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arXiv2018

NIEL DOSE and DLTS Analyses on Triple and Single Junction solar cells irradiated with electrons and protons

Space solar cell radiation hardness is of fundamental importance in view of the future missions towards harsh radiation environment (like the Jupiter missions) and for the new spacecraft using Electrical Propulsion. In this paper we report the radiation data for triple junction (TJ) solar cells and related component cells. Triple junction solar cells, InGaP top cells and GaAs middle cells degrade after electron radiation as expected. With proton irradiation, a high spread in the remaining factors was observed, especially for the TJ and Ge bottom cells. Radiation results have been analyzed by means of the Displacement Damage Dose method and DLTS spectroscopy. In particular with DLTS spectroscopy it was possible to analyze the nature of a few defects introduced by irradiation inside the GaAs sub cell observing a strong correlation with the Displacement Damage Dose.

physics.ins-det↗

arXiv2018

Biomimetic potassium selective nanopores

Reproducing the exquisite ion selectivity displayed by biological ion channels in artificial nanopore systems has proven to be one of the most challenging tasks undertaken by the nanopore community, yet a successful achievement of this goal offers immense technological potential. Here we show a strategy to design solid-state nanopores that selectively transport potassium ions and show negligible conductance for sodium ions. The nanopores contain walls decorated with 4'-aminobenzo-18-crown-6 ether and ssDNA molecules located at one pore entrance. The ionic selectivity stems from facilitated transport of potassium ions in the pore region containing crown ethers, while the highly charged ssDNA plays the role of a cation filter. Achieving potassium selectivity in solid-state nanopores opens new avenues toward advanced separation processes, more efficient biosensing technologies and novel biomimetic nanopore systems.

physics.app-ph↗

arXiv2018

Detection and spatial mapping of conductive filaments in metal/oxide/metal cross-point devices

A simple means of detecting and spatially mapping volatile and nonvolatile conductive filaments in metal/oxide/metal cross-point devices is introduced and its application demonstrated. The technique is based on thermal discolouration of a thin photoresist layer deposited on the top electrode of the cross-point device and relies on the increase in temperature produced by local Joule heating of an underlying conductive filament. Finite element modelling of the temperature distribution and its dependencies shows that the maximum temperature at the top-electrode/photoresist interface is particularly sensitive to the top-electrode thickness. The technique is demonstrated on NbOx based metal-oxide-metal cross-point devices with a 25 nm thick top (Pt) electrode, where it is used to undertake a statistical analysis of the filament location as a function of device area. This shows that filament formation is heterogeneous. The majority of filaments form preferentially along the top-electrode edge and the fraction of these increases with decreasing device area. Transmission electron microscopy of the top and bottom electrode edges is used to explain this observation and suggests that it is due to a reduction in the effective oxide thickness in this region.

physics.app-ph↗

arXiv2018

A dynamic classifying approach for materials

Considering the time-varying of the poriness of the materials in the loading process, a dynamic classifying method for the materials is proposed based on the strain factor defined from the strain relationship in three orthogonal directions. Comparing with the current classifying approach based on the Poisson's ratio, the method presented in this work is a dynamic approach that can reflect the real-time strain relationship in three orthogonal directions as well as the varying specific molecular structures partly in the loading process of the materials.

physics.app-ph↗

arXiv2018

Effects of microstructure and growth conditions on quantum emitters in Gallium Nitride

Single-photon emitters in gallium nitride (GaN) are gaining interest as attractive quantum systems due to the well-established techniques for growth and nanofabrication of the host material, as well as its remarkable chemical stability and optoelectronic properties. We investigate the nature of such single-photon emitters in GaN with a systematic analysis of various samples produced under different growth conditions. We explore the effect that intrinsic structural defects (dislocations and stacking faults), doping and crystal orientation in GaN have on the formation of quantum emitters. We investigate the relationship between the position of the emitters (determined via spectroscopy and photoluminescence measurements) and the location of threading dislocations (characterised both via atomic force microscopy and cathodoluminescence). We find that quantum emitters do not correlate with stacking faults or dislocations; instead, they are more likely to originate from point defects or impurities whose density is modulated by the local extended defect density.

