Top Electrode Research Articles

Numerical Study of Acoustic‐Electric Signal Conversion for GIS Discharge Detection and Structure Optimization for pMUT

With the development of microelectromechanical technology, the piezoelectric micromechanical ultrasonic transducer (pMUT) for ultrasonic detection of gas‐insulated switchgear (GIS) breakdown discharge has become possible. In this article, the ultrasonic‐solid‐pMUT coupling simulation model was established to investigate the propagation process of ultrasonic waves in GIS enclosure and the conversion between ultrasonic and electric signals through pMUT. The output electric signals for the unsimplified top electrode and simplified top electrode model were compared, and the structure of receiving pMUT was optimized according to the main ultrasonic frequency range of GIS breakdown discharge and the key performance parameters of pMUT. First, it is found that the simulation results of the ultrasonic‐solid‐pMUT coupling model are reliable, and the frequency of the output electric signal of receiving pMUT is consistent with the input signal, while the amplitude of output signal attenuates greatly. Second, simplifying the top electrode of pMUT may lead to relatively large errors in simulation results. The output response of simplified pMUT is obviously weakened, and the inherent frequency is higher. Furthermore, it is noted that the pMUT inherent frequency (f0) is inversely proportional to the radius (R) of the piezoelectric layer and directly proportional to the substrate thickness (dsi). When the coverage rate of the top electrode is between 30% and 80%, the pMUT effective electromechanical coupling coefficient () is high and has a maximum value. Finally, it is indicated that the pMUT with the optimum structure is suitable for the detection of GIS breakdown discharge ultrasonic signals, and the can be improved by 106.7%.

Guided‐Growth Ultrathin Metal Film Enabled Efficient Semi‐Transparent Organic Solar Cells

AbstractSemi‐transparent organic solar cells (STOSCs) have great potential in power‐generating windows for building facades and automobiles. At present, the evaporated metal thin film is widely used as the transparent top electrode in STOSCs owing to its relatively high conductivity. However, its transmittance in the visible range is sacrificed for fulfilling the thickness requirement of the electrical percolation threshold. Herein, a facile approach of introducing pre‐located Ag nanoparticles (NPs) with an optimized amount of ligands is demonstrated to promote the high‐quality and ultrathin evaporated Ag film formation for high‐performance transparent electrodes beyond that of merely evaporated electrodes. With the pre‐located and ligand‐optimized Ag NPs, the growth of evaporated Ag clusters can be guided to form high‐quality transparent electrode. Equally important, the approach also reduces the mis‐stacking defects of the electron transport layer and thus favors the carrier transportation/extraction to the electrode. By using these Ag NPs/7 nm Ag with a sheet resistance less than 15 Ω sq−1 and average transmittance of 59.30% in the visible region as the main structure in the top electrode, a PM6:L8‐BO based STOSC achieves light utilization efficiency of 4.422% with a remarkable power conversion efficiency of 12.80%. This work provides a facile strategy to not only realize high‐quality and transparent ultrathin electrode with detailed understanding, but also to promote the practical applications of semi‐transparent photonic devices.

Highly Sensitive Tactile Sensor With Wide Dynamic Range Based on Multicontact Mechanism

High-performance tactile sensors have become the important part of human–machine interaction, intelligent medical monitoring, industrial robots, and many other fields. However, high sensitivity and wide range are considered as contradictory aspects, so that the trade-off is always made between parameters. How to achieve the balance between them is a huge challenge. Herein, a novel tactile sensor is introduced based on the multicontact structure, which consists of two parts: the deformable top electrode with conductive spherical surface and the bottom electrode distributed with graphite nanosheets/polyimide (GNSs/PI) resistor array. Thereafter, the sensing mechanism of this new structure is systematically analyzed, which originates from changes in the contact resistance between overlapping electrode pairs and the expansion of the parallel equivalent circuit. Notably, tactile performances can be optimized by adjusting the curvature and conductivity of the top electrode, and the distribution pattern and density of the bottom electrode, leading to an outstanding sensitivity of 15.65 kPa−1 for low pressure (0–200 kPa) and 2.99 kPa−1 for high pressure (200–1400 kPa) with linearity <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}^{\,2}$ </tex-math></inline-formula> higher than 0.95 in both intervals. Furthermore, the sensor shows relatively low detection limit (1 g), short response time (170 ms), and the excellent repeatability and durability advantageous to prolonged use. It is proven that the sensor design concept can be used as a preferred choice for accurate collection of tactile information and gives full support for the relevant applications of intelligent electronics.

