Thickness Of Top Layer Research Articles

Analytical Evaluation of a Coplanar Interdigitated Sensor Capacitance for 1-N-1 Multilayered Structure

In this article, an analytical expression (model) for the capacitance of coplanar interdigitated (IDT) capacitive sensors of four 1- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n$ </tex-math></inline-formula> -1 patterns (such as 1-1-1, 1-3-1, 1-5-1, and 1-11-1) based on conformal mapping (CM) techniques has been obtained. An important feature of the proposed model is that it is universal and independent of any specific application. The proposed model can be used for any number of layers (variable permittivity and thickness), finger width, and the gap between the fingers. In general, the capacitance of a particular IDT sensor depends on finger (electrode) length, the number of fingers, potential distribution of the fingers, dielectric permittivity (substrate and sensitive layers), and geometric parameters (nondimensional): 1) the ratio between the finger widths and gap and 2) the ratio between the top sensitive layer thickness and the sensor wavelength (spatial). Keeping these parameters in mind, comparisons with finite element analysis and experimental results have been made. Also, to confirm the effectiveness of the proposed model, different patterns of 1- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n$ </tex-math></inline-formula> -1 structured IDT sensors have been developed to measure the water droplets on the leaf canopy by a noncontact method. The proposed sensor structure can be said to be a noncontact leaf wetness sensor (LWS), generally employed to quantify the leaf wetness duration (LWD).

A parametric study of embankment supported by geosynthetic reinforced load transfer platform and timber piles tip on sand

This paper presents the results of finite element parametric study on the performance of pile-supported embankment system built over soft soil utilizing geosynthetic-reinforced load transfer platform (GLTP), where timber piles tipped on sand. The studied parameters included thickness of top soft layer, lateral extent of GLTP, embankment height (H), pile spacing, GLTP configuration, and different soil profiles. Three performance criteria were considered: maximum geosynthetic tensile strain, settlement, and slope stability. The results show that extending GLTP to 1.5H for low embankment load and full extent of 2.0H for medium and high embankment loads satisfies slope stability. The required number of geosynthetic layers varies from 3 layers for low and intermediate embankment loads to 7 layers for high embankment load with large pile spacing. Increasing the thickness of top soft layer results in increasing the strains along geosynthetics. However, increasing the number of geosynthetic layers and thickness of GLTP results in lower strains along geosynthetics but have little effect on settlement. Increasing 2 geosynthetic layers results in 15–20 % reduction in geosynthetic strains, but <2 % reduction in settlement. Meanwhile, increasing GLTP thickness by 50 % results in 10 % reduction in settlement, and 20–25 % reduction in geosynthetic strains.

Influence of optical aberrations on depth-specific spatial frequency domain techniques.

Spatial frequency domain imaging (SFDI) and spatial frequency domain spectroscopy (SFDS) are emerging tools to non-invasively assess tissues. However, the presence of aberrations can complicate processing and interpretation. This study develops a method to characterize optical aberrations when performing SFDI/S measurements. Additionally, we propose a post-processing method to compensate for these aberrations and recover arbitrary subsurface optical properties. Using a custom SFDS system, we extract absorption and scattering coefficients from a reference phantom at 0 to 15mm distances from the ideal focus. In post-processing, we characterize aberrations in terms of errors in absorption and scattering relative to the expected in-focus values. We subsequently evaluate a compensation approach in multi-distance measurements of phantoms with different optical properties and in multi-layer phantom constructs to mimic subsurface targets. Characterizing depth-specific aberrations revealed a strong power law such as wavelength dependence from ∼40 to ∼10 % error in both scattering and absorption. When applying the compensation method, scattering remained within 1.3% (root-mean-square) of the ideal values, independent of depth or top layer thickness, and absorption remained within 3.8%. We have developed a protocol that allows for instrument-specific characterization and compensation for the effects of defocus and chromatic aberrations on spatial frequency domain measurements.

