bluuebunny/crossref_metadata_2025_split huggingface dataset:
(Keynote) Nitrogen-Ion Implantation Doping of Ga2O3 and Its Application to Transistors
Authors: Higashiwaki Masataka , Wong Man Hoi , Goto Ken , Murakami Hisashi , Kumagai Yoshinao 
Published: February 2020

As a key ultra-wide bandgap semiconductor, gallium oxide (Ga2O3) has been attracting much interest for power device applications due to its excellent material properties based on an extremely large bandgap of 4.5 eV and the availability of high-quality, large-diameter wafers produced from bulk single crystals synthesized by melt growth methods. Despite having received only little attention, the ease of both n- and p-type ion implantation doping is another very attractive and important feature for Ga2O3 device technologies.
Recently, we succeeded in developing nitrogen (N)-ion implantation doping technology to form p-type Ga2O3 [1]. Note that it is almost impossible to obtain p-type Ga2O3 with effective hole conductivity as for conventional semiconductors. This is not only due to a lack of shallow acceptors with moderate activation energies but also because the valence band structure of Ga2O3, which is composed of O 2p orbitals, is characterized by a very large hole effective mass and conduces to self-trapping of holes with associated characteristic lattice distortions. Therefore, p-Ga2O3 is only useful for engineering large energy barriers in the form of p-n junctions. We have experimentally confirmed that a N-ion implanted p-Ga2O3 region formed in n-Ga2O3 can be utilized as a current blocking layer.
In this talk, we first discuss the material properties of p-Ga2O3 formed by N-ion implantation doping. Then, the device process and characteristics of vertical normally-on Ga2O3 MOSFETs fabricated by using multiple Si- and N-ion implantations are presented [2].
This work was partially supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Next-generation power electronics” (funding agency: New Energy and Industrial Technology Development Organization).
[1] M. H. Wong, C.-H. Lin, A. Kuramata, S. Yamakoshi, H. Murakami, Y. Kumagai, and M. Higashiwaki, Appl. Phys. Lett. 113, 102103 (2018).
[2] M. H. Wong, K. Goto, H. Murakami, Y. Kumagai, and M. Higashiwaki, IEEE Electron Device Lett. 40, 431 (2019).
(Invited) GaN-Based Multiple 2DEG Channel BRIDGE (Buried Dual Gate) HEMT Technology for High Power and Linearity
Authors: Shinohara Keisuke , King Casey , Regan Eric , Gomez M P , Bergman Joshua , Carter Andrew , Arias Andrea , Urteaga Miguel , Brar Berinder , Page Ryan , Chaudhuri Reet , Islam Moudud , Xing Huili Grace , Jena Debdeep 
Published: February 2020

A unique combination of high mobility, high velocity and high sheet density of the 2DEG formed in GaN-based heterostructures has enabled GaN-based HEMTs to be used in a wide range of applications from RF power amplifiers to efficient power converters. Today’s complex communication systems require transceivers to process RF signals efficiently with large bandwidth and high fidelity. While GaN-based HEMT technology has advanced to reach higher power densities, it has not fundamentally changed the power requirements for the linearity performance.
To address fundamental limitations of HEMT’s power/linearity/efficiency/frequency tradeoff, we proposed a transistor structure called BRIDGE FET (buried dual gate FET) where gate electrodes are buried into AlGaN/GaN heterostructures and contact laterally with multiple 2DEG channels [1]. A deliberate elimination of a conventional top-contact gate leads to a unique device operation principle and performance advantages for improved linearity and efficiency at large signal operations; (1) The drain-source current is controlled solely by modulating the width of the 2DEG channels by the lateral gate electric field while maintaining the 2DEG density. (2) The MESFET-like device operation enables gradual pinch-off, greatly reducing g
m derivatives near pinch-off. (3) Lack of density modulation with V
gs leads to a constant electron velocity at high electric field, eliminating a typical g
m roll-off at high V
gs. This results in a constant gain along a resistive load line. (4) The buried gates forms Schottky contacts to the GaN channels below the 2DEG layers. This enhances electron confinement and improves electrostatic isolation between the source and drain, significantly reducing g
d at high V
ds. (5) Elimination of the top-contact gate prevents electrons from being trapped on the surface, suppressing current collapse at high voltage operations. (6) An absence of inverse piezoelectric effect due to the reduced vertical electric field at the drain-side of the gate improves device reliability under high voltage stress.
In this presentation, current status of our development of multi-2DEG channel BRIDGE FETs will be discussed. This work was sponsored by DARPA-MTO DREaM program under DARPA/CMO Contract No. FA8650-18-C-7807. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the Defense Advanced Research Projects Agency or the U.S. Government.
[1] K. Shinohara et al., IEEE EDL, vol. 39, no. 3, p. 417, March 2018.
Electrochemical Behaviour of Expanded Graphite for Thermogalvanic Cells and Thermally-Charged Supercapacitors Applications in Ionic Liquids
Authors: Torres Bautista Blanca Estela , Bhattacharya Kakoli , Bonetti Marco , Roger Michel , Nakamae Sawako 
Published: February 2020

