bluuebunny/crossref_metadata_2025 huggingface dataset:
Simulating an Applied Voltage in Molecular Dynamics Using Charge Optimized Many Body (COMB3) Potentials
Authors: Antony Andrew Charles , Akhade Sneha A. , Liang Tao , Janik Michael J. , Maranas Janna K , Sinnott Susan B.
Published: February 2020

Technological development of energy sources such as fuel cells and batteries has been enhanced by the progress of computational models and simulations used to study these electrochemical systems, more specifically, the electrode/electrolyte interface.  Here, we present a methodology for applying a voltage during a dynamic charge molecular dynamics (MD) simulation.  Our method is developed within the third generation charge optimized many body (COMB3) potential [1] framework and implemented in the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) software package [2].  The COMB3 potential incorporates the electronegativity-equalization (EE) principle in its dynamic charge equilibration scheme.  The EE principle requires the electronegativity at all atomic sites be equal, assuming fixed atomic positions.  The partial charge on each atom is dynamically equilibrated by solving Euler-Lagrange equations of motion, a method developed by Rick et al. [3].  Only changes in the charge equations of motion are shown for brevity, but a complete description of the charge equilibration steps in COMB3 can be found in ref. 1.  Electrode COMB3 (ECOMB3) development includes a simplistic view of varying the atomic charge equations of motion during the charge equilibration steps to simulate an applied voltage for various electrochemical systems.  In ECOMB3, the mean electronegativities at the cathode and anode differ by an applied voltage, V.  This electronegativity imbalance generates a new set of charge equations of motion for the anode and cathode while the electrolyte (non-electrode) equation of motion remains unchanged.  Using the ECOMB3 method, we simulate an applied voltage for a system of liquid water between two Cu electrodes.  The aim of these simulations is to study the differing dynamical and structural properties of water molecules at the Cu/H2O interface as opposed to molecules in the bulk.  More specifically, we analyze the charge density distribution across the interface to demonstrate the practicality of using a charge transfer scheme in such a simulation.  We also report density distribution profiles for atomic oxygen and hydrogen in water, indicating the ordering of molecules near the interface.

References

[1]         T. Liang et al., Materials Science & Engineering R-Reports 74(2013).

[2]         S. Plimpton, Journal of Computational Physics 117(1995).

[3]         S. W. Rick, S. J. Stuart, and B. J. Berne, Journal of Chemical Physics 101 (1994).
Computational Analysis of Solvent Effects on Electron Transfer in Organic Radical Battery Cathode Materials
Authors: Kemper Travis , Larsen Ross E. , Gennett Thomas
Published: February 2020

Organic polymers and molecules containing stable radical groups are currently of interest in a variety of electronic and energy storage applications. However, in many of these systems very little is understood about the fundamental mechanisms by which charge is transported. This includes electron transport, ionic migration as well as molecular rearrangements and solvent reorganization.  This work focuses on utilizing modeling, specifically large scale molecular dynamics simulations and electronic structure calculations to understand charge transport at a molecular level for the prototypical nitroxide radical containing polymer poly(2,2,6,6-tetramethyl-1-piperidinyloxy-4-yl methacrylate) (PTMA). Transport in PTMA films occurs homogeneously between the radical-containing groups, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). Considering the Marcus formalism, that the rate of transfer depends on several properties of the molecular system: the energy of an electron at each TEMPO site; the electronic coupling between pairs of TEMPO sites; and the molecular reorganization energy associated with the homogeneous electron transfer reaction.  Furthermore we performed simulations to evaluate the electron transfer process in a pure PTMA film and a PTMA film swelled by acetonitrile, similar to what would happen in solution. This was accomplished by quantum mechanical calculations to find the site energy of each TEMPO group in the film and then compute the electronic density of states and the related distribution of charge-transfer reaction enthalpies. In addition, pairs of TEMPO groups were extracted and the inter- and intra-chain electronic coupling was also calculated. Utilizing this data, we found that based on the inter-TEMPO coupling values and the distances between sites the electronic-coupling-weighted radial distribution function to predict the effective electron transfer lengths as a function of solvent concentration. Finally, the implications of these results for how transport occurs within other organic radical polymer materials will be discussed in greater detail.
(Invited) Thermodynamics of Deposition Flux Dependent Intrinsic Film Stress
Authors: Rost Marcel J.
Published: February 2020

The growth of polycrystalline films at temperatures above ~0.2 of the melting temperature is accompanied by compressive stress development after film closure.

Mysteriously, a significant part of this stress has a reversible nature: it disappears when the deposition is stopped and re-emerges upon resumption. Chason [1] even showed the corresponding inverse behavior during electrochemical etching

All these observations clearly point towards a steady-state, flux (etching rate) dependent equilibrium phenomenon, which should, therefore, be treated properly by thermodynamics.

