bluuebunny/crossref_metadata_2025 huggingface dataset:
Water Desalination By Pseudocapacitive Deionization
Authors: Kim Taeyoung , Gorski Christopher A. , Logan Bruce E.
Published: February 2020

Capacitive deionization (CDI) is an electrochemical desalination process based on electrochemical principles that involves storing ions inside pores of capacitive, carbon-based electrodes. Although there have been intensive efforts for improving CDI performance, its broader application has been limited due to the low charge storage capacities of carbon-based electrodes. However, pseudocapacitive electrode materials could provide a higher capacity for salt removal because they can undergo both faradaic ion insertion and surface capacitive reactions. For example, sodium manganese oxide can double the amount of removed ions per mass of both electrodes (mg/g) compared to activated carbon. Despite its promising performance, little work has been done mainly because most known pseudocapacitive materials undergo reactions with cations, but not anions (e.g., Cl−). Therefore, precious silver metal or anion-exchange membrane-assisted carbon electrodes were necessary, which increased costs and reduced salt adsorption capacities. To overcome these limitations, we developed a system that uses two identical pseudocapacitive electrodes in two channels divided by an anion-exchange membrane. Based our success in using this system for power generation from salinity gradients, and supported by theoretical and experimental demonstrations of this concept for water desalination by others, we conducted tests to show that our system could improve desalination performance compared to previous capacitive approaches. Tests with this flow cell produced a salt adsorption capacity of the electrodes of ~40 mg/g, which is much higher than typical values reported using carbon electrodes (<20 mg/g). The cell voltage is also less than a half of typical CDI systems. Based on this high salt adsorption capacity and low applied cell voltage, this type of pseudocapacitive system could represent the next generation of low energy electrochemical process for water desalination.
Electrocatalytic Biomass Valorization Integrated with Hydrogen Production
Authors: Sun Yujie
Published: February 2020

Electrocatalytic water splitting has been widely considered as a promising approach to produce clean hydrogen (H2). The anodic half reaction of water splitting, O2 evolution reaction (OER), is the kinetic bottleneck of the overall water splitting process while its product O2 does not possess high market value. A novel strategy to replace OER with thermodynamically more favorable anodic reactions, such as the oxidative valorization of biomass-derived intermediates, represents an attractive approach, which not only reduces the overall voltage input but also produces two valuable products (H2 and biomass products) at both electrodes. This talk will present our recent work in electrocatalytic biomass valorization integrated with H2 production under ambient conditions. A series of biomass-derived intermediates, including 5-hdyroxymethylfurfural, furfural, furfuryl alcohol, and benzyl alcohol, were employed as representative organic substrates to be oxidized to value-added acids. 1st-row transition metal-based electrocatalysts were developed to catalyze biomass oxidation and H2 evolution. Results of long-term electrolysis demonstrated the bifunctionality of our electrocatalysts and nearly 100% Faradaic efficiencies for both H2 production and biomass oxidation reactions.
Composite Photoelectrocatalysts for Tempo-mediated Alcohol Oxidations
Authors: Chadderdon David J , Wu Ivy , Panthani Matthew G , Li Wenzhen
Published: February 2020

The transformation of primary alcohols to higher valuable carbonyls or acids has attached enormous attention from both academic and industrial viewpoints, but typically requires stoichiometric oxidants, which are costly and generate potentially toxic waste. Previously, we have studied selective electrocatalytic oxidation of biomass-derived alcohols and 5-hydroxymethylfurfural (HMF) on noble metal catalysts.[1-3] Electroactive homogeneous catalysts such as (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) represent an alternative to expensive metal catalysts.[4] Conversion of biorenewable feedstock in a photoelectrochemical cell (PEC) [5] is potentially one of the most environmentally-friendly chemical production routes available, which takes advantage of renewable carbon sources from the earth, renewable photons from the sun, and renewable electrons from wind, geothermal, or photovoltaic sources. However, there is a need to develop new composite materials which can simultaneously facilitate light absorption, charge separation, charge transport, and electrochemical reactions. We have investigated the use of composite photoanodes for efficient TEMPO-mediated oxidations. Bismuth vanadate (BiVO4) semiconductor films modified with an earth-abundant cobalt phosphate (Co-Pi) layer facilitated TEMPO oxidation at overpotentials almost 500 mV lower than unmodified BiVO4, while simultaneously suppressing the undesired oxygen evolution reaction. The photoanode materials were characterized using XRD, XPS, SEM, EDS, UV/vis and photoelectrochemical measurements. The composite photoanodes achieved complete oxidation of HMF to 2,5-furandicarboxylic acid (FDCA) with 88% yield, whereas an unmodified BiVO4 only reached partial oxidation intermediates (FDCA yield <1%) under the same conditions. Transient photocurrents revealed the role of Co-Pi for promoted TEMPO oxidation is two-fold: (1) to efficiently remove photogenerated charges from the semiconductor and (2) promote charge transfer across the catalyst-electrolyte interface for TEMPO oxidations.

References:

(1) Zhang, Z.Y. et al., Appl. Catal. B, (2012), 119, 40-48.

(2) Chadderdon, D. J. et al., Green Chem. (2014), 16 3778–3786.

