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.
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.
Laser induced transient gratings are used to study carrier generation and recombination properties via free carrier nonlinearity in differently grown GaAs and CdZnTe samples. Simulation of free carrier, photorefractive, and absorptive optical nonlinearities for 10-ns pulses and various illumination intensities allowed us to reveal conditions for the efficient transient quenching of EL2 defect at room temperature in semi-insulating GaAs. In addition, the straightforward coupling of nonlinear degenerate four wave-mixing signal at 1.06 μm with the steady-state charge states of EL2 defect is shown to allow a rough estimation of a crystal compensation ratio by EL2 defect. This novel method was applied to liquid-encapsulated Czochralski and Bridgeman-grown samples and compensation values ranging from 0.1 to approximately 0.6 have been derived. Also, feasibility of nanosecond- and picosecond-dynamic grating techniques for control of
GaAs wafer quality is shown. The first one allowed fast and highly sensitive mapping of EL2 defect distribution and its charge state; the second one has proved a presence of a fast traps in the vicinity of dislocation conglomerations. Analogous mapping of CdZnTe wafers has shown very high spatial homogeneity of the samples, and revealed areas with the non-photoactive absorption or scattering of light.
In this work we describe 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 analyze 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, current-temperature, photocurrent spectra, high-speed time response, and on-wafer frequency domain 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); C-V measurements highlight the low geometrical capacitance values; the photocurrent spectrum shows a clear peak at 850 nm wavelength, while time response measurements give a 3 dB bandwidth of about
30 GHz. Small signal model based on frequency domain data is also extracted in order to facilitate future photoreceiver
design. Furthermore, two-dimensional numerical simulations have been carried out in order to predict the electrical properties of these detectors.
Combination of very low dark current and capacitance, fast response, wavelength selectivity, and compatibility with high electron mobility transistors makes these devices especially suitable for the above-mentioned applications.
Even though it is recognized that the study of polarization from cosmic high-energy sources can give very important information about the nature of the emission mechanism, to date very few measurements have been attempted. For several years we have proposed the use of a thick CdTe array as a position sensitive spectrometer for hard X- and soft gamma-ray astronomy, a design which is also efficient for use as a polarimeter at energies above approximately 100 keV. Herein we describe the preliminary results of our study of a polarimeter based on 4096 CdTe microcrystals that we would like to develop for a high altitude balloon experiment. We present the telescope concept with a description of each subsystem together with some results on activities devoted to the optimization of the CdTe detector units' response. Furthermore we give an evaluation of the telescope performance in terms of achievable spectroscopic and polarimetric performance. In particular we will show the results of Monte Carlo simulations developed to evaluate the efficiency of our detector as a hard X ray polarimeter.
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.
KEYWORDS: Monte Carlo methods, Sensors, X-rays, Gallium arsenide, Electrons, X-ray detectors, Energy efficiency, Signal detection, Signal attenuation, Astatine
We have developed a Monte Carlo simulator for semi- insulating GaAs detectors which gives the energy spectra of x-ray radiations. The simulated spectra are analyzed in terms of: shaping time, trapping properties of the material, and applied reverse voltage. The main features of the spectra as well as the associated charge collection efficiency and the energy resolution of the photoelectric peak are interpreted in physical terms for the whole range of applied voltages covering under- and over-depleted conditions. The results of the simulations provide a general interpretation scheme which is satisfactorily tested with experimental results.
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