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This paper presents a systematic approach to transduction optimization of resonant piezo-based MEMS devices. The method is based on inverse design approach using direct piezoelectric effect and exploits mechanical action of desired mode to calculate the polarization map of the piezoelectric ceramics. Optimal transduction is achieved by matching the excited electric field distribution to the inversely retrieved polarization map using the electrode segmentation and topology optimization technique. The proposed method was applied to improve the design of existing state-of-the-art piezoelectric MEMS mirrors. The optimized devices feature 70% increase in the optical scan angle (θopt) as well as 47 % reduction in power consumption compared to original design. At the same optical scan angle, the optimized devices required 2.28-fold less driving voltage amplitude. [2020-0342] [ABSTRACT FROM AUTHOR]

The dependence of electromechanical behavior on strain in ferroelectric materials can be leveraged as parameter to tune ferroelectric properties such as the Curie temperature. For van der Waals materials, a unique opportunity arises because of wrinkling, bubbling, and Moiré phenomena accessible due to structural properties inherent to the van der Waals gap. Here, we use piezoresponse force microscopy and unsupervised machine learning methods to gain insight into the ferroelectric properties of layered CuInP2S6 where local areas are strained in-plane due to a partial delamination, resulting in a topographic bubble feature. We observe significant differences between strained and unstrained areas in piezoresponse images as well as voltage spectroscopy, during which strained areas show a sigmoid-shaped response usually associated with the response measured around the Curie temperature, indicating a lowering of the Curie temperature under tensile strain. These results suggest that strain engineering might be used to further increase the functionality of CuInP2S6 through locally modifying ferroelectric properties on the micro- and nanoscale. [ABSTRACT FROM AUTHOR]

An electromechanically coupled modeling framework for the simulation of electric current flow in ZnO varistors is developed. The model is based on an equivalent circuit representation of the varistor microstructure with grain boundaries represented by nonlinear resistor branches of the circuit. This approach extends on previous circuit models by including the effect of mechanical stress on grain boundary conductivity. In this article, we consider, in particular, the effect of residual thermal stress in polycrystalline structures on electrical conductivity. The distribution of this stress throughout the material is computed by finite element method (FEM) simulations. Then, each grain boundary conductivity is determined by applying a self-consistent model for the trapped interface charge induced by piezoelectric polarization. Finally, the electric current flow patterns and the bulk conductivity of the material are computed using the nonlinear circuit model. The simulated I–V characteristics reveal a significant sensitivity of the electrical conductivity to internal thermal stress within the material. Furthermore, for realistic ZnO microstructures, the simulations demonstrate the effect of current concentration along thin conducting paths depending on the mechanical stress condition of the material. [ABSTRACT FROM AUTHOR]

In deep-ultraviolet (DUV) light-emitting diodes (LEDs), it is difficult to obtain both efficient carrier confinement and high light extraction, which are quite sensitive to optical polarization and other physical parameters. In this paper, characteristics of DUV LEDs with various n-AlGaN layers and quantum barriers (QBs), and various widths of quantum wells (QWs) are investigated. Specifically, the capability of carrier confinement and properties of optical polarization are analyzed in detail. The simulation results show that LED structure with Al0.64Ga0.36N QBs, n-Al0.7Ga0.3N layer, and 4-nm-thick QWs, which has a peak emission wavelength of 284.5 nm at 60 mA, exhibits high internal quantum efficiency of 25% and high degree of optical polarization of 0.874. [ABSTRACT FROM AUTHOR]

This paper derives theoretical solutions for three radially layered piezoceramic/epoxy cylindrical composite transducers, which are composed of a solid epoxy disk, two axially polarized piezoceramic rings, and two epoxy rings. Two piezoceramic rings are the functional components, which can actuate and adjust the composite’s performance. According to different functions, three typical transducers are developed. The first one involves both of the two piezoceramic rings acting as actuating elements with parallel connections electrically. The other two involve only one piezoceramic ring as an actuating element, while the other ring that is connected to a resistor acts as a sensing element to adjust the electromechanical characteristics. Based on the plane stress assumption, theoretical solutions of these three transducers in radial vibration are derived, and performance differences of their electromechanical characteristics are analyzed and discussed. Furthermore, the solutions are validated by comparing with the ANSYS simulation results and the experimental data. The simulated and the measured first resonance and antiresonance frequencies are in a good agreement with the theoretical results, which validates the accuracy of the directed solution. This paper contributes to a comprehensive understanding of the proposed cylindrical composite’s electromechanical performance, which is helpful for further application in underwater sound and ultrasonic fields. [ABSTRACT FROM AUTHOR]