physics.app-ph↗

arXiv2018

Thermal fuctuations and dynamic modelling of a dAFM cantilever

We discuss the Brownian thermal noise which affects the cantilever dynamics of a dAFM (dynamic atomic force microscope), both when it works in air and in presence of water. Our scope is to accurately describe the cantilever dynamics, and to get this result we deeply investigate the relationship between the cantilever thermal fluctuations and its interactions with the surrounding liquid. We present a relatively simple and very easy-to-use analytical model to describe the interaction forces between the liquid and the cantilever. The novelty of this approach is that, under the assumption of small cantilever oscillations, by using the superposition principle we found a very simple integral expression to describe fluid-cantilever interactions. More specifically we note that, beside including fluid inertia and viscosity (which is common to many existing models in the literature) an additional diffisivity term needs to be considered, whose crucial influence for the correct evaluation of the cantilever response to the thermal excitation is shown in the present paper. The coefficients of our model are obtained by using numerical results for a 2D fluid flow around a vibrating rectangular cross-section, and depend on the distance from the wall. This allowed us to completely characterize the dynamics of a dAFM cantilever also when it operates in tilted conditions. We validate the analytical model by comparing our results with numerical and experimental dAFM data previously presented in literature, and with experiments carried out by ourselves. We show that we can provide extremely accurate prediction of the beam response up and beyond the second resonant peak.

physics.ins-det↗

arXiv2018

Effect of YIG Nanoparticle Size and Clustering in Proximity-Induced Magnetism in Graphene/YIG Composite Probed with Magnetoimpedance Sensors: Towards Improved Functionality, Sensitivity and Proximity Detection

Proximity-induced magnetism (PIM) in graphene (Gr) adjacent to magnetic specimen has raised great fundamental interests. The subject is under debate and yet no application is proposed and granted. In this paper, toward accomplishment of fundamental facts, we first explore the effect of particle size and clustering in the PIM in Gr nanoplates (GNPs)/yttrium iron garnet (YIG) magnetic nanoparticle (MNP) composite. Microscopic analyzes suggest that fine MNPs distributed uniformly on the GNPs have higher saturation magnetization due to the PIM in Gr. We propose that such magnetic plates can thus be used to shield the stray field generated on the surface of magnetic sensors and play a role as a magnetic lens to prevent the field emanating outside the body of magnetic specimen. The GNPs/YIG composites are coated on a magnetic ribbon and proposed for application in magneto-impedance (MI) sensors. We show that such planar magnetic flakes enhance the MI response against the external applied magnetic field significantly. The suggested application can be furthermore developed toward bio-sensing and magnetic shielding in different magnetic sensors and devices.

physics.app-ph↗

arXiv2018

Invited Article: 4D Printing as a New Paradigm for Manufacturing with Minimum Energy Consumption

4D printing is a new manufacturing paradigm that combines stimuli-responsive materials, mathematics, and multi-material additive manufacturing to yield encoded multi-material structures with intelligent behavior over time. This emerging field has received growing interests from various disciplines such as space exploration, renewable energy, bioengineering, textile industry, infrastructures, soft robotics, and so on. Here, as a first attempt, we consider the energy aspect of 4D printing. By a thermodynamic analysis, we obtain the theoretical limit of energy consumption in 4D printing and prove that 4D printing can be the most energy-efficient manufacturing process. Before that, we clearly underpin 4D printing as a new manufacturing process and identify its unique attributes.

physics.app-ph↗

arXiv2018

Characterization and Modeling of 0.18μm CMOS Technology at sub-Kelvin Temperature

Previous cryogenic electronics studies are most above 4.2K. In this paper we present the cryogenic characterization of a 0.18μm standard bulk CMOS technology(1.8V and 5V) at sub-kelvin temperature around 270mK. PMOS and NMOS devices with different width to length ratios(W/L) are tested and characterized under various bias conditions at temperatures from 300K to 270mK. It is shown that the 0.18μm standard bulk CMOS technology is still working at sub-kelvin temperature. The kink effect and current overshoot phenomenon are observed at sub-kelvin temperature. Especially, current overshoot phenomenon in PMOS devices at sub-kelvin temperature is shown for the first time. The transfer characteristics of large and thin-oxide devices at sub-kelvin temperature are modeled using the simplified EKV model. This work facilitates the CMOS circuits design and the integration of CMOS circuits with silicon-based quantum chips at extremely low temperatures.

physics.app-ph↗

arXiv2018

Optimization of quantum well number of AlGaN/AlGaN deep-ultraviolet light-emitting diodes

In this work, performance and characteristics of AlGaN/AlGaN deep-ultraviolet light-emitting diodes (DUV LEDs) with varied number of quantum-well (QW) are investigated numerically. From our simulation, 1-QW structure give the best performance at low injection current. However, at higher injection current, 2-QWs structure give the largest power output due to its higher total radiative recombination rate and internal quantum efficiency (IQE) compared to other structures. The 2-QWs structure also has less serious efficiency droop at high current than 1-QW, which makes it an optimum structure for high-power LEDs.

physics.app-ph↗

arXiv2018

An HAM-Based Analytic Modeling Methodology for Memristor Enabling Fast Convergence