Thermal transport in metal-NbOx-metal cross-point devices and its effect on threshold switching characteristics.

Volatile threshold switching and current-controlled negative differential resistance (NDR) in metal-oxide-metal (MOM) devices result from thermally driven conductivity changes induced by local Joule heating and are therefore influenced by the thermal properties of the device-structure. In this study, we investigate the effect of the metal electrodes on the threshold switching response of NbOx-based cross-point devices. The electroforming and switching characteristics are shown to be strongly influenced by the thickness and thermal conductivity of the top-electrode due to its effect on heat loss from the NbOx film. Specifically, we demonstrate a 40% reduction in threshold voltage and a 75% reduction in threshold power as the thickness of the top Au electrode is reduced from 125 nm to 25 nm, and a 24% reduction in threshold voltage and 64% reduction in threshold power when the Au electrode is replaced by a Pt electrode of the same thickness of NbOx film, due to its lower thermal conductivity. Lumped element and finite element modelling of the devices show that these improvements are due to a reduction in heat loss to the electrodes, which is dominated by lateral heat flow within the electrode. These results clearly demonstrate the importance of the electrodes in determining the electroforming and threshold switching characteristics of MOM cross point devices.

Iontronic memories based on ionic redox systems: operation protocols.

A recently developed, new ionic device called the ionic voltage effect soft triode (IVEST) was optimized, tuned and embedded into a memory application concept. The device is an electrochemical micro-cell, consisting of a top electrode and two bottom electrodes. The device controls the concentration and diffusion of ions via the voltage applied on the top electrode. The device showed a memory effect lasting up to 6 hours. Despite the remarkably large stability time, the memory contrast was small in the first device versions. Now, we have increased the memory contrast by introducing a new external electrical circuit layout combined with a new operation protocol. This new investigation also reveals peculiarities of the memory and shows that the IVEST can be used in memory applications. These iontronic memories show a secondary information storage connected with the read-out frequency.

Effects of solvent vapor annealing on the optical properties and surface adhesion of conjugated D : A thin films.

A diketopyrrolopyrrole (DPP) and perylene diimide (PDI)-based molecule, denoted as PDI-DPP-PDI, was investigated as an electron acceptor material in bulk heterojunction (BHJ) solar cells, with poly[[4,8-bis [5-(2-ethylhexyl)-2-thienyl]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl] [2-(2-ethyl-1-oxohexyl)thieno[3,4-b]thiophenediyl]] (PBDTTT-CT) as an electron donor. The donor polymer and the acceptor molecule have complementary absorption spectra, which is an essential feature for energy collection in organic solar cells. However, AFM images indicated the presence of isolated and microsized PDI-DPP-PDI domains along the surface of the films, which reduced the power conversion efficiency. Therefore, to improve the homogenization of the acceptor along the film, a post-deposition treatment, denoted as solvent vapor annealing (SVA), was performed in a saturated atmosphere containing the vapour of an organic solvent for 3-10 minutes. This procedure changed the optical and morphological properties of the PBDTTT-CT : PDI-DPP-PDI active layer, resulting in increased power conversion efficiency values by more than 2.5 times (reaching 5.1%). Theoretical simulation pointed out that the experimental absorbance band localized at 580 nm, which appeared after SVA treatment, is possibly related to an intense simulated band with a maximum at 572 nm, resulting from a pair of transitions starting in the copolymer and ending in PDI-DPP-PDI, in regions where both are stacked at about 3 Å. The most significant natural transition orbitals (NTOs) related to these transitions indicated charge transfer character. Moreover, analyses carried out by power spectrum density (PDS) of images acquired from the SVA-treated film indicated that in the region of larger frequencies, across the length scale at around 30-70 nm, an additional fractal region appeared with a Ds of 0.95, indicating a flattened region, possibly related to changes in the overall conformation after SVA treatment. This indicates an improvement in the molecular packing, a feature not observed in the as-cast film. The analyses by force curve spectroscopy pointed out increased adhesion forces and adhesion energy in the PBDTTT-CT : PDI-DPP-PDI film after SVA treatment; this feature enhanced the interfacial interaction with the top electrodes, reflecting improved charge extraction in the photovoltaic device.

