Angel Cuadras

Control of tunnel oxide thickness in Si-nanocrystal array memories obtained by ion implantation and its impact in writing speed and volatility

The injection and storage of charge in Si nanocrystals obtained by ion implantation and annealing have been studied for different tunnel oxide thicknesses. The energy of the ions was kept fixed at 15 keV, which is compatible with most ion implanters used in Si technology, and the distance between the Si nanocrystals and the substrate was controlled by using gate oxides with different thicknesses. The processing conditions were adjusted for precipitating all the Si excess and for having Si–SiO2 interfaces free of defects. Consequently, reliable structures were obtained working in the direct tunneling injection regime, which show unprecedented longer retention times. Furthermore, it is shown that by changing only the oxide thickness it is possible to engineer devices with a tradeoff between writing speed and retention time.

Multimodal piezoelectric wind energy harvesters

We investigated energy harvesting from wind to electrical power using piezoelectric films. Commercial films with different areas (from 3 to 30?cm2) and thicknesses (40?64??m) were used. The generated energy was rectified through a diode bridge and delivered to a storage capacitor. Two different wind flows were investigated: laminar flow from a wind tunnel and turbulent flow from a dryer using three different setups and two wind incidences (parallel and normal). Piezofilms oscillating in wind flows were recorded using video cameras and electrical signals monitored with an oscilloscope. They were found to be stressed by travelling waves of different frequencies induced by wind turbulences. We propose a simple model based on sinusoidal current generators working at different frequencies. We studied the geometrical dependence of the piezofilm on the power generation. Power generation was found to be of the order of 0.2??W for these particular sensors. Guidelines to significantly improve power generation are provided, taking into account a convenient geometrical design to match the piezofilm resonance frequency to the vortex generation frequency.

Smart Interfaces for Low Power Energy Harvesting Systems

An effective energy management plays a key role in environmental energy harvesting systems due to the low energy densities involved. The energy flow inside an energy management interface should be organized in order to guaranty the operation of the system and minimize energy losses. An efficient interface for low power solar cells is proposed. It combines a current-to-voltage converter with a storage unit, a super-capacitor, optimizing the energy flow from the solar cell to a load. Circuit simulations of the interface agree with experimental results.

Energy Harvesting from PZT Pyroelectric Cells

PZT pyroelectric elements were designed as thermal energy harvested sources, describing the best technological parameters to achieve large currents. It was demonstrated that currents followed the pyroelectric model for hot air heating. Large voltages were achieved in capacitor electrodes, what allowed charging and storaging energy in 1 muF capacitors. Moreover, it was also demonstrated cell parallel association to provide larger powers to a load

SoC Li-ion battery monitoring with impedance spectroscopy

The state of charge (SoC) of Li-ion coin batteries is investigated as a function of impedance variation. Batteries are submitted to highly stressed conditions to simulate the operation of active loads and impedance is measured in real time both for charge and discharge. In particular, modulus and phase dependence as a function of SoC is studied with impedance spectroscopy in the range 40 Hz to 20 kHz. Phase is found to be the most sensitive parameter. It decreases with battery discharge and increases with battery charge. Impedance is simply modelled with a 3 parameters circuit, two resistors and a constant phase element (CPE). Selected frequencies are then proposed to estimate the SoC of the battery.

Determination of heart rate using a high-resolution temperature measurement

A high resolution temperature measurement system able to measure temperature fluctuations down to 0.1 mdegC was developed. It was based on thermistors, which were fed with an ac signal to ensure a good signal-to-noise ratio (SNR). Two different configurations were tested, one unipolar and the other differential. The final system was inexpensive, easily portable and performed a noninvasive determination of temperature. It measured either volume or surface temperatures. A good SNR for both configurations using different types of thermistors was achieved. To evaluate a practical application of this system, temperature fluctuations were measured on the skin, in the proximity of different arteries, from where the heart rate was determined. The system may have many potential applications, in fields ranging from biomedicine to aerospace engineering

Oxidation of Si1−x−yGexCy strained layers grown on Si: kinetics and interface properties

Abstract We have investigated the thermal oxidation of strained Si1−xGex and Si1−x−yGexCy layers and the influence of the thermal process on the structure of the layers. Using XPS and SIMS depth profiles, we have found a germanium pile-up in the epitaxial layer near the oxide-layer interface. Using transmission electron microscopy (TEM), we have observed that crystallinity is well conserved, but additionally, we have found SiC precipitates. A qualitative model for the oxidation of this kind of binary and ternary alloys is presented. The model is based on the strain development of the samples and depends on germanium and carbon compositions and on the temperature of the process.

Thermal dry oxidation of Si 1-x-y Ge x C y strained layers grown on silicon

Abstract The incorporation of C into strained Si 1-x Ge x to form partially compensated Si 1-x-y Ge x C y layers improve their critical thickness and thermal stability against relaxation. Thus, these ternary alloys are attractive for the realization of MOS-gated HFETs with the gate grown by thermal oxidation. For this purpose, we present a detailed study of the growth kinetics of SiO 2 in the thin oxide regime for tensile and compressive layers. The oxides have been analyzed by FTIR. The modification of the Si 1-x-y Ge x C y layers after oxidation has been studied by FTIR (substitutional carbon, β-SiC precipitation) and SIMS (Ge and C depth profiles). From these analyses, suitable process windows for dry thermal growth of oxides with good quality are defined. Preliminary results of the electrical characterization performed on test capacitors are shown.

Optical characterization of thermally oxidized Si1−x−yGexCy layers

Abstract The oxidation kinetics and the optical properties of the new SiO 2 /Si 1− x − y Ge x C y system (grown epitaxially on Si) have been studied in detail by ellipsometry (EL) and infrared spectroscopy (FTIR). While EL is straightforwardly applied to silicon, the thermal oxidation process affects the structure and refractive index of the underlying non-oxidized Si 1− x − y Ge x C y . Thus, to determine the oxidation kinetics, part of the oxide must be etched-away to monitor the variation of the refractive index of the Si 1− x − y Ge x C y layer after oxidation. The EL data has been interpreted as having a Ge-rich superficial layer and β-SiC precipitates embedded in a SiGeC matrix. This model have allowed us to correlate the EL results with the characterization performed by SIMS and TEM (Ge pile-up at the interface with the oxide and β-SiC nanoprecipitates in the bulk of the semiconductor). These changes are driven by rejection of Ge from the oxidation front and carbon leaving its substitutional site (C s ). The inverse evolution of the C s and β-SiC content has been quantified by monitoring their IR bands at 607 and 820 cm −1 , respectively. Accurate results on dry thermal oxidation kinetics of Si 1− x − y Ge x C y in the thin oxide regime are presented.

Irreversible entropy model for damage diagnosis in resistors

We propose a method to characterize electrical resistor damage based on entropy measurements. Irreversible entropy and the rate at which it is generated are more convenient parameters than resistance for describing damage because they are essentially positive in virtue of the second law of thermodynamics, whereas resistance may increase or decrease depending on the degradation mechanism. Commercial resistors were tested in order to characterize the damage induced by power surges. Resistors were biased with constant and pulsed voltage signals, leading to power dissipation in the range of 4–8 W, which is well above the 0.25 W nominal power to initiate failure. Entropy was inferred from the added power and temperature evolution. A model is proposed to understand the relationship among resistance, entropy, and damage. The power surge dissipates into heat (Joule effect) and damages the resistor. The results show a correlation between entropy generation rate and resistor failure. We conclude that damage can be conveniently assessed from irreversible entropy generation. Our results for resistors can be easily extrapolated to other systems or machines that can be modeled based on their resistance.