density 2-D electron gas (2DEG). Through localized field-assisted manipulation of the 2DEG density, a twice larger equilibrium capacitance and a minimum capacitance, less than the geometric capacitance of a conventional MSM, are achieved. Moreover, the maximum capacitance increases through a Batman-shaped capacitance enhancement at a threshold voltage. Therefore, giant TR > 2000 is attained while maintaining quality factors of up to 30. Capacitance-voltage characteristics exhibit a switched-capacitor behavior with S as high as 350 that is due to localized transitions from a dense 2DEG to a complete depletion. This MSM 2-D varactor combines the unconventional features of 2DEG with superior electrical properties of MSMs.

Capacitance of capacitors in which one or both platesare made of a two-dimensional charge system (2DCS) can be increasedbeyond their geometric structural value. This anomalouscapacitance enhancement (CE) is a consequence of manipulationof quantum mechanical exchange and correlation energies in theground state energy of the 2DCS. Macroscopically, it occurs atcritical charge densities corresponding to transition from an interacting"metallic" to a noninteracting "insulator" mode in the 2-Dsystem. Here, we apply this concept to a metal-semiconductor-metal capacitor with an embedded two-dimensional hole system(2DHS) underneath the plates for realization of a capacitancebasedphotodetector. Under sufficient illumination, and at criticalvoltages the device shows a giant CE of 200% and a peak-tovalleyratio of over 4 at probe frequencies larger than 10 kHz.Remarkably, the light-to-dark capacitance ratio due to CE atthis critical voltage is well over 40. Transition of the 2DHSfrom insulator to metallic, enforced by charge density manipulationdue to light-generated carriers, accounts for this behavior,which may be used in optical sensing, photo capacitors, and phototransistors.

Digital holography (DH) is a well-established interferometric tool in optical metrology allowing the investigation of engineered surface shapes with microscale lateral resolution and nanoscale axial precision. With the advent of charged coupled devices (CCDs) with smaller pixel sizes, high speed computers and greater pixel numbers, DH became a very feasible technology which offers new possibilities for a large variety of applications. DH presents numerous advantages such as the direct access to the phase information, numerical correction of optical aberrations and the ability of a numerical refocusing from a single hologram. Furthermore, as an interferometric method, DH offers both a nodestructive and no-contact approach to very fragile objects combined with flexibility and a high sensitivity to geometric quantities such as thicknesses and displacements. These features recommend it for the solution of many imaging and measurements problems, such as microelectro-optomechanical systems (MEMS/MEOMS) inspection and characterization. In this work, we propose to improve the performance of a DH measurement on MEMS devices, through digital filters. We have developed an automatic procedure, inserted in the hologram reconstruction process, to selectively filter the hologram spectrum. The purpose is to provide very few noisy reconstructed images, thus increasing the accuracy of the conveyed information and measures performed on images. Furthermore, improving the image quality, we aim to make this technique application as simple and as accurate as possible. © 2014 SPIE.

In this paper, shunt capacitive RF microelectromechanical systems (MEMS) switches are developed in III-V technology using tantalum nitride (TaN) and tantalum pentoxide (Ta2O5) for the actuation lines and the dielectric layers, respectively. A compositional, structural, and electrical characterization of the TaN and Ta2O5 films is preliminarily performed, demonstrating that they are valid alternatives to the conventional materials used in III-V technology for RF MEMS switches. Specifically, it is found that the TaN film resistivity can be tuned from 0.01 to 30 ohmcm . cm by changing the deposition parameters. On the other hand, dielectric Ta2O5 films show a low leakage-current density of few nanoamperes per square centimeter for E similar to 1 MV/cm, a high breakdown field of 4 MV/cm, and a high dielectric constant of 32. The realized switches show good actuation voltages, in the range of 15-20 V, an insertion loss better than -0.8 dB up to 30 GHz, and an isolation of similar to -40 dB at the resonant frequency, which is, according to bridge length, between 15 and 30 GHz. A comparison between the measured S-parameter values and the results of a circuit simulation is also presented and discussed, providing useful information on the operation of the fabricated switches.

We develop shunt capacitive RF MEMS switches in III-V technology making use of materials which can be alternative to the ones commonly used, in order to overcome some technological constraints concerning the RF MEMS reliability. Specifically, we evaluate the potential of tantalum nitride (TaN) and tantalum pentoxide (Ta2O5) to be used for the switches actuation pads and dielectric layers, respectively. To this scope, a compositional, structural and electrical characterization of TaN and Ta2O5 films as a function of the deposition parameters, such as the substrate temperature and the sputtering mixture composition, is performed. The realized switches show good actuation voltages, in the range 15- 20 V, an insertion loss better than -0.8 dB up to 30 GHz, and an isolation of ~ -40 dB at the resonant frequency. A comparison between the measured S-parameter values and the results of a circuit simulation is also presented and discussed, providing useful information on the operation of the fabricated switches.

