bluuebunny/crossref_metadata_2025 huggingface dataset:
Realization of Stable Cathode-Electrolyte Interfaces in DMSO Based Li-O2 Batteries: Experimental and Theoretical Perspectives
Authors: Noked Malachi , Schroeder Marshall A , Kumar Nitin , Pearse Alexander J , Leung Kevin , Lee Sang Bok , Rubloff Gary W
Published: February 2020

One of the primary challenges impeding realization of the non-aqueous Li-O2 battery is finding a solvent that is chemically and electrochemically stable under cell operating conditions. Dimethyl sulfoxide (DMSO) is an attractive candidate for rechargeable Li-O2 battery studies; however, there is still significant controversy regarding its stability on the Li-O2 cathode surface.  

We report here results from a model  cathode system featuring various atomic layer deposited (ALD) catalysts including Ru, RuO2, and Pt on a mesoporous CNT sponge to study the OER/ORR behavior in DMSO-based Li-O2 cells. We performed multiple experiments (in-situ XPS, FTIR, Raman, and XRD) which assess the stability of the DMSO-Li2O2 interface and report perspectives on previously published studies. Our electrochemical experiments demonstrate long term, stable cycling of DMSO-based Li-O2 cells.

This work is complemented by density functional theory calculations of DMSO degradation pathways on Li2O2.

Both experimental and theoretical evidence strongly suggest that oxidation of DMSO on the surface of Li2O2 is very unlikely to spontaneously occur and will take place only under certain conditions and to a minor extent under controlled operating voltages and in cell environments free of acidic function groups (either in the electrolyte or porous scaffold).

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Numerical Study of the Effect of Stress on Diffusion in Silicon Nanowires
Authors: Mashayek Farzad , Ramasubramanian Ajaykrishna , Khounsary Ali , Shahbazian-Yassar Reza , Najafi Ali
Published: February 2020

Silicon is widely recognized as a promising anode material for a new generation of high capacity lithium (Li)-ion batteries due to its high energy density. However, silicon anodes have limited applications due to the large volume change (about 300%) associated with the lithiation process. This large swelling causes cracking and pulverization of the anode resulting in a loss of electrical contact and sudden fading of capacity. Recent research has revealed that the problems caused by the electrode deformations can be remedied through better electrode geometry design. Numerical models will greatly facilitate this design process. Although from a theoretical perspective, different macroscopic scale models have been proposed regarding the stress evolution in these electrodes, the effect of stress on the kinetics of lithium diffusion is poorly understood. Thus a clear understanding of the stress evolution on kinetics of diffusion in such electrodes is very crucial. Here, we report on the development of a coupled - diffusion / expansion model which is solved using finite element analysis. The model incorporates the effect of stress on the diffusion rate, using a non-linear diffusivity relationship. The effect of stress on the kinetics of lithiation is well captured and is discussed in detail. The - structural changes in different electrode geometries such as nanospheres, nanowires, and thin films are demonstrated, are shown to be in good agreement with the experiments results in the literature.
Nanostructured Electrodes Via Electrostatic Spray Deposition for Energy Storage System
Authors: Chen Chunhui , Wang Chunlei
Published: February 2020

Energy storage system (such as Li-ion battery and supercapacitor) is an important technology that has been widely used for portable electronics, electric vehicles and renewable energy generations. Lots of research has focused on improving its performance, however, many crucial challenges need to be addressed to obtain high performance for further applications. Recently, an electrostatic spray deposition (ESD) technique could be one of the promising technique for electrode formation due to its many advantages, such as simplicity, cost-effective setup, non-vacuum, low temperature deposition conditions, good control for tailoring film composition and morphology, and high deposition efficiency.  It provides a simple but versatile strategy for generating single or multi-component materials with varied morphologies, such as sponge-like, dense, porous and fractal, etc. Figure 1 shows the ESD set-up and some typical ESD derived electrode structures and performance. It is belived that controllable morphology, phase and crystalline structure is crucial for the optmization of electrochemical performance in terms of buffering volume change, increasing contact area, shorting ion diffusion passway, etc.. In this presentation, our recent research outcomes related to various ESD derived electrodes for Li-ion batteries and supercapacitors will be summarized and discussed.

Figure 1. (a-b) Schematic drawing and actual set-up of ESD; (c) CNT grow on C-MEMS structure; (d-e) porous Sn@carbon composites; (f) porous Li4Ti5O12; (g) NiO confined in cut-CNTs; (h) NiO/graphene composite; (i) cross section of rGO-CNT composite; (j) rate capability of ESD- Li4Ti5O12 electrode.

Figure 1
High Energy Density Battery Electrodes with Low and Anisotropic Tortuosity
Authors: Amin Ruhul , Delattre Benjamin , Tomsia Antoni P , Chiang Yet-Ming
Published: February 2020

The high inactive materials content (e.g., separators, current collectors, conductive additives, binder and packaging) of current nonaqueous battery designs contributes directly to high battery cost and reduces specific energy and energy density. Area specific capacity, which is perhaps the most objective measure of design performance, cannot be increased arbitrarily by increasing electrode thickness and/or density, due to kinetic limitations. In the limit of high density and large thickness, salt depletion within the electrolyte-filled porosity typically becomes rate-limiting. In order to reach higher usable energy at practical C-rates from thick, dense electrodes, it is essential to tailor the topology of the pore structure to reduce tortuosity anisotropy.