A versatile mass-sensing platform based on the nonlinear dynamical response of a microcantilever embedded in a self-excitation feedback loop is proposed. It is experimentally shown that the delay imposed in the feedback loop by an adjustable phase-shifter can be used to finely tune this system to work in three different modalities, according to the desired mass sensing application: 1) as a continuous mass sensor, where the oscillation frequency smoothly responds to changes in the mass added to the resonator; 2) as a threshold sensor, where a sudden change in the oscillation frequency is triggered by an arbitrarily small change of mass added to the cantilever; and 3) as a stable microresonator, whose oscillation frequency is almost not affected by environmental conditions, such as changes in added mass, or in density/viscosity of the surrounding fluid. This variety of dynamical responses was registered for a wide range of added masses, in the form of beads individually attached to the cantilever. A complete analytical model to explain the observed experimental results is derived and shows a strong agreement with the measured data. The high resolution and signal-to-noise ratio, as well as the threshold and stable sensing modalities obtained with this closed-loop technique, are not available in the current open-loop microcantilever-based mass sensors. [ABSTRACT FROM AUTHOR]

In III–nitride nanowire (NW)-based solar cells, efficient utilization of polarization charges is essential in order to achieve high performance. In this paper, the polarization behavior of a GaN/InxGa1–xN/GaN core/shell/shell triangular NW solar cell with {000-1} or {0001} as one of the facets is intensively studied through numerical simulations. An analysis of defect density, strain profile, and polarization charges at different facets of NWs with different “In” compositions is considered carefully. It is observed that an NW solar cell with a N-polar {000-1} facet is good enough to enhance carrier collection efficiency as compared with Ga-polar {0001} facet. This numerical study provides an innovative aspect of implementation of fundamental device physics with respect to recent growth techniques in order to realize the application of III–nitride NWs toward photovoltaic applications. The effect of an InxGa1–xN layer with different “In” compositions in all crystallographic orientations of NW solar cells is also discussed. It is interesting to observe that a maximum conversion efficiency of ~3.85% with the 93.38% fill factor is achieved from an n-GaN/i-InxGa1–xN/p-GaN nanowire solar cell considering 10% of “In” content under one Sun AM1.5 illumination. [ABSTRACT FROM PUBLISHER]

The objective of this paper is to understand the characteristics of the phenomena in piezo devices in a better manner, and to demonstrate a novel application for triage. That is, we focus not only the piezo effect, but also the pyroelectric effect of devices, which may expand the possibility of their application fields. To accomplish this, instead of the conventional equivalent electric circuit, we use a lumped-mass mathematical model of the device based on electrostatics and mechanics using a block diagram and transfer functions. The proposed model shows that the piezo device has internal feedback, indicating that it can function as a multiple-input/multiple-output sensor and/or actuator with reversible characteristics. These characteristics allow us to apply piezo devices to triage to detect the vital signs of heartbeat and respiration when the device is set inside a mask. We developed a simple and robust low-power, mask-type triage sensor that visualizes the respiration and heartbeat. [ABSTRACT FROM PUBLISHER]

ZnO is receiving a considerable attention for the development of novel cost-effective nanostructures with outstanding functional properties for applications in electronics and energy. In this paper, we investigate the effects of the nonlinear piezoelectricity, that has been recently observed in ZnO nanostructures, on the piezotronic effect of ZnO nanowires (NWs). We insert a physically-based model of the nonlinear direct piezoelectric effect into a fully-coupled thermo-mechanical-electric scheme to study the current–voltage characteristic of ZnO NWs under a purely vertical compressive/tensile strain. Our results show for the first time that the nonlinear piezoelectricity deeply affects the current transport processes inside the NW and the behavior of devices for piezoelectric-piezotronic applications and provides remarkable insights into the underlying physics. [ABSTRACT FROM PUBLISHER]