Analogous to a drop exciting a wave in a reservoir that is detected more rapidly than the drop's transport by current flow, charge plasma confined in a semiconductor can transfer energy, hence respond much faster than the electric field-induced carrier drift current. Here we construct an optoelectronic device in which charge reservoirs respond to excitation with a speed that is impossible to achieve by transport of charge. In response to short optical pulses, this device produces electrical pulses that are almost 2 orders of magnitude shorter than the same device without the charge reservoirs. In addition to speed, the sensitivity of this process allowed us to measure, at room temperature, as low as 11 000 photons. These micro plasma devices can have a range of application such as optical communication with a fraction of a microwatt power compared to the present tens of milliwatts, ultrasensitive light detection with cryogenic cooling, photovoltaic devices capable of harvesting dim light, THz radiation detectors, and charged particle detectors.

In this work, the bridge imaging and the reliability of surface-micromachined capacitive RF MEMS switches in III-V technology are presented. A low cost scanning technique allowed us to image the shape of the moveable bridge with a micrometer spatial resolution, thus quantitatively valuating its lowering as a function of the applied voltage. The reliability of the switches was tested under the application of different unipolar and bipolar voltage waveforms, showing that a significant improvement of the switch operation and lifetime can be achieved by applying high frequency bipolar square pulses with suitable durations. © 2013 CMP.

Digital holographic microscopy is an important interferometric tool in optical metrology allowing the investigation of engineered surfaces with microscale lateral resolution and nanoscale axial precision. In particular, microelectromechanical systems (MEMS) surface analysis, conducted by holographic characterization, requires high accuracy for functional testing. The main issues related to MEMS inspection are the superficial roughness and the complex geometry resulting from the several fabrication steps. Here, an automatic procedure, particularly suited in the case of high-roughness surfaces, is presented to selectively filter the spectrum, providing very low-noise reconstructed images. The numerical procedure is based on Butterworth filtering, and the obtained results demonstrate a significant increase in the images' quality and in the accuracy of the measurements, making our technique highly applicable for quantitative phase imaging in MEMS analysis. Furthermore, our method is fully tunable to the spectrum under investigation and automatic. This makes it highly suitable for real-time applications. Several experimental tests show the suitability of the proposed approach. (C) 2015 Optical Society of America

Shunt capacitive radio-frequency microelectromechanical system (RF MEMS) switches were fabricated on silicon substrate and characterized in the RF domain. Various switch typologies were obtained by three different approaches, which are: (1) the change of the bridge geometric parameters, (2) the covering of the actuator with a floating metal, and (3) the deposition of the bridge directly on the actuator. The S parameters of the fabricated switches were measured in the up and down states, observing the impact on the RF performance of the variation of the geometric parameters and the fabrication process. The electromagnetic modelling of the fabricated switches was used to interpret the measured RF behaviour, allowing to elucidate the drawbacks of the non-perfect conforming of the bridge on the actuator. Finally, the reliability of the fabricated RF MEMS switches under a bipolar voltage excitation was evaluated by cycling tests. Hence, the study presented here provides guidelines to solve some issues of the tight correlation between design, fabrication, performance, and reliability of RF MEMS switches, in view of a large-scale development of these devices.

Shunt capacitive RF MEMS switches were developed on GaAs substrate, using a III-V technology process that is fully compatible with standard MMIC fabrication. The switches show an insertion loss lower than 0.8 dB and isolation better than 30 dB with resonance frequencies in K-band, according to the switch geometric parameters. Reliability limits due to dielectric charging were overcome by applying suitable fast bipolar actuation waveforms, making the developed switches good candidates for both redundancy (always on/off) and cycled applications.

The electrostatic actuation behaviour of the gold bridge in capacitive radio frequency microelectromechanical system switches, fabricated on GaAs substrate, is investigated. An unconventional imaging technique, based on the out-of-focus reflection, was used to evaluate the topographic profile of the suspended bridge and its lowering as a function of the voltage. Important parameters for the switch actuation, such as the pull-down voltage and the air gap between the bridge and the actuator, are estimated. Capacitance-voltage curves allow to evaluate the capacitance associated to the bridge in the up and down states as well as the dielectric constant of the Si<inf>3</inf>N<inf>4</inf> layer, which covers the actuator. The experimental values of the pull-down voltage and the dielectric constant are used to extract from the theoretical equations the residual stress of the fabricated gold membrane. Finally, the current through the dielectric Si<inf>3</inf>N<inf>4</inf> layer was measured as a function of the voltage applied to the actuator, finding that the Poole-Frenkel effect is the dominant conduction mechanism when the switch is actuated.