Modeling and Characterization of Pre-Charged Collapse-Mode CMUTs

Recently, the applications of ultrasound transducers expanded from high-end diagnostic tools to point of care diagnostic devices and wireless power receivers for implantable devices. These new applications additionally require that the transducer technology must comply to biocompatibility and manufacturing scalability. In this respect, Capacitive Micromachined Ultrasound Transducers (CMUTs) have a strong advantage compared to the conventional PZT based transducers. However, current CMUTs require a large DC bias voltage for their operation, which limits the miniaturizability of these devices. In this study, we propose a pre-charged collapse-mode CMUT for immersive applications that can operate without an external bias by means of a charge trapping Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> layer embedded in the dielectrics between the top and bottom electrodes. The built-in charge layer was analytically modeled and four layer stack combinations were investigated and characterized. The measurement results of the CMUTs were then used to fit the model and to quantify the amount and type of trapped charge. It was found that these devices polarize due to the ferroelectric-like behavior of the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , and the amount of charge stored in the charge-trapping layer was estimated to be approximately 0.02C/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Their acoustic performance shows a transmit and receive sensitivity of 8.8 kPa/V and 13.1V/MPa respectively. In addition, we show that increasing the charging temperature, the charging duration, and the charging voltage results in a higher amount of stored charge. Finally, results of ALT tests showed that these devices have a lifetime of more than 2.5 years at body temperature.

Proton irradiation induced damage effects in CH&lt;sub&gt;3&lt;/sub&gt;NH&lt;sub&gt;3&lt;/sub&gt;PbI&lt;sub&gt;3&lt;/sub&gt;-based perovskite solar cells

Perovskite solar cells (PSCs) have a great potential for space applications due to their high specific power, low cost and high defect tolerance. PSCs used in space will be subjected to high-energy particle irradiation, especially proton irradiation, resulting in the decline of photovoltaic (PV) performance. However, the research on proton irradiation effects in PSCs is still in its infancy stage. In this work, the CH&lt;sub&gt;3&lt;/sub&gt;NH&lt;sub&gt;3&lt;/sub&gt;PbI&lt;sub&gt;3&lt;/sub&gt; (MAPbI&lt;sub&gt;3&lt;/sub&gt;) thin films and their PSCs are irradiated by protons with energy of 0.1, 2, 10, 20 MeV, etc. Irradiation-induced changes in PV parameters of the PSCs are studied as a function of proton fluence. The structural and surface morphological changes of the irradiated MAPbI&lt;sub&gt;3&lt;/sub&gt; films and Au electrode layers of PSCs are characterized by X-ray diffraction and scanning electron microscopy. In addition, UV spectrophotometer is also employed to analyze the transmission loss in glass substrate induced by proton irradiation. It is found that PSCs exhibit superior resistance against proton irradiation. The PV properties of the PSCs don’t degrade after 0.1 MeV (2 MeV) proton irradiation up to a fluence of 1×10&lt;sup&gt;13&lt;/sup&gt; p/cm&lt;sup&gt;2&lt;/sup&gt; (1×10&lt;sup&gt;14&lt;/sup&gt; p/cm&lt;sup&gt;2&lt;/sup&gt;). The irradiation-induced damage in the charge transport layers may be the main cause for the performance degradation of PSCs. The gaseous products (NH&lt;sub&gt;3&lt;/sub&gt; and CH&lt;sub&gt;3&lt;/sub&gt;I) of perovskite decomposition eventually lead to exfoliation of the top Au electrode from the PSCs. Regarding 10 and 20 MeV proton irradiation with larger projected ion ranges, the irradiations create color center defects in glass substrate of PSCs, which results in a decrease in light transmission of visible spectrum. However, the color center defects, specifically non-bridging oxygen hole centers, will be partly annealed at room temperature or 100 ℃, reducing the transmission loss in glass. The reported results may help predict the performance degradation of PSCs in space irradiation environment.