Molecular dynamics simulation of oxide-nitride bilayer etching with energetic fluorocarbon ions

In 3D NAND devices, the number of stacked layers dictates the storage capacity. In the fabrication of such devices, hole channels with a high-aspect ratio (HAR) are etched through these layers. The higher the aspect ratio becomes, the more difficulties the HAR etching faces. In this study, molecular dynamics simulation was performed to examine the etching of silicon dioxide (SiO2), silicon nitride (SiN), and oxide-nitride (ON) stacked layers by energetic fluorocarbon ions. Good agreement of etching yields obtained from the simulations and ion beam experiments was observed for the etching of SiO2 and SiN by CF3+ ions for the incident ion energy ranging from 200 to 2000 eV. As to the etching of the ON bilayer, the SiO2 and SiN layers were observed to be etched with their own etch rates at low ion incident energy. However, at sufficiently high incident ion energy, the oxide and nitride layers were mixed by energetic ion impact and the depth of the mixing layer exceeded the thickness of the top SiO2 layer thickness, resulting in a single etch rate limited by the etch rate of a much deeper underlying SiN material. This suggests that if the incident ion energy is high enough such that the thicknesses of the multilayers are lower than the ion penetration depth, the ON stacked layer exhibits a single etch rate determined by the mixed material of the oxide and nitride.

To realize a variety of structural color adjustments via lossy-dielectric-based Fabry–Perot cavity structure

Abstract Structural colors with tunable properties have extensive applications in surface decoration, arts, absorbers, and optical filters. Planar structures have more advantages over other forms studied to date due to their easy manufacturability. Metal-insulator-metal-based structures are one of the known methods to fabricate structural colors where colors can be tuned mainly by the thickness of the intermediate lossless insulator layer. However, generating colors by MIM structure requires a thin metallic layer on top, and the top metals’ abrasiveness and/or oxidation may degrade the colors quickly. Thus, we propose a lossy dielectric layer to replace the top metallic layer as a solution to ensure the structure’s durability by preventing scratches and oxidation. Herein, CrON/Si3N4/Metal structures have been studied where theoretical investigations suggest that highly saturated colors can be generated in the lossy-lossless dielectric structures. Experimental data validated such simulations by revealing a range of vivid colors. Furthermore, these structures can easily achieve strong light absorption (SLA) even for a thick top layer of ∼100 nm. The colors realized by these structures are appeared due to a combination of the interference effect of the asymmetric Fabry–Perot cavity structure and the absorption rate in the CrO x N1−x layer.

Load-Settlement Curve and Subgrade Reaction of Strip Footing on Bi-Layered Soil Using Constitutive FEM-AI Coupled Techniques

This study presents a hybrid Artificial Intelligence-Finite Element Method (AI-FEM) predictive model to estimate the modulus of a subgrade reaction of a strip footing rested on a bi-layered profile. A parametric study was carried out using 2D Plaxis FEM models for strip footings with width (B) and rested on a bi-layered profile with top layer thickness (h) and bottom layer thickness (H). The soil was modeled using the well-known Mohr-Coulomb’s constitutive law. The extracted load-settlement curve from each FEM model is approximated to hyperbolic function and its factors (a, b) were determined. The subgrade reaction value (Ks) is the (stress/settlement), hence (1/Ks = a·Δ + b). Both inputs and outputs of the parametric study were collected in a single database containing the geometrical factors (B, h &amp; H), soil properties of the top and bottom layers (c, φ &amp; γ) and the extracted hyperbolic factors (a, b). Finally, three AI techniques—Genetic Programming (GP), Evolutionary Polynomial Regression (EPR) and Artificial Neural Networks (ANN)—were implemented to develop three predictive models to estimate the values of (a, b) using the collected database. The three developed models showed different accuracy values of (50%, 65% and 80%) for (GP, EPR and ANN), respectively. The innovation of the developed model is its ability to capture the degradation of a subgrade reaction by increasing the stress (or the settlement) according to the hyperbolic formula.

