Shunt capacitive RF MEMS switches have been developed using III-V technology and employing (tanta... more Shunt capacitive RF MEMS switches have been developed using III-V technology and employing (tantalum pentoxide) Ta 2 O 5 thin films as dielectric layers. In order to evaluate the potential of the Ta 2 O 5 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, Ta 2 O 5 films are found to show a leakage current density of few nA/cm 2 for E ∼ 1 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.
In this paper we describe the characterization of a family of optical devices based on heterojunc... more In this paper we describe the characterization of a family of optical devices based on heterojunction and heterodimensional structures and we investigate their static and dynamic properties. Such devices are good candidates, due to their high performance, for utilization as the sensing element for the realization of sensors in the fields of telecommunications, remote sensing, LIDAR and medical imaging. First, we present a Heterostructure Metal-Semiconductor-Metal (HMSM) photodetectors that employ a uniformly doped GaAs/AlGaAs heterojunction for the dual purpose of barrier height enhancement and creating an internal electric field that aids in the transport and collection of the photogenerated electrons. In this first family of devices, two doping levels are compared showing the direct effect of the aiding field due to modulation doping. Subsequently, we analyse a novel Resonant-Cavity-Enhanced (RCE) HMSM photodetector in which a Distributed Bragg Reflector (DBR) is employed in order to reduce the thickness of the absorption layer thus achieving good responsivity and high speed as well as wavelength selectivity. Current-voltage, photocurrent spectra and highspeed time response measurements point out the better performance of this last family of detectors, as they can operate in tens of Giga-Hertz range with low dark current and high responsivity. Particularly, the I-V curves show a very low dark current (around 10 picoamps at operative biases) and the photocurrent spectrum shows a clear peak at 850 nm wavelength. Combination of very low dark current wavelength selectivity, and compatibility with high electron mobility transistors makes these devices especially suitable for the above-mentioned applications.
Vapor phase epitaxy growth of CdTe epilayers for RT x-ray detectors
We report on the growth of thick CdTe layers on ZnTe/(100) GaAs hybrid substrates by the novel H2... more We report on the growth of thick CdTe layers on ZnTe/(100) GaAs hybrid substrates by the novel H2 transport vapor phase epitaxy (H2T-VPE) method. High crystalline quality (100)-oriented CdTe single crystal epilayers can be fabricated under atmospheric pressure and at growth temperatures (TD) in the 600 - 800 degree Celsius interval. Double crystal X-ray diffraction measurements performed on epilayers thicker than 30 micrometer show CdTe (400) peaks with FWHM < 59 arcsec. Samples grown under optimized conditions exhibit mirror-like surfaces. Nominally undoped epilayers grown < 650 degrees Celsius are p-type and low resistive, but they turn n-type above 650 degrees Celsius, as a result of donor (likely Ga) diffusion from the substrate. RT resistivities ((rho) ) approximately 106 (Omega) (DOT)cm are obtained for 675 degrees Celsius < TD < 700 degrees Celsius, but (rho) decreases for higher temperatures and thinner samples. Layers grown under these conditions show RT electron concentrations in the 1014 - 1011 cm-3 range. The detection capability of H2T-VPE grown CdTe is demonstrated by time- of-flight measurements performed at RT on Au/n-CdTe/n+- GaAs diode structures under reverse bias conditions. The present results show the potentials of H2T-VPE for the growth of detector-grade CdTe.
A generic requirement for any sensor is to respond to the particular stimulus since all other sti... more A generic requirement for any sensor is to respond to the particular stimulus since all other stimuli are considered noise, so the heart of any sensor is its sensing element. AN other components of the sensor system work within the constraints established by the sensing element, that is, the fundamentalphysical limit of a sensor system is established by the element whose physicalproperties are modified by the stimulus to be sensed In thispaper we will treat with thepropem'es concerning a fami& of optical devices based-on heterojunction and heterodimensional structures and we will investigate on their main performances (in terms of modelling, dark current, light response, high-speed measurements). The implementation of device architectures that relies on structures of diflerent physical dimensions, has the purpose to maximize the physical benefits of the lower dimensional systems. Advantages of electron transport in a reduced dimensional systems make these devices good candidates for integrated optoelectronic applications. The performance of these kind of devices was simulated using ISE-TCAD simulator. Physical models used in the simulations include Schokley-Read HaN recombination, Auger recombination, jieId-dependent mobili@, thermionic effect, heterointerfaces and surface density of state-effects, considering also the specific quantized nature of the two-dimensional electron gas. We compared the simulated characteristics of AIGaAdGaAs material system basedon device with measurements and obtained a good match between our simulation results and measurements data. The results show that the chosen physical model applied by the two-dimensional devices simulation is viable for a study and development of device performances. We took into account also the photoresponse measurements of AlGaAs and InGaAs materials-based on devices, in order to evaluate the behavior of devices under light. High speed time response measurements aregiven in the lastpart of this paper that indicate higher peak transient response is achieved in presence of higher 2DEG density without degradation of temporal behavior. Microwave measurements taken in the frequency domain conclude the picture of characterizations, allowing us to extract the elements of the equivalent circuit.