Comparison of numerical and experimental results on melt electrovortex flow driven by direct current supplied over top and bottom electrodes

Comparison of numerical and experimental results on melt electrovortex flow driven by direct current supplied over top and bottom electrodes

Impact of NaCl concentration in crystalline nanocellulose for printed ionic dielectrics

While electronics continue to be an integral part of nearly every aspect of our lives, there is a growing need for more environmentally friendly electronic materials and manufacturing processes. The use of crystalline nanocellulose (CNC) as a biodegradable dielectric material has recently gained interest due to its high capacitance at low voltages and compatibility with additive manufacturing techniques, such as printing. The addition of salt to the CNC has shown improved performance in transistor applications, but a thorough study optimizing the addition of NaCl in CNC capacitors has not been completed. We hypothesized that increasing the NaCl concentration altered the charging characteristics of the CNC. In this work, we verified this hypothesis by measuring the charging and discharging characteristics for CNC capacitors with varying NaCl concentrations and extracting the resistance-capacitance (RC) delay and charging current. These aerosol jet printed capacitors consisted of graphene top and bottom electrodes and a CNC dielectric sandwiched between the electrodes. The results reveal a minimum charging current at a NaCl concentration of 0.05%, independent of the applied voltage, and a minimized RC delay at NaCl concentrations above 0.1% with applied voltages below 2 V. These findings show that there are trade-offs that must be managed between RC delay and charging current, thus the NaCl concentration must be optimized for each application. This work is another significant step in understanding the electrical properties of CNC so that this environmentally friendly material may one day be utilized in commercially available electronics.

Interface roughness effects and relaxation dynamics of an amorphous semiconductor oxide-based analog resistance switching memory.

The analog resistive switching properties of amorphous InGaZnOx (a-IGZO)-based devices with Al as the top and bottom electrodes and an Al-Ox interface layer inserted on the bottom electrode are presented here. The influence of the electrode deposition rate on the surface roughness was established and proposed as the cause of the observed unusual anomalous switching effects. The DC electrical characterization of the optimized Al/a-IGZO/AlOx/Al devices revealed an analog resistive switching with a satisfactory value for retention levels, but the endurance was found to decrease after 200 cycles. The predominant conduction mechanism in these devices was found to be thermionic emission. An in-depth analysis was performed to explore the relaxation kinetics of the device and it was found that the current has a lower decay rate. The current level stability was tested and found reliable even after 5 h. The cost-effective and precious metal-free nature of the a-IGZO memristor investigated in this study makes it a highly desirable candidate for neuromorphic computing applications.

Stress analysis of circular membrane-type MEMS microphones with piezoelectric read-out

In this study, the impact of residual tensile stress on a membrane-type piezoelectric MEMS (Micro Electromechanical Systems) microphone is investigated both theoretically by FEM (finite element method) simulations and experimentally by stress measurements, by manufacturing devices and by recording the acoustic performance. Based on these findings, a design is proposed composing of segmented electrodes which are arranged close to the silicon frame as well as on the inner part of the membrane. The MEMS microphone features an aluminum nitride layer which allows a piezoelectric read-out, and multiple metal bottom and top electrodes distributed radially and azimuthally. FEM simulations were done over a large range of different layer thicknesses, membrane radii, and tensile stress levels, which indicate the existence of a sweet-spot when targeting a maximum output signal. The data showed, that thicker membranes perform better under tensile stress than thinner membranes. Furthermore, a relation between output signal and residual stress level was established, which showed that the FEM predictions agree well with the measurement results of piezoelectric microphones. • Thicker MEMS membranes preferred over thinner counterparts when tensely pre-stressed • Finite element analyses revealed sweet spot for maximizing piezoelectric output • Design propositions for stressed closed membranes without stress relaxing measures • Microphone measurements and finite element methods results agree well