Hydrogen-Assisted Molecular Beam Epitaxy of SiGeSn

Since its first synthesis in 2003 [1], the ternary alloy semiconductor SiGeSn has seen an increasing interest due to its unique properties in the field of Group-IV semiconductors [2–6]. SiGeSn not only allows the decoupling of its bandgap and lattice-constant by what it is predestined for the epitaxy of strain-reduced heterostructures on the Si platform. Furthermore, SiGeSn becomes a direct bandgap semiconductor at high Sn concentrations. Considering these properties, SiGeSn is an attractive material system for optoelectronic applications. An exemplary heterojunction optoelectronic device is the single confinement heterostructure laser diode, the still missing key component for the monolithic integration of the optical on-chip communication on Si.However, one of the greatest challenges faced by many Group-IV researchers is the epitaxy of SiGeSn bulk alloys with high crystal quality. The temperature induced segregation of Sn results in interstitial Sn atoms and subsequently in acceptor-like defect states, as it was already reported for GeSn [7].In this work, we show the molecular beam epitaxy (MBE) of SiGeSn structures under Hydrogen ambient. First of all, we determined the concentration of the acceptor-like defect states of undoped SiGeSn NA,defect on the basis of capacitance-voltage (C-V) measurements of SiGeSn pin diodes. Furthermore, we investigated Hydrogen-assisted epitaxy as an approach for the reduction of the acceptor-like defect states.All epitaxy experiments were performed in a 6” MBE system, where Si, Ge and Sn are used as matrix materials and B and Sb as dopants respectively. In particular, we investigated the influence of molecular (H2) as well as atomic (H) Hydrogen on the epitaxy process and therefore the quality of the grown structures. For this purpose, the MBE system was upgraded with a Hydrogen atom beam source for the generation of an atomic Hydrogen flux.Since the Sn segregation is affected mostly by the substrate temperature, this critical growth parameter is measured in-situ using an infrared pyrometer, which allows the observation of the dynamic processes on the sample surface [8] and the precise control of the substrate temperature at TSub = 200 °C during the SiGeSn epitaxy.All SiGeSn structures are based on Ge virtual substrates (Ge-VS) on Si(001) [9], followed by a 400 nm thick additional Ge buffer layer. In order to fulfil lattice matching of SiGeSn on Ge, a constant ratio of Si and Sn of Si/Sn = 3.67 was kept for all structures. The Sn concentration was varied in the range of 5 % < cSn < 10 %. For material characterization, 300 nm thick undoped SiGeSn layers were grown on this substrate layer stack (see Fig. 1a). A detailed characterization of the crystallinity and the surface roughness was performed using X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. The comparison of AFM micrographs of SiGeSn layers, grown with and without Hydrogen assistance (see Fig. 1b) show a clear reduction of the surface roughness due to Hydrogen assisted epitaxy.In order to determine NA,defect, C-V measurements on SiGeSn pin diodes (see Fig. 1c) were performed. The diode layer structure, also based on the previously described substrate layer stack, starts with a 400 nm thick p-doped SiGeSn bottom layer with an acceptor concentration of NA = 5x1019 cm-3, followed by an 300 nm thick undoped SiGeSn layer and a closing 200 nm thick n-doped SiGeSn top layer with a donor concentration of ND = 5x1019 cm-3. Subsequently, pin diodes were fabricated using a single mesa process consisting of four basic steps: Mesa structuring using inductive coupled plasma reactive ion etching (ICP-RIE) with HBr, mesa passivation with plasma enhanced chemical vapor deposition (PECVD) grown SiO2, oxide window opening using RIE with CHF3 and metallization using sputtered Al. An exemplary device can be seen in the scanning electron microscopy (SEM) image in Fig. 1d.The SiGeSn pin diodes were electrically characterised by means of C-V measurements using a Keithley 4200 semiconductor characterisation system. For further evaluation, the C-V characteristics of the pin diodes show linear behaviour when plotted on a C-2-V-scaled diagram (Fig. 1e). Under the assumption of a strongly asymmetric pn junction, (ND >> NA,defect), the slope m of the C-2-V characteristics is proportional to NA,defect. In this manner, we determined NA,defect for several SiGeSn pin diodes in dependence of the alloy composition. The results, as shown in Fig. 1f, show an acceptor-like defect concentration between 1017 cm-3 < NA,defect < 1018 cm-3, which is comparable with results reported by other groups for GeSn [7].In our presentation, we will discuss Hydrogen-assisted epitaxy as a promising approach for improving the SiGeSn crystal quality. Figure 1

High-flux ceramic membrane derived from UV-curable slurry for efficient separation of nanoparticles suspension