(3) Chadderdon, D. J. et al., ACS Catal. (2015), 5 6926−6936.

(4) Rafiee, M.; Miles, K. C.; Stahl, S. S., JACS (2015), 137 14751−14757.

(5) Cha, H. G.; Choi, K. S., Nature Chem. (2015), 7 328–333.
Electrochemical Flow Cell for the Upgrading of Biomass Derived Streams
Authors: Diaz Luis A. , Lister Tedd E.
Published: February 2020

Abstract

 

The upgrading of biomass derived streams (BDS) provides a unique opportunity for the synthesis of fuel and chemicals completely derived from renewable sources. In order to achieve a complete greenhouse gas neutral fuel, the upgrading process needs to comply with high energy efficiencies and be able to totally rely in renewable resources as inputs and feedstock. However, one of the main challenges in the utilization of BDS comes from the nature and composition of the original sources, specifically, from the large oxygen content present in the carbohydrate polymers that compose the lignocellulose biomass in comparison with fossil sources [1]. The high concentration of oxygen, carbonyls, and phenolic groups present, directly translate in instability and poor physicochemical properties for their incorporation in the current refinery infrastructure. This drives the need to develop upgrading processes which are tailored to future energy schemes.

Electrochemical reduction is the only technology that can be directly powered by renewable energy sources, and do not require external addition of hydrogen, as this can be generated in-situ. In this work cathode formulations and membrane electrode assembly architectures have being engineered to allow a selective and efficient upgrading of BDS in a flow cell electrolyzer. Experimental results with surrogates and biomass derived streams will be presented.

 

 

References

[1] D. Mohan, C.U. Pittman, P.H. Steele, Pyrolysis of Wood/Biomass for Bio-oil: A Critical Review, Energy & Fuels, 20 (2006) 848-889.
Room Temperature Photoluminescence Characterization of Silicon Wafers for In-Line Monitoring Applications
Authors: Yoo Woo Sik , Ishigaki Toshikazu , Kang Kitaek
Published: February 2020

Performance variations and yield loss in advanced Si devices are closely related to the quality of the semiconductor surface and interfaces. These are very significant factors for successful development of advanced devices. In addition, proper implementation of manufacturing processes is critical as devices scale to smaller size and complexity of device structures increase. The importance of proper understanding and control of surface cleaning and passivation of silicon and dielectric/Si interface quality, is more important than ever.  Characterization techniques for determining the effectiveness of Si cleaning, surface passivation and the dielectric/Si interface include Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), high resolution cross-sectional transmission electron microscopy (HRXTEM), non-contact I-V, C-V and carrier lifetime measurements. Various chemical analysis techniques are also used for determining contamination sources in manufacturing. While the conventional characterization techniques have been providing useful information on the properties of the dielectric/Si interface, they cannot sufficiently provide substantive clues for many puzzling problems of Si surface cleanliness, Si surface passivation and the dielectric/Si interface.

In this paper, overall quality of the Si surface and dielectric/Si interface (including cleanness, passivation and surface recombination characteristics) were characterized using room temperature photoluminescence (PL) spectroscopy. The spatial resolution of PL spectroscopy is tens of microns. To understand the depth distribution of electrically active defects/traps, at or near the Si surface and the dielectric/Si interface, different excitation wavelengths (with different probing depths) were used as the light source. Significant variations in electrically active defects, traps, and contaminants, at or near the surface, and the dielectric/Si interface were found from multiwavelength PL spectroscopic studies of Si with native oxide. Non-contact, multiwavelength PL spectroscopy was able to reveal electrically active defects and characteristics induced by the cleaning process used, which cannot be properly investigated using conventional chemical analysis and dielectric/Si interface characterization techniques.
Stability of SiGe(100) Surfaces after Ammonium Sulfide Passivation
Authors: Heslop Stacy Lynn , Peckler Lauren , Muscat Anthony J.
Published: February 2020

Germanium and silicon germanium are attractive channel materials for transistors. Germanium has a smaller band gap and higher hole mobility than silicon, and varying the Ge concentration is one means of tuning the properties of SiGe alloys. Although these materials have many potential benefits, they are challenging to work with. The native oxide, GeO2, is water soluble and therefore cannot protect the surface during aqueous processes. One option is to deposit a layer that chemically passivates the surface after cleaning to protect it from reoxidation. Sulfur could passivate the surface due to its size and valency. Since the dominant cleaning technology is wet, aqueous ammonium sulfide could be a viable option. While ammonium sulfide processes deposit sub-monolayer coverages of sulfur on germanium, the surface reactions on SiGe surfaces are vastly different.

Our study focuses on the treatment of Si-rich SiGe surfaces (Si1-xGex x=0.25) with aqueous (NH4)2S. We varied the concentration of (NH4)2S in solution with and without added acid and characterized the surfaces using X-ray Photoelectron Spectroscopy (XPS). Immersions in (NH4)2S alone oxidized the surface and did not deposit a sulfide layer. Instead the surface oxidized as a function of the (NH4)2S concentration. In order to remove the oxide and slow down or stop the oxidation, small volumes of HF and HCl were added to the mixture. Addition of halogen acids dropped the pH of the solution from 10 to 8 and sulfides were detected with XPS in the S 2p region. Sulfur coverage increased with increasing concentrations of HF and HCl, however oxides increased as well. The simultaneous deposition of sulfur and oxygen suggests that oxidized sulfur, such as sulfates and thiosulfates, deposited on the surface.

