Anna Persano

Photoconduction Properties in Aligned Assemblies of Colloidal CdSe/CdS Nanorods

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.

Capacitive RF MEMS Switches With Tantalum-Based Materials

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 Ω · 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 ~ 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 ~-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.

Reliability Enhancement by Suitable Actuation Waveforms for Capacitive RF MEMS Switches in III–V Technology

In this paper, the reliability of shunt capacitive radio frequency microelectromechanical systems switches developed on GaAs substrate using a III-V technology fabrication process, which is fully compatible with standard monolithic microwave integrated circuit fabrication, is investigated. A comprehensive cycling test is carried out under the application of different unipolar and bipolar polarization waveforms in order to infer how the reliability of the realized capacitive switches, which is still limited with respect to the silicon-based devices due to the less consolidation of the III-V technology, can be improved. Under the application of unipolar waveforms, the switches show a short lifetime and a no correct deactuation for positive pulses longer than 10 ms probably due to the charging phenomena occurring in the dielectric layer underneath the moveable membrane. These charging effects are found to vanish under the application of a waveform including consecutive positive and negative voltage pulses, provided that proper durations of the positive and negative voltage pulses are used. Specifically, a correct switch deactuation and a lifetime longer than 1 million cycles, being this value limited by the duration of the used testing excitation, are achieved by applying a 1-kHz waveform with 20-μs-long positive and negative consecutive pulses.

Polarization anisotropy of individual core/shell GaAs/AlGaAs nanowires by photocurrent spectroscopy

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 ρ (varying from ∼0.1 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.

Charge carrier transport in thin films of colloidal CdSe quantum rods

Phototransport properties of organically capped colloidal CdSe quantum rod thin films deposited by spin coating are studied in air at room temperature in planar electrode configuration. Under optical excitation, the observed current-voltage characteristics and current transients are well described by a resonant tunneling model. A significant and irreversible current quenching of the photoresponse occurs with either the aging of the samples or the flowing of the current itself when above few picoamperes. The process, which is still interpreted in the frame of the model, can be attributed to the charge trapping by the defect states at the barrier between rods with a consequent increase in the barrier height.

Transport and charging mechanisms in Ta2O5 thin films for capacitive RF MEMS switches application

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/cm2 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, ...

A highly tunable heterostructure metal-semiconductor-metal capacitor utilizing embedded 2-dimensional charge

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.

An Unconventional Hybrid Variable Capacitor With a 2-D Electron Gas

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 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.

Anomalous Capacitance Enhancement Triggered by Light

Capacitance of capacitors in which one or both plates are made of a two-dimensional charge system (2DCS) can be increased beyond their geometric structural value. This anomalous capacitance enhancement (CE) is a consequence of manipulation of quantum mechanical exchange and correlation energies in the ground state energy of the 2DCS. Macroscopically, it occurs at critical charge densities corresponding to transition from an interacting “metallic” to a noninteracting “insulator” mode in the 2-D system. 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 capacitance-based photodetector. Under sufficient illumination, and at critical voltages the device shows a giant CE of 200% and a peak-to-valley ratio of over 4 at probe frequencies larger than 10 kHz. Remarkably, the light-to-dark capacitance ratio due to CE at this critical voltage is well over 40. Transition of the 2DHS from insulator to metallic, enforced by charge density manipulation due to light-generated carriers, accounts for this behavior, which may be used in optical sensing, photo capacitors, and photo transistors.

Photocurrent properties of single GaAs/AlGaAs core–shell nanowires with Schottky contacts

Conductivity and photoconductivity properties of individual GaAs/AlGaAs core-shell nanowires (NWs) are reported. The NWs were grown by Au-assisted metalorganic vapor phase epitaxy, and then dispersed on a substrate where electrical contacts were defined on the individual NWs by electron beam induced deposition. Under dark conditions, the carrier transport along the NW is found to be limited by Schottky contacts, and influenced by the presence of an oxide layer. Nonetheless, under illumination, the GaAs/AlGaAs core-shell NW shows a significant photocurrent, much higher than the bare GaAs NW. The spatial dependence of the photocurrent within the single core-shell NW, evaluated by a mapping technique, confirms the blocking behavior of the contacts. Moreover, local spectral measurements were performed which allow one to discriminate the contribution of carriers photogenerated in the core and in the shell.