In the present times of ever-increasing energy demands, escalating fuel prices and environmental concerns such as global warming, thermal energy harvesting emerges as a promising source of clean and sustainable energy. Thermogalvanic effect observed in liquid electrolyte-based thermoelectrochemical cells (TECs) offers direct heat to electricity conversion with zero gas emission. TECs in comparison to conventional solid-state thermoelectric materials are reliable, cost effective and require little maintenance, as they have no moving parts.
Furthermore, these TECs can also serve as potential storage devices for waste-heat energy, which can be later recovered and converted into electrical energy. The objective of the present work is to study the combinations of different expanded graphite electrodes with different ionic liquid based electrolytes using traditional electrochemical methods such as open circuit potential, cyclic voltammetry and electrochemical impedance spectroscopy, coupled with the thermoelectric characterizations. The results allow a better understanding of the thermoelectrochemical energy conversion and stored charge efficiency in thermogalvanic cells and thermally-charged supercapacitors.
This work is supported by European Union’s Horizon 2020 research and innovation programme under the grant agreement No 731976 (MAGENTA).
(Invited) Nanoengineered Thermoelectric Energy Devices for IoT Sensing Applications
Authors: Ono Takahito , Nguyen Trung Huu , Samat Khairul Fadzli , Li Jinhua , Toan Nguyen Van 
Published: February 2020

Flexible thermoelectric micro power generators have been developed for wearable devices such as body area network. Also in order to store the generated power, a solid state micro-supercapacitor using a thin film composed of graphene nanowall are developed.
The flexible thermoelectric micro power generator is fabricated by electrochemical deposition of Bi2Te3 and Sb2Te3, which is embedded by PDMS as shown in Fig. 1 (1). A power generation of 4 μW/cm2 are demonstrated from human body temperature. In order to improve the power generation performance, nanocomposites processes of Bi2Te3 are developed. In this method, nanomaterials such as metal nanoparticles and carbon nanomaterials are included in the film by a co-electrodeposition method (Fig. 2).
For the fabrication of the solid state micro super capacitor, plasma-enhanced chemical vapor deposition is used to deposit graphene nanowall on a spiral-shaped silicon electrode pattern, as shown in Fig. 3. On the graphene nanowall Ru oxide or Ni oxide is deposited for electrochemical charge storage layer. Areal energy density of ~15 μWh/cm2 and areal power density of ~2.49 mW/cm2 are achieved (2).
References
(1) T. N. Huu, T. Nguyen, and T. Ono, "Fabrication of π-type flexible thermoelectric generators using an electrochemical deposition method for thermal energy harvesting applications at room temperature" Journal of Micromechanics and Microengineering 27, (2017) 125006.
(2) J. Li, M. Zhu, Z. An, Z. Wang, M. Toda, T. Ono, Constructing in-chip micro-supercapacitors of 3D graphene nanowall/ruthenium oxides electrode through silicon-based microfabrication technique, Journal of Power Sources 401, (2018) 204-212.