We have previously reported the time response of a 6-doped heterostructure metal-semiconductor-me... more We have previously reported the time response of a 6-doped heterostructure metal-semiconductor-metal photodetector (HMSM-PD) with FWHM of l l p s for a 4 pm finger gap device 111. Comparison of the 6-doped and undoped devices shows improved performance due to the 6-doping. In this paper, we simulate the device dynamics behavior using Ramo's theorem. Results show that the introduction of a &-doping layer significantly changes the two dimensional potential and field profiles from horizontally oriented to vertically oriented ones due to the screening effect of the highly crowded two dimensional electron gas (ZDEG) formed along the interface of heterojunction, which reduces the carrier travel distance hence the transit time of carriers in the device. With this knowledge, we expect to achieve high speed performance without further scaling down of the device.
Optoelectronics, Displays, and Imaging-Optical and Electrical Characterization of GaAs-Based High-Speed and High-Sensitivity Delta-Doped Resonant Cavity-Enhanced HMSM Photodetector
Imaging and reliability of capacitive RF MEMS switches in III-V technology
ABSTRACT In this work, the bridge imaging and the reliability of surface-micromachined capacitive... more ABSTRACT 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.
Overcoming Photodetector Transit Time Limitations with Collective Excitations
ABSTRACT Incorporating InGaAs/GaAs/AlGaAs heterostructures creates a bi-layer electron and hole p... more ABSTRACT Incorporating InGaAs/GaAs/AlGaAs heterostructures creates a bi-layer electron and hole plasma producing a planar, top-illuminated photodetector with &lt;2.5-ps time response and sensitive to &lt;1-microWatt optical power, and can operate without applied bias.
The homoepitaxy of n-CdTe:I layers is reported as a technological step towards the fabrication of... more The homoepitaxy of n-CdTe:I layers is reported as a technological step towards the fabrication of CdTe-based p-i-n diode nuclear radiation detectors. CdTe:I layers were grown at 330°C on detector-grade (111)-oriented CdTe crystals by metalorganic vapour phase epitaxy. To ensure CdTe homoepitaxy, as-received substrates were treated before growth by etching in Br 2 -methanol and in-situ H 2 heatcleaning at 350°C. (111)-oriented layers with fairly good surface morphology were obtained on as-prepared substrates by growing under Cd-rich vapour conditions. I-doped samples turned out to be n-type with resistivity values around a few Ω·cm and electron concentration ~10 16 cm -3 , but substantial electrical compensation of I donors occurs in the material, likely due to the formation of unintentional V Cd -I Te acceptor centres. Homoepitaxial n-CdTe:I/i-CdTe samples were used to fabricate a preliminary Pt/i-CdTe/n-CdTe:I/Al device structure. Improvement of electrical insulation between back and front electrodes in this M-i-n device was achieved by reactive ion etching of the CdTe:I layer around the Al electrode. This treatment turned out to be effective in reducing the current flowing through the device under reverse bias conditions by more than one order of magnitude.
We have investigated the photocurrent transient response of planar metal-semiconductor-metal (MSM... more We have investigated the photocurrent transient response of planar metal-semiconductor-metal (MSM) structures containing a single layer of self-assembled InAs quantum dots (QDs) embedded in different matrices of GaAs or InGaAs. Growth conditions are such that ground state transition energies correspond to the wavelength range of 1.2-1.3 mm. Electron-hole pairs are created in the QDs, due to a quasi-resonant interband excitation, allowing for direct observation of the carrier dynamics under lateral electric field. Results prove the reasonable detection capability of one layer of QDs in a common photodetector structure with time responses of the order of 10 ps. A long tail, about 100 ps, but at a small fraction of the peak response amplitude is also observed. r
Physica Status Solidi (A) Applications and Materials
Among spintronic materials, mixed-valence manganite La 0.7Sr0.3MnO3 (LSMO) is widely investigated... more Among spintronic materials, mixed-valence manganite La 0.7Sr0.3MnO3 (LSMO) is widely investigated due to its half-metal nature. LSMO thin films were grown by pulsed laser deposition (PLD) onto amorphous silica substrates heated at nearly 600 °C. An ArF excimer laser was chosen to induce ablation due to its more energetic photons compared to the other quoted excimer laser sources. Different oxygen pressures were considered in order to study the influence of oxygen on the LSMO optical and electrical properties. In this respect, the visible transparency percentage of the deposited films is found good enough for spin-OLED applications. The absorption coefficient shows an absorption band tunable as a function of the oxygen content. Its energetic location and evolution with the oxygen content demonstrate it originates from radiative transitions between the spin-majority bands separated by the Jahn–Teller distortion. All of this lets relate the deposition oxygen pressure to the Mn3+ ion co...
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Papers by Adriano Cola