Incorporating bi-layer electron and hole plasma, we produce a novel planar, top illuminated photodetector with time response of < 2.5 ps sensitive to fraction of ?W optical power, and capable of operation without applied bias. © OSA 2013.

Low temperature growth of GaAs (LT-GaAs) near 200 °C results in a recombination lifetime of nearly 1 ps, compared with approximately 1 ns for regular temperature ~600 °C grown GaAs (RT-GaAs), making it suitable for ultra high speed detection applications. However, LT-GaAs detectors usually suffer from low responsivity due to low carrier mobility. Here we report electro-optic sampling time response measurements of a detector that employs an AlGaAs heterojunction, a thin layer of LT-GaAs, a channel of RT-GaAs, and a vertical electric field that together facilitate collection of optically generated electrons while suppressing collection of lower mobility holes. Consequently, these devices have detection efficiency near that of RT-GaAs yet provide pulse widths nearly an order of magnitude faster-~6 ps for a cathode-anode separation of 1.3 ?m and ~12 ps for distances more than 3 ?m. © 2013 by the authors; licensee MDPI, Basel, Switzerland.

The optical response by NO2 gas adsorption at different concentrations has been investigated, at room temperature, in ZnO nanostructured films grown by controlled vapor phase deposition. The variation (quenching) in the photoluminescence signal from excitonic and defects bands, due to the interactions between the oxidizing gas molecules and the sample surface, has been detected and dynamic responses and calibration curves as a function of gas concentration have been obtained and analyzed for each band. We showed that the sensing response results larger in excitonic band than in defect one and that the emission signal rises from two different quenchable and unquenchable states. A simple model was proposed in order to explain the quenching processes on the emission intensity and to correlate them to the morphological features of the samples. Finally, the reversibility of the quenching effects has also been tested at high gas concentration. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3700251]

In this paper, the reliability of shunt capacitive radiofrequency microelectromechanical systems switches developed onGaAs substrate using a III-V technology fabrication process,which is fully compatible with standard monolithic microwaveintegrated circuit fabrication, is investigated. A comprehensivecycling test is carried out under the application of different unipolarand bipolar polarization waveforms in order to infer howthe reliability of the realized capacitive switches, which is stilllimited with respect to the silicon-based devices due to the lessconsolidation of the III-V technology, can be improved. Underthe application of unipolar waveforms, the switches show a shortlifetime and a no correct deactuation for positive pulses longerthan ~10 ms probably due to the charging phenomena occurringin the dielectric layer underneath the moveable membrane. Thesecharging effects are found to vanish under the application ofa waveform including consecutive positive and negative voltagepulses, provided that proper durations of the positive and negativevoltage pulses are used. Specifically, a correct switch deactuationand a lifetime longer than 1 million cycles, being this value limitedby the duration of the used testing excitation, are achieved byapplying a 1-kHz waveform with 20-?s-long positive and negativeconsecutive pulses.

RF Micro-Electro Mechanical Systems (MEMS) have been developed during the last decade withseveral applications for commercial as well as space and military sub-systems [1-6]. Technologicalprocesses encompassing the utilization of typical microelectronic substrates like alumina, GaAs andSilicon have been considered for the realization of micro-switches by surface micromachining. Truetime delay lines (TTDL), single port multiple throw (SPMT), matrices and phase shifters takebenefit from the low insertion loss, all passive and distortion-free environment offered by RFMEMS. Antenna re-configuration through digital phase shifters based on RF MEMS switches arecurrently studied.

The objective of this paper is to study the morphology, structure, and composition, as well as the thermal-induced morphological, structural, and chemical changes of copper(Cu)/titanium nitride(TiN) bilayers versus Cu single layers, deposited on silicon substrates for microelectronic applications. These characterizations aimed to assess the reliability of Cu metallization for local interconnect and to investigate the barrier capability of TiN against Cu diffusion into the silicon (Si) substrate. Moreover, this paper provides a fundamental study of the temperature-induced interactions between Cu and Si, intermediated by the presence of a thin TiN layer. Cu thin films were sputtered onto Si substrates, with and without the interposition of thin TiN layers, and were successively annealed at temperature as high as 600. C. Different nitrogen flux percentages in the sputtering mixture (Ar + N-2) were used for the deposition of the barriers. X-ray diffraction (XRD) analyses were carried out in order to study the structural evolution of the layers, before and after the annealing. Scanning electron microscopy (SEM) observations gave information about the surface and cross section morphology, and spatially resolved energy dispersive X-ray Spectroscopy (EDS) profiles provided chemical information about the cross-sectional distribution of the atomic species and their possible interdiffusion. The barrier efficacy has been demonstrated by comparing the morphological and chemical modifications of the annealed Cu layers, with and without the presence of the TiN layer, and their effects on the electrical properties of the Cu films.