Here we report on the fabrication, characterization of tortuosity, and electrochemical testing of thick, dense electrodes made by the freeze-casting of aqueous suspensions followed by drying/sintering. This shaping technique allows design of oriented and interconnected structures that have tortuosity anisotropy, low absolute tortuosity, and high ionic transport. We report on the characterization of tortuosity of directionally fabricated electrodes with similar porosity while varying the solidification rate that resulted in different microstructural features. The electrochemical performance of the fabricated electrodes in half cells exhibits high areal capacity. Hybrid pulse power characterization (HPPC) and dynamic stress tests (DST) were also performed.
(Invited) The Roadmap to Applications of Graphene and Related Materials
Authors: Ferrari A C.
Published: February 2020

Disruptive technologies are usually characterised by universal, versatile applications, which change many aspects of our life simultaneously, penetrating every corner of our existence. In order to become disruptive, a new technology needs to offer not incremental, but dramatic, orders of magnitude improvements. Moreover, the more universal the technology, the better chances it has for broad base success. The Graphene Flagship has brought together universities, research centres and companies from most European Countries. At the end of the ramp-up phase significant progress has been made in taking graphene, related layered materials and hybrid systems from a state of raw potential to a point where they can revolutionize multiple industries. I will overview the progress done thus far and the future roadmap.
(Invited) Review of Recent Progress in Nanoscaled Thermoelectric Thin Films
Authors: Baumgart Helmut , Chen Xin , Lin Pengtao , Zhang Kai
Published: February 2020

In recent years, thermoelectric materials have received increasing attention due to their ability to generate electricity from waste heat recovery. The efficiency of thermoelectric materials systems to produce thermoelectric power is related to the figure of merit ZT and the power factor. The dimensionless thermoelectric figure of merit is expressed as ZT = S2σT/k, where S is the Seebeck coefficient, σ is the electrical conductivity, and k is the thermal conductivity. In the quest to enhance ZT values, a key strategy involves reduction in thermal conductivityk, resulting from phonon scattering by numerous interfaces in low dimensional structures. Promising advances have been achieved with phononic crystal (PnC) nanostructures in thermoelectric materials, because the thermal conductivity of PnC samples is lower compared to non-patterned thermoelectric samples due to phonon-boundary scattering. Metal telluride based compounds such as bismuth telluride (Bi2Te3) and antimony telluride (Sb2Te3) alloys have reported some of the highest figures of merit and work best for thermoelectric devices used for the temperature range of 200 to 400 K, while lead chalcogenides such as PbTe, PbS, and PbSe are ideal for the temperature range of 350 to 600 K. Here we review advances in the ALD synthesis of composite thermoelectric nanolaminates of alternating Bi2Te3 and Sb2Te3 thin films, and PbSe and PbTe films. During our investigations, we have observed that PbTe/PbSe nanolaminates grown on porous templates have higher Seebeck coefficients than the ones grown on regular planar silicon wafers. The higher Seebeck values result from the lower thermal conductivity k in porous structures, which in turn lead to a higher figure of merit ZT. We demonstrate the feasibility to enhance the figure of merit ZT further by modulating the size and periodicity of the pattern and the thickness of the thermoelectric film in relation to the mean free path (MFP) of the phonons of the thermoelectric material. In our study PbTe and PbSe thin films and nanolaminates were synthesized by Atomic Layer Deposition (ALD) technology on regular planar silicon wafers and on porous templates. Various approaches to enhance the figure of merit ZT and the Seebeck coefficient for ALD PbTe & PbSe films are discussed that result in an effective reduction of the thermal conductivity. In this work, we report on the physical characterization and electrical characterization focusing on Seebeck coefficient measurements in PbTe/ PbSe nanolaminate structures synthesized by ALD on planar Si wafers and on porous Si templates.
(Invited) Manipulating Electrical and Thermal Transport in Nanowire Based Nanocomposite through Doping for Thermoelectric Applications
Authors: Wu Yue
Published: February 2020

In this talk, we will discuss our work on how to use doping and porosity effects to manipulate the electrical and thermal properties in nanowire based bulk nanocomposites. Our success in increasing the efficiency of p-type BixSb2-xTe3 at higher temperatures can be largely explained by the increase in the optical band gap, according to the Burstein-Moss shift, due to doping, thus delaying the onset of bipolar conduction, which can shift the operation temperature to over 500K with a much improved ZT over 1. In addition, we will discuss the scalable synthesis of hollow nanostructures and the subsequent sintering of them into a highly porous thermoelectric nanocomposite. The as-sintered material has high porosity and holds a record low thermal conductivity, however, its zT is comparable to or even better than the state-of-the-art. The relative density leads to the less cost in raw material and better portability. We foresee that this large-scale approach can be extended to other types of thermoelectric materials and will inspire the utilization of hollow nanostructure in other fields.
(Invited) Nanocomposite-Based Nanogenerators for Thermal Energy Harvesting and Self-Powered Sensing Applications
Authors: Lin Zong-Hong
Published: February 2020

