bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Substrate Type on CoSi2 Formation in Rapid Thermal Annealing
Authors: Kim Eun Jung , Lee Jin Yul , Kim Sang Deok , Song Han Sang , Yeom Seung Jin , Ishigaki Toshikazu , Kang Kitaek , Yoo Woo Sik
Published: February 2020

Low resistivity junctions and contacts are essential for fabricating high performance devices with advanced device nodes. Significant effort has been expended towards the development of low resistivity junctions. However, low resistivity contact formation has not received significant attention. The silicide of choice has been changed from high temperature silicide to low temperature silicide (i.e., WSi → TiSi2 → CoSi2 → NiSi) depending on the allowed maximum process temperature and required minimum line width for the device node. Once the candidate silicide material is chosen for the device node, relatively small improvements in resistivity and small changes in the silicide formation process have been made in recent years, despite silicidation process optimization. This includes fine tuning of silicide formation conditions for types of Si (i.e., doped poly Si, NMOS and PMOS), modifications of process steps and integration schemes.

In this study, we have investigated the effect of substrate type on CoSi2 formation in rapid thermal annealing (RTA). The RTA temperature and time sensitivity of the resulting resistivity of the Co2Si contacts with P-doped poly-Si, NMOS source/drain (S/D) and PMOS S/D were investigated for process window determination using a single wafer furnace (hot wall)-based RTA system.

The type of Si had a significant effect on the resulting CoSi2 formation temperature and contact resistance was observed. With the increase of RTA time, the CoSi2 formation temperature was reduced and the difference of CoSi2 formation temperature for different Si substrates became larger. Depending on CoSi2 integration schemes (i.e., one step (for all CoSi2 in poly-Si gate and S/D) or two step (one for CoSi2 for poly-Si gate and the other for CoSi2 for S/D)), the CoSi2 formation RTA conditions must be modified. Detailed effects of substrate type on CoSi2 formation and process windows will be reported.   

Figure 1
Hot-Wire Chemical Vapour Deposition for Silicon Nitride Waveguides
Authors: Domínguez Bucio Thalía , Tarazona Antulio , Khokhar Ali Z. , Mashanovich Goran Z. , Gardes Frederic Y.
Published: February 2020

SiN layers are typically deposited by low-pressure chemical vapour deposition (LPCVD) and plasma-enhanced chemical vapour deposition (PECVD). The issue with these deposition techniques is that they require high temperature processing (>400ºC) in order to produce waveguides with low propagation losses. As a result, they are incompatible with multilayer platforms in which the integration of active devices is intended, because the fabrication of these devices requires processes that are sensitive to high temperatures such as metallisation and doping. To address this situation, we are proposing hot wire chemical vapour deposition (HWCVD) as a means to deposit SiN layers at temperatures below 400ºC. This technique has already been used in solar cell applications [1,2] to produce amorphous SiN layers with low hydrogen content and high mass density [3]. In this study, several deposition recipes were defined with a design of experiments methodology (DoE) in which different combinations of deposition parameters were investigated to understand how they affect the material structure of the SiN layers in order to optimise the quality and the losses of the deposited materials. The physical, chemical and optical properties of the obtained SiN layers were characterised using ellipsometry, scanning electron microscopy (SEM), Fourier transform infrared spectrometry (FTIR) and atomic force microscopy (AFM).  Also, waveguide structures were fabricated on the deposited materials to measure their propagation losses at 1310 and 1550nm using the cutback method. SiN layers with good uniformity, roughness as low as 0.61nm, H concentration as low as 10.4x1021 atoms/ cm3 and losses of 7.1dB/cm at 1310nm and 12.3dB/cm at 1550nm were obtained.

[1]  H. D. Goldbach, C. H. M. van der Werf, J. Löffler, A. Scarfo, A. M. C. Kylner, B. Stannowski, W. M. Arnoldbik, A. W. Weeber, H. Rieffe, W. J. Soppe, J. K. Rath, and R. E. I. Schropp,“Hot-wire chemical vapor deposition of silicon nitride for multicrystalline silicon solar cells,” in Conference Record of the Thirsy-first IEEE Photovoltaic Specialists Conference, pp. 1249–1252, IEEE, January 2005.

[2]  J. K. Holt, D. G. Goodwin, A. M. Gabor, F. Jiang, M. Stavola, and H. A. Atwater, “Hot-wire chemical vapor deposition of high hydrogen content silicon nitride for solar cell passivation and anti-reflection coating applications,” Thin Solid Films 430, pp. 37–40, April 2003.

[3]  V. Verlaan, R. Bakker, C. H. M. van der Werf, Z. S. Houweling, Y. Mai, J. K. Rath, and R. E. I. Schropp, “High-density silicon nitride deposited at low substrate temperature with high deposition rate using hot wire chemical vapour deposition,” Surface and Coating Technologies 201, pp. 9285–9288, September 2007.
Determination of Antiphase Domain Boundary Annihilation Rate in GaAs on Si(001) and the Influence of MOCVD Growth Temperature
Authors: Barrett Caleb Shuan Chia , Martin Thomas Patrick , Bao Xin-Yu , Martin Patrick , Sanchez Errol , Jones Kevin Scott
Published: February 2020