Ceramic membranes with high performance have been increasingly applied in the process industry. In this study, a rapid construction process was proposed for preparing high-flux ceramic membranes by UV-curable slurry. Two different alumina powders were well dispersed to enable the dip-coating of interlayer and top layer, respectively, on a 3D printed microporous support. After the coating of interlayer and top layer, a rapid UV curing process was applied, which was completed within 30 s, as opposed to the traditional drying process of more than 10 h for each layer. Additionally, the dual-layer was formed by a one-step co-sintering instead of conventional separate sintering. The effects of coating time on the thickness of interlayer and top layer, as well as the performance of ceramic membranes, were systematically studied. The optimized dual-layer ceramic membrane exhibited a uniform pore size distribution with an average pore size of about 108 nm, and high pure water permeance of 1220 L·m−2·h−1·bar−1. The high-flux ceramic membranes exhibited a good performance in the separation of nanoparticles from water suspensions. In the filtration of CeO2 suspension, the permeance was as high as 850 L·m−2·h−1·bar−1, and the dispersed nanoparticles were almost completely removed.

When and Where Are Multiple Snow Layers Important for Simulations of Snow Accumulation and Melt?

AbstractMany watershed‐scale and land surface models incorporate snowmelt modules with simplified representations of the snowpack with three or fewer layers. These modeling choices were traditionally made to reduce model complexity and computational demand while still being able to simulate large model domains. However, these simple snow layering schemes may not always simulate snow processes and the effects of climate change across a range of climatic and geographic conditions. Here we evaluate simple snow layering schemes (having two to five layers, commonly found in watershed‐scale and land surface models) against a synthetic benchmark with up to 100 layers at three locations with different climate conditions using the SUMMA modeling framework. We evaluate 10 different layering configurations of two to five layers with variable thicknesses and show that the effect of the layering scheme varies with site conditions. We find that the layer configuration is more important at a cold high elevation site in the Sierra Nevada, California (∼1.4°C annual average temperature), and at a warm site in the French Alps (∼6.5°C), and less important at a site in Idaho (∼5.0°C). The top layer thickness of the simpler snow layering configurations also influences the simulated snow surface and snowpack temperatures and timing of snowmelt. Our tests showed that the five‐layer model with thin layers near the surface was closest to the benchmark (median NSE = 0.99), and therefore we recommend using multiple snow layers for reliable simulations of snow accumulation and melt across a range of climates.

Free vibration analysis of a porous functionally graded, truncated conical shell with a magneto-rheological fluid core

Vibration absorbers are vital to resisting shocks in many structures. This study analyzed the effects of magnetic field, porosity and geometry on the natural frequency and loss factor of a conical shell made of smart porous materials. Natural frequency declined when the length, radius and thickness ratios increased. The loss factor and porosity coefficients increased concurrently. The magnetic intensity had little effect on the natural frequency. Increasing the apex angle or porosity, raised the loss factor but reduced the natural frequency. Raising the core’s top layer thickness reduced the natural frequency but increasing the L/R1 or R1/h ratio reduced the frequency.

Two-layer analytical model for estimation of layer thickness and flow using Diffuse Correlation Spectroscopy.

Diffuse correlation spectroscopy (DCS) has been widely explored for its ability to measure cerebral blood flow (CBF), however, mostly under the assumption that the human head is homogenous. In addition to CBF, knowledge of extracerebral layers, such as skull thickness, can be informative and crucial for patient with brain complications such as traumatic brain injuries. To bridge the gap, this study explored the feasibility of simultaneously extracting skull thickness and flow in the cortex layer using DCS. We validated a two-layer analytical model that assumed the skull as top layer with a finite thickness and the brain cortex as bottom layer with semi-infinite geometry. The model fitted for thickness of the top layer and flow of the bottom layer, while assumed other parameters as constant. The accuracy of the two-layer model was tested against the conventional single-layer model using measurements from custom made two-layer phantoms mimicking skull and brain. We found that the fitted top layer thickness at each source detector (SD) distance is correlated with the expected thickness. For the fitted bottom layer flow, the two-layer model fits relatively consistent flow across all top layer thicknesses. In comparison, the conventional one-layer model increasingly underestimates the bottom layer flow as top layer thickness increases. The overall accuracy of estimating first layer thickness and flow depends on the SD distance in relationship to first layer thickness. Lastly, we quantified the influence of uncertainties in the optical properties of each layer. We found that uncertainties in the optical properties only mildly influence the fitted thickness and flow. In this work we demonstrate the feasibility of simultaneously extracting of layer thickness and flow using a two-layer DCS model. Findings from this work may introduce a robust and cost-effective approach towards simultaneous bedside assessment of skull thickness and cerebral blood flow.