Optimization of HIL-HTL--Free Naphthalimide:Isoquinoline TEOLEDs with BZO Top Electrode

Optimization of HIL-HTL--Free Naphthalimide:Isoquinoline TEOLEDs with BZO Top Electrode

Influence of the upper electrode material on the electrophysical properties of MDM structures based on ferroelectric films

Ferroelectric films of the composition Ba0.8Sr0.2TiO3 (BST) were synthesized on platinized silicon substrates by RF sputtering. Top electrodes made of nickel, aluminum, copper and chromium were formed on the grown films by electron beam sputtering. The results of studies of the electrophysical properties of the obtained metal-dielectric-metal (MDM) heterostructures (Me-BST-Pt-Si) are presented. Keywords: ferroelectric BST films, metal-dielectric-metal (MDM) structures, microstructure, electrophysical properties.

Parameters for ferroelectric phase stabilization of sputtered undoped hafnium oxide thin films

In this work, various stabilization factors for the ferroelectric phase of undoped hafnium oxide prepared by physical vapour deposition were investigated. The capping of the thin films with platinum top electrodes before annealing, as well as the amount of oxygen during sputter deposition and subsequent annealing was shown to have a significant influence on the resulting ferroelectric properties and phase of the HfO x layer. When the prepared films were not capped, only one specific set of process parameters was found to lead to the formation of the ferroelectric phase. We conclude that capping is a crucial condition for stabilizing the ferroelectric phase. Furthermore, it is shown that the amount of oxygen supply during all fabrication processes determines the resulting ferroelectric phase fraction for capped samples. Increasing the oxygen flow during sputtering and annealing results in a larger monoclinic phase fraction and thus a decrease of the remanent polarization.

Electrochemically decorated gold nanoparticles on CVD graphene ChemFET sensor for the highly sensitive detection of As(III)

In this paper, we report a novel reusable receptor-free chemically sensitive field-effect transistor (ChemFET) sensor for the sensitive detection of As(III) using electrochemical doping of Au nanoparticles (AuNPs)-chemical vapour deposition (CVD) graphene (Gr) sensing film (AuNPs-Gr). The sensor consists of a single-gap gold electrode and a single layer of CVD graphene as a conducting/sensing channel decorated with a thin layer of electrochemically deposited AuNPs to catalyze the electroreduction of As(III) ions onto the sensing channel. A new detection strategy was proposed by measuring the resistance change rate of the AuNPs-Gr sensing channel modified between the source electrode and drain electrode, wherein the resistance change was caused by the electrochemical doping of As(0). The results indicated that the electrochemical doping of As(0) onto the sensing channel led to an increase in resistance through the doping interaction among As(0), AuNPs and graphene, enabling the proposed sensor to have an extraordinary detection performance with a high sensitivity. Moreover, the proposed sensor can be recycled by removing the As(0) deposited on the sensing film with the aid of a simple electrochemical oxidization process. Additionally, the mechanism of the sensing strategy based on the electrochemical n-doping of As(0) into the AuNPs-Gr sensing film was investigated using a top liquid gate electrode. The As(III) concentration detected by the AuNPs-Gr ChemFET sensor was as low as 0.001 μg/L, demonstrating that such sensors with a compatible sensing strategy can be used to detect As(III) with high sensitivity, easy fabrication, fast response and can be recycled.

