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.

A method for reconstructing the spatial profile of the electric field along the thickness of a generic bulk solid-state photodetector is proposed. Furthermore, the mobility and lifetime of both electrons and holes can be evaluated contextually. The method is based on a procedure of minimization built up from current transient profiles induced by laser pulses in a planar detector at different applied voltages. The procedure was tested in CdTe planar detectors for X- and Gamma rays. The devices were measured in a single-carrier transport configuration by impinging laser light on the sample cathode. This method could be suitable for many other devices provided that they are made of materials with sufficiently high resistivity, i.e., with a sufficiently low density of intrinsic carriers. (C) 2014 AIP Publishing LLC.

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.

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.

The perturbation behaviour of Ohmic and Schottky CdTe detectors under strong optical pulses is investigated. To this scope, the electric field profiles and the induced charge transients are measured, thus simultaneously addressing fixed and free charges properties, interrelated by one-carrier trapping. The results elucidate the different roles of the contacts and deep levels, both under dark and strong irradiation conditions, and pave the way for the improvement of detector performance control under high X-ray fluxes.

We study charge transients induced by optical pulses in CdTe detectors. Different carrier dynamics and collection properties are observed in the signals induced by optical excitation in the wavelength range 500-1650 nm, depending on the absorption and on the transport mechanism involved. A systematic comparison between charge transients, by irradiation through cathode and anode contacts, allows to point out the role of defects near the surface, instability effects, deep level transitions into the bulk, and internal photoelectric effects at the contacts.

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.

Schottky CdTe X-ray detectors exhibit excellent spectroscopic performance but suffer from instabilities. Hence it is of extreme relevance to investigate their electrical properties. A systematic study of the electric field distribution and the current flowing in such detectors under optical perturbations is presented here. The detector response is explored by varying experimental parameters, such as voltage, temperature, and radiation wavelength. The strongest perturbation is observed under 850 nm irradiation, bulk carrier recombination becoming effective there. Cathode and anode irradiations evidence the crucial role of the contacts, the cathode being Ohmic and the anode blocking. In particular, under irradiation of the cathode, charge injection occurs and peculiar kinks, typical of trap filling, are observed both in the current-voltage characteristic and during transients. The simultaneous access to the electric field and the current highlights the correlation between free and fixed charges, and unveils carrier transport/collection mechanisms otherwise hidden.

CdTe Schottky diodes for X- and ?-ray detection exhibit excellent spectroscopic performance, even though these are not stable under operative voltages. Improvements require the comprehension of the main carrier transport mechanisms, presently unclear. We address this issue by correlating the internal electric field and the flowing current. Depending on the temperature and applied voltage, different mechanisms become dominant where the deep levels always play a central role. Indeed, the partial ionization of deep levels directly controls the electric field distribution. Transient measurements show how, under high voltages, the field at the contacts controls the current flowing through the detector. © 2013 American Institute of Physics.

In this paper we report on the effects of the insertion of Cr atoms on the electrical and optical properties of indium tin oxide (ITO) films to be used as electrodes in spinpolarized light-emitting devices. ITO films and ITO(80 nm)/Cr-doped ITO(20 nm) bilayers and Cr-doped ITO films with a thickness of 20 nm were grown by pulsed ArF excimer laser deposition. The optical, structural, morphological wand electrical properties of ITO films and ITO/Cr-doped structures were characterized by UV-Visible transmission and reflection spectroscopy, transmission electron microscopy (TEM), atomic force microscopy (AFM) and Hall-effect analysis. For the different investigations, the samples were deposited on different substrates like silica and carbon coated Cu grids. ITO films with a thickness of 100 nm, a resistivity as low as similar to 4 x 10(-4) Omega cm, an energy gap of similar to 4.3 eV and an atomic scale roughness were deposited at room temperature without any post-deposition process. The insertion of Cr into the ITO matrix in the upper 20 nm of the ITO matrix induced variations in the physical properties of the structure like an increase of average roughness (similar to 0.4-0.5 nm) and resistivity (up to similar to 8x10(-4) Omega cm). These variations were correlated to the microstructure of the Cr-doped ITO films with particular attention to the upper 20 nm.

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.

The electronic transport and gating characteristics in GaAs and Ge nanowires (NWs) are altered significantly following either indirect or direct exposure to a focused Ga+ ion beam (FIB), such as that used to produce Pt electrical contacts to NWs. While these results challenge the assumptions made in some previously reported work relating to the electronic properties of semiconductor NWs using FIB-assisted production of contacts and/or their leads, local electron beam induced deposition is shown to be a reliable and facile route for producing robust electrical contacts to individual vapor phase-grown NWs in a manner that enables study of their actual carrier transport properties.