In addition to chemical characterization of the surface, metal-insulator-semiconductor capacitors (MISCAPs) were fabricated after three different surface treatments: 1) clean and dry (control), 2) cleaning and immersion in (NH4)2S and 3) cleaning and immersion in (NH4)2S with added acid. Capacitance-voltage measurements were obtained for all three surface treatments. The density of interface defects (Dit) was calculated for each process from conductance and capacitance data. Processes that yielded the highest sulfur coverage (ammonium sulfide with added acid) showed a reduction in Dit (1.4 x 1012 cm-2 eV-1) compared to samples that were cleaned and dried only (6.4 x 1012 cm-2 eV-1) and samples treated with aqueous ammonium sulfide (4.4 x 1012 cm-2 eV-1).
Nanometric Particles Removal during Photoresist Stripping
Authors: Lallart Adeline , Garnier Philippe , Pizzetti Christian , Lorenceau Elise , Cartellier Alain , Charlaix Elisabeth
Published: February 2020

Microelectronics devices dimension have followed the Moore’s law in the last decades. In order to keep a high yield during integrated circuits production, a continuous improvement has been carried out in the particles cleaning area, switching from brushes and acoustic cleans to high velocity sprays. With the device nodes shrink, tinier and tinier particles are to be removed. Even though their adhesion and removal are well understood and documented for years [1], our industry still misses robust solutions to clean nanoparticles, keeping finest features integrity, and extremely low material consumption [2].

Well chosen resist stripping conditions enable an excellent particles removal without materials consumption nor damaging 14nm node polysilicon gates. The cleaning mechanism is investigated and key parameters involving the photoresist and wet stripping will be discussed. The aim of this work is to transfer this knowledge into an industrial optimized sequence. Previous studies deal with the fluids viscoelasticity role on the particles removal from a substrate [3]. It is thought non newtonian properties are involved during the resist removal process, with the fluid generated on the wafer through the well-known interface gel formation.

This paper will present results obtained through the use of 300 mm wafers intentionally contaminated with 45nm silica particles, then coated by different polymers, stripped thanks to various methods. The particles counting before and after the complete resist removal step is assigned to a KLA Surfscan SP3 blue laser diffraction tool.

The stripping conditions are a key parameter to enable an excellent defects removal. Indeed the PRE (Particles Removal Efficiency) varies from a few % to 70% when switching from SPM (Sulfuric acid / hydrogen Peroxide Mixture) conditions: too chemically active (radicals rich) with a stripping rate of few hundreds of nanometers per seconds on a single wafer platform to mild conditions, requiring few minutes to remove the resist on a batch spray tool, enabling the resist transition into a gel like phase (figure 1). Two samples have also been used as reference: first a resist removal by plasma ashing that doesn’t remove any particle, proving the particles removal doesn’t occur during the resist coating step and another one without resist, with the wet stripping to quantify the PRE brought by the SC1 (Standard Clean 1). Further tests are done with various solvents to substitute the SPM, but keeping the gel phase, required for the particles lift off.

Not only results on blanket but also challenging patterned wafers will be presented, showing absence of fine feature damage and PRE data, with a fair comparison with high velocity spray methods.

Eventually a basic mechanism will tentatively be proposed. Moreover, such results tend to prove an interest in continuing using resist stripping in wet instead of plasma ashing.

Figure 1
Reduced Anisotropy in Tetramethylammonium Hydroxide Based Silicon Etchants
Authors: Westwood Glenn , Hsu Chien-Pin Sherman
Published: February 2020

At sub-10 nm technology nodes, silicon etching is critical to FEOL processes such as FinFET thinning, gap filler removal, pre-III-V deposition etch, and nanowire release etching. The key to developing solutions for these applications is being able to control the anisotropy of the Si etch, or the etch selectivity between Si (111), Si (100), and Si (110). For this study, we focused on the impact of modifications to tetramethylammonium hydroxide (TMAH) on Si etch selectivity, with the ultimate goal of developing an isotropic Si etchant based on TMAH.   

Initial studies focused on etch selectivity testing of TMAH based chemistries on Si (SOI) wafers of varying orientations. Etch rates were based on measuring thickness changes by cross-sectional SEM. Differences in surface roughness were also characterized by atomic force microscopy. Finally, solutions were evaluated for anisotropy by performing an etch of patterned pSi wafers.