Figure 1
(Invited) Hybrid Monolithic Nanomanufacturing of 3D Liquid-Solid Heterojunction Devices for Self-Powered Smart Skin
Authors: Wu Wenzhuo 
Published: February 2020

The seamless and adaptive interactions between functional devices and their environment are critical for advancing emerging technologies, e.g. wearable devices, consumer electronics, and healthcare. The state-of-the-art electronics are based on planar and rigid structures, limiting their integration with three-dimensional soft biological systems. Mechanically deformable devices incorporating confined liquids present an ideal platform for enabling the conformal coverage of electronics on curved and soft surfaces in related applications. However, to date, liquid-based devices have been limited to merely incorporate liquid components as the passive electrodes given the difficulty in the fabrication of liquid-based heterojunctions. The heterogeneous interfaces between functional materials are the central constituents in the state-of-the-art electronics and optoelectronics. Moreover, there is a lack of the fundamental understanding and technological capability of monolithically integrating liquid structures (e.g. electrodes) with functional materials, in particular, the inorganic semiconductors that offer desired electronic and optoelectronic properties in the anticipated applications.
Here, we demonstrate for the first time a versatile platform for the monolithic integration of liquid-solid heterojunction devices through the hybrid manufacturing of self-assembled two-dimensional inorganic semiconductor nanostructures on additively-manufactured liquid electrodes. This new class of wearable devices are conformable to human skins and can sustainably perform non-invasive physiological functions, e.g. detection of pulses and vocal vibration, by harvesting the operation power from the human body. This research is expected to have a positive impact and immediate relevance to many societally pervasive areas, e.g. biomedical monitoring, consumer electronics, and human-machine interface.
Design of Electric Field-Induced High Thermal Conduction Routes in Polymer-Based Nanocomposites with Low Amount of Boron Nitride Nanosheets (≤20 vol%)
Authors: Cho Hong-Baek , Nakayama Tadachika , Choa Yong-Ho 
Published: February 2020

Polymeric nanocomposites with high thermal conductivity with and without electrical insulating properties are great potential for thermal interface materials (TIMs) in semiconductor industry, in particular, in the field of high-power-density electronics and optoelectronics. Conventional materials require a large amount, over 70 vol%, of electrically conducting fillers such as carbon allotropes to attain thermal conductivities more than 1 W/mK. However, due to the incorporation of large quntities of expensive fillers and enevitible increase of viscosity are inefficient cost-effectiveness and processibility.
Here, we introduce electric field-induced application techniques to control the assembly of inorganic micro- or nanofillers at a low level of filler loading less than 20 vol% and to achieve enhancement of thermal conductivity in polymer matrix. Boron nitride (BN) nanosheets have been incorporated into electrically insulating polysiloxane matrix to generate high thermally conductive routes inside the nanocomposites. The homogeneous suspension of filler particles and prepolymer mixture was cast woth 200~300 μm thickness and subjected to diﬀerent electric ﬁelds. Analysis showed that BN nanosheets were fabricated in the hybrid ﬁlms with oriented or linear assembles perpendicular to the ﬁlm surface with high anisotropy. In particular, the thermal conductivity of composites with controlled assembly of BN nanosheets could be increased to 1.56 W/mK only using 15 vol% of BN fillers which was over 500% of composite with random distribution. Mechanisms for the ﬁeld-induced fabrication and structural variation of the two dimensional filler assembly in the polymer matrix are elucidated in relation to enhancement of the thermal properties. This technique provides a facile, controllable approach that can be extended to the synthesis of other TIMs in based on polymer-based nanocomposites.
(Invited) Temperature-Dependent Thermal Diffuse Scattering for Scanning Transmission Electron Microscope Thermometry
Authors: Wehmeyer Geoff 
Published: February 2020