Increasing research efforts have been devoted to renewable energy owing to the large energy consumption in recent years. Comparing to fossil fuel, renewable energy with the advantages of reduced carbon emission and secure long-term energy supply is more mandatory for the sustainable development of the world. Thermoelectric generator, which works based on thermoelectric effect, is capable of converting waste heat into electricity with a temperature difference relative to environmental temperature. Many studies have demonstrated that nanomaterials can enhance their ZT values by suppressing the thermal conductivity compare to their bulk counterparts. In our research, we developed thin, light-weight, and flexible thermoelectric generators based on the nanocomposites of inorganic (Te, Ag2Te, MoS2, and Graphene) nanomaterials and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). These properties result in those nanocomposites are good candidates to fabricate thermoelectric generators to harvest tiny-scale thermal energy and show their potential to be applied in the collection of heat energy from vehicles, aircrafts, and power plants.
(Invited) The Electrochemical Impedance Spectra of BiTe Thermoelectric Device
Authors: Lu Cian-Tong , Hsueh Kan-Lin , Li Wen-Jin , Liu Chia-Mei
Published: February 2020

Thermoelectric material can be used for energy harvesting that converting thermal energy into electricity. It also can be used as solid cooler by passing electricity thought the element. Many TE array device were manufactured and their performance were tested. Some of the TE device property is not consisted with others. Performance degradation of TE devices was observed. Impedance spectrum was proposed as the non-invasive diagnostic tool.

The impedance measurement was carried out on a single semiconducting element of N-type or P-type. Impedance data was measured over a range of dc bias and they were fitted with an equivalent RC circuit. The parameter value of the RC circuit was calculated.

The EIS data were measured for three different TE array devices over bias voltage ranged from – 1.0 V to 1.0 V and temperature ranged from room temperature to 90 oC. An equivalent circuit Rs (CPE-C) (R1-C1) (R2-C2) was found that is capable of described the EIS data of these samples over the bias voltage and temperature ranges. Values of these parameters were calculated. The EIS technique can be used as an analytic tool for the diagnosis of TE array.
(Invited) Resistive States in Strontium Titanate Thin Films
Authors: Kubicek Markus
Published: February 2020

Recently, resistive switching devices have attracted a lot of attention, especially for use as non-volatile memories being an alternative to classic transistor technology. Hysteretic I-V characteristics allow modulations between high and low resistive states with potential for fast switching speed, high scalability and low switching energies. Perovskite oxides are an interesting class of active switching materials: Their interplay of band structure, electronic and ionic defects, and interface effects control the switching process. The processes on the atomic scale (e.g. reordering of anions or cations) to accomplish the changes between resistive states require attention. Here, local high electric fields and/or locally elevated temperatures are enabling resistive switching. Studies on strontium titanate (SrTiO3) thin films deposited by pulsed laser deposition (PLD) are presented, focusing on understanding defect chemical and mechanistic implications and requirements of the switching process.

Defect chemical differences between the changes of resistive states at room temperature and at elevated temperatures were investigated. Here we use the temperature change to switch between two regimes in which oxygen vacancies are either mobile or immobile. By using combined electric DC and Electrical Impedance Spectroscopy (EIS) techniques we were able to develop and test a model which can explain different hysteretic I-V responses or pseudo-inductive loops found in impedance spectra as a consequence of the change of resistive states.

Also the influence of PLD parameters on the conductivity of SrTiO3 thin films as well as on their resistive switching behavior was studied in detail. By using microelectrodes, it was investigated how certain areas of a SrTiO3 thin film either required a forming process for reversible resistive switching or not. We propose critical defects distributed in the thin films to explain experiments with differently sized microelectrodes.

Additionally, the influence of the PLD process itself is critically discussed. For this purpose, we use a newly developed in-situ technique combining pulsed laser deposition and electrical DC or electrochemical impedance measurements. Recent results show that interactions with the plasma plume and/or the UV light has a strong impact on the electrical properties of oxide thin films. Interestingly, PLD interactions were even found to affect strontium titanate substrates used for deposition.
(Invited) Beyond Electrostatic Effects at Oxide Hetero-Interfaces: Electrochemical Phase Change, Strong Electric Fields, and Elastic Strain
Authors: Yildiz Bilge
Published: February 2020