III-V compound semiconductors are currently being investigated as replacements for Si in future metal-oxide-semiconductor field effect transistor (MOSFET) technologies. GaAs is a good candidate as a buffer layer material for integration of alternative channel materials such as InGaAs and InAs on Si wafers. One barrier to good quality III-V epitaxy on Si is the formation of antiphase domain boundaries (APBs) due to the polar-on-nonpolar growth. High-temperature pre-growth bakes and off-cutting of the wafer typically used to eliminate APBs in III-V layers grown on Si are incompatible with conventional CMOS processing. Thus, the influence of growth conditions on APB propagation to facilitate self-annihilation is critical to understand. It has been shown that a higher bulk growth temperature can influence the preferred plane of propagation for APBs in GaP on Si(001) leading to enhanced annihilation of APBs. However, this effect has yet to be examined for GaAs on Si(001) and the effect on APB annihilation rate as a function of growth temperature has not been quantified. In this study, nominally 800 nm thick GaAs films were grown on exact-cut 300 mm Si(001) wafers by an Applied Materials III-V metal-organic chemical vapor deposition (MOCVD) system. The films were grown in a multi-step manner. The initial growth stages were identical for all samples in order to obtain similar starting APB densities. The growth temperature of the final stage was varied to investigate the effect on APB annihilation. Depth profiles of APB density for each sample were obtained using a combination of a bulk GaAs chemical etchant and an APB-selective chemical etchant and examination with a scanning electron microscope to determine the respective APB annihilation rates. The evolution of APBs in cross-section was examined using transmission electron microscopy. It was found that increasing the growth temperature of the final stage by 80 °C increased the annihilation rate of APBs in the GaAs films by approximately a factor of 5. The associated activation energy for the increase in APB annihilation rate was calculated to be 1.1 eV. There was no significant difference in APB annihilation rate after the 80 °C increase. This result is of relevance for the selection of appropriate growth conditions and considerations of thermal budgets for future CMOS processing with III-V based devices.
(Invited) Basic Properties of h-BN Epilayers
Authors: Jiang Hongxing , Lin Jingyu
Published: February 2020

III-nitride wide bandgap semiconductors have been at the center stage of the semiconductor R & D effort over the last two decades.  Photonic/electronic devices based on III-nitrides, including UV/blue/green/white light emitting diodes (LEDs), violet/blue laser diodes (LDs), UV detectors, and high power/temperature transistors, have been successfully realized and commercialized.

Among the members of the III-nitrides, BN is the least studied and understood. BN exists in three crystalline forms (hexagonal, cubic and wurtzite). The sp2-bonded hexagonal form (h-BN) is the most stable phase when synthesized at any temperature under ambient pressure. Its extraordinary physical properties, such as ultra-high chemical stability, thermal conductivity, electrical resistivity, and band gap (Eg ~ 6.5 eV) make h-BN the material of choice for many emerging applications. Due to its close lattice match to graphite, h-BN is being recognized as the most suitable substrate/dielectric/separation layer for 2D electronics and optoelectronics. As such, much efforts in the synthesis of h-BN have been largely focused on mono- or a few layers of h-BN in the past.

In this presentation, I will summarize recent progress in the growth of wafer-scale h-BN epilayers with a few um in thickness by metal-organic chemical vapor deposition (MOCVD). The basic properties of h-BN epilayers will be discussed in comparison with other nitride semiconductors, in particular AlN.  H-BN shows the unique qausi-2D nature with exceptionally high density of states (DOS) and large exciton binding energy (around 740 meV), which results in high optical absorption and emission intensity. P-type conduction in h-BN and diode behaviors in the p-n junction structures consisting of p-type h-BN and n-type Al-rich AlGaN heterostructures have been demonstrated, which could potentially address the intrinsic problem of low p-type conductivity in Al-rich AlGaN for DUV photonic devices. The potential applications of h-BN for deep UV emitters and detectors, 2D templates/dielectric/separation layers, and solid-state neutron detectors will also be discussed.
(Invited) Diamond - the Unknowns and Challenges to Make It Work for GaN Electronics
Authors: Kuball Martin , Anaya Julian , Liu Dong , Baranyai Roland , Sun Huarui , Pomeroy James W
Published: February 2020

Diamond has demonstrated excellent properties for thermal management of GaN electronics. In this presentation, we review our latest results on the diamond properties near the GaN-on-diamond interface, and the mechanical stability of the GaN-on-diamond interface and its properties.
(Invited) The Use of Sub-Bandgap Optical Pumping to Identify Defects in AlGaN/GaN High Electron Mobility Transistors
Authors: Ren Fan
Published: February 2020

AlGaN/GaN high electron mobility transistors (HEMTs) showed its promising features in high power and high frequency applications such as inverter units in hybrid electric vehicles, advanced radar systems, and satellite-based communication networks with its high density of sheet carrier concentration, high electron mobility, and radiation hardness. However, intrinsic defects and impurities in the channel and gate-to-drain regions degrades HEMT performance through phenomena such as drain current collapse, high gate leakage, and lower rf power efficiency. Moreover, subsequent electrical stressing of the HEMTs during operation leads to creation of more traps and further device degradation through various mechanisms, including gate contact sinking, shallow trap formations, and the inverse piezoelectric effect. It is clearly desirable to have non-destructive methods available that can identify the activation energies and spatial location of trap states in GaN-based heterostructure devices.