As expected, aqueous TMAH solutions showed a high level of anisotropy (111/100/110 = 1:5:5) and significant level of surface roughening. In contrast, some of the modified TMAH solutions tested had decreased surface roughening and an decreased level of anisotropy (111/100/110 = 1:2:2). Pattern etching on implanted pSi wafers is used to highlight the differences in anisotropy between TMAH and the different modifications.
Dielectric-MoS2 Interfaces Grown By Atomic Layer Deposition
Authors: Letourneau Steven , Mane Anil U. , Elam Jeffrey W , Graugnard Elton
Published: February 2020

Molybdenum disulfide (MoS2) has become a prototypical transition metal dichalcogenide (TMDC) atomic-layered material because of its unique materials properties. For example, bulk MoS2 exhibits an indirect band-gap of 1.3 eV, while a single monolayer has a direct band-gap of 1.8 eV. Recently, nanometer scale transistor devices have been made with MoS2 and graphene, yet much of this work relies on layered materials prepared using chemical vapor deposition (CVD) and mechanically exfoliation. Multiple reports have demonstrated the growth of MoS2 via CVD, but only a few studies have reported MoS2 growth using atomic layer deposition (ALD), which offers potential advantages for high volume semiconductor manufacturing. Here, we report the ALD of few-layer MoS2 films using MoF6 and various sulfur precursors on various ALD-grown dielectric surfaces and the ALD of metal oxides on the ALD MoS2 films at temperatures between 100-300 °C. In particular, we used in-situ quartz crystal microbalance measurements to investigate the nucleation of MoS2 ALD on oxide surfaces and vice versa. In general, self-limiting growth of MoS2 was observed on a wide range of dielectric surfaces including alumina, magnesia, and hafnia. The ALD MoS2 films were amorphous as-deposited, but crystallized into a layered atomic structure upon annealing at 800 °C. Using in-situ spectroscopy and electrical measurements, growth inhibition of MoS2 was seen depending on the growth substrate. These studies provide insight into the low-temperature ALD of MoS2 and provide guidance for the ALD of additional TMDC films.
(Invited) Area-Selective Atomic Layer Deposition: Role of Surface Chemistry
Authors: Mameli Alfredo , Karasulu Bora , Verheijen Marcel A. , Mackus Adriaan J.M. , Kessels W. M. M. , Roozeboom Fred
Published: February 2020

Area-selective atomic layer deposition (AS-ALD) is being considered as the next paradigm shift in future device fabrication. Given the chemical nature of the deposition technique, ALD is very sensitive to surface functional groups and deposition conditions. This characteristic often leads to different nucleation delays when growing a material on one surface (growth surface) as opposed to another (non-growth surface). This feature has been exploited by several research groups to obtain area-selective growth of various materials.1,2 The approaches that have been developed for achieving AS-ALD can be categorized as either area-activation1 or area-deactivation3. When no subtractive steps are employed, area-activation is referred to as direct-write (i.e. maskless) ALD.

In this contribution, results will be presented based on recently developed direct-write ALD approaches for two relevant metal oxides: In2O3:H, and ZnO. The method exploits the difference in nucleation delay on OH- and H-terminated Si surfaces. As shown in Figure 1, direct-write ALD can be achieved by locally activating an H-terminated Si starting surface using an O2-fed µ-plasma printer or by electron beam induced deposition (EBID) of a SiO2 seed layer. This allows to define OH-rich regions with dimensions ranging from a few microns (µ-plasma) down to tens of nanometers (EBID). Therefore, high-quality materials provided by ALD can be combined with the patterning capabilities of direct-write techniques to obtain patterns in a bottom-up approach.

The area-selective characteristics for these two different processes was probed by X-ray photoelectron spectroscopy and electron dispersive X-ray spectroscopy for micro- and nanoscale patterns, respectively. As shown in Figure 2, in-situ spectroscopic ellipsometry was used to investigate the nucleation behaviours of the ALD processes on the growth and non-growth surfaces. Finally, the key reasons for the selectivity have been unveiled using density functional theory (DFT) simulations.

References

1. Mameli, A. et al. Area-Selective Atomic Layer Deposition of In2O3:H Using a µ-Plasma Printer for Local Area Activation. Chem. Mater.
29, 921–925 (2017)

2. Longo, R. C. et al. Toward Atomic-Scale Patterned Atomic Layer Deposition: Reactions of Al2O3 Precursors on a Si(001) Surface with Mixed Functionalizations. J. Phys. Chem. C
120, 2628–2641 (2016)

3. Chen, R. and Bent, S. F. Chemistry for Positive Pattern Transfer Using Area-Selective Atomic Layer Deposition. Adv Mater
18, 1086–1090 (2006)

Figure 1
(Invited) Epitaxial Ge/GeSn High Mobility Channel Transistors
Authors: Liu C. W. , Wong I-Hsieh , Lu Fang-Liang , Huang Yu-Shiang
Published: February 2020

To boost the drive current of transistor and/or reduce power consumption, new channel materials with high carrier mobility such as SiGe/Ge/GeSn have been considered as the promising candidates for the continuous scaling of Moore’s law. SiGe channel have been used in the industry for pFETs, and the addition of Sn into Ge to further enhance the hole mobility is the attractive area of research. The strained GeSn on Ge/Si grown by CVD provides higher hole mobility than Ge due to the biaxial compressive strain and effective mass reduction by Sn incorporation.