Transition metal oxide hetero-interfaces are interesting due to the distinctly different properties that can arise from their interfaces, such as memristive behavior, superconductivity, high catalytic activity and magnetism. These interfaces are the source for local heterogeneities in composition, atomic structure and electronic structure. Classically, defect redistribution is quantified at the continuum level by concurrent solution of Poisson’s equation for the electrostatic potential and the steady-state equilibrium drift-diffusion equation for each defect. It is possible to inform this level of modeling with first principles calculations of band off-sets, and defect formation and segregation energies at thermodynamically relevant conditions. This approach had numerous successful implementations, including the quantification of charge transport properties at surfaces and grain boundaries. In this talk, I will discuss three phenomena that also need to be considered in a broader framework of defect structures and distributions at oxide hetero-interfaces. 1) Presence of strong electric fields that can cause polarization of defective systems and affect the defect abundance and structure. We have assessed this effect on neutral oxygen vacancies in simple binary oxides from first principles calculations. 2) Phase change under the effect of local electrostatic potential because of a change in the electrochemical potential of oxygen. We have assessed the ability to trigger phase change electrochemically in two classes of oxides, SrCoOx and VOx, and have quantified the phases and the corresponding distinctly different electronic properties by combining in operando x-ray diffraction and x-ray photoelectron and absorption spectroscopy. The results have implications both for oxide hetero-interfaces and for oxide electronic devices that aim to control properties electrically. 3) Elastic strain, that affect the stability and mobility of defects. In this recent work, we have focused on the stability of electronic defects, specifically the electron polarons versus free electrons SrTiO3, as a function of temperature and hydrostatic stress, by combining first principles calculations and quasi harmonic approximation. Our results demonstrate that it is possible to control the type of electronic defect, and so the transport properties, by means of electro-chemo-mechanics.
(Invited) The Non-Volatile Redox Transistor for Neuromorphic Computing
Authors: Talin Albert Alec
Published: February 2020

Inspired by the efficiency of the brain, CMOS-based neural architectures and memristors are being developed for pattern recognition and machine learning. However, the volatility, design complexity and high supply voltages for CMOS architectures, and the stochastic and energy-costly switching of memristors complicate the path to achieve the interconnectivity, information density, and energy efficiency of the brain using either approach. In my talk I will describe a non-volatile redox transistor (NVRT): a device with a resistance switching mechanism fundamentally different from existing memristors, based on the concept of reversible, electrochemical reduction/oxidation of a material to tune its electronic conductivity. The first type of NVRT that I will describe is based upon the intercalation of Li-ion dopants into a channel of Li1−xCoO2. This Li-ion synaptic transistor for analog computing (LISTA) switches at low voltage (mVs) and energy, displays hundreds of distinct, non-volatile conductance states within a 1V range, and achieves high classification accuracy when implemented in neural network simulations1. The second type of NVRT I will describe operates on a similar principle but is based on the polymer system PEDOT:PSS, and which we call the electrochemical neuromorphic organic device (ENODe)2. Plastic ENODes are fabricated on flexible substrates enabling the integration of neuromorphic functionality in stretchable electronic systems. Mechanical flexibility makes ENODes compatible with three-dimensional architectures, opening a path towards extreme interconnectivity comparable to the human brain.

(1) Fuller, E. J.; El Gabaly, F.; Leonard, F.; Agarwal, S.; Plimpton, S. J.; Jacobs-Gedrim, R. B.; James, C. D.; Marinella, M. J.; Talin, A. A. Li-Ion Synaptic Transistor for Low Power Analog Computing. Advanced Materials
2017, 29, 1604310.

(2) van de Burgt, Y.; Lubberman, E.; Fuller, E. J.; Keene, S. T.; Faria, G. C.; Agarwal, S.; Marinella, M. J.; Talin, A. A.; Salleo, A. A non-volatile organic electrochemical device as a low-voltage artificial synapse for neuromorphic computing. Nature Materials
2017, 16, 414.

This work was supported by the Sandia Laboratory Directed Research and Development (LDRD) Program. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
(Invited) Epitaxial Random Access Memory (epiRAM) Towards Large-Scale Synaptic Arrays
Authors: Kim Jeehwan
Published: February 2020

Epitaxial random access memory (epiRAM) towards large-scale synaptic arrays

Shinhyun Choi, Scott Tan, Yunjo Kim, and Jeehwan Kim

Massachusetts Institute of Technology

 

Abstract

Neuromorphic computing has recently emerged as a non-Von Neuman computing method. Because its analog switching ability to represent multiple synaptic weights by varying conductance in the vertical filaments formed in the switching medium, a memristor has been considered as a suitable neuromorphic hardware platform. Conventional memristors typically utilize an defective amorphous solid as a switching medium for defect-mediated formation of conducting filaments. However, the imperfection of the switching medium also causes stochastic filament formation leading to spatial and temporal variation of the devices. In this talk, we introduce a silicon-based epitaxial random access memory (epiRAM), where we precisely confined the conducting paths in the single-crystalline films resulting in unprecedented device performances. MIT’s epiRAM exhibits extremely low temporal/spatial variation, linear synaptic weight update, high on/off ratio (250 for analog/10,000 for digital), great endurance (>109), long retention time (2 days at 85oC), and self-selectivity. This will allow large-scale neural network hardware, paving the way for computing beyond the conventional von Neumann architecture.
Ferroelectric Heterostructures for Synapse Devices
Authors: Choi Taekjib
Published: February 2020