              In this work, a sub-bandgap optical pumping technique using 671, 532 and 447 nm lasers was used to determine the trap locations in AlGaN/GaN HEMTs. Using different photon fluxes, the traps with different activation energies can be identified. This indicates that the defects originated at different physical locations in the HEMT heterostructures. The locations of the traps were confirmed by companion gate pulse measurements under optical pumping.
(Invited) Improved GaN Based Hydrogen Sensors
Authors: Baik Kwang Hyeon , Kim Jimin , Jang Soohwan
Published: February 2020

Recently, there are many interests in hydrogen as alternative energy source and carrier. Hydrogen is emission-free fuel when reacted with oxygen to produce the heat of combustion of 141.9 kJ/g while 47.0 and 45.0 kJ/g are for gasoline and diesel. Electrochemical cells and combustion in internal engines are used to transport hydrogen-generated energy to power vehicles and electric devices. Great efforts have been devoted on the researches to produce hydrogen in efficient and economical ways such as photoelectrochemical water splitting, photocatalytic water splitting, radiolysis, and photobioligical water splitting.

Hydrogen is consumed in the petrochemical plant for hydrodealkylation, hydrodesulfurization, and hydrocracking, as a coolant in power stations due to its highest specific thermal conductivity of all gases, as an energy carrier in fuel-cell vehicles, in the propulsion of spacecraft, and in the semiconductor industry. Hydrogen gas is colorless, odorless, extremely reactive with oxygen, and has low ignition energy. When it leaks from pressurized container, its temperature rises up due to its negative Joule-Thomson coefficient, which can induce spontaneous flammable ignition. Therefore, hydrogen gas detection is very important issue in various hydrogen related industrial processes and equipments. Not only for early flammability alarm, a hydrogen sensor is also essential to monitor feed stream in the fuel-cell vehicle for efficient energy conversion.

GaN based material system is very suitable to construct hydrogen sensor.  Wide bandgap of GaN enable devices to operate at high temperature and harsh radiative environment, and mechanical and chemical robustness of GaN ensures reliability and durability of the device. Many types of GaN based hydrogen sensing devices including Schottky diode, metal oxide semiconductor (MOS) diode, GaN nanowire and AlGaN/GaN high electron mobility transistors (HEMTs) which employ platinum or palladium film as a catalytic active layer have been developed for fast and sensitive detection of hydrogen .

In this study, effect of crystal polarity and plane on hydrogen sensing characteristics of GaN based diodes was investigated. Also, AlGaN/GaN HEMT hydrogen sensors with dramatically increased active sites on platinum nanostructure and GaN diode sensors with etched large surface area will be introduced as an approach to improve the sensitivity of hydrogen sensors.
(Invited) Photon Managements By Employing Nanostructures for Light Emission Diodes
Authors: He Jr-Hau
Published: February 2020

It is of current interest to develop the photon management with nanostructures since the ability to suppress the reflection and light trapping over a broad range of wavelengths and incident angles plays an important role in the performance of light-emitting diodes (LEDs). Superior light-extraction characteristics of nanostructures for LEDs are discussed in this talk. These advantages are mainly attributed to the subwavelength dimensions of the nanowires, which makes the nanostructures behave like an effective homogeneous medium with continuous gradient of refraction index, significantly reducing the reflection through destructive interferences. The relation between the geometrical configurations of nanostructures and the light-harvesting characteristics is discussed. We also demonstrated their applications in solar cells and photodetectors. This research paves the way to optimize the nanostructured optoelectronic devices with efficient light management by controlling structure profile of nanostructures.
Anchoring Molecules on the Surface of ZnO and Effects on Intergranular Transport Properties
Authors: Gamon Jacinthe , Barboux Philippe , Giaume Domitille , Le Mercier Thierry , Le Bahers Tangui
Published: February 2020

Improving electrical conductivity in thin film semiconductor devices such as solar cells, thermoelectric modules, or transparent conducting devices, remains a key point for better performances. As polycrystalline films have the favor of the industry for their lower fabrication cost, the limitation of the current is most of the time attributed to charge scattering at the grain boundaries due to high energy barriers at the interface between grains.(1)

This work focuses on the understanding of intergranular electrical conductivity in the view of improving thin film semiconductor performances deposited via a soft chemistry deposition routes. In this goal, ZnO:Al was chosen as a model doped semiconductor for our investigations as it is a reference material in many semiconductor applications (sensors, solar cells, transparent conducting oxides)(2). Many studies have already demonstrated the role of adsorbed species onto ZnO thin films and nanostructures surfaces to improve sensor selectivity or charge transfer in dye sensitized solar cells for instance (3).

With this in mind, we have performed liquid phase adsorption studies of organic molecules onto ZnO particles through solution conductimetry and IR. A theoretical modeling of the energy level of anchoring molecules was performed in order to fit the LUMO level to the conduction band. Conjugated and thiophene based molecules with different substituents and lengths were selected. Conductivity on pressed powders was studied as a function of the applied pressure to show the effect of the grain boundaries and of the powder compaction. A conclusion was drown on the participation of these molecules and their LUMO levels in the electrical conductivity of ZnO:Al.

This study allows quantitative determination adsorbates on ZnO surface and puts into perspective the investigation of better matches between LUMO and conduction band.

Literature:

1.    Greuter F, Blatter G. Electrical properties of grain boundaries in polycrystalline compound semiconductors. Semicond Sci Technol. 1990;5(2):111.

2.    Klaus Ellmer, Andreas Klein, Bernd Rech. Transparent Conductive Zinc Oxide. Springer Series in Material Science. 2008.