To ensure the gate stack quality, and reduce the scattering of holes in the GeSn channel by oxide/interface charges and surface roughness, the epitaxy Ge cap with significant ΔEV has been integrated. The Ge-cap/GeSn/Ge quantum well structure with [Sn] up to 9% is grown by CVD on SOI, and the low temperature process is used to fabricate the devices without Sn segregation/diffusion. By optimizing the cap thickness and thermal budget of gate stack formation, the high mobility (~430cm2/V-s) of the CVD-grown GeSn quantum well p-MOSFETs is achieved to demonstrate the possible integration of GeSn material in industrial fabrication. The high hole mobility and reduced low frequency noise are obtained using quantum well structures with optimal cap thickness of 1 nm. The Ge cap also reduces the defects formed by Sn diffusion into dielectric. To improve the gate controllability and increase drive current with same footprint, the gate-all-around (GAA) pFETs using GeSn channels will be reported.

For Ge nFETs, junctionless GAAFETs with high drive current and high Ion/Ioff are investigated. To boost the drive current and to suppress short channel effects, the optimized doping profile is needed. Since the subthreshold characteristic of JL FET is highly related to the channel cross-section and channel doping, the channel doping of 1.2x1019cm-3 is used. However, it is too low for the S/D region, and heavily doped Ge (~1.2x1020 cm-3) is used in S/D by selective laser annealing with NiGe contact to reduce the parasitic resistance. The optimized doping profile is achieved by selective laser annealing in the S/D, with Pt as hard mask on channel region to reflect the laser light in the channel. The demonstrated Ge JL nGAAFET with fin width (Wfin) down to 7 nm, EOT = 2.2 nm, and Lch = 60 nm has Ion = 1146 μA/μm, Ion/Ioff = 2×106, and SS = 95 mV/dec. The Ion can be further boost to 1235 μA/μm with external uniaxial tensile strain of 0.16%. The self-heating effect is responsible in part for such high Ion, because the high device temperature can reduce the dominant impurity scattering in the channel. Note that the increasing mobility with increasing temperature indicates the impurity scattering is dominant.

With Ge/GeSn CVD growth and optimized process, GAA transistors with high mobility channels can be realized as promising candidates for future CMOS scaling.
(Invited) Past, Present and Future Challenges of Thermal Management of Electronics Packaging
Authors: Refai-Ahmed Gamal
Published: February 2020

The present and future use of the electronics devices of 7nm and beyond shall be supporting a complete heterogonous integration. This heterogonous integration will be more addressing the end customers need from system level point of view.

In the past few years there has been a tremendous increase in the power dissipation of the electronic devices especial general purpose GPU, CPU, as well as, FPGA in order to address the growing applications and support requirements for IoT features. In this talk, more discussion on the challenges of enabling heterogonous integration from thermal prospective will be given. This will be given based on the present thermal management devices of the present and future generation will be identified. The intent of this talk is to challenge the researchers on hot issues which need to be addressed on the coming five years.
(Invited) Thermoelectric Thin Films and Nanocomposites Synthesized Via Solution-Phase Chemistry
Authors: Wang Robert
Published: February 2020

Solution-phase processing routes to thermoelectric materials have the potential to decrease costs and enable novel device architectures. The best thermoelectric materials are crystalline inorganic semiconductors, which makes finding solution-phase routes to these materials of high interest. Unfortunately, inorganic semiconductors are generally insoluble due to their strong covalent bonds. One way around this hurdle is to create soluble semiconductor precursors that can be transformed into crystalline semiconductors after deposition. Furthermore, these precursor chemistries can be combined with colloidal nanocrystals to create nanostructured thin films with controlled composition and microstructure.

We first present the synthesis of CuSeS and Ag-doped CuSeS thin films via soluble precursors and then report their thermoelectric properties. Specifically, we study the effect of Cu vacancies, Se:S ratio, and Ag-doping. We find that Ag-doping leads to appreciable improvements in thermoelectric performance by increasing Seebeck coefficient and decreasing thermal conductivity, while having little to no change in electrical conductivity. Overall, we find that the room temperature thermoelectric properties of these solution-processed materials are comparable to measurements on CuSeS alloys made via conventional thermoelectric material processing methods. Achieving parity between solution-phase processing and conventional processing is an important milestone and demonstrates the promise of this approach to making thermoelectric materials.

Next, we present new chemical syntheses for soluble PbSe, PbTe, SnSe, and SnTe precursors. These precursors are especially relevant because PbSe and PbTe are the best-performing conventional thermoelectric materials in the moderate-to-high temperature regime. This chemical discovery is also important because soluble precursor routes to PbSe and PbTe thin films have not been reported in the literature. In addition, SnSe has the highest thermoelectric figure of merit of any known material, which makes our SnSe synthesis of interest as well.