Ferroelectric polarization related charge conduction have shown intriguing properties of ferroelectric heterostructure devices, such as ferroelectric tunnel junction, switchable diode, and ferroelectric memristor. Recently, ferroelectric heterostructure has been proposed for synapse device exhibiting a high on/off ratio, low power consumption and short process time. In ferroelectric-based synapse devices, gradual electroresistive modulation is associated with change of ferroelectric domain configuration, providing the controllability of the electric conductance for the effective operation of synapse devices. In this work, we demonstrated a ferroelectric-field-effect transistor based synapse devices with channel layer of metal to semiconducting transition (MST) oxide. Ferroelectric polarization switching can modulate the amount of applied energy to MST oxide phase transition and channel conductance at the same time. Our ferroelectric heterostructure with analog-like electroresistive modulation is highly desirable for realizing a efficient synapse device.
I-V Characteristics of NiO Nanowire Based Resistive Change Memory
Authors: Takase Kouichi , Aono Takashige , Sugawa Kosuke , Shimizu Tomohiro , Shingubara Shoso
Published: February 2020

1. Introduction

Resistive switching random access memory (ReRAM) has been expected to a next generation memory due to high-speed response, non-volatility, low power consumption [1]. However, poor reproducibility of a switching voltage hinders the practical application. According to a filament model, many kinds and number of filaments made at the soft breakdown of an insulator are thought to be the origin. In our previous studies, we employed anodic porous alumina (APA) with ordered nanoholes as the insulator of ReRAM because nanoholes are expected to inhibit the growth of conductive filaments spacially. In the result, we succeeded in suppression of the switching voltage variation [2].Looking at the top view of SEM image of APA (Fig. 1), we notice no restriction along the dashed lines. It means that there exist a division indicating a bulk property. In this study, a nanowire based insulator has been focused on to confine the filaments omnidirectionally.

2. Experiment

Nickel nanowires were buried into nanoholes of APA using electroplating method, where nickel sulfate was used as an electrolyte solution. The exposed Ni nanowires were obtained by chemical etching the alumina layer. The nanowires were oxidized by oxygen plasma for 1, 3, 5 min. The surface morphology of nanowires was investigated by a field emission scanning electron microscope (FE-SEM). For a contact preparation, Indium solder was used against the top electrode. The bottom electrode was a residual aluminum substrate. The I-Vcharacteristics of NiO nanowires were measured by a standard two-probe method at room temperature, where the current was limited to 1 mA in a SET process guard the device from a complete dielectric breakdown.

3. Results

Figure 2 (a) ~ (c) shows the cross-sectional SEM images of NiO nanowires. The typical diameter and the length were about 40 nm and 600 nm, respectively. Significant oxidation time dependence was not found in the shape. We can see clear switching behaviors for all samples. In case of 1 min oxidation, a relatively wide distribution of the switching voltage was measured. The distribution width became smaller in 3 min oxidation and widens again in 5 min oxidation. The relatively high current found in the RESET process in case of 3 min oxidation. It might be due to poor crystallinity of the NiO thin film because oxygen plasma is expected to make the film amorphous by the high kinetic energy of oxygen plasma. Longer oxidation time might help to formation of many filaments. So, this leads to widen the switching voltage distribution again in 5 min oxidation.

References

[1] M. J. Lee et al. Nano Lett. 9, 1475 (2009).

[2] Y. Tanimoto et al. Jpn. J. Appl. Phys 54, 06FH05(2015).

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(Invited) Critical Recent Advancement of GaN Technology and MMICs for Millimeter-Wave Applications
Authors: Chen Shuoqi , Witkowski Larry , Gu Xing
Published: February 2020

In this paper, we will present the in-development of Qorvo’s 90nm GaN technology imbedded within Qorvo’s existing 3-Metal-Interconnect (3MI) MMIC process flow, to make active and passive devices fully compatible with state-of-the-art GaN MMIC process, for V- and W-band MMIC High Power Amplifier (HPA) applications. By applying an advanced epitaxial material structure, dedicated T-gate configuration and process refinement, as well as 50-mm SiC substrate with Individual Source Via (ISV) FET construction, this technology demonstrates GaN FET devices with >145GHz Ft and >850mS/mm Gm, enables circuit designers to utilize this technology to pursue millimeter-wave market opportunities. Using this technology, a Technology characterization vehicle (TCV) MMICs using 5-stage 85GHz GaN power amplifiers have been demonstrated through circuit design, fabrication and measurements. This MMIC achieves greater than 38 dB small signal gain at 85GHz (8dB small signal gain per stage), and greater than 30 dB small signal gain over 72 – 90 GHz bandwidth. Further test results for both FET device and MMIC, including DC, small signal and power characteristics, will be reported in conference. This emerging GaN on SiC transistor technology is well suited for next generation millimeter-wave HPA, wideband and multi-function MMIC applications and beyond.

This work was funded by ONR (Program Manager: Dr. Paul A. Maki, ONR N00014-13-C-0071).