Effect of a Discontinuous Ag Layer on Optical and Electrical Properties of ZnO/Ag/ZnOStructures

ZnO/Ag/ZnO nanolaminate structures were deposited by consecutive RF sputtering at room temperature.The optical transparency, sheet resistance, and figure of merit are determined in relation to the deposition time of Ag and to the film thickness of the ZnO top layer. An improved transmittance has been found in the visible spectral range of the ZnO/Ag/ZnO structure compared to ZnO multilayers without Ag. High transmittance of 98% at 550 nm, sheet resistance of 8 Ω/sq, and figure of merit (FOM) of 111.01 × 10−3 Ω−1are achieved for an optimized ZnO/Ag/ZnO nanolaminate structure. It is suggested that the good optical and electrical properties are due to the deposition of the discontinuous Ag layer. The electrical metallic type conductivity is caused by planar located silver metal granules. The deposition of a discrete layer of Ag nano-granules is confirmed by atomic force microscopy (AFM) and cross-section high-resolution transmission electron microscopy (HRTEM) observations.

Strong Coupling of Tamm Plasmons and Fabry-Perot Modes in a One-Dimensional Photonic Crystal Heterostructure

The coupling between multiple modes in subwavelength scale has attracted a lot of attention due to the interesting physics favored by the enhancement of light-matter interaction. In this contribution, we propose a planar hybrid plasmonic-photonic crystal structure, which enables the tunable coupling between Tamm plasmon-polariton and Fabry-Perot cavity modes. Then, a strong coupling can be achieved through evoking the Tamm-mode dependence of the top-layer thickness of the hybrid structure, which is indicated by an anticrossing with a spectrum splitting of 9.298 meV. The effect enables the spatial confinement of polaritons and the formation of hybrid one-dimensional plasmon-polariton modes. In order to account for the splitting of the orthogonally polarized resonances, a detailed investigation on the TE-TM splitting is also performed through the angular-dependence analysis. We note that the value of the TE-TM splitting strongly depends on the thicknesses of dielectric layer and the distance between two distributed Bragg reflector (DBR) stacks. We show that the cavity modes are extremely sensitive to the distance between the DBR stacks and propose using this feature for strain sensing. Thus, our structure may not only offer a platform for study of interesting physics (e.g., spin optronics), but may also provide a route for high-accuracy strain sensors.

Penetration Behavior of the Footing of Jack-Up Vessel of OWTs in Thin Stiff over NC Clay

This study investigated the behavior of the spudcan foundation of jack-up vessels of offshore wind turbines during the undrained vertical penetration into thin stiff-over-normally consolidated clay. Large deformation finite element (LDFE) analyses were used to simulate the continuous spudcan penetration into the seabed surface. Detailed parametric analysis was performed to explore a range of normalized soil properties and layer geometry and roughness of the soil–spudcan interface. The results were validated against previously reported data. The LDFE results were consistent with those of centrifuge tests. The evolving soil-failure patterns revealed soil backflow and the trapping of stronger top-layer material beneath the spudcan. The plug shape was influenced by the top layer thickness, the strength gradient of the bottom layer, and the relative strength ratio, which also affected the penetration resistance of soils. In this study, an expression was derived to quantify the plug shape with the aim of providing a theoretical basis for the design of spudcan footings with penetration resistance suitable for thin stiff-over-soft clay.