3.    Galoppini E. Linkers for anchoring sensitizers to semiconductor nanoparticles. Coord Chem Rev. 2004 juillet;248(13–14):1283–97.
TiO2 Nanoparticles Synthesis from a Ti Alloy Foil By Electrochemical Method in Aqueous Electrolyte
Authors: Ortega-Díaz David , Fernandez Dennys , Pelaez-Abellan Ernesto
Published: February 2020

In recent years, catalysis and photocatalysis processes using TiO2 nanoparticles (TNPs) have received great attention due to their effectiveness in degrading and mineralizing organic and inorganic compounds. Nanocrystalline TiO2 powder with different crystallinity and phase structures were synthesized using a Ti alloy as anode and stainless steel as cathode by the galvanostatic anodization method in alcoholic solution of chloride. Sodium dodecyl sulphate (SDS) surfactant was used as additive for to control the particle size. The X-ray diffraction (XRD), RAMAN spectroscopy and transmission electron microscopy (TEM) results, showed that a 50:50 anatase and brookite mixture phase was obtained with crystal sizes 7.2 nm and 7.5 nm respectively without any annealing. The size of the particles and the material purity are measured using scanning electron microscopy (SEM), dynamic light scattering (DLS) and energy dispersive X-ray analysis (EDX) indicated that a particle size smaller than 40 nm and a free impurity materials. The Brunauer–Emmett–Teller (BET) results show a specific surface area bigger than 200 m2/g. The band gap energy of the resulting TNPs were determined by diffuse reflectance measurements according to the Kubelka Munk theory, revealing a value of 3.24 eV. Therefore, the results indicate the success of this method to create TiO2 nanoparticles in aqueous medium with a good properties to be use like photocatalyst in a very economical and environmentally benign way.
(Invited) Electrostatic and Electrochemical Nature of Liquid-Gated Electric-Double-Layer Transistors Based on Oxide Semiconductors
Authors: Yuan Hongtao
Published: February 2020

The electric-double-layer (EDL) formed at liquid/solid interfaces provides a broad and interdisciplinary attraction in terms of electrochemistry, photochemistry, catalysts, energy storage, and electronics because of the large interfacial capacitance coupling and their ability for high-density charge accumulation. Much effort has recently been devoted to the fundamental understanding and practical applications of such highly-charged EDL interfaces. For example, electric field control of carrier density with EDL in a transistor configuration has been used as a powerful tool to tune the electronic states of condensed matter. However, the intrinsic nature of the EDL charging, whether it is electrostatics or electrochemistry, and how to distinguish them are far from clear. Here, by combining electrical transport measurements with electrochemical impedance spectroscopy (EIS), we studied the charging mechanisms of highly-charged EDL interfaces between an ionic-liquid and oxide semiconductor, ZnO. The direct measure for mobile carriers from the Hall effect agreed well with that from the capacitance-voltage integration at 1 Hz, implying that the pseudo-capacitance does not contribute to carrier transport at EDL interfaces. The temperature-frequency mapping of EIS was further demonstrated as a “phase diagram” to distinguish the electrostatic or electrochemical nature of such highly-charged EDL interfaces with densities of up to 8×1014 cm-2, providing a guideline for electric-field induced electronic phenomena and a simple method for distinguishing electrostatic and electrochemical charging in EDLTs not only based on a specific oxide semiconductor, ZnO, but also commonly applicable to all types of EDL interfaces with extremely high-density carrier accumulation.

1. H. T. Yuan, H. Shimotani, J. T. Ye, S. J. Yoon, H. Aliah, A. Tsukazaki, M. Kawasaki, and Y. Iwasa, Electrostatic and Electrochemical Nature of Liquid-Gated Electric-Double-Layer Transistors on Oxide Semiconductors,
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Am

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Chem. Soc.
132, 18402 (2010).

2. H. T. Yuan, H. Shimotani, A. Tsukazaki, A. Ohtomo, M. Kawasaki, and Y. Iwasa, Hydrogenation- Induced Surface Polarity Recognition and Proton Memory Behavior at Protic-Ionic-Liquid/Oxide Electric-Double-Layer Interfaces,
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Chem. Soc.
132, 6672 (2010).
(Invited) A Frequency Domain Optofluidics Dissolved Oxygen Sensor with Enhanced Sensitivity for Water Monitoring
Authors: Mahoney Eric , Du Fei , Hsu HuanHsuan , Selvaganapathy Ravi , Fang Qiyin
Published: February 2020

Dissolved oxygen (DO) is an important indicator for water quality and is also used in point of care applications.  Fluorescence signal from Ruthenium (Ru(dpp)3
2+ ) based fluorophores can be quenched by oxygen such that the reduction of fluorescence signal is proportionally to oxygen concentration. This mechanism allows an optical method to measure DO in liquid.  Although optical DO measurements offer high sensitivity and stability, they are also expensive, bulky, and difficult to use due to its complex design and requirement for specialized optical instruments such as light sources and spectrometers. Recently, the advances in microfabricated optical component including waveguides, lenses and mirrors, light sources, and photo detectors enable the integration of optical sensing and microfluidics sampling platform. We report the development of a total internal reflection assisted (TIRA) optical DO sensor for sensitivity enhancement.  The excitation light is conducted down the water channel direction in the microfluidic device, while being confined within glass slide by total internal reflection. In addition to steady state intensity measurements, oxygen quenching is measured in the frequency domain using modulated light sources and synchronized detection systems. Experimental results show that optical sensitivity can be increased up to an order of magnitude in TIRA sensors. These results suggest the potential of TIRA scheme as a sensing and characterizing platform for optofluidic sensors.
Single- and Few-Layers MoSe2 Nanoflowers: Synthesis, Characterization, and Their Piezoresponse
Authors: Wu Mei Hsuan , Wu Jyh Ming
Published: February 2020

We demonstrate a hydrothermal process at temperature of 180oC to synthesize a single to few layers MoSe2 nanoflowers which exhibit a high surface-to-volume ratio and is stable under room temperature in ambient air. The crystal structure of the MoSe2 nanoflowers has been confirmed by X-ray diffraction pattern, Raman, and High-resolution transmission electron microscopy (HRTEM). On the basis of the scanning electron microscope (SEM) image, the size of the MoSe2 nanoflowers is in the range of the 300-500nm. HRTEM image reveals that the MoSe2 nanoflowers possess a great number of the single- and few-layers, which further confirms that our nanoflowers display the plentiful amount of active surface sites. In addition, the lattice fringes between each single layer is about 0.66 nm, which is slightly larger than the reported value of 0.62 nm for the MoSe2. The piezoresponse force microscopy image further shows that the MoSe2 nanoflowers exhibits significantly piezoelectric potential which is generated from the active surface sites of the petals in the MoSe2 nanoflowers.