In this work, the evolution of the Au assisted-growth of ZnO nanorods deposited by vapour phase deposition both on sapphire and on indium-tin-oxide on glass (ITO-glass) substrates has been studied. Our investigation demonstrates that the growth proceeds first as a 3D growth, giving rise to a buffer layer, few microns thick, formed by ZnO grains with different orientation. Then a 1D transition occurs with the nucleation of a dense array of vertically aligned nanorods. A different degree of crystalline order and nanorods alignment was found between the samples grown on ITO-glass and sapphire substrates, which was ascribed to the different morphology that the Au seed layer acquires on the two different substrates. A semi-quantitative analysis of the ZnO crystalline orientation was carried out by X-ray diffraction (XRD) measurements performed at fixed incidence configuration and supported by high resolution scanning electron microscopy (HR-SEM) investigations on focused ion beam (FIB) prepared cross-sections.

Shunt capacitive RF MEMS switches have been developed using III-V technology and employing (tantalum pentoxide) Ta2O5 thin films as dielectric layers. In order to evaluate the potential of the Ta2O5 thin films for the considered application, the compositional, structural, and electrical characterization of the deposited films has been performed, demonstrating that they are good candidates to be used as dielectric layers for the fabrication of RF MEMS switches. Specifically, Ta2O5 films are found to show a leakage current density of few nA/cm2 for MV/cm and a high dielectric constant of 32. Moreover, the charging process has been investigated, finding that it follows a stretched exponential law. The fabricated switches show actuation voltages in the range 15-20 V, an insertion loss better than -0.8 dB up to 30 GHz, and an isolation of ~-40 dB at the resonant frequency which is around 25 GHz.

Shunt capacitive radio-frequency microelectromechanical (RF MEMS) switches were modelled, fabricated and characterized in the K-band domain. Design allowed to predict the RF behaviour of the switches as a function of the bridge geometric parameters. The modelled switches were fabricated on silicon substrate, using a surface micromachining approach. In addition to the geometric parameters, the material structure in the bridge-actuator area was modified for switches fabricated on the same wafer, thanks to the removal/addition of two technological steps of crucial importance for RF MEMS switches performance, which are the use of the sacrificial layer and the deposition of a floating metal layer on the actuator. Surface profilometry analysis was used to check the material layer structure in the different regions of the bridge area as well as to investigate the mechanical behaviour of the moveable bridge under the application of a loaded force. The RF behaviour of all the fabricated switches was measured, observing the impact on the isolation of the manipulation of the bridge size and of the variations in the fabrication process.

The potential of sputtered Ta2O5 thin films to be used as dielectric layers in capacitive radio frequency microelectromechanical system switches is evaluated by investigating two factors of crucial importance for the performance of these devices which are the transport mechanisms and the charging effects in the dielectric layer. We find that Ta2O5 films show good electrical and dielectrical properties for the considered application in terms of a low leakage current density of 4 nA/cm(2) for E=1 MV/cm, a high breakdown field of 4 MV/cm and a high dielectric constant of 32. For electric fields lower than 1 MV/cm the conduction mechanism is found to be variable-range hopping in the temperature range 300-400 K, while nearest-neighbor hopping is observed at higher temperatures. For fields in the range 1-4 MV/cm Poole-Frenkel becomes the dominant conduction mechanism. Current and capacitance transients used to investigate the charging effects show a decay which is well described by the stretched-exponential law, thus providing further insights on capture and emission processes.

In this work, a thin-film packaging was developedto be used for radio-frequency microelectromechanicalsystem configurations. The fabricated packages aresuspended membranes in the multilayer SixNy/aSi/SixNy onconductive coplanar waveguides (CPWs) of differentlength. Several geometric parameters of the membranes,which are the length, the curvature radius at the vertices ofthe rectangular base, the density and the diameter of holeson the capping surface, were also varied. The mechanicalproperties of the suspended membranes were investigatedby mechanical simulations and surface profilometry measurementsas a function of the geometric parameters. RFcharacterization was performed to evaluate the impact ofthe package on the CPW performance. Finally, networkanalysis was carried out, allowing to clarify the origin ofthe RF losses measured for the fabricated microdevices.

In this work, thin film packages were developed for radio-frequency microelectromechanical system (RF MEMS) configurations. The fabricated packages are suspended membranes in the multilayer SixNyHz/aSi/SixNyHz on conductive coplanar lines of different length. Several geometric parameters of the membranes, which are the length, the curvature radius at the vertices of the rectangular base, the density and the diameter of holes, were also varied. The mechanical properties of the suspended membranes were investigated by surface profilometry as a function of the geometric parameters. Finally, the RF characterization was performed to evaluate the impact of the package on the coplanar line performance. Hence, the proposed study provides results of crucial importance for the application of thin-film suspended microstructures for the packaging of RF MEMS devices.