Scanning transmission electron microscopy (STEM) thermometry techniques open new opportunities for mapping temperature (T) with high spatial resolution. Existing STEM thermometry methods based on measuring thermally-induced strains must contend with small thermal expansion coefficients (<10 parts per million (ppm)/K) for some materials, as well as potentially non-local relationships between strain and temperature. In contrast, the well-known mechanism of thermal diffuse scattering (TDS) offers promise for inherently local T measurements, and Debye-Waller theory predicts that many materials should display temperature coefficients >1000 ppm/K at room temperature. This T-dependent TDS has not been leveraged for STEM thermometry, however, because the Debye-Waller effect on the Bragg peak intensity is typically overwhelmed by the effects of thermal tilts and thermal drift.
Here, we demonstrate quantitative TDS measurements using STEM by measuring the diffuse background intensity (rather than the Bragg peak intensity) in energy-filtered scanning electron nanodiffraction patterns. Applying virtual apertures to these diffraction patterns during post-processing allows us to quantify the T-dependent TDS in the diffuse background between the Bragg spots; previous TEM work (with the beam in flood mode) showed that this diffuse signal is relatively insensitive to thermal tilts and drift. Using this diffuse signal, we measure a position-averaged temperature coefficient of 2400±400 ppm/K for a single-crystal gold film averaged between T=100 K and T=300 K, and compare our results with the predictions of Debye-Waller theory. The measurements display typical temperature uncertainties of 8 K and temperature sensitivities of 51 K Hz-1/2. This TDS-based STEM thermometry demonstration provides a step towards the goal of non-contact nanoscale temperature mapping of thin nanostructures.

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(Amazon Catalyst at ECS Grant Winner) Active Control of Heat Flow in Quantum Computing Applications through Piezoelectric Induced Mechanical Strain
Authors: Rajan Aravindh , Kommandur Sampath 
Published: February 2020

Current technologies focused on manipulating heat flow primarily employ materials that exhibit metal-insulator transitions. However, for these materials to be efficient, the operating temperature of the device must be close to the transition temperature. Since the transition temperature is not easily tunable, the solution involves developing and studying new materials that have desirable transition temperatures which often is an inhibitor. Furthermore, as temperature and heat are strongly coupled, the ability to actively control heat flow can be limited.
We propose a solution for active control of heat flow that involves altering the thermal conductivity of a material through mechanical strain. Stretching (compressing) a material leads to an increase (decrease) in the spacing between the atoms, results in a decrease (increase) in thermal conductivity of the material. Our approach involves achieving the required strain by applying an electrical voltage, referred to as the piezoelectric effect. In this study, the relationship between mechanical strain and thermal conductivity is experimentally studied on thin films. Both inorganic and organic films, fabricated using solution processing and sputtering techniques will be used to explore this effect. Thermal conductivity measurements will be carried out using 3-omega, which is a well-established electro-thermal technique. There will also be a thrust to quantify anisotropy in thermal conductivities of the piezoelectric films. In this talk, we report data from preliminary experiments that demonstrate the reversible and tunable thermal conductivity of piezoelectric thin films such as ZnO.
(Invited) Impact of Stoichiometry and Interface Configuration on the Time Stability and the Speed-Limiting Step in Memristive SrTiO3 Cells
Authors: Dittmann Regina , Bäumer Christoph , Siegel Sebastian , Hensling Felix V. E. , Heisig Thomas , Gutsche Alexander , Menzel Stephan 
Published: February 2020