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1.         Duerloo, K.-A.N., M.T. Ong, and E.J. Reed, Intrinsic piezoelectricity in two-dimensional materials. The Journal of Physical Chemistry Letters, 2012. 3(19): p. 2871-2876.

2.         Kaasbjerg, K., K.S. Thygesen, and A.-P. Jauho, Acoustic phonon limited mobility in two-dimensional semiconductors: Deformation potential and piezoelectric scattering in monolayer MoS 2 from first principles. Physical Review B, 2013. 87(23): p. 235312.

3.         Alyörük, M.M., et al., Promising Piezoelectric Performance of Single Layer Transition-Metal Dichalcogenides and Dioxides. The Journal of Physical Chemistry C, 2015. 119(40): p. 23231-23237.

4.         Qi, J., et al., Piezoelectric effect in chemical vapour deposition-grown atomic-monolayer triangular molybdenum disulfide piezotronics. Nature communications, 2015. 6.

5.         Wu, T. and H. Zhang, Piezoelectricity in Two
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Dimensional Materials. Angewandte Chemie International Edition, 2015. 54(15): p. 4432-4434.

6.     Chhowalla, M., et al., The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets. Nature chemistry, 2013. 5(4): p. 263-275.
(Invited) Layered Semiconductor Materials and Device Applications
Authors: Jena Debdeep
Published: February 2020

Several innovative device technologies have been proposed with 2D layered semiconductor materials, of which I will disucss a small sampling that we are investigating in our group. I will present preliminary work investigating semiconducting, and superconducting properties of layered materials and their heterostructures.  Several topologically non-trivial electronic states can potentially be realized in such materials, which will be discussed in the framework of device applications.  Finally, I will present experimental progress in our group on the epitaxial growth, and device applications of 2D layered materials and their heterostructures.
Band Alignment in the Single-Layer MoS2/WSe2 Heterojunction
Authors: Li Lain-Jong , Chiu Ming-Hui , Tseng Wei-Hsuan , Shiu Hung-Wei , Chen Chia-Hao , Wu Chih-I
Published: February 2020

The emergence of transition metal dichalcogenides as two-dimensional electronic materials has stimulated proposals of novel electronic and photonic devices based on the heterostructures of transition metal dichalcogenides (TMDs). The band alignment for TMD heterostructures is practically important for applications. We examined the feasibility of Electron Affinity Rule (EAR) on TMD heterojunction band alignment by stand-alone X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Our results suggest that the EAR can successfully predict the band alignment of TMD heterostructures.
Perovskite Solar Cells with Two Dimensional Materials As Hole Extraction Layer
Authors: Kim Soo Young , Kim Yu Geun , Oh Jeong Hyeon
Published: February 2020

Organometallic halide perovskites have attracted much attention as a promising material for high-efficiency absorber in photovoltaic devices. It is based on organic-inorganic light absorbing semiconducting material with a perovskite polycrystalline structure, CH3NH3PbX3, where X is halogenatoms (Cl, Br, I, or combination of some of them). The poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was usually used as a hole extraction layer in planar type perovskite photovoltaic cells. However, PEDOT:PSS showed several disadvantages such as hygroscopic properties and a highly acidic suspension, which result in poor stability in air.

In this respect, we have investigated the possibility of two-dimensional materials including MoS2, WS2, and graphene oxide (GO) as a hole extraction layer in perovskite photovoltaic cells. MoS2 and WS2 layers with a polycrystalline structure were synthesized by a chemical deposition method using uniformly spin-coated (NH4)MoS4 and (NH4)WS4 precursor solutions. GO was made by following Hummer’s method. Raman spectra, transmission electron microscope image, and x-ray diffraction data confirmed the well-synthesis of two-dimensional materials. The device structure is indium tin oxide/PEDOT:PSS or MoS2 or WS2 or GO/CH3NH3PbIxCl3-x/bathocurione/LiF:Al. The PCE values of PEDOT:PSS, GO, MoS2 and WS2-based one are measured to be 9.93, 9.62, 9.53 and 8.02 %, respectively. These result indicated that two-dimensional materials could replace the PEDOT:PSS layer for the improvement of device stability.
(Invited) Growth of Hexagonal Boron Nitride and Its in-Plane Heterostructures with Graphene
Authors: Shin Hyeon Suk
Published: February 2020