(Invited, Digital Presentation) Approach to Neuromorphic Computing with Ferroelectric Schottky Barrier FETs

Neuromorphic computing inspired by neural network systems of the human brain enables energy efficient computing as a solution of the von Neumann bottleneck. A neural network consists of thousands or even millions of neurons which communicate with each other through connected synapses. Synapses can memorize and process the information simultaneously. The plasticity of a synapse to strengthen or weaken its activity over time make it be capable of learning and computing. Thus, artificial synapses which can emulate functionalities and the plasticity of bio-synapses form the backbone of a neuromorphic computing system.Non-volatile memories with two-terminals, like resistive random-access memory (ReRAM), phase change memory (PCM), are attractive candidates for artificial synapses. However, signal processing and learning cannot be performed simultaneously in these two-terminal synapses. FeFET, similar to a MOSFET structure using CMOS compatible HfO2 based ferroelectrics as gate oxide forms three-terminal synapses offering high endurance, good performance and high energy efficiency. In contrast to two-terminal devices, three terminal FeFET based synapses can perform processing and learning at the same time. In order to maintain the ferroelectric properties of an HfO2 based ferroelectric film, high temperature annealing should be avoided after the ferroelectric layer deposition. In this paper, we present ferroelectric NiSi2 source/drain Schottky barrier (SB) MOSFET (FE-SBFET) structure (Fig.1a), which requires neither ion implantation nor thermal activation of source/drain contacts at high temperatures.FE-SBFETs were fabricated on SOI substrates with a boron-doped (1016 B/cm-3), 55 nm thick top Si layer and a 145 nm thick buried oxide (BOX) layer. Very thin (9 nm) single crystalline NiSi2 layers which offer superior properties of uniform and stable SB contacts on Si are used at source/drain regions. A gate stack consisting of 10 nm thick Hf0.5Zr0.5O2 (HZO) layer and a 40 nm thick TiN layer are deposited by ALD and sputtering, respectively. A rapid thermal annealing at 500 °C is performed to crystallize the HZO into a ferroelectric phase before the gate patterning. The fabricated device has a channel length of 10 µm and a gate width of 10 µm. The overlap between the top gate and NiSi2 is 6 µm along the channel and 10 µm wide on each side. The ferroelectric polarization modulates both the SB at the source/drain contacts as well as the potential in the channel, thus changing the carrier injection through the SB. The Id-Vg transfer characteristics of a p-type FE-SBFET shows a clockwise hysteresis which is caused by the ferroelectric polarization switch.The excitatory post-synaptic current (EPSC), one of the typical short-term synaptic plasticity features for biologic synapses, is characterized by measuring the transient drain currents for a voltage pulse on the gate of a FE-SBFET (Fig.1b). The amplitude of the pulse VAM changes from -0.2 to -1.2 V with a fixed pulse width tpw=1 μs. We found that the EPSC peak value increases linearly with VAM. It shows a very low energy/spike consumption of 2fJ/spike at VAM=-0.2 V, demonstrating a very high energy efficiency. From the EPSC measurements with repeated gate voltage pulses paired-pulse facilitation/depression (PPF/PPD) are characterized showing an exponential decay, similar to biological synapses.The long-term synaptic plasticity of FE-SBFET synapses is characterized by a series repeated identical or non-identical pulses. The later can improve the long-term potentiation/depression (LTP/LTD) symmetry and linearity. The measurements show a large Gmax/Gmin ratio, very high endurance and small cycle-to-cycle (CTC) variation (1.06%) due to the perfect contact of NiSi2 (Fig.1c). The biological neuron-like spike-timing-dependent plasticity (STDP) is characterized for the FE-SBFET synapse. The results show an asymmetric anti-Hebbian STDP, which is one of the biological STDP functionalities (Fig.1d).In conclusion, the fabricated FE-SBFET synapse exhibits multiple synaptic functions with high endurance and small variations. The ultra-low energy/spike consumption indicates a high potential for low power neuromorphic computing applications. Acknowledgement: This work was supported by the Federal Ministry of Education and Research (BMBF, Germany) in the project NEUROTEC (16ME0398K). Figure 1

Morphological Instability of Lithium Electrodeposition Due to Stress-Driven Interface Diffusion