Simultaneous photoluminescence (PL) and external quantum efficiency (EQE) confocal mapping is used to investigate the correlation between the local PL and the EQE in a regioregular poly(3-exylthiophene): poly(9,9-dioctylfluorene-co-benzothiadiazole) inverted bulk heterojunction solar cell. We show that the charge generation and charge collection are strongly non-uniform on a length scale up to 100 mu m. Our results evidence that organic solar cells optimization requires not only the control of the submicrometric active materials arrangement but also the control of the large scale device uniformity.

We report on the transmission of a terahertz (THz) radiation through prototype structures based on a p-type silicon substrate. In particular, the bare substrate and progressively more complicated multilayer structures were investigated, allowing to address the effect on the transmission of different factors, such as the orientation of interdigitated contacts with respect to the polarized beam, the temperature, and the current flowing through a conductive SnO2 nanorods layer. A suitable experimental set-up was developed for the direct spectral measurement of transmission in the range of 0.75-1.1 THz at room and low temperatures. A simple Drude-Lorentz model was formulated, finding a quantitative agreement with the experimental transmission spectrum of the bare substrate at room temperature. For the multilayer structures, the spectra variations observed with temperature are well accounted by the corresponding change of the mobility of holes in the silicon p-type substrate. The influence of the contact orientation is consistent with that of a polarizing metallic grating. Finally, Joule heating effects are observed in the spectra performed as a function of the current flowing through the SnO2 nanorods layer. The experimental results shown here, together with their theoretical interpretation, provide insights for the development of devices fabricated on conductive substrates aimed to absorb/modulate radiation in the THz range. (C) 2014 AIP Publishing LLC.

1 MeV copper ions were implanted in polycarbonate (PC) matrices with fluences ranging from 5 x 10(13) ions cm(-2) to 1 x 10(17) ions cm(-2) in order to modify the optical and electrical properties of the polymer host. Increasing the ion fluence, an increase of the overall absorption and a redshift of the optical band gap were observed, from the initial value of 3.40 eV for the pristine PC to 0.80 eV measured for 1 x 10(17) ions cm(-2). For fluences above 5 x 10(14) ions cm(-2) a broad optical absorption bands at 450-475 nm and 520 nm were observed and, from 1 x 10(16) ions cm(-2), an additional band appeared at 570 nm. Both bands redshift when the fluence is increased. On the contrary, the optical response of the highest fluence sample is characterized by an overall band at 580 nm. The chemical modifications observed in the polymer range from induced -OH stretching, C=O and -C=C- double bonds and -C C and CH triple bonds formation, as the ion fluence increases. The implantation process affects the electrical properties of the polymer leading to a strong reduction in sheet resistance when ion fluence exceeds 5 x 10(16) ions cm(-2). A value of similar to 7.1 x 10(7) Omega/sq has been obtained for the highest fluence, i.e. about 10 order of magnitude lower than the pristine PC.

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.

We report on photoconduction and optical properties of aligned assemblies of core-shell CdSe/CdS nanorods prepared by a seeded growth approach. We fabricate oriented layers of nanorods by drop casting the nanorods from a solution on substrates with prepatterned, micrometer-spaced electrodes and obtain nanorod alignment due to the coffee stain effect. The photoconductivity of the nanorod layers can be improved significantly by an annealing process under vacuum conditions. The spectral response of the photocurrent shows distinct features that can be assigned to the electronic level structure of the core-shell nanorods and that relate well to the spectra obtained by absorption measurements. We study assemblies of nanorods oriented parallel and perpendicular to the applied electric field by the combined use of photocurrent and photoluminescence spectroscopy. We obtain consistent results which show that charge carrier separation and transport are more efficient for nanorods oriented parallel to the electric field. We also investigate the light polarization sensitivity of the photocurrent for the oriented nanorod layers and observe higher conductivity in the case of perpendicular polarization with respect to the long axis of the nanorods.

The photoconduction properties of individual GaAs/AlGaAs core-shell nanowires under uniform and local optical excitation are investigated, allowing the external quantum efficiency, the polarization anisotropy, and the role of the nanocontacts to be valuated. © OSA 2013.

Conductivity and photoconductivity properties of individual GaAs/AlGaAs core-shellnanowires (NWs) are reported. The NWs were grown by Au-assisted metalorganic vaporphase epitaxy, and then dispersed on a substrate where electrical contacts were defined on theindividual NWs by electron beam induced deposition. Under dark conditions, the carriertransport along the NW is found to be limited by Schottky contacts, and influenced by thepresence of an oxide layer. Nonetheless, under illumination, the GaAs/AlGaAs core-shell NWshows a significant photocurrent, much higher than the bare GaAs NW. The spatialdependence of the photocurrent within the single core-shell NW, evaluated by a mappingtechnique, confirms the blocking behavior of the contacts. Moreover, local spectralmeasurements were performed which allow one to discriminate the contribution of carriersphotogenerated in the core and in the shell.