Resistive switching oxides are promising candidates for future non-volatile memory and for functional units of neuromorphic computing. Epitaxial SrTiO3 thin films can be regarded as model system for resistive switching thin films due to their well-known defect chemistry and the absence of grain boundaries. We have recently shown that so-called eightwise switching in SrTiO3 can be attributed to the release and reincorporation oxygen at the interface to a Pt electrode [1,2]. In the light of this knowledge, we studied in detail the impact of the Sr/Ti stoichiometry on the filament formation process, the filament stability [3] and the switching kinetics. We observed that Sr rich thin films exhibit an improved retention of the low resistive state (LRS) at low current compliance values with respect to the stoichiometric thin films. In order to identify the underlying processes, we investigated the filament formation process of SrTiO3 thin films with different stoichiometry by photoelectron emission microscopy. Stoichiometric thin films form a stable LRS state as soon as the current compliance is sufficient to trigger SrO segregation at the surface [4]. We attribute this to the impeded reoxidation of the oxygen deficient filament by the presence of the SrO at the surface. We could mimic this effect by intentionally depositing additional SrO, or other oxygen blocking layers such as Al2O3 on the surface of stoichiometric thin films and were thereby able to improve the memory window as well as the LRS retention.
We furthermore investigated the switching kinetics of the SET process for different Sr/Ti stoichiometries and interface layers. We compared these results to analytical models for the switching kinetics based on ion movement, modeled by the Mott-Gurney-Law, and an oxygen exchange reaction at the interface, modeled by the Butler-Volmer-Equation, as the rate-determining steps in combination with simulated temperature and electrical potential data from finite element simulations, as shown in Figures 1 ((a) stoichiometric, (b) Sr rich). For stoichiometric SrTiO3 cells, oxygen transfer at the oxide-metal interface is determined as the rate-determining step. In contrast, for Sr-rich STO cells we propose oxygen diffusion within the SrO layer at the metal-STO interface as rate-limiting. We will discuss possible reasons for the different rate-determining steps, such as different vacancy concentrations and exchange probabilities.

References

[1] D. Cooper et al. Adv. Mat.. 29, 1700212 (2017)
[2] T. Heisig et al., ,Adv. Mater. 30, 1800957 (2018)
[3] F.V.E. Hensling, et al., Solid State Ionics 325, 247 (2018)
[4] Bäumer et al., Nature Commun. 6, 8610 (2015)

Figure 1
Electrical Pulse Driven Insulator-Metal Transition of VO2 Devices As Leaky-Integrate-and-Fire Artificial Neurons
Authors: Xu Zhen , Fan Zhaoyang 
Published: February 2020

Both artificial neurons and synapses are needed to efficiently implement artificial neural networks directly in hardware. Memristors have been actively investigated to implement synaptic functions. In contrast, the implementation of artificial neurons are still limited to Si-based circuit components, while single device based artificial neuron has not been well explored.
In this study, we show that VO2 two-terminal devices, driven under electrical pulses, can be used to implement the leaky, integrate and fire function of spiking neurons. Since the thermally driven metal-insulator transition of VO2 is volatile and electrical pulses can produce Joule heating, while the heating and cooling have delay times, VO2 devices exhibit characteristic firing time and relaxation time, corresponding to timing difference of the exciting pulse and the resistance switching. In addition, the integrating effect of several small pulses can also trigger the firing from high resistance to low resistance transition. These characteristics suggest the potential using the volatile phase transition of VO2 for artificial spiking neuron. Experimental measurements are further elaborated by a 2-D heterogeneous resistor network based modelling. The insulator-metal transition in VO2 under electrical bias was simulated to confirm the dynamic formation of current filament between two electrodes, and the formation of this path is a flashing process during a single pulse in the pulse train when the integrate effect of these pulses arrives the threshold.
Parallel Programming of an Ionic Floating-Gate Memory Array for Scalable Neuromorphic Computing
Authors: Fuller Elliot James , Keene Scott T , Melianas Armantas , Wang Zhongrui , Agarwal Sapan , Li Yiyang , Tuchman Yaakov , James Conrad D. , Marinella Matthew J. , Salleo Alberto , Talin A. Alec 
Published: February 2020

Neuromorphic computers could overcome efficiency bottlenecks inherent to conventional computing through parallel programming and read out of artificial neural network weights in a crossbar memory array. However, selective and linear weight updates and <10nA read currents are required for learning that surpasses conventional computing efficiency. We introduce an ionic floating-gate memory array [1] based upon a polymer redox transistor connected to a volatile conductive-bridge memory (CBM). Selective and linear programming of a transistor array is executed in parallel by overcoming the bridging threshold of the CBMs. Synaptic weight readout with currents <10nA is achieved by diluting the conductive polymer in an insulating channel to decrease the conductance. The redox transistors endure > 109 ‘read-write’ operations and support > 1MHz ‘read-write’ frequencies.
[1] Fuller et. al. Science, 2019 (in press)