Memristor has great application prospects in various high-performance electronic systems, such as memory, artificial intelligence, and neural networks, due to its fast speed, nano-scale dimensions, and low-power consumption. However, traditional nonanalytic models and lately reported analytic models for memristor have the problems of nonconvergence and slow convergence, respectively. These problems lay great obstacles in the analysis, simulation and design of memristor. To address these problems, a modeling methodology for analytic approximate solution of the state variable in memristor is proposed in this work. This methodology solves the governing equation of memristor by adopting the Homotopy Analysis Method (HAM) for the first time, and the convergence performance of the methodology is enhanced by an optimized convergence-control parameter in the HAM. The simulation results, compared with the reported analytic models, demonstrate that the HAM-based modeling methodology achieves faster convergence while guaranteeing sufficient accuracy. Based on the methodology, it can be simultaneously revealed that highly nonlinearity is the potential sources of slow convergence in analytic models, which is beneficial for the analysis and design guidance. In addition, a Spice subcircuit is constructed based on the obtained HAM model, and then it is integrated into an oscillator to verify its applicability. Due to the generality of HAM, this methodology may be easily extended to other memory devices.

physics.app-ph↗

arXiv2018

Compact Graphene Plasmonic Slot Photodetector on Silicon-on-insulator with High Responsivity

Graphene has extraordinary electro-optic properties and is therefore a promising candidate for monolithic photonic devices such as photodetectors. However, the integration of this atom-thin layer material with bulky photonic components usually results in a weak light-graphene interaction leading to large device lengths limiting electro-optic performance. In contrast, here we demonstrate a plasmonic slot graphene photodetector on silicon-on-insulator platform with high-responsivity given the 5 um-short device length. We observe that the maximum photocurrent, and hence the highest responsivity, scales inversely with the slot gap width. Using a dual-lithography step, we realize 15 nm narrow slots that show a 15-times higher responsivity per unit device-length compared to photonic graphene photodetectors. Furthermore, we reveal that the back-gated electrostatics is overshadowed by channel-doping contributions induced by the contacts of this ultra-short channel graphene photodetector. This leads to quasi charge neutrality, which explains both the previously-unseen offset between the maximum photovoltaic-based photocurrent relative to graphenes Dirac point and the observed non-ambipolar transport. Such micrometer compact and absorption-efficient photodetectors allow for short-carrier pathways in next-generation photonic components, while being an ideal testbed to study short-channel carrier physics in graphene optoelectronics.

physics.app-ph↗

arXiv2018

Fabrication technology for high light-extraction ultraviolet thin-film flip-chip (UV TFFC) LEDs grown on SiC

The light output of deep ultraviolet (UV-C) AlGaN light-emitting diodes (LEDs) is limited due to their poor light extraction efficiency (LEE). To improve the LEE of AlGaN LEDs, we developed a fabrication technology to process AlGaN LEDs grown on SiC into thin-film flip-chip LEDs (TFFC LEDs) with high LEE. This process transfers the AlGaN LED epi onto a new substrate by wafer-to-wafer bonding, and by removing the absorbing SiC substrate with a highly selective SF6 plasma etch that stops at the AlN buffer layer. We optimized the inductively coupled plasma (ICP) SF6 etch parameters to develop a substrate-removal process with high reliability and precise epitaxial control, without creating micromasking defects or degrading the health of the plasma etching system. The SiC etch rate by SF6 plasma was ~46 μm/hr at a high RF bias (400 W), and ~7 μm/hr at a low RF bias (49 W) with very high etch selectivity between SiC and AlN. The high SF6 etch selectivity between SiC and AlN was essential for removing the SiC substrate and exposing a pristine, smooth AlN surface. We demonstrated the epi-transfer process by fabricating high light extraction TFFC LEDs from AlGaN LEDs grown on SiC. To further enhance the light extraction, the exposed N-face AlN was anisotropically etched in dilute KOH. The LEE of the AlGaN LED improved by ~3X after KOH roughening at room temperature. This AlGaN TFFC LED process establishes a viable path to high external quantum efficiency (EQE) and power conversion efficiency (PCE) UV-C LEDs.

physics.app-ph↗

arXiv2019

Acoustic waveguide filters made up of rigid stacked materials with elastic joints

The acoustic dispersion properties of monodimensional waveguide filters can be assessed by means of the simple prototypical mechanical system made of an infinite stack of periodic massive blocks, connected to each other by elastic joints. The linear undamped dynamics of the periodic cell is governed by a two degree-of-freedom Lagrangian model. The eigenproblem governing the free propagation of shear and moment waves is solved analytically and the two dispersion relations are obtained in a suited closed form fashion. Therefore, the pass and stop bandwidths are conveniently determined in the minimal space of the independent mechanical parameters. Stop bands in the ultra-low frequency range are achieved by coupling the stacked material with an elastic half-space modelled as a Winkler support. A convenient fine approximation of the dispersion relations is pursued by formulating homogenised micropolar continuum models. An enhanced continualization approach, employing a proper Maclaurin approximation of pseudo-differential operators, is adopted to successfully approximate the acoustic and optical branches of the dispersion spectrum of the Lagrangian models, both in the absence and in the presence of the elastic support.