It has been suggested that the variation of the surface chemical potential upon starting/stopping of the deposition may cause adatoms to diffuse in/out of the grain boundaries leading to the development/relaxation of the intrinsic compressive film stress.

However, film growth involves a myriad of atomic processes such that the mystery is not yet solved and new mechanisms and ideas are still published frequently.

Here we present an analytical derivation, in which we address, for the first time, the reason
why
additional atoms want to get incorporated in the grain boundaries. This background delivers the required driving force independently of the precise way the atoms get incorporated. In this sense we are not proposing a new model, but delivering the basis for all different models proposed.

To derive the driving force, we calculate the variation of the chemical potential of the surface, the grain boundaries, and the film. Surprisingly, using pure thermodynamic arguments, our results fully agree with the magnitude of the reversible compressive stress:

the tremendous stress levels observed in the experiments can indeed be explained by the flux induced density variations in the extremely dilute adatom gas on the surface!

[1] Shin and Chason, Phys. Rev. Lett. 103, 056102 (2009)

E-mail:rost@physics.leidenuniv.nl

Web: www.physics.leidenuniv.nl/rost
(Invited) Electrodeposition of Fe-Pt Magnetic Films and Multilayers with Largemagnetic Anisotropy for Magnetic Recording and Microsystems
Authors: Ge Siyuan , Liang Defu , Zangari Giovanni
Published: February 2020

Alloys of Pt with metals of the Fe group form various phases with large magnetic anisotropy (above 1 MJ/m3), in particular the L10 phases of equiatomic Fe-Pt and Co-Pt. These alloys are promising candidates for permanent magnet components in magnetic MEMS or magnetic storage media.

Electrochemical growth of magnetic films presents unique advantages, such as the close control of the driving force for film growth to finely tune properties via various deposition modalities, and the ability to form micro- or nano-structures of high aspect ratio via purely additive processes. This talk will discuss the electrochemical synthesis, structural characterization and magnetic properties of equiatomic Fe-Pt alloy films, highlighting the unique features afforded by electrochemical processing. 

As-deposited equiatomic Fe-Pt alloys are obtained by an underpotential codeposition process; these films exhibit a metastable face-centered cubic structure in the as-deposited condition, but formation of the tetragonal L10 phase and coercivities of up to 14 kOe can be achieved via thermal annealing of the films at only 450C [1, 2]. The composition of Fe-Pt alloys can be easily varied during growth by potential pulsing, enabling the formation of multilayers with a compositional gradient. Thermal annealing of Fe75Pt25/Fe50Pt50 bi- and multi-layers in forming gas results in a significant acceleration of the phase transformation to the L10 phase, enabling the formation of highly coercive films at lower annealing temperature. The quality of the multilayers is inferred by the observation of exchange-coupled behavior at about 5 nm sublayer thickness.

[1] D Liang, JJ Mallett, G Zangari, ACS Appl Mater Interfaces 2 (4) 961-964 (2010);

[2] D. Liang, S. Ge, G. Zangari, IEEE Trans Magn. DOI: 10.1109/TMAG.2015.2394326 (2015)
(Invited) The Electrodeposition of Crystalline Gallium Antimonide Using Electrochemical Liquid-Liquid Solid Deposition (ec-LLS)
Authors: DeMuth Joshua James , Ma Luyao , Maldonado Stephen
Published: February 2020

Gallium antimonide (GaSb) is a III-V semiconductor that has potential use in a variety of applications including thermophotovoltaics, tandem solar cells and photodetectors.1 Due to its conceivable use in semiconductor devices several thin film growth methods have been established for crystalline GaSb. Electrodeposition of crystalline GaSb films has been previously attempted 2,3 but the deposits contained large amounts of excess antimony and required post-deposition annealing steps. Electrochemical liquid-liquid solid deposition (ec-LLS) is a method in which an electrochemical potential gradient is used to achieve a supersaturation within liquid metal electrode/solvent in order to promote the growth of crystalline semiconductors.4 In this study stoichiometric films of crystalline GaSb were electrodeposited from an Sb­2O3 precursor in aqueous electrolyte via ec-LLS at mild temperatures (40o-90oC) and ambient pressure. In addition, the growth conditions were varied and the observed effects on the resultant materials crystallinity and composition were used to obtain insight on the proposed mechanism of ec-LLS.  