Lead metaniobate PbNb2O6 (PN) has a unique combination of high piezoelectric anisotropy; relatively low dielectric permittivity and high Curie temperature; and a low Q-factor, near 20. The very low Q-factor is the most intriguing PN property among the piezoelectric materials, and as shown in this research, this internal high dissipation and damping effect is directly related to the presence of silicon oil in the porous PN structure; consequently, it is dependent on the oil properties. To the contrary, the quality factor of PN not saturated with oil was found to be as high as nearly 400. Full sets of PN electro-mechanical constants, transient resonance and dissipation characteristics, and their temperature dependencies were determined under both conditions: PN conventionally saturated with oil and PN not saturated with oil. As was experimentally shown, at higher temperatures particularly after a 260?C soak for several days, a transition from the ?with oil? state to the ?no oil? state takes place in the conventional PN properties; this effect is a consequence of the phase transition in the silicon oil from liquid to solid state. [ABSTRACT FROM AUTHOR]

Here, we report a facile fabrication of flexible piezoelectric sensors based on highly aligned poly(vinylidene fluoride) (PVDF) nanofibers and printed silver electrodes without any additional poling processes. One-dimensional (1-D) piezoelectric PVDF nanofibers are directly produced onto stamped and sprayed interdigitated electrodes by electrospinning. We studied here the effect of contact area between nanofibers and electrodes by controlling the surface roughness of electrode. PVDF sensors show increased piezoelectric responses with larger actual contact area of the interdigitated electrodes. The measured output voltage under periodic bending stress is up to about 1 V. It is expected that this facile method to fabricate the PVDF sensors can be used to integrate the energy harvesters into flexible and stretchable functional electronic devices. [ABSTRACT FROM PUBLISHER]

Boron nitride (BN) films were deposited on different metal buffer layer (Cu, Al, Pt) using RF magnetron sputtering. Microstructure and piezoelectric properties of BN films were characterized by FTIR and piezoresponse force microscopy (PFM). After optimizing the deposition condition, the 95% cubic phase volume fraction from FTIR results indicates that BN films with the highest cubic phase is obtained under 120 V bias voltages. Based on the PFM measurements results, the images of the piezoelectric response, the butterfly curve, and the hysteresis loop of a certain grain in the films and polarization image of c-BN/Cu/SiO2/Si structure are confirmed. Compared with Pt and Al, Cu buffer layer is more suitable for depositing c-BN films with better piezoelectric properties by conventional fabrication process. [ABSTRACT FROM PUBLISHER]

The effect of temperature-dependent electroluminescence (EL) on nitride-based light-emitting diodes (LEDs) with different thicknesses of quantum barrier are studied and demonstrated. It was found that quantum confined stark effect (QCSE) of 6-nm thick barrier was more slightly than that of 9- and 12-nm thick barrier. The results indicated that the polarization field is independent of ambient temperature due to no clearly change of blue-shift value. The results also pointed out that the polarization field within the active region of 12-nm thick barrier was stronger than the others due to larger variation of the wavelength transition position (i.e. blue-shift change to red-shift) from 300 to 350 K, and thus it needed more injection carriers to complete the screening of QCSE. In this study, we reported a simple method to provide useful comparison of electrostatic fields within active region in nitride-based LEDs, specifically for structures consisting of identical active regions with different barrier thicknesses. [ABSTRACT FROM PUBLISHER]

This paper investigates the optimization of PECVD \alpha -SiN-passivated AlGaN/GaN heterostructures to achieve higher 2-D electron gas (2DEG) densities and lower the electric fields residing in the AlGaN barrier layer. A model is developed to calculate the 2DEG density of passivated AlGaN/GaN heterostructures taking into account the piezoelectric and spontaneous polarization effects together with strain relaxation. The model is validated with 2DEG measurements and data from the literature. The optimized passivation layer thickness is calculated for different Al mole fractions and AlGaN thicknesses to achieve targeted 2DEG densities and polarization electric field. Fabrication of samples with different \alpha-SiN thicknesses and effects of postannealing on 2DEG density are reported. The model accurately predicts the experimental results. [ABSTRACT FROM AUTHOR]