Hexagonal boron nitride (h-BN) is an attractive two-dimensional (2D) dielectric material and thus its heterostructures with graphene are gaining a great deal of attention for potential applications in 2D materials. In this talk, I first present the growth of single-layer and multilayer h-BN in large area by a low-pressure chemical vapor deposition (LPCVD) method. Both of single-layer and multilayer h-BN are characterized to be highly crystalline by high-resolution transmission electron microscopy. Next, I demonstrate a new chemical route for heterostructures of h-BN and graphene on Pt foil by LPCVD, in which the Pt substrate plays a catalytic role. I propose that this reaction proceeds through h-BN hydrogenation on Pt step edges; subsequent graphene growth quickly replaces the initially etched region. Importantly, this conversion reaction enables the controlled formation of patterned in-plane graphene/h-BN heterostructures, without needing the commonly employed protecting mask, simply by using a patterned Pt substrate.
(Invited) Predictive Modeling in 2D Materials: Morphology, Defects, Synthesis
Authors: Yakobson Boris I.
Published: February 2020

It is of great interest and importance for materials design to uncover, through computational and theoretical modeling, the following relationships: {basic atomic interactions → structure/morphology → functionality (including electronic)}. We will discuss recent examples from low-dimensional materials, where we seem to achieve satisfactory degree of understanding, mostly focusing on nucleation and islands shapes [1], grain boundaries and dislocations [2], heterojunctions [3], catalysis [4], and even predictive synthesis [5].

[1] V. Artyukhov et al. Phys. Rev. Lett. 114, 115502 (2015); V. Artyukhov, Z. Hu et al. unpublished.

[2] X. Zou et al. Nano Lett. 15, 3495 (2015); A. Aziz et al. Nature Comm. 5, 4867 (2014); Y. Liu et al. Nano Lett.14, 6782 (2014).

[3] Y. Gong et al. Nature Mater. 13, 1135 (2014); A. Kutana, H. Yu et al. unpublished; F. Xu, et al. Nano Lett. DOI: 10.1021/acs.nanolett.5b02430 (2016).

[4] Y. Liu et al. Phys. Rev. Lett. 113, 028304 (2014); X. Zou et al. Acc. Chem. Res.48, 73 (2015).

[5] Y. Liu et al. Angew. Chemie, 52, 3156 (2013); Z. Zhang et al., Angew. Chemie, 54, 13022 (2015).
(Invited) Spin-Valley Coupling in Monolayer Transition Metal Dichalcogenides
Authors: Cui Xiaodong
Published: February 2020

The monolayers of group VI transition metal dichalcogenides feature a valence band spin splitting with opposite sign in the two valleys located at corners of 1st Brillouin zone. This spin-valley coupling, particularly pronounced in tungsten dichalcogenides, can benefit potential spintronics and valleytronics with the important consequences of spin-valley interplay and the suppression of spin and valley relaxations. In this talk we discuss the spin-valley coupling in monolayers and multilayers WS2. We explored the interplay of spin and valley degrees of freedom with photocurrent experiments. We experimentally demonstrate that this giant spin-valley coupling, together with the valley dependent physical properties, may lead to rich possibilities for manipulating spin and valley degrees of freedom in these atomically thin 2D materials.
(Invited) Direct Template-Less Synthesis of Oriented Sub-10 Nm Semiconducting Graphene Nanoribbons with Smooth Armchair Edges on Ge(001)
Authors: Arnold Michael S
Published: February 2020

The rational synthesis of graphene nanoribbons that are semiconducting with sub-10 nm width, controlled crystallographic orientation, and well-defined edges on non-metallic substrates has been a significant challenge. The growth of nanoribbons on metal substrates precludes their direct use in semiconducting electronics due to the conductive substrate, and the direct synthesis of nanoribbons in solution is complicated by challenges of their post-synthetic assembly.

In this talk, we demonstrate the direct, scalable synthesis of graphene nanoribbons via chemical vapor deposition (CVD) on Ge(001).[1] Low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) show that the ribbons are self-orienting ±2.9° from the Ge[110] directions and are self-defining. The nanoribbons have predominately smooth armchair edges that give rise to electron interference patterns indicative of high quality edges. By tuning the precursor flux, growth time, and growth temperature, the ribbon anisotropy and growth kinetics can be tailored to yield ribbons with controlled width < 10 nm and aspect ratio > 60. Compared to previous low aspect ratio crystals of graphene obtained on Ge, we find that in order to realize high aspect ratio nanoribbons, it is critical to operate in a regime in which the growth rate is especially slow, on the order of 5 nm/h in the width direction.

This work is important because unlike continuous two-dimensional graphene, which is semimetallic, one-dimensional graphene nanoribbons can be semiconducting, allowing for the substantial modulation of their conductance and enabling their application in semiconductor logic, optoelectronics, photonics, and sensors. Moreover, the direct synthesis of ultranarrow and smooth graphene nanoribbons on Ge demonstrated here provides a scalable, high throughput pathway for integrating semiconducting graphene directly on conventional large-area semiconductor wafer platforms that are compatible with planar processing.

[1] Jacobberger RM, Kiraly B, Fortin-Deschenes M, Levesque PL, McElhinny KM, Brady GJ, Rojas Delgado R, Singha Roy S, Mannix A, Lagally MG, Evans PG, Desjardins P, Martel R, Hersam MC, Guisinger NP, Arnold MS, Direct Oriented Growth of Armchair Graphene Nanoribbons on Germanium, NATURE COMMUNICATIONS, 6, 8006 (2015)
(Invited) Liquid-Phase Exfoliated Two-Dimensional Nanosheets: From Large Scale Production to Energy Storage Applcations
Authors: Nicolosi Valeria
Published: February 2020

Graphene has generated huge interest in recent years due to its unique physics properties [1]. We have shown that high-quality monolayer graphene can be produced at significant yields by non-chemical, solution-phase exfoliation of graphite in certain organic solvents. Until a few years ago the standard procedure used to make graphene was micromechanical cleavage [2], which is a very low yield production method. In order to fully exploit graphene’s outstanding properties, a mass production method was necessary and the development of a method to exfoliate cheap, commercial graphite in organic solvents down to large area single graphene flakes with high yield was one major achievement [3]. Recently this work has been extended to a wide range of two-dimensional inorganic nanomaterials [3,4]. These are potentially important because they occur in >100 different types with a wide range of electronic properties, varying from metallic to semiconducting. The liquid-phase exfoliation method has now been up-scaled to produce grams of a variety of exfoliated materials per day [5].