Lastly, we combine these precursors with colloidal nanocrystals to create nanostructured thin films with controlled composition and microstructure. Colloidal nanocrystals are made via solution-phase chemistry and consist of an inorganic core with short ligand molecules bound to its surface. These nanocrystals can be made with excellent control over size, shape, and composition. We demonstrate that our precursors can replace the typical organic ligands used in colloidal nanocrystal synthesis. This creates new pathways to inorganic nanostructured thermoelectric thin films with controlled composition and microstructure, and we report on the thermoelectric properties of said films.
Simulation of Thermoelectric Generator Composed of Nanopatterned Thermoelement
Authors: Chen Xin , Lin Pengtao , Zhang Kai , Baumgart Helmut
Published: February 2020

The conversion efficiency of thermoelectric (TE) devices is determined by the dimensionless figure of merit ZT, which is expressed as ZT = S2σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute temperature, respectively. The key strategy to enhance ZT is through decreasing the thermal conductivity of thermoelectric materials. In this study, finite element analysis was used to investigate the effect of stripe-patterned Si substrates on thermal properties of a thermoelectric thermoelement, which consists of PbSe and PbTe thin film layers grown on the periodic patterned Si substrates. The width and depth of the stripe patterns were varied from 10 µm to 0.5 µm to determine their effects on the heat transport in the thermoelement. The calculated results reveal that the depth and width of the stripe patterns play an important role in decreasing the thermal conductivity of the thermoelectric module. The effect of the dimension of the patterns on the effective thermal conductivity of the TE thermoelement was shown in figure 1. The thermal conductivity is lower with respect to the deeper and narrower stripe patterns. The decreased thermal conductivity results from the enhanced phonon-boundary scattering due to the presence of periodic patterns. The phonons can be scattered more extensively when the dimension of the patterns is comparable to the mean free path of TE materials. Therefore, we modeled a thermoelectric generator (TEG) composed of stripe-patterned TE thermoelements taking advantage of its lower thermal conductivity. The dimension of the patterns was optimized selected to generate to achieve a higher power output of TEG. This work demonstrated the feasibility of using nanopatterned thermoelectric (TE) thermoelements to improve TE generator performance.

Figure 1
Transferable Memristive Nanoribbons Comprising Solution-Processed Strontium Titanate Nanocubes
Authors: Wang Jiaying , Choudhary Satyan , Harrigan William L. , Crosby Alfred J. , Kittilstved Kevin R. , Nonnenmann Stephen S.
Published: February 2020

Memristors, often comprising an insulating metal-oxide film between two metal electrodes (MIM), constitute a class of two-terminal devices that possesses tunable variations in resistance based on the applied bias history. Intense research remains focused on the metal-insulator interface, which serves as the crux of coupled electronic-ionic interactions and dictates the underpinning transport mechanisms at either electrode. Top-down, ultrahigh vacuum (UVH) deposition approaches for MIM nanostructures yield highly crystalline, heteroepitaxial interfaces, but limit the number of electrode configurations due to a fixed bottom electrode. Here we report on the convective self-assembly, removal, and transfer of individual nanoribbons comprising solution-processed, single-crystalline strontium titanate (STO) perovskite oxide nanocrystals to arbitrary metallized substrates. Nanoribbon transferability enables changes in transport models ranging from interfacial trap-detrap to electrochemical metallization processes. We also demonstrate the endurance of memristive behavior, including switching ratios up to 104, after nanoribbon redeposition onto polyethylene terephthalate (PET) flexible substrates. The combination of ambient, aerobic prepared nanocrystals and convective self-assembly deposition herein provides a pathway for facile, scalable manufacturing of high quality, functional oxide nanostructures on arbitrary surfaces and topologies.
(Invited) Atomistic Simulations for Understanding Microscopic Mechanism of Amorphous-Tantalum-Oxide-Based Resistive Switching Devices
Authors: Watanabe Satoshi , Xiao Bo , Li Wenwen
Published: February 2020

Among oxide-based resistive switching devices, amorphous-TaOx based ones show good endurance and switching performances. Their switching behaviors are understood based on the formation/rupture of conductive filaments (CFs). However, the atomistic structures of CFs and their microscopic formation/rupture processes have not been sufficiently clarified yet. In this talk, we present the results of our atomistic simulations (most of which are with the density functional theory (DFT)) and discuss several aspects related to the nature and formation processes of CFs as follows.

1) CFs in the Cu/amorphous-Ta2O5/Pt heterostructure: Our simulations reveals that single Cu chains in Ta2O5 cannot work as conductive filaments (CFs), while Cu nanowires with a diameter of three atoms or larger can work as CFs. The stability of the Cu nanowires has been checked by ab initio molecular dynamics [1].

2) CFs in the Pt/TaOx/Pt heterostructure with x < 2.5: Our results suggest that not O vacancy chain but the Ta-Ta bonding mainly contributes to the CFs [2].

3) Difference in the atomistic features between Cu/ Ta2O5 and Pt/Ta2O5 interfaces: In the former, considerable portion of interface Cu atoms tend to migrate to the amorphous Ta2O5 layer, while similar behavior is not seen in the latter [3].

4) Ion migration behaviors in TaOx: Our results of ab initio molecular dynamics and nudged elastic band calculations show that the diffusion coefficient in TaOx strongly depends on x [3].

5) Construction of neural network interatomic potentials: We constructed interatomic potential combining the DFT calculations and neural network for simulations of Cu migration behavior in amorphous-Ta2O5 to achieve computation speed and reliability simultaneously. The pathways and barrier energies for Cu diffusion calculated using the NN potential agree well with those obtained from DFT calculations [5].

 The final part of the present works was partly supported by the Support Program for Starting up Innovation hub from Japan Science and Technology Agency (JST), and CREST-JST, Japan.

[1] B. Xiao, T. Gu, T. Tada and S. Watanabe, J. Appl. Phys. 115, 034503 (2014).