Morphological instability of the lithium-electrolyte interface is a critical problem limiting the development of lithium-metal negative electrodes for batteries. At high current densities approaching the diffusion-limited current density, dendrites form due to depletion of Li+ ions near the electrode surface (1). At lower current densities, unstable deposition produces whiskers (2). Whiskers are separated by typically several micrometers, and in contrast to dendrites grow by addition of Li atoms to their base or "root" (3). Experimental evidence indicates that whisker growth is fed by large-scale interface or grain boundary diffusion, and that whiskers relieve compressive stress in the metal generated by electrodeposition (4-7).The present study proposes that Li electrodeposition is destabilized by interface diffusion driven by compressive stress due to incorporation of Li atoms at grain boundaries. The competition between stress and stabilizing surface energy effects generates a surface pattern which determines (in part) whisker sites. A morphological instability model is formulated based on the Asaro-Tiller-Grinfel'd (ATG) surface instability on elastically stress solids (8). The model applies to deposits less than 1 micron thick for which elastic deformation is expected to dominate (9,10). The Li electrode is depicted by a three-layer elastic model consisting of a stress-free substrate (current collector) layer, a Li layer with uniform diffusion-induced in-plane stress, and top layer. The top layer can simulate submicron thickness solid-electrolyte interface (SEI) layers, or macroscopically thick polymer separators and solid electrolytes. The Li-top layer interface deforms by diffusion. Out-of-plane normal stress is included to simulate the effect of applied stress on the instability (11,12).For model calculations, the interface stress was estimated from neutron-depth-profiling measurements of Li diffusion into Cu current collectors (13). The measured Li incorporation was found to be consistent with a whisker spacing of several microns, in agreement with experimental results (3,6,14). Calculations showed that the instability is inhibited significantly by the use of substrates with elastic modulus much greater than that of Li. This substrate stiffness effect is consistent with experimental observations of Sn whiskers (15). The effect of a stress-free SEI layer on the instability was found to be negligible, due to its small thickness. Whisker growth was suppressed by macroscopically thick top layers with elastic modulus at least 10 times that of Li. No significant whisker inhibition was found at applied stress levels of ~ 1 MPa, which are found experimentally to stabilize deposition in Li films significantly exceeding 1 micron thickness (11,12). This effect may be due to an instability associated with viscoplastic rather than elastic deformation (16).REFERENCES P. Bai et al., Energy Environ. Sci., 9, 3221(2016).L Frenck et al., Front. Energy Res., 7, 115 (2016)A. Kushima et al., Nano Energy, 32, 271 (2017).J. H. Cho et al., Energy Storage Mater., 24, 281 (2020).X. Wang et al., Nat. Energy, 3, 227 (2018).A. A. Rulev et al., J. Phys. Chem. Lett., 11, 10511 (2020).E. Chason et al., Prog. Surf. Sci., 88, 103 (2013).B. J. Spencer et al., J. Appl. Phys., 73, 4956 (1993).C. Xu et al., Proc. Nat. Acad. Sci., 114, 57 (2017).L. Q. Zhang et al., Nat. Nanotechnol., 15, 94 (2020).A. J. Louli et al., J. Electrochem. Soc., 166, A1291 (2019).K. L. Harrison et al., ACS Appl. Mater. Interfaces, 13, 31668 (2021).S. Lv et al., Nat. Commun., 9, 2152 (2018).J. Steiger et al., J. Power Sources, 261, 112 (2014).B. Hutchinson et al., Mater. Sci. Forum, 467-470, 465 (2004).S. Narayan and L. Anand, J. Electrochem. Soc., 167, 040525 (2020).

Ohmic contact morphology improvement with reduced resistance using Si/Au/Ti/Al/Ni/Au (AlGaN) and Si/Au/Ti/Al/Ni/Au (InAlN) stack layers in III-Nitride HEMTs

This article reports a Ti/Al-based ohmic contact utilizing a thin interfacial Au layer for improved morphology, edge acuity and low contact resistance for applications to III-Nitride high electron mobility transistors (HEMTs). Conventional Ti/Al contacts are based on a metal stack of the form Ti/Al/X/Au, where X is any suitable barrier layer such as Ni. The formation of ohmic contact in GaN and its alloy systems is governed by the formation of TiN during annealing; Au inter-diffusion during annealing also assists the formation of TiN islands and aids in reducing the contact resistance. Furthermore, inter-metallic phase formation between Ti and Al lowers the contact resistance. It is observed that the contact surface morphology strongly depends on the top Au layer thickness. Insertion of a very thin Au layer (2–3 nm) has drastically improved the surface quality with the achievement of contact resistance as low as 0.36 Ω mm and a specific contact resistance of 2.4 × 10−6 Ω cm2 on AlGaN/GaN HEMT. The reduction in contact resistance is due to the formation of more conducting intermetallic phases during annealing, which is assisted due to the thin Au layer insertion. Further reduction in the contact resistance to 0.13 Ω mm was also achieved with the introduction of Si as a dopant layer in the reported metal scheme. The measured rms surface roughness was reduced to ∼8 nm from 0.12 µm in comparison to the conventional ohmic contact.