Figure 1
Surface Analysis of AlGaN Channel Hemts
Authors: Douglas Erica A. , Brumbach Michael , Klein Brianna , Baca Albert G. , Allerman Andrew A.
Published: February 2020

High electron mobility transistors (HEMTs) with AlGaN channels are excellent candidates for high-power applications due to increased bandgap and electric field as compared to GaN channel HEMTs. An increase in critical electric field is achieved by increasing the Al mole fraction of both the barrier and channel layer of the HEMT to AlyGa1-yN/AlxGa1-xN, where y>x≥0.3. However, the effects of the fabrication process on the surface of high Al content AlGaN has yet to be studied.

As AlyGa1-yN/AlxGa1-xN HEMTs gain interest for a variety of applications, unintended modifications to the surface of the AlGaN can have unintended consequences to device performance. X-ray photoelectron spectroscopy (XPS) was performed on Al0.45Ga0.55N/Al0.3Ga0.7N heterostructure. XPS indicated that native surface oxidation of Al0.45Ga0.55N occurs in as-grown material stored at ambient of approximately 8 atomic %. A standard low power microwave O2 ash is a standard process performed to remove organic contamination and descum after photolithography. Though an Al0.45Ga0.55N sample exposed to an O2 ash indicated a slight decrease in C, an increase in the surface oxidation from approximately 8 atomic % (as grown) to approximately 20 atomic % after O2 plasma was observed (Fig. 1). XPS spectra show that after exposure to an O2 ash, oxygen is binding to both Ga and Al. This has substantial implications for device performance and integration schemes for heterostructures with increasing Al content. As the presence of an oxide interlayer can have large implications to Schottky contacts, the effect of an HCl clean to remove the native oxide was investigated. Even after exposure to a 35% HCl dilute in H2O, the O atomic % decreased from 8% to only 6%. Presuming that lower oxygen surface content is desirable, the HCl clean offers a tangible benefit for the surface of Al0.45Ga0.55N/Al0.3Ga0.7N heterostructures.

This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

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(Invited) Advances in SiC and GaN Power Device Development and Future Directions
Authors: Anderson Travis J , Hobart Karl D , Hite Jennifer K , Luna Lunet E , Kub Francis J
Published: February 2020

As wide bandgap semiconductors, devices based on SiC and GaN represent critical next-generation medium voltage and high voltage power switch technology. SiC device technology has matured rapidly over the past two decades, owing to advances in substrate quality, a fundamental understanding of epitaxial growth to characterize defects and identify and eliminate performance-limiting defects such as basal plane dislocations, and device breakthroughs such as the junction barrier Schottky (JBS) diode and DMOSFET. While lateral GaN-based HEMT technology has been highly successful for RF power amplifiers and is well-positioned to supersede GaAs-based MMICs, advances in vertical power device technology has been limited due to the lack of high quality native substrates. As large area substrates have become available by hydride vapor phase epitaxy (HVPE) and ammonothermal growth, several fundamental vertical power devices have recently been realized, including Schottky diodes with edge termination, trench MOSFETs, and CAVETs. However, significant fundamental materials challenges currently limit device performance, specifically the realization of repeatable thick drift layers with low background doping as well as a reliable selective area planar process for p-type doping.

In this presentation, an overview of SiC and GaN device development at the U.S. Naval Research Laboratory will be summarized, first looking at SiC devices including the development of novel materials characterization techniques to understand the role of defects on device performance, reliability and failure analysis of devices, including BPD kinetics in PiN and MPS diodes, and novel device designs including grayscale termination processes, temperature compensated PiN diodes, and hybrid Si/SiC modules. Recent developments in GaN vertical device technology will also be presented, including the development of an ion implanted p-type doping process and preliminary materials characterization to understand the relationship between substrate quality, epitaxial layer quality, and device performance. Future directions for GaN device development enabled by the availability of bulk substrates will also be discussed, including implications for lateral devices, novel lateral device designs, and fundamental materials work based on the lessons learned from SiC.
(Invited) Thermo-Mechanical Reliability Challenges in the Heterogeneous Integration and Packaging of Wide Bandgap Semiconductors
Authors: Jiang Tengfei
Published: February 2020

Wide band gap semiconductors such as GaN possess many favorable properties comparing to Si, which make them especially suitable for high power and high temperature applications. This brings about unique opportunities and challenges for the development of robust and reliable packaging technologies. In this talk, we will focus on the thermo-mechanical aspects of packaging reliability challenges, where thermal stress can be a main reason causing failure of power modules. We will discuss the source of the thermal stress from materials interaction during temperature excursion, and will discuss some of the stress induced failure mechanisms such as delamination and fatigue. Characterization techniques for high resolution and in situ measurement will also be reviewed. We will point out the similarity and unique challenges of these problems as compared with Si based devices to show that packaging reliability of wide band gap semiconductor devices presents an exciting opportunity to explore new packaging materials and designs.
Bi-Parabolic Graded Buffer Layers for Metamorphic Devices
Authors: Song Yifei , Kujofsa Tedi , Ayers John E
Published: February 2020