[2] B. Xiao and S. Watanabe, Nanoscale 6, 10169 (2014).

[3] B. Xiao and S. Watanabe, ACS Appl. Mater. Interfaces 7, 519 (2015).

[4] B. Xiao and S. Watanabe, Extended abstract of SSDM2014 (September 8-11, 2014, Tsukuba, Japan).

[5] W. Li, Y. Ando and S. Watanabe, submitted.
(Invited) Local Redox Reaction By Ion Transport Probed By in-Situ PES Measurements
Authors: Yamaguchi Shu
Published: February 2020

It is now evident that ion migration in oxides governs the memristive behavior of oxide thin film. After various approaches, still there are some uncertainties about the relation between ion migration and memristive behaviors. Complexities lie in the the ion migration behavior and redox reaction in oxides and at interface between oxide electrolyte and electrode. The mode change from frost-like to filament growth of reduced domains are of another issue to be discussed. In the present study, the author employed in-situ measurement of photoemission spectroscopy (PES) using synchrotron radiation to look at the redox reaction in oxide just underneath the electrode. The author employed thin Au electrode with 3-5 nm in thickness, using hard X-ray, for the excitation of photoelectrons 3-5nm underneath the Au electrode. Various type of “electrolytes,” Gd-doped ceria(GDC), amorphous TaOx(a-TaO
x
), amorphous silica(a-SiO2), and so on, are employed to look at the electrochemical redox reaction in the vicinity of electrode. In case of GDC, XAS measurements have been also made to look at the redox reaction in deeper region from the electrode/oxide interface.

In-situ PES measurements on various oxide thin film systems examined with MIM-type two-electrode cells revealed the variation of Fermi level upon electrochemical polarization, suggesting variations of local electrochemical redox state by the ion migration. Surprisingly narrow electrochemical windows were suggested limited by a pinning at the donor state of component cations and a chemical pinning due to oxygen evolution, in contrast to the ultra-wide band gap of these almost insulating oxides systems. The importance of the reversibility and how to control the kinetics at the anodic electrode should be emphasized. One important conclusion is the system is identical to the simple two-electrode cell composed of the electrolyte that shows variation of Fermi level within an electrochemical window of oxide system by ion migration or electrochemical reaction and modulated overpotential at electrolyte/electrode interfaces, the latter of which gives huge influence to the cell behaviors. These suggest that oxide memristor is governed by the electrochemical properties of the cell. Finally, the author proposes both a new Pourbaix-type E-pO diagram to understand the redox and acid/base reaction in addition to new proposal of the definition of basicity of complex oxide based on electronic band structure, as a necessary framework of solid state electrochemistry for oxide systems.
(Invited) What's Next after GaN? How about More GaN!
Authors: Meyer David J. , Downey Brian P. , Hardy Matthew T. , Storm David F. , Katzer D. Scott , Ancona Mario G. , Nepal Neeraj , Roussos Jason A.
Published: February 2020

Over the course of the past few years, transistors based on gallium nitride (GaN) and related III-N materials have proven to be clear contenders for solid-state power amplifier applications in the commercial and military market spaces. In microwave technology nodes of lateral GaN high-electron-mobility transistor (HEMT) technology (with gate lengths greater than 0.25 microns), the material-driven electrical performance of GaN has been demonstrated with exceptional reliability in production-released processes.

While a power density of several W/mm can be easily achieved at S- and X-band with GaN HEMTs based on an AlGaN/GaN heterojunction, it generally becomes more difficult to obtain the same output power level at higher frequency nodes. This is primarily due to limitations on the operating voltage that can be used; as device dimensions scale shorter, internal electric fields increase and field management techniques, such as source-connected field plates, generally are not used because they introduce undesirable parasitic capacitance. To address these shortcomings, significant technical research and development has been carried out over the past few years focusing on extending GaN HEMT frequency performance into the millimeter wavelength (MMW) range of 30 – 300 GHz.

To prevent the onset of short-channel effects in short gate length devices, vertically-scaled barrier heterostructures have been investigated. In a number of examples, materials growth development of novel III-N materials has enabled heterojunctions with desirable two-dimensional electron gas transport properties. In this talk, we will review a number of recent developments in the community related to MMW GaN HEMT technology. In addition to reported literature, we will discuss NRL’s internal materials and MMW device development using molecular beam epitaxy. Novel HEMT heterostructures based on AlN, ScxAl1-xN, and InxAl1-xN (metal- and N-polar) will be discussed.
(Invited) Towards a Si Foundry-Compatible, GaN-on-Si Mmic Process on 200 Mm Si with a Cu Damascene BEOL
Authors: Laroche J. , Ip K. , Kennedy T. , Soirez L. , Trimble T. , Kazior T.
Published: February 2020

In 2013 Raytheon began to integrate deep submicron (≤0.25 µm gate) 200mm GaN on Si HEMT processes within a commercial CMOS Si Foundry environment. When fully realized, these processes will demonstrate multi GHz GaN on Si MMICs by leveraging a fully subtractively processed transistor coupled with multi level copper based back end of line (Cu BEOL) processes. This work provides a status update on the progress towards that goal.
(Keynote) Advances in Ga2O3 Processing and Devices
Authors: Yang Jiancheng , Carey Patrick , Ahn Shihyun , Ren Fan , Jang Soohwan , Kim Jihyun , Hays David , Pearton Stephen J. , Kuramata Akito
Published: February 2020

