In the present study, I-V curves and EFM-Phase measurement for surface polishing treatments are used in order to obtain electrical properties like breakdown voltages, leakage currents and Schottky Diode Barrier Height (SBH) calculated by Thermionic Emission model (TE) and parabolic model of EFM-Phase. The results showed that a polishing treatment with 0.03 μm grain size alumina powder solution induces a higher SBH, lower leakage current and higher breakdown voltage compared to that with 1 μm alumina solution.
Periodic quantum dot solids are new materials in which quantum dots are periodically distributed in space. The properties of these materials are explored to find applications in new photovoltaic devices. In order to shed light on these future applications, investigating carrier transport and light absorption are of paramount importance. The first step to investigate these systems is to compute their electronic structures. In this work we present the electronic structure of one- (1D), two- (2D) and three-dimensional (3D) quantum dot solids made of 12 Å radius InAs quantum dots. Two different stoichiometries are studied. In the first case (system A) the quantum dot has an In atom in its center and the interdot contact surfaces along the (111) directions in the arrays are mainly arsenic atoms. In the second case (system B) the atomic positions are inverted, and therefore the contact surfaces between neighbours are mainly indium atoms. The influence of dimensionality and stoichiometry in these quantum dot solids are discussed.
Nowadays, bio-inspiration is driving novel sensors designs, beyond vision sensors. By taking advantage of their compatibility with standard CMOS technologies, the integration of giant magneto-resistance (GMR) based magnetic sensors within such event-driven approaches is proposed. With this aim, several topologies of such GMR sensors have been designed, fabricated and characterized. In addition, integrated circuit interfaces of a standard CMOS technology are also proposed. Their suitability for this approach is then demonstrated by means of Cadence IC simulations.
In this work we study the self-heating effect (SHE) in nanoscale Silicon on Insulator Junctionless (SOI JL) FinFET transistor with fin cross section in rectangular, trapeze and triangle form. The lattice temperature dependence on the channel length as well as on buried oxide thickness is considered. It is shown that for considered transistor structure the lattice temperature in the middle of the channel is lower than at lateral sides, near source and drain. Also, we have found at the same conditions the lattice temperature depends on shape of channel cross section too.
Virtual Ge substrates fabricated by direct deposition of Ge on Si have become a pathway with high potential to attain high-efficiency III-V multijunction solar cells on Si. We study the development of III-V triple junction solar cells using two types of Ge|Si virtual substrates. The first uses a thick (2–5 μm) Ge layer grown by CVD, which acts as the bottom Ge subcell. The second, grown by low-temperature RT-PECVD, has a thickness of a few tens of nanometres, with the Si substrate acting as Si bottom cell. We discuss the challenges related to each design (formation of cracks, parasitic absorption in the Ge layer, dislocations, …), present the theoretical design and show the experimental results obtained. Finally, an advanced approach using embedded porous Si layers as buffer layers for crack mitigation is also presented.
Laser-based surface texturing treatments have been investigated as a promising option for innovative low-cost concepts to improve the light absorption of silicon heterojunction solar cells manufactured from unconventional silicon wafers. A great advantage of using a laser as a processing tool is its high precision, which lead to selected and well-controlled morphologies. This is a particularly interesting feature for multicrystalline silicon wafers, where the large number of grain boundaries makes it difficult to obtain high light-trapping morphologies by other methods. The investigations described in this work include testing different patterns on the wafer surface in order to define the best morphology to improve the light absorption. A significant decrease in reflectance (R<;9%) has been achieved by direct-laser texturization and has been compared with acid-chemical etching with average reflectance above 20%. This result suggests the enormous potential of direct laser texturization for this type of wafers, without chemical residues and its easy incorporation to the manufacturing of low-cost silicon heterojunction solar cells.
In this work, the effect of 60Co gamma radiation on the electrical properties of TiN/Ti/HfO2/W RRAM devices is investigated through systematic and extensive measurements of biased and unbiased devices during irradiation. For this purpose, first an experiment has been carried out in fresh devices in order to verify that gamma radiation did not permanently damage the HfO2 after several cumulative radiation doses. Then, the resistive switching behavior of the RRAMs has been assessed by comparing the performance of the devices prior and after irradiation up to a total dose of 22 Mrad(Si). Furthermore, data retention experiments have been carried out by in-situ measuring the device resistance under radiation exposure, for both the low and the high resistance states up to a dose of 7.9 Mrad(Si). The results clearly show that the electrical response of HfO2-based RRAM devices prior and after the formation of an oxygen deficient conductive filament is not significantly affected by ionizing radiation and its corresponding damage. The observed radiation hardness of the devices against gamma radiation in the studied doses is a promising result to extend the application of the HfO2-based RRAM technology to the space industry and other harsh environments.