We investigate the dependence of dipole matrix elements for InGaN/GaN single quantum well structures on the indium surface segregation (ISS). Obtained results show that the influence of the surface segregation on the dipole matrix element is not equal for all optical transition. This effect results from the joint action of the piezoelectric polarization and ISS that change selection rules. In addition, surface segregation at each interface of the quantum well has different impact on optical characteristics depending on the direction of the piezoelectric polarization. The effect of the surface segregation has been estimated applying the global sensitivity analysis in the frame of six-band approximation for the valence band and parabolic approximation for the conduction band. [ABSTRACT FROM PUBLISHER]

Theoretical analysis for different active layer structures is performed to minimize the laser threshold current of the ultraviolet GaN/AlGaN multiple-quantum-well laser diodes by using advanced device simulation. The simulation results show that the lower threshold current can be obtained when the number of quantum wells is two or three and the aluminum composition in the barrier layer is about 10%-12%. This result is attributed to several different effects including electron leakage current, nonuniform carrier distribution, interface charge density induced by spontaneous and piezoelectric polarization, and optical confinement factor. These internal physical mechanisms are investigated by theoretical calculation to analyze the effects of quantum-well number and different aluminum compositions in barrier layer on laser threshold properties. Furthermore, the effect of quantum-well thickness is discussed as well. It is found that the optimal quantum-well thickness is about 3 nm due to the balance of the advantages of a large confinement factor against the disadvantages of significant quantum-confined Stark effect (QCSE). [ABSTRACT FROM PUBLISHER]

The influence of spontaneous and strain-induced piezoelectric polarizations on the photovoltaic characteristics of an \Al0.14\Ga0.86\N/\In0.21 \Ga0.79\N superlattice (SL) solar cell is numerically investigated. The simulation results show that, compared with the conventional p-i-n structure, the \Al0.14\Ga0.86\N/\In0.21\Ga0.79\N SL solar cell has better photovoltaic characteristics and is less sensitive to the effect of internal polarization. Therefore, the use of an SL structure in the III-nitride solar cells is advantageous if both the polarization effect and the crystal quality are of major concern. [ABSTRACT FROM AUTHOR]

The influence of piezoelectric polarization on the performance of p-on-n (0001)-face GaN/InGaN p-i-n solar cells is investigated. Simulation results show that the energy band is tilted into the direction detrimental for carrier collection due to the polarization-induced electric field. When the indium composition of InGaN layer increases, this unfavorable effect becomes more serious which, in turn, deteriorates the device performance. This discovery demonstrates that, besides the issue of crystal quality, the problem caused by the polarization effect needs to be overcome for the development of GaN-based solar cells. [ABSTRACT FROM AUTHOR]

We investigate the effect of built-in spontaneous and piezoelectric polarization on the internal device physics of current-injected GaN-based vertical-cavity surface-emitting lasers (VCSELs) with strained InGaN quantum wells. Advanced device simulation is applied to a previously manufactured device design featuring dielectric mirrors and an indium-tin-oxide current injection layer. Contrary to common perception, we show: 1) that only a small fraction of the built-in quantum-well polarization is screened at typical injection current densities and 2) that the polarization of the AlGaN electron stopper layer has a strong effect on the VCSEL threshold current which can be partly compensated for by higher p-doping. [ABSTRACT FROM PUBLISHER]

Textured ceramic fibers were fabricated from the tungsten-bronze-type piezoelectric (KSr2Nb5O15) . Development of crystallographic texture was evaluated through rocking curve analysis and a texture with Lotgering factor of 0.97 was achieved. 1-3 piezocomposites were prepared from the textured fibers and their electrical properties were evaluated. Highly enhanced properties that are 2 to 3 times higher than the random case were observed with increasing texture fraction with Pr and d33 reaching 20 μC/cm2 and 75 pC/N, respectively. [ABSTRACT FROM AUTHOR]