Metamorphic semiconductor devices such as high electron mobility transistors (HEMTs), light-emitting diodes (LEDs), laser diodes, and solar cells are grown on mismatched substrates and typically exhibit a high degree of lattice relaxation. In order to minimize the incorporation of threading defects it is common to use a linearly-graded buffer layer to accommodate the mismatch between the substrate and device layers. However, some work has suggested that buffer layers with non-linear grading could offer higher performance in terms of limiting the surface density of threading defects. In this work, we compare bi-parabolic graded buffer layers to linear and S-graded buffer layers in terms of the surface threading dislocation density by considering a uniform InGaAs layer grown on a GaAs substrate with an intermediate graded InGaAs buffer layer. The threading dislocation densities were calculated using the LMD model, in which the coefficient for second-order annihilation and coalescence reactions between threading dislocations is considered to be equal to the length of misfit dislocations.
Surface Sensitivity of Fluoroethylene Carbonate Breakdown on Lithium Silicide Surfaces
Authors: Sprowl Lynza H. , Árnadóttir Líney , Chan Maria K. Y.
Published: February 2020

Silicon anodes are a promising material for lithium ion batteries due to their large theoretical capacity for storing lithium. When the battery is charging, the anode works by alloying lithium ions with the silicon anode where the lithium ions are reduced. In addition to the lithium ions being reduced, unwanted side reduction reactions of the electrolyte also occur. When the electrolyte reductively decomposes at the anode surface, a solid-electrolyte interphase (SEI) is formed. Fluoroethylene carbonate (FEC) is added to the electrolyte to make a more robust SEI which prevents further electrolyte decomposition. From FEC reductive decomposition, the primary reduction products are CO2, CH2CHO, and F-. While the decomposition products have been discovered, the lowest energy surface facet which promotes the breakdown of FEC into those products is still unknown.

Here FEC and its reduction products are being studied on different lithium silicide surfaces using density functional theory. The lithium silicide phases under consideration are the crystalline LiSi phase with a low lithium concentration and the crystalline Li15Si4 phase with a higher lithium concentration. All unique low index surface facets are considered for each lithium silicide phase. The interactions of FEC and its dissociation products are analyzed on the different anode surface facets to determine the surface sensitivity of FEC breakdown. From the binding energies of the FEC reactant and the CO2, CH2CHO, and F- products on the lithium silicide surface facets, FEC dissociation reaction energies are calculated. The facet on which SEI formation begins to form is determined from the reaction energies for both the low and high concentration lithium silicide anodes.

This material is based upon work supported by the U.
 
S. Department of Energy,
 
Office of Science, Office of Workforce Development for Teachers and Scientists,
 
Office of Science Graduate Student Research (SCGSR) program. The SCGSR program is administered by the Oak Ridge Institute for Science and Education (ORISE) for the DOE. ORISE is managed by ORAU under contract number DE-SC0014664.
 
Use of the Center for Nanoscale Materials, an Office of Science user facility, is
 
supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under
 
contract number
 
DE-AC02-06CH11357.
Polysulfide Trapping Agent (PTA) Coated Directly Doped Sulfur Architectures (DDSA) – New Platforms for High Capacity Li – S Batteries
Authors: Murugavel Shanthi Pavithra , Jampani Prashanth , Gattu Bharat , Kumta Prashant N
Published: February 2020

Increasing demands for high energy and high power storage systems for use in electric vehicles and electronic devices has made it imperative to develop high performance rechargeable batteries. This stringent need has attracted widespread attention worldwide. Lithium – sulfur (Li – S) batteries has been consequently, extensively studied recently as a potential rechargeable electrochemical power sources due to its high theoretical capacity (1672 Ah kg−1 sulfur)1 and high energy density (2500 Wh kg−1 sulfur)2
based on the complete reduction of elemental sulfur to form the end product of lithium sulfide (Li2S).

However, commercialization pathway for Li – S batteries is still fraught with high self – discharge and the short cycle life problems of the Li – S rechargeable system. These limitations are largely related to the poor ionic and electronic conductivity of elemental sulfur3, formation of a series of sulfur reduction intermediates during discharge (Li2Sx, x > 2)4
,
5 that are highly soluble in organic electrolyte and the high reactivity of the dissolved polysulfides with the Li anode4.

Several approaches including the use of a conductive additive6, encasing sulfur into conductive porous host materials7 and replacing the organic electrolyte with a polymer electrolyte8 has been shown to improve the performance by improving the ionic conductivity of sulfur and reducing polysulfide dissolution by avoiding direct contact with the liquid electrolyte. However, despite these advances, complete prevention of polysulfide dissolution has not yet been accomplished due to lack of fundamental understanding about the mechanism of polysulfide dissolution.

In this work, directly doped sulfur architectures (DDSA) with sulfur loadings greater than 16mg/cm2 were created using simple electrochemical techniques. The cathodes were studied chemically and electrochemically to understand the mechanism of polysulfide dissolution in these structures. The DDSA electrodes are then coated with a polysulfide trapping agent (PTA) to chemically prevent the dissolution of polysulfide species. These PTA coated DDSA showed a high initial capacity of 1305 mAhg-1 with stable performance of 1208 mAhg-1 for over 30 cycles (Figure 1). Mechanism of polysulfide dissolution by PTA will be presented and discussed.