In this research work, noteworthy progress has been achieved to fabricate stable inverted polymer solar cells using spray pyrolysis technique to deposit a thin film of ZnO as interfacial layer. A standard inverted polymer solar cells were fabricated by spin coating technique as reference devices. It was interesting to observe that the performance of the inverted polymer solar cells fabricated by ZnO-spray pyrolysis were quite similar to the reference samples fabricated by the ZnO-spin coating technique. However, the devices fabricated using the ZnO- spray pyrolysis showed higher current density as well as higher stability more than the ZnO- spin coating ones. Hence, this promising spray pyrolysis technique might be a breakthrough forward step for commercializing the inverted polymer solar cell based on mass production scale.
In this work, an energy-based analysis has been performed for the reset transition of Resistive switching RAM (ReRAM) memristive devices. A voltage ramp input with different slopes has been considered and assumed that these ramps are slower than the physical mechanisms inside the memristor. The analysis has been done in the flux-charge space, instead of the usual voltage-current one. The effects of changing the slope on the reset point have been shown, and a method to estimate the new parameters has been introduced, assuming that the important parameters are those that describe the process in the flux-charge space more than those in the voltage-current domain. In any case, it has been shown that the total energy up to the reset point is dependent on the input ramp, thus strongly hinting at a thermally driven degradation mechanism, as with a slower input signal more energy will be dissipated to the ambient.
The effect of traps on DC and high-frequency behavior of a short channel single-layer graphene field-effect transistor (GFET) is discussed thoroughly in the present work. Trap-induced hysteresis is evident when a standard staircase measurement technique is applied while it is diminished when an opposing-pulse method is used. In both cases, forward and backward gate voltage (VGS) sweeps are utilized. A recently proposed analytical compact model accounting for traps activated both by vertical electric field and high-lateral electric field enabled by hot carriers, is accurately validated with both trap-affected and trap-reduced data. Important high-frequency figures of merit (FoM) such as cut-off and maximum oscillation frequencies as well as the intrinsic gain of the GFET under test, are also derived from the model, and exhibit a strong trap dependence through the DC operating point of the device. These FoM not only demonstrate VGS shifts, but also, they exhibit magnitude alterations due to traps impact even when the Dirac voltage of the GFET under test coincides in both forward and backward staircase measurement schemes.
A semiempirical memdiode model of resistive switching devices is proposed. This model is a modification of the quasi-static memdiode model (QMM). It is based on the incorporation of time dependencies in the QMM parameters, as well as on the empirically observed asymmetries between the reset and set transition. The model considerably improves the prediction of the response of resistive switching devices to arbitrary input stimuli.
In this work we have explored the growth by high pressure sputtering (HPS) of materials intended for novel selective contacts for photovoltaic cells. This technique shows promise for the low-damage low-temperature deposition of PV materials. We studied the deposition of ITO, MoOx and TiOx using pure Ar and mixed Ar/O-2 atmospheres as well as ceramic or metallic targets. We show that HPS deposition of these materials is feasible. The growth rate is greatly reduced when oxygen is added to the argon sputtering atmosphere. The best sputtering RF power was 20-45 W for the pressure range studied. Finally, as-deposited films present high surface recombination, but a mild hot plate anneal at 200 degrees C recovers long effective lifetimes.
In this work we present the results obtained on the simulation and nanofabrication of photonic crystals based on silicon nanopillars. The simulations show the formation of photonic band gaps within 1.31 and 1.89μm, with a gap-to-midgap ratio approaching 40%. The introduction of waveguides and cavities prove the adaptability of these structures to tune the wavelengths allowed to be transmitted through the system within the photonic band gaps. On the other hand, thanks to the use of advanced nanofabrication techniques, the modelled structures have been successfully fabricated.
Germanium hyperdoped with deep level donors, such as tellurium, would lead to dopant-mediated sub-band gap mid-infrared photoresponse at room temperature. We use a combination of non-equilibrium techniques to supersaturate Ge with Te via ion implantation followed by pulsed laser melting (PLM). Typically, liquid N 2 (77K) temperatures are used to avoid implantation-induced Ge surface porosity. In this work, alternatively, we report on the use of slightly higher implantation temperatures (143 K) together with an amorphous Si (a-Si) capping layer. We demonstrate that the solid solubility limit of Te in Ge is overcome upon recovering the crystallinity of the material after laser processing.