Charge Transport Measured Using the EGaIn Junction through Self-Assembled Monolayers Immersed in Organic Liquids

This paper describes measurements of charge transport by tunneling through molecular junctions comprising a self-assembled monolayer (SAM) supported by a template-stripped metal bottom electrode (MTS), which has been immersed in an organic liquid and contacted by a conical Ga2O3/EGaIn top electrode. These junctions formed in organic liquids are robust; they show stabilities and yields similar to those formed in air. We formed junctions under seven external environments: (I) air, (II) perfluorocarbons, (III) linear hydrocarbons, (IV) cyclic hydrocarbons, (V) aromatic compounds, (VI) large, irregularly shaped hydrocarbons, and (VII) dimethyl siloxanes. Several different lengths of SAMs of n-alkanethiolates, S(CH2)n-1CH3 with n = 4-18, and two different kinds of bottom electrodes (AgTS or AuTS) are employed to assess the mechanism underlying the observed changes in tunneling currents. Measurements of current density through junctions immersed in perfluorocarbons (II) are comparable to junctions measured in air. Junctions immersed in other organic liquids show reductions in the values of current density, compared to the values in air, ranging from 1 (III) to 5 orders of magnitude (IV). We interpret the most plausible mechanism for these reductions in current densities to be an increase in the length of the tunneling pathway, reflecting the formation of thin (0.5-1.5 nm) liquid films at the interface between the SAM and the Ga2O3/EGaIn electrode. Remarkably, the thickness of the liquid film─estimated by the simplified Simmons model, measurements of electrical breakdown of the junction, and simulations of molecular dynamics─is consistent with the existing observations of structured liquid layers that form between two flat interfaces from measurements obtained by the surface force apparatus. These results suggest the use of the EGaIn junction and measurements of charge transport by tunneling as a new form of surface analysis, with the applications in the study of near-surface, weak, molecular interactions and the behavior of liquid films adjacent to non-polar interfaces.

The effect of oxygen affinity electrode Ti on the performance of vanadium oxide-based valence change resistive random access memory

Two top electrode (TE) metals, Pt and Ti, are deposited on VOX/Pt to investigate the effect of oxygen affinity TE on resistive switching (RS). The oxygen affinity electrode Ti significantly improves the variability and durability of the devices compared to the inert TE Pt. The TiOY interfacial layer (IL) plays a vital role in RS as an oxygen reservoir between the Ti TE and the dielectric layer. During repeated RS, a rapid endurance degradation is observed for Pt/VOX/Pt. However, the resistance value in the high-resistance state (HRS) gradually decreases over the RS cycles for Ti/VOX/Pt. The oxygen ions in VOX are consumed and stored in TE Ti owing to the high oxygen affinity of Ti. This process causes an improvement in the uniformity and endurance for Ti/VOX/Pt.

Miniature Deformable MEMS Mirrors for Ultrafast Optical Focusing.

Here, we introduce ultrafast tunable MEMS mirrors consisting of a miniature circular mirrored membrane, which can be electrostatically actuated to change the mirror curvature at unprecedented speeds. The central deflection zone is a close approximation to a parabolic mirror. The device is fabricated with a minimal membrane diameter, but at least double the size of a focused optical spot. The theory and simulations are used to predict maximum relative focal shifts as a function of membrane size and deflection, beam waist, and incident focal position. These devices are demonstrated to enable fast tuning of the focal wavefront of laser beams at ≈MHz tuning rates, two to three orders of magnitude faster than current optical focusing technologies. The fabricated devices have a silicon membrane with a 30-100 μm radius and a 350 nm gap spacing between the top and bottom electrodes. These devices can change the focal position of a tightly focused beam by ≈1 mm at rates up to 4.9 MHz and with response times smaller than 5 μs.

Increased fatigue resistance in transparent lead zirconate titanate thin films with interdigitated electrodes

This work compares the fatigue behavior of fully transparent lead zirconate titanate films with a composition at the morphotropic phase boundary on fused silica glass or sapphire, respectively, fitted with coplanar interdigitated top electrodes and no bottom electrodes to that of the same systems in a traditional metal–insulator–metal parallel plate capacitor geometry. While the polarization hysteresis in the latter geometry decreases rapidly during bipolar electrical cycling, systems with coplanar electrodes show no notable fatigue up to 107 cycles. The difference is explained based on the columnar microstructure of the films: for coplanar electrode geometries, the grain boundaries of the columnar structure act as trapping sites for oxygen vacancies, preventing the build-up of agglomerates that cause fatigue by domain wall pinning.