This talk will first discuss the galaxy of existent layered materials, with emphasis on synthesis and liquid-phase exfoliation to finish off with some key applications recently developed in our laboratories.  

A. K. Geim, Science 324, 1530 (Jun, 2009).

S. Stankovich et al., Nature
442, 282 (Jul, 2006).

Y. Hernandez et al., Nature Nanotechnology, 3, 9 (2008) 563

J. N. Coleman et al., Science,
331, pp. 568-571 (2011)

V. Nicolosi et al., Science, 340, 6139, 1420 (2013)

K. R. Paton et al., Nature Materials, 13, 6, 624-30 (2014)
Observation of Two Dimensional Metallic Surface States and Metal to Semiconductor Transition on Low Dimensional Polar Thin Films
Authors: Wu Fei , Faghaninia Alireza , Lo Cynthia , Banerjee Parag
Published: February 2020

Two dimensional (2D) metallic conductivity has been studied for more than a decade because the unusual surface metallic states of ultrathin films may result in applications for 2D superconductors, spintronics and optoelectronic devices. Surface metallic states were predicted on ZnO polar surfaces, which is caused by charge transfer from O-terminated (000-1) surface to Zn-terminated (0001) surface to stabilize the polar instability.1 This charge transfer in polar thin films was also found to cause abrupt phase transition (i.e. ferromagnetism,2 and semiconductor to semimetal transition3), opening a new way to design functional devices by engineering surface or interface of ultrathin films.

2D metallic conductivity on ZnO has not been previously demonstrated in experiments. The charge transport properties of 2D surfaces have proved to be difficult to measure because of the dominant effect of bottom, bulk conduction channels. In our study, we reduce the ZnO thickness to several nanometer to suppress the bulk conduction channels and first experimentally show 2D metallic states on polar ZnO surfaces through surface conductivity measurements on SiO2 substrates. Then, introduction of a high density of oxygen vacancies to the polar ZnO surfaces can cause metal to semiconductor transition. These experimental results are also confirmed by the first principle calculations.

[1] Wander, A.; Schedin, F.; Steadman, P.; et al., Phys. Rev. Lett.
2001, 86, (17), 3811-3814.

[2] Wang, X. R.; Li, C. J.; Lu, W. M.; et al., Science
2015, 349, (6249), 716-719.

[3] Kim, J.; Baik, S. S.; Ryu, S. H.; et al., Science
2015, 349, (6249), 723-726.
(High Temperature Materials Division J. Bruce Wagner, Jr. Award) Oxygen Non-Stoichiometry of La, Yb, Y, and Ca Doped Cerium Oxide and the Stoichiometric Expansion Coefficient of Yb, Y, and Ca Doped Cerium Oxide
Authors: Bishop Sean R.
Published: February 2020

Ionic materials, particularly oxides, are often used in energy conversion and storage applications such as solid oxide fuel cells or batteries, and as a support material for high temperature catalysis. In many of these applications, high surface areas and high densities of interfacial contact points are desirable to speed overall reaction rates, and thus these ionic materials are less than one micrometer thick in at least one dimension. Recent observations by our group and others have found that point defect concentrations can differ from the bulk at the nanoscale, thus altering both electrochemical and mechanical properties. In this presentation, we examine the coupling between point defects, interfacial transport, and mechanical properties (electrochemomechanics) of model ionically conductive oxides including (Pr,Ce)O2-δ and Pr2CuO4-δ. In particular, we employ in situ methods including high temperature nanoindentation, x-ray diffraction, wafer curvature, and impedance spectroscopy to elucidate correlations among elastic modulus, chemically driven dilation (chemical expansion), and oxygen transport across interfaces as a function of point defect chemistry.
Oxygen Reduction Reaction Kinetics of PrBaCo2O5+
As SOFC Cathode
Authors: Zhang Nan , Li Wenyuan , Liu Xingbo
Published: February 2020

PrBaCo2O5+
δ double-perovskite has been identified as promising cathode for intermediate temperature SOFC. Research has shown that oxygen diffusivity and surface defects sites that promote the surface reactivity can be greatly enhanced due to the ordered structure and non-stoichiometry of such material.  However, there have been very few detailed investigations on the ORR mechanisms and kinetics of PrBaCo2O5+
δ, including identifying the rate-limiting steps. In this work, oxygen reduction reaction mechanisms of PrBaCo2O5+
δ cathode were studied. The ORR kinetics was evaluated through EIS toward the PrBaCo2O5+
δ /GDC/ PrBaCo2O5+
δ symmetric cell configuration. The complex nonlinear least square (CNLS) fitting results shows that oxygen reduction on porous PrBaCo2O5+
δ electrode is limited by surface chemical exchange and solid state diffusion not the traditionally TPB charge transfer process under the limit of semi-infinite electrode. According to the reaction order under different temperature and Po2, oxygen surface incorporation is supposed to be the main rate-limiting step for the overall reaction rate with the certain microstructure.
Chromium Poisoning Effects on Surface Exchange Kinetics of La0.6Sr0.4Co0.2Fe0.8O3-X
Authors: Huang Yi-Lin , Pellegrinelli Christopher , Xiong Chunyan , Wachsman Eric D.
Published: February 2020