Tetrapod-shaped CdSe(core)/CdTe(arms) colloidal nanocrystals, capped with alkylphosphonic acids or pyridine, were reacted with various small molecules (acetic acid, hydrazine and chlorosilane) which induced their tip-to-tip assembly into soluble networks. These networks were subsequently processed into films by drop casting and their photoconductive properties were studied. We observed that films prepared from tetrapods coated with phosphonic acids were not photoconductive, but tip-to-tip networks of the same tetrapods exhibited appreciable photocurrents. On the other hand, films prepared from tetrapods coated with pyridine instead of phosphonic acids were already highly photoconductive even if the nanocrystals were not joined tip-to-tip. Based on the current-voltage behavior under light we infer that the tunneling between tetrapods is the dominant charge transport mechanism. In all the samples, chemically-induced assembly into networks tended to reduce the average tunneling barrier. Additionally, pyridine-coated tetrapods and the tip-to-tip networks made out of them were tested as active materials in hybrid photovoltaic devices. Overall, we introduce an approach to chemically-induced tip-to-tip assembly of tetrapods into solution processable networks and demonstrate the enhancement of electronic coupling of tetrapods by various ligand exchange procedures.

We investigate the photodetection properties of individual core/shell GaAs/AlGaAs nanowires (NWs) and, in particular, their behavior under linearly polarized light. The NWs are grown by Au-assisted metalorganic vapor phase epitaxy and electrical contacts are defined on NWs by electron beam induced deposition. The spectral photocurrent of the single NW is measured and the dependence of the polarization anisotropy rho (varying from similar to 0.1 to similar to 0.55) on the absorption wavelength is found to be clearly affected by the core/shell structure. High quantum efficiency values (10% at 600 nm) are obtained which are attractive for a wide range of optoelectronic devices.

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.

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.

The potential of InAs quantum-dot (QD) photodetectors for room-temperature high-speed operation at wavelengths near 1.3 mu m is evaluated. Specifically, planar metal-semiconductor-metal structures on GaAs substrates containing one absorption layer of self-assembled InAs QDs embedded in Ga(In)As matrices are fabricated, characterized, and analyzed. Light absorption, optically generated carrier transport, and collection mechanisms are studied. The role of the QD embedding matrix in the lateral transport of the photogenerated carriers is also studied by comparing structures with QDs in GaAs and In0.15Ga0.85 As matrices. Devices show low dark currents in tens of nanoamperes and high light sensitivity when adjusted to QD volumes, whereas external quantum efficiency remains in the range 10(-5)-10(-4) for all fabricated samples. The time response of the fabricated devices is obtained using an excitation wavelength resonant with QD interband transitions, thus allowing the photogeneration of electron-hole pairs inside the dots. Results prove detection capability of a single layer of QDs in a common photodetector structure with a full-width half-maximum time response on the order of 10 ps. A long tail, about 100 ps, but at a small fraction of the peak response amplitude, is also observed, suggesting mechanisms for charge transport and collection.

Great efforts are being presently devoted to the development of CdTe and CdZnTe detectors for a large variety of applications, such as the basic, medical, industrial, and space research.The purpose of this work is to present the spectroscopic properties of some CZT crystals grown by the boron oxide encapsulated vertical Bridgman method, which has been recently implemented at IMEM-CNR. By this technique the crystal, during the growth, is fully encapsulated by a thin layer of liquid boron oxide, so that the crystal-crucible contact is prevented, thus allowing larger single grains with lower dislocation density to be obtained.Several detectors were realized about 4 mm x 4 mm x 1 mm in size, with two planar gold contacts on both the surfaces realized by an electroless technique.The behavior of these detectors was studied as a function of the bias voltage, irradiation geometry and energy of the interacting photons. Good electron charge collection properties have been demonstrated and electric field distribution has been investigated using the Pockels effect.

The efficiency optimization of bulk heterojunction solar cells requires the control of the local active materials arrangement in order to obtain the best compromise between efficient charge generation and charge collection. Here, we investigate the large scale (10-100 ?m) inhomogeneity of the photoluminescence (PL) and the external quantum efficiency (EQE) in inverted all-polymer solar cells (APSC) with regioregular poly(3-hexylthiophene) (P3HT):poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) active blends. The morphology and the local active polymer mixing are changed by depositing the active layer from four different solvents and by thermal annealing. The simultaneous PL and EQE mapping allowed us to inspect the effects of local irregularities of active layer thickness, polymer mixing, polymer aggregation on the charge generation and collection efficiencies. In particular, we show that the increase of the solvent boiling point affects the EQE non-uniformity due to thickness fluctuations, the density non-uniformity of rrP3HT aggregate phase, and the blend components clustering. The thermal annealing leads to a general improvement of EQE and to an F8BT clustering in all the samples with locally decrease of the EQE. We estimate that the film uniformity optimization can lead to a total EQE improvement between 2.7 and 6.3 times.

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.