physics.app-ph↗

arXiv2018

Tracking picosecond strain pulses in heterostructures that exhibit giant magnetostriction

We combine ultrafast X-ray diffraction (UXRD) and time-resolved Magneto-Optical Kerr Effect (MOKE) measurements to monitor the strain pulses in laser-excited TbFe2/Nb heterostructures. Spatial separation of the Nb detection layer from the laser excitation region allows for a background free characterization of the laser-generated strain pulses. We clearly observe symmetric bipolar strain pulses when the excited TbFe2 surface terminates the sample and a decomposition of the strain wavepacket into an asymmetric bipolar and a unipolar pulse, when a SiO2 glass capping layer covers the excited TbFe2 layer. The inverse magnetostriction of the temporally separated unipolar strain pulses in this sample leads to a MOKE signal that linearly depends on the strain pulse amplitude measured through UXRD. Linear chain model simulations accurately predict the timing and shape of UXRD and MOKE signals that are caused by the strain reflections from multiple interfaces in the heterostructure.

physics.app-ph↗

arXiv2018

Basic and extendable framework for effective charge transport in electrochemical systems

We consider basic and easily extendible transport formulations for lithium batteries consisting of an anode (Li-foil), a separator (polymer electrolyte), and a composite cathode (composed of electrolyte and intercalation particles). Our mathematical investigations show the following novel features: (i) \emph{complete and very basic description of mixed transport processes} relying on a neutral, binary symmetric electrolyte resulting in a non-standard Poisson equation for the electric potential together with interstitial diffusion approximated by classical diffusion; (ii) \emph{ upscaled and basic composite cathode equations allowing to take geometric and material features of electrodes into account}; (iii) \emph{the derived effective macroscopic model can be numerically solved with well-known numerical strategies for homogeneous domains} and hence does not require to solve a high-dimensional numerical problem or to depend on a computationally involved multiscale discretisation strategies where highly heterogeneous and realistic, nonlinear, and reactive boundary conditions are still unexplored. We believe that the here proposed basic and easily extendible formulations will serve as a basic and simple setup towards a systematic theoretical and experimental understanding of complex electrochemical systems and their optimization, e.g. Li-batteries.

physics.app-ph↗

arXiv2018

Influence of symmetry breaking on Fano-like resonances in high Figure of Merit planar terahertz metafilms

It is well established that nearly all high-quality (Q) Fano-like resonances in terahertz (THz) metasurfaces broaden as asymmetry increases, resulting in a decline of Q-factor and an increase in the resonance intensity. Therefore, in order to determine the optimal design for applications in THz sensing, a Figure of Merit (FoM) is required. Previous studies have identified the asymmetry regimes at which the peak FoM occurs for various, specific unit cell geometries. However to date, there is no systematic comparison of the resulting FoMs for common and novel geometries. Here, a THz planar metafilm featuring split ring resonators with four distributed capacitive gaps is investigated to compare three unique methods of implementing asymmetry: (1) adjacent L-bracket translation, (2) capacitive gap translation and (3) increasing gap width. The results obtained find that by translating two gaps and increasing the bottom gap width of the unit cell, the high-Q Fano-like resonances are $6 \times$ higher than the FoM for the fundamental dipole mode. This work further informs the design process for THz metasurfaces and as such will help to define their applications in photonics and sensing.

physics.app-ph↗

arXiv2018

Pure voltage controlled magnetic nano-oscillator

Nanomagnetic oscillator is a key component for radio-frequency (RF) signal generation in many nano-scale spintronic devices. However, the actuation mechanisms of nanomagnetic oscillators are mostly current-based, which is energy inefficient at nanoscale due to Joule heating. In this study, we present a new actuation mechanism for nanomagnetic oscillator with pure voltage input using a multiferroic structure. An AC voltage with a DC bias is applied to the piezoelectric substrate, and steady perpendicular magnetic oscillation is achieved in the attached Ni disk when the frequency of the voltage matches the ferromagnetic resonance (FMR) of the Ni disk. The FMR can be tuned by simply changing the voltage bias, therefore, the oscillation frequency has a wide range. A systematic simulation study is conducted to investigate the impact of the voltage amplitude, frequency, waveform, as well as the thickness of the magnet on the magnetic oscillation. This opens new possibilities of designing energy efficient nanomagnetic oscillators using multiferroics that have large amplitude and wide frequency range.

physics.app-ph↗