Adriano Cola

Increase of charge carriers density and reduction of Hall mobilities in oxygen-plasma treated indium–tin–oxide anodes

We report investigations of the electronic transport properties carried out by means of the Hall technique for indium–tin–oxide thin films on glass after a variety of surface treatments. We find that oxygen-plasma treatments induce a significant increase in the carrier concentration, and a less significant decrease of mobilities with respect to “as-received” or aquaregia treated substrates. We consider that this is indicative of an increased concentration of defects, as a result of the plasma exposure.

The polarization mechanism in CdTe Schottky detectors

Schottky CdTe nuclear detectors are affected by bias-induced polarization phenomena when operating at room temperature. A space charge buildup occurs at the blocking contact causing the degradation in detection performance. By means of Pockels effect, we study the electric field distribution inside the detector and its variation with time and temperature. The analysis of the space charge has allowed us to point out the role of the Schottky contact and of carrier detrapping from deep levels in the polarization mechanism. Moreover, measured current transients have been quantitatively accounted for by the increase in the electric field at the blocking junction.

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.

Electric Field Properties of CdTe Nuclear Detectors

Recently, diode-like In/CdTe/Pt detectors have been realized which look very promising thanks to their enhanced spectroscopic performance. Scope of this work is to investigate the electric field distribution inside these detectors and its temporal evolution by means of the Pockels effect. The implemented set-up allows us to map the electric field and thus to extract the field profiles between the contacts. The analysis shows that the field is mainly confined below the anode. After applying the bias, these detectors are not very stable at room temperature, and they present a pronounced degradation in spectroscopic performance. This degradation is correlated in this work to the evolution of the electric field which has been observed to dasiamovepsila towards the anode. We attribute this dasiapolarizationpsila behavior in CdTe detectors to the concomitance of two factors: the presence of deep levels in the bulk material, and the high hole barrier height of In on CdTe.

Study on Instability Phenomena in CdTe Diode-Like Detectors

Diode-like In/CdTe/Pt detectors are widely used thanks to their excellent spectroscopic performance. However, when operated at room temperature they are not stable, and their performance degrades with time. The aim of this paper is to investigate in detail the physical mechanisms underlying this effect, by studying the evolution of the space charge inside the detector. Our approach makes use of the Pockels effect, by looking at the evolution of the electric field distribution inside the detector at different temperatures. The results show that a negative space charge accumulation occurs at the anode and that the process is thermally activated. The effect is attributed to a midgap acceptor which, under reverse bias, increases its ionization because of hole detrapping. Moreover, we compare the results with those obtained on stable detectors realized from nominally same crystals but having ldquostandard,rdquo i.e., not rectifying contacts. Consistently with the detection performance stability of this kind of detectors, the electric field is relatively stable. Furthermore, it decreases from cathode and it shows peculiar features which will be discussed in conjunction with charge transport properties.

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.

Field-assisted capture of electrons in semi-insulating GaAs

We present a drift-diffusion modeling of the electric-field profile in semi-insulating n-GaAs detectors accounting for hot-carrier dynamics and the associated kinetics of electrical active traps. From the fitting of the detector active thickness we infer unambiguous evidence of a field-enhanced capture cross section from the two deepest electron traps we attribute to EL2 and EL3 centers.

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.

Electric fields and dominant carrier transport mechanisms in CdTe Schottky detectors

CdTe Schottky diodes for X- and γ-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.

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.