Solid oxide fuel cells (SOFC) have been demonstrated with high efficiency in energy conversion through electrochemical reactions. However, the durability of SOFC cathodes under real operating conditions is still an important issue for widespread implementation. Chromium-containing interconnect alloys have been reported to cause severe degradations and restrain cathode performance. Also, the presence of both chromium and water vapor have been shown to accelerate the degree of chromium poisoning. Therefore, fundamental understanding of chromium effects, as well as cross-contamination effects, on the oxygen reduction reaction (ORR) and the cathode degradation mechanism are important to preventing chromium poisoning. Here we utilize in-situ gas phase isotopic oxygen exchange to explore effect of chromium on the ORR mechanisms. Gas phase isotopic oxygen exchange is a powerful technique to investigate surface reaction kinetics and catalytic activity from a molecular point of view. To investigate Cr effects on the surface kinetics of cathodes, La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF) powders had been aged in contact with Cr-containing alloy (Crofer APU 22) with the presence of water. Aged La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF) powders were examined under different atmospheres and the effects of chromium on LSCF as a function of temperature with or without the presence of water vapor were explored. The use of gas phase isotopic oxygen offers separate examination of Cr effects on the surface dissociation step and the overall surface exchange on LSCF. The changes in active surface sites of aged LSCF could also be quantified. Cross-contamination and the lifetime of intermediate adsorbed species were further determined to elucidate multiple gas-gas and gas-solid reactions on cathodes. Results of surface kinetics show correlations with surface configurations of LSCF.  A possible degradation mechanism has been proposed to illuminate chromium poisoning effects on the ORR.
Poisoning of Proton Conducting SOFC By Hydrogen Sulfide and Carbon Dioxide Fuel Contaminants
Authors: Sun Shichen , Cheng Zhe
Published: February 2020

It is well known that conventional solid oxide fuel cells (SOFCs) with oxygen ion conducting electrolyte and nickel (Ni) anode are susceptible to poisoning by trace amount of hydrogen sulfide (H2S) while not significantly impacted by presence of carbon dioxide (CO2) in the fuel stream.  In comparison, the impacts of H2S and CO2 as fuel contaminants on proton-conducting SOFCs remain unexplored.  Here we report our study aimed at revealing the poisoning behavior caused by H2S and CO2 for proton-conducting SOFCs.  Anode-supported proton-conducting SOFCs with BaZe0.1Ce0.7Y0.1Yb0.1O3 (BZCYYb) electrolyte and Ni-BZCYYb anode and La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) cathode were subject to ppm-level H2S or percentage-level CO2 as fuel contaminants in hydrogen fuel. The response in cell electrochemical behavior and the change in materials microstructure and composition are characterized.  Preliminary results suggest that, contrary to conventional SOFCs that show “sulfur poisoning” and “CO2 tolerance”, such Ni-BZCYYb anode/BZCYYb electrolyte/LSCF cathode proton-conducting SOFC button cells show good tolerance to H2S up to ~5 ppmv while severe poisoning by CO2 at even low percentage level.  Experimental results aimed at understanding such different electrochemical behaviors for proton-conducting SOFCs versus conventional oxygen ion conducting SOFCs will be presented and their implications on the fundamental mechanism for anode hydrogen electrochemical oxidation reaction in proton conducting SOFCs will be discussed.
Sensitivity Studies of Fuel Characteristics and State of Health Investigation of a Planar SOFC Short Stack Operating on Alternative Fuel By Electrochemical Impedance Spectroscopy
Authors: Das Debanjan , Lvov Serguei N.
Published: February 2020

An approach based on Electrochemical Impedance Spectroscopy (EIS) was formulated and investigated to understand the most efficient fuel conditions for suitable long term operation of a solid oxide fuel cell stack under power generation conditions. The procedure helped to reflect the individual effects of CO/H2 volumetric ratio, S/C ratio and fuel utilization under the presence of a simulated alternative fuel at 0.4 A cm-2. Signature impedance plots were obtained with variation of each parameters and subsequent circuit modelling was used to calculate the electrochemical parameters associated with the plots. It was seen that the impedance plots show an additional arc at lower frequencies attributed to the diffusion of CO during the water gas shift reaction near the anode (CO + H2O → H2 + CO). Tests with various CO/H2 ratio helped to narrow down the operating regimes where CO electro-oxidation was expected to take an important part with H2 oxidation. S/C ratios less than 2 were tested to understand the limits of non-carbon interaction regions with the anode, without reducing system efficiency. Optimum fuel utilization was obtained for cell operation, highlighting regions having a negative effect on performance. Finally, 8 different combinations of the optimized three parameters were tested long term with the stack, and the most efficient blend was determined in terms of degradation of power in 500 h.
(Invited) Special Sites in Electrocatalysis for Oer and ORR
Authors: Rossmeisl Jan
Published: February 2020

So far most electrocatalyst has been designed only based on reactivity of the surface. This single parameter is not sufficient to optimize the activity and selectivity. This means that activities and selectivity can only be optimized to a certain point. The activity as function of the surface reactivity is often represented in volcano curves. Electrocatalytic activity and selectivity is ultimately determined by the atomic and electronic structure of the catalyst surface. By controlling the atomic structure of the catalyst surface it is possible provide more parameters besides the reactivity to tune activity and selectivity beyond the normal limits. I will show some examples on atomic scale design of special electrocatalytic sites for oxygen evolution and oxygen reduction for better selectivity and activity.