Figure 1
Hair-Based Flexible All-Solid-State Supercapacitor with Wide Operating Voltage and Ultra-High Rate Capability
Authors: Liu Wenwen , Feng Kun , Lui Gregory , Tjandra Ricky , Lim Lucas , Yu Aiping
Published: February 2020

Fiber-shaped supercapacitors (FSCs) are a promising candidate as power source or energy storage unit in wearable/stretchable electronics. However, it is still a significant challenge to design FSCs with excellent electrochemical performance while maintaining good flexibility to meet the requirement of wearable/stretchable electronics. Here, a human hair-based flexible all-solid-state asymmetric FSCs has been rationally designed and successfully prepared. Importantly, the as-obtained FSCs show extraordinary flexibility and outstanding electrochemical performance with a wide potential window, excellent rate capability (up to 20,000 mV·s-1), fast frequency response (τ0=55 ms), high volumetric energy density, and long cycle stability. The strategy presented here not only provides a reference for the construction of high-performance flexible FSCs, but also paves a new way to explore the next-generation portable/wearable energy storage devices.
Long-Cycling Ferrocene/MV Aqueous Organic Redox Flow Battery (AORFB)
Authors: Hu Bo , DeBruler Camden , Liu Tianbiao
Published: February 2020

Redox flow batteries (RFBs) are a viable technology to store renewable energy in the form of electricity that can be supplied to electricity grids. However, widespread implementation of traditional RFBs, such as vanadium and Zn-Br2 RFBs, is limited due to a number of challenges related to materials, including low abundance and high costs of redox-active metals, expensive separators, and corrosive and hazardous electrolytes. To address these challenges, we demonstrate an aqueous organic redox flow battery (AORFB), named FcNCl/MV AORFB,1 featuring a high theoretical energy density, 45.5 Wh/L, the highest value known for AORFBs. The battery is built from earth abundant elements (C, H, N, and Fe), specifically consisting of a newly designed organic catholyte based on a highly water soluble ferrocene compound, (ferrocenylmethyl)trimethylammonium chloride(FcNCl), a methyl viologen anolyte which was studied in our previous work,2 non-flammableand non-corrosive NaCl supporting electrolyte, and low cost anion exchange membrane. This is the first application of a water-soluble ferrocene compound in aqueous RFBs. The batteries exhibit extraordinary electrochemical performances, 700 cycles with 99.99% capacity retention per cylce and 65% energy efficiency at 60 mA/cm2, and delivered power density up to 125 mW/cm2. The presented AORFB technology underlines great promise of soluble redox-active organic molecules for green energy storage.

Reference

1. Hu, B.; DeBruler, C.; Rhodes, Z.; Liu, T. L., J. Am. Chem. Soc.,
2017, 139, 1207.

2. Liu, T. B.; Wei, X. L.; Nie, Z. M.; Sprenkle, V.; Wang, W., Adv. Energy. Mater.,
2016,
6, 1501449.
Direct Electrochemical Compression of Hydrogen and Oxygen Via PEM Water Electrolysis
Authors: Ayers Katherine E , Dalton Luke , Parker Michael
Published: February 2020

One of the biggest challenges in applying hydrogen to energy storage and vehicle fueling applications is to efficiently store the hydrogen in a small volume with high reliability equipment. High pressure hydrogen applications that are used at a high duty cycle, including vehicle fueling demonstrations, repeatedly show that mechanical compression is one of the highest maintenance devices in the system. Electrochemical compression requires no moving parts within the cell, resulting in less maintenance due to wear, and also is highly efficient, with very low voltage penalty. The primary efficiency loss in electrochemical compression of hydrogen at high pressure is due to back diffusion of the compressed gas to the other cell chamber. Still, the overall efficiency still competes very well with mechanical compression to thousands of psi, as will be shown in this talk.

Proton exchange membrane (PEM)-based electrolysis enables combining the generation of hydrogen through water-splitting with the direct electrochemical compression of the gas. These membrane-based systems also have the advantage that they are capable of differential pressure operation. While there are applications for both pressurized oxygen and hydrogen, the differential pressure capability is especially attractive for generating high pressure hydrogen while maintaining the water-oxygen loop at ambient pressure, due to the inherent safety risks of high pressure oxygen.

Proton OnSite, the market leader in PEM electrolysis, has been increasing the proven output pressure of PEM electrolyzer devices, with demonstration of 350-bar (5,000 psi) hydrogen generation and 250-bar (3600 psi) oxygen generation, both in full differential pressure mode (see Figure). The hydrogen cell stack has been scaled to a capacity of 4.5 kg/day and is certified to meet the relevant codes and standards. This talk will address the challenges and successes for both hydrogen and oxygen compression at Proton, and potential best fits for these technologies.

Figure 1