In this work, with the help of a semi-classical two-dimensional Monte Carlo (MC) simulator, we study the DC current-voltage curves of Self-Switching Diodes (SSDs) fabricated on an AlGaN/GaN heterostructure from 100 K up to room temperature. Due to the very narrow channel of the SSDs, the presence of surface effects plays a key role not only on their DC behavior but also on their RF detection performance. The evolution with temperature of the negative surface charge density σ at the etched sidewalls of the SSD is the key quantity to explain the measurements. At 300 K, MC simulations with a constant value of σ are able to replicate very satisfactorily the experiments. However, to reproduce the shape of the I-V curve at low temperatures, a more realistic approach, where σ depends not only on T, but also on the applied bias V, is necessary.
Aerosol assisted chemical vapour deposited ZnO nanostructured films integrated into Si-based transducing platforms are modified with preformed Au nanoparticles (NPs) via impregnation. The morphological, structural, and chemical characterization of these films using different characterisation techniques shows the incorporation of well-distributed and stable Au nanoparticles (NPs) at the surface of ZnO. Photoactivated gas sensing tests at room temperature (RT) demonstrate enhanced sensitivity and better speed of response for the Au modified ZnO films (AuZn) providing 3 times higher response to ethanol and acetone as compared to the non-modified ZnO films (Zn).
Ultrasonic sensors have demonstrated great potential for non-destructive testing (NDT) of materials, being widely applicable in health care/monitoring (e.g. biomedical, muscle recovery, cancer early detection), industry, and defence (e.g. proximity sensors used in unnamed aerial vehicles - UAV; detection of submarines). Most conventional ultrasonic sensors are based on monolithic piezoelectric ceramic materials (e.g. PZT, PbTiO 3 or PMN-PT) which are too bulky and nonconforming to enable their integration on flexible substrates. To address these drawbacks, ZnO thin films have emerged as an alternative piezoelectric material for low profile and high-frequency ultrasonic transducers due to properties such as high piezoelectric coefficient, great tuneability of working frequency, large bandwidth, low-cost of materials and manufacturing, compatibility with flexible substrates, and biocompatibility. This work analyses glancing angle deposition (GLAD) of ZnO thin films at different reactive sputtering conditions optimised to meet dual requirements of highly crystalline c-axis orientation while controlling the inclined angle of resulting nanostructured films for their application as piezoelectric material in ultrasonic sensors. Characteristics of ZnO nanostructured films, including morphology, crystallinity, and composition, are analysed as a function of GLAD conditions (gas flux angle with respect the substrate surface (α) and plasma conditions (plasma power, substrate position, substrate temperature, total gas-flow, and processing/reactive gas ratio). The obtained piezoelectric values for β angles of α=88° present d33 values of 33.1±1.7 pm/V, surpassing the piezoelectric coefficient found in ZnO bulk 12.4 pm/V. The influence of film titled angle (β) on piezoelectric performance for ultrasound sensing applications will be studied.
This paper presents a generic voltage amplifier intended to be used in a platform for acquisition of electrophysiological signals through a multi-channel microelectrode array (MEA). The design is compatible with commercially available MEAs and is intended to be used in any experimentation platform with multiple parallel channels for signal acquisition. The design and the preliminary experimental results, which confirm the feasibility of the design, are presented.
Hafnium oxide based memristors were fabricated and multilevel programming driven by a capacitor discharge current through the device was performed. Furthermore, the dynamic memdiode model was used for modeling and analyzing the experimental data.
A Y-function based method (YFM) is used here to extract the contact resistance R-c of different two-dimensional (2D) field-effect transistor (FET) technologies. The methodology relies on individual transfer characteristics, at a single drain-to-source voltage, of devices from a same technology with different channel lengths. In contrast to the widely used transfer length method where a global-back gated test structure is required, the YFM presented here can be applied to 2D-FETs regardless the gate architecture. This method does not require the fabrication of dedicated test structures and hence it can be a useful and immediate tool for device characterization and scaling studies. R-c is extracted here for graphene-, black phosphorus-, WS2 and MoS2-FETs using the proposed methodology and considering the mobility degradation coefficient in the underlying model. The extracted values are in good agreement with the ones obtained with other approaches. An accurate description of the experimental drain current and channel resistance is achieved by using the extracted parameters in the corresponding equation.