Radiation-hardened silicon photonic passive devices on a 3 µm waveguide platform under gamma and proton irradiation.

Silicon photonics is considered to be an ideal solution as optical interconnect in radiation environments. Our previous study has demonstrated experimentally that radiation responses of device are related to waveguide size, and devices with thick top silicon waveguide layers are expected to be less sensitive to irradiation. Here, we design radiation-resistant arrayed waveguide gratings and Mach-Zehnder interferometers based on silicon-on-insulator with 3 µm-thick silicon optical waveguide platform. The devices are exposed to 60Co γ-ray irradiation up to 41 Mrad(Si) and 170-keV proton irradiation with total fluences from 1×1013 to 1×1016 p/cm2 to evaluate performance after irradiation. The results show that these devices can function well and have potential application in harsh radiation environments.

Broadband diffuse optical spectroscopy of two-layered scattering media containing oxyhemoglobin, deoxyhemoglobin, water, and lipids.

We investigated the relationship between chromophore concentrations in two-layered scattering media and the apparent chromophore concentrations measured with broadband optical spectroscopy in conjunction with commonly used homogeneous medium inverse models. We used diffusion theory togenerate optical data from a two-layered distribution of relevant tissue absorbers, namely, oxyhemoglobin, deoxyhemoglobin, water, and lipids, with a top-layer thickness in the range 1–15 mm. The generated data consisted of broadband continuous-wave (CW) diffuse reflectance in the wavelength range 650–1024 nm, and frequency-domain (FD) diffuse reflectance at 690 and 830 nm; two source-detector distances of 25 and 35 mm were used to simulate a dual-slope technique. The data were inverted using diffusion theory for a semi-infinite homogeneous medium to generate reduced scattering coefficients at 690 and 830 nm (from FD data) and effective absorption spectra in the range 650–1024 nm (from CW data). The absorption spectra were then converted into effective total concentration and oxygen saturation of hemoglobin, as well as water and lipid concentrations. For absolute values, it was found that the effective hemoglobin parameters are typically representative of the bottom layer, whereas water and lipid represent some average of the respective concentrations in the two layers. For concentration changes, lipid showed a significant cross-talk with other absorber concentrations, thus indicating that lipid dynamics obtained in these conditions may not be reliable. These systematic simulations of broadband spectroscopy of two-layered media provide guidance on how to interpret effective optical properties measured with similar instrumental setups under the assumption of medium homogeneity.

Finite-Element Analysis of the Lateral Load Response of Monopiles in Layered Sand

Most of the existing design methods for laterally loaded monopiles were developed for uniform soil profiles. The application of such design methods in cases involving layered sand profiles can lead to incorrect capacity estimations. In order to investigate the impact of the soil layering on the response of monopiles to lateral loads and to develop a design method that is applicable to layered soil profiles, a series of three-dimensional (3D) finite-element (FE) analyses were performed on laterally loaded monopiles embedded in layered sand profiles using an advanced two-surface-plasticity constitutive model. The analyses take into consideration a wide range of pile geometries (pile diameter, slenderness ratio, and wall thickness), load eccentricities, sand types and relative densities, and layered soil profiles. The study examines the change of the lateral capacity of a monopile in a two-layer sand profile as the top layer thickness varies, as well as the impact of the presence of a thin loose-sand layer in a dense-sand profile on the pile lateral capacity. Based on the results of these analyses, a set of design equations is proposed to estimate the lateral capacity and load-rotation response of monopiles in layered sand profiles. The proposed method accounts for the 3D pile–soil interaction and provides estimates of the lateral capacity that are in close agreement with those obtained from the 3D FE analyses.