Ga2O3 is a promising wide bandgap semiconductor for applications including power electronics and photodetectors and is available in large diameter, high quality bulk crystalline form. There have been many recent advances on both the materials and processing/device fronts and in this chapter we discuss some developments in Ohmic contacts, plasma etching, understanding of the role of hydrogen and high breakdown rectifier structures. Ga2O3 appears to have a role in expanding applications for high-speed or high-power semiconductor electronic devices. The high-power/high-voltage market is currently primarily served by Si and SiC devices. Ga2O3 is the leading candidate to address the ultra-high power market (>1kW). We will discuss recent progress in developing Ohmic contacts using interlayers of ITO or AZO, high density plasma etching using ICP discharges and the resultant etch rates, morphologies and near-surface damage, behavior of hydrogen incorporated during ion implantation or plasma exposure, measurement of band offsets with dielectrics and application of these processes to vertical and lateral transistor structures.
(Keynote) Accelerating Commercialization of SiC Power Devices through Low-Cost US Manufacturing
Authors: Veliadis Victor
Published: February 2020

The U.S Department of Energy launched the PowerAmerica (PA) Institute under the initiative of “National Network of Manufacturing Institutes” to commercialize SiC and GaN wide bandgap (WBG) power devices. PowerAmerica started operations in 2015 with $140M funds over 5 years, and is managed by North Carolina State University in Raleigh, NC. PA’s mission is to create U.S. high-tech manufacturing jobs and save energy through accelerated large-scale adoption of WBG semiconductor devices in power electronics systems. To achieve its goals, PowerAmerica funds strategic projects and brings together a range of companies – from startups to major corporations – as well as national labs and universities that work on every facet of the supply chain, from research and development to manufacturing and commercialization. Through membership in the PowerAmerica institute, industry members grow their business by accelerated wide bandgap product introduction to market, and University members gain by engaging in collaborative projects with industry.

Projects launched in 2016 covered low cost WBG device fabrication and a range of promising power electronics applications including a faster circuit breaker, a fast charger, electric vehicle traction drives, photovoltaic inverters, more efficient data centers, and heavy-duty vehicle electrification.

PowerAmerica’s largest project to date has focused on lowering the barrier to market entry for fabless SiC device companies, thereby contributing to building a stronger overall manufacturing ecosystem for this technology. The PA Institute partnered with Texas-based X-Fab to form the world’s first “open foundry” capable of supporting 6-inch SiC wafers. By leveraging mature Si fabrication practices and qualifying SiC specific processes, XFAB has enabled high-yield volume manufacturing at the economy scale of silicon. This capability enables fabless semiconductor companies to economically fabricate silicon carbide devices and compete in new ways. The conversion of an existing silicon factory into one capable of additionally fabricating SiC has produced significant benefits for the SiC ecosystem.

In addition to facilitating volume manufacturing, PA is accelerating WBG adoption by training the workforce through short course offerings, and preparing the next generation of high-skilled WBG technologists through hands-on projects at leading US Universities.

The presentation will outline the model for low cost SiC manufacturing in the US, and describe select projects that exemplify the performance advantages of SiC based power electronic systems.
(Invited) Dirac Fermions in Layered Topological Material ZrTe5
Authors: Jiang Zhigang , Jiang Yuxuan , Smirnov Dmitry
Published: February 2020

Finding a large band gap topological insulator (TI) is crucial to future room-temperature (corresponding to ~25 meV) device applications. Recently, monolayer ZrTe5 has been predicted to be new functional two-dimensional (2D) TI [1], with a large (bulk) band gap far surpassing their predecessors, e.g., HgTe/CdTe and InAs/GaSb quantum wells. However, its electronic structure in the bulk is currently under heated debate, with interpretations ranging from weak/strong TI to Dirac semimetal.

Here, we report on a “bulk-sensitive” magneto-infrared transmission study of ZrTe5 thin flakes in magnetic fields up to 17T. At zero magnetic field, our samples exhibit graphene-like optical absorption, which signifies their 2D nature. In a magnetic field, we observed a series of inter-band Landau level (LL) transitions that can be described by a massive Dirac fermion model with a small mass. More interestingly, we observed a four-fold splitting of low-lying LL transitions. Thanks to the newly developed circular polarized magneto-infrared capability, we were able to separate the σ+ and σ- active transitions. These observations enable further exploration of the origin in the splitting: (i) the band asymmetry breaks the degeneracy of the dipole allowed inter-band LL transitions, and (ii) the remaining two-fold degeneracy is due to a combined effect of large g-factor and a small energy gap in this system. Our results support a 2D Dirac semimetal interpretation, consistent with recent electronic transport studies [2,3]. Finally, we compare the LL structure of Dirac fermions in ZrTe5 and in other topological materials that we systematically investigated over last few years, such as HgCdTe and InAs/GaSb [4, 5, 6].

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5. F. Teppe et al. Nature Communications
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6. Y. Jiang et al. Physical Review B
95, 045116 (2017).