References

(1) Kumar, V. et al; J. Appl. Phys., 1997, 81, 5821-5870

(2) McChesney, J et al., Optical Materials, 1996, 6, 63-67

(3) Musiani, M et al., J. Appl. Electrochem., 1989, 20, 868-873

(4) Fahrenkrug, E.; Gu, J.; Maldonado, S. J. Am. Chem. Soc.,2013, 
135
, 330-339
Electrodeposition and Characterization of Selective Coatings Based on Black Cobalt for the Conversion of Solar-to-Thermal Energy
Authors: Herrera Zamora Dallely Melissa , Lizama-Tzec Francisco Ivan , Arés Muzio Oscar Eduardo , Oskam Gerko
Published: February 2020

Solar-thermal energy conversion is achieved by selective coatings on a metallic substrate. The selective coating consist of a thin selectively absorbing film on a highly IR reflective underlying metal. The optical performance of the selective coating depends on the thickness of the respective films, which can be controlled through the electric charge applied during the electrodeposition process. Selective coatings have been deposited on copper substrates by the electrodeposition method. The coating consists of an intermediate reflective layer of electrodeposited bright nickel and an absorbing film of black cobalt. Different times were used for the galvanostatic deposition of black cobalt in order to optimize the optical properties. In order to evaluate the thermal stability, the coatings were exposed to temperature of 300oC.  The coatings were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The visible and IR film reflectance was evaluated before and after the thermal treatment through UV-Vis-NIR spectrophotometer and a Perkin Elmer FTIR spectrophotometer (Frontier MIR/NIR) with an integrating sphere from 2 µm to 15 µm.   The spectrophotometry results showed little change in the film reflectance after of the thermal treatment, suggesting a good thermal stability.
Influence of Structural Properties on the Performance of Dye-Sensitized Solar Cells Based on Electrodeposited ZnO
Authors: Canto Aguilar Esdras Josué , Anta Juan Antonio , Oskam Gerko
Published: February 2020

ZnO is an interesting material for application in the dye-sensitized solar cell (DCS) related to the large variety of low-temperature synthesis methods that allow for easy manipulation of the morphology and texture of the material. In particular, electrodeposition of crystalline ZnO can be achieved at room temperature, making it an excellent method for the preparation of solar cells based on flexible plastic substrates.

We will present results on the performance of dye-sensitized solar cells based on electrodeposited ZnO, describing the electrodeposition method in detail, and compare the results to ZnO-based solar cells prepared using sol-gel synthesis and CVD.  The morphology and texture are expected to influence the performance of the dye solar cell via a variety of effects. For instance, the exposed crystal facets may affect the dye bonding, dye coverage, and injection efficiency, the electron transfer kinetics to the solution, and the trap state distribution [1]. Using these materials, we have compared the electron transport and recombination properties using small-signal perturbation techniques [2]. We have also addressed the influence of solution additives on the performance of ZnO-based dye-sensitized solar cells and rationalized the results. The influence of surface facet exposure on dye bonding is highlighted.

References

 1. Anta, J. A.; Guillén, E.; Tena-Zaera, R. ZnO-based dye-sensitized solar cells. J. Phys. Chem. C 116, 11413-11425 (2012).

2. Vega-Poot, A.G.; Macías-Montero, M.; Idígoras-León, J. A.; Borrás, A.; Barranco, A.; Gonzalez- Elipe, A. R.; Lizama-Tzec, F. I.; Oskam, G.; Anta, J. A. Mechanisms of electron transport and recombination in ZnO nanostructures for dye-sensitized solar cells. ChemPhysChem. 15, 1088-1097 (2014).
Properties of Cabon Fibers with Electrochemically Formed ZnO Nanorods
Authors: Lee Kyu Hwan , Lim Dongchan , Choi Sung Mook
Published: February 2020

We presented that aligned arrays of ZnO nanorods grown on surface of carbon fibers can lead to significantly enhanced properyies, such as interfacial strength, wettability of epoxy resin and electric conductivity. ZnO were grown by electrochemical process. We made carbon composit to evaluate interlaminar shear stress. We also observed electrical property and piezo property. Finally we demonstrate continuous formation of ZnO on carbon fiber using semi-pilot scale bath. 

Figure 1
(Invited) Toward the Formation of Ordered Nanoparticle Films By Electrophoretic Deposition and the Subsequent Assessment of Order Via Voronoi Tessellation Analysis
Authors: Dickerson James H.
Published: February 2020

Electrophoretic deposition (EPD) is a liquid phase, materials casting approach whose technological advantages range from ease of application to size scalability, cost effectiveness, and rapid performance.  Material coatings, casts, and films assembled by EPD can be fabricated in just a few minutes and can easily be produced in nanometer to centimeter thicknesses across square nanometer to square meter surfaces.  EPD and other liquid-phase casting techniques can yield various degrees of order among the fabricated supercrystals.  Recently, we observed the evolution of order among nanoparticles (NPs) of cobalt ferrite.  We uncovered evidence that the NPs not only arranged into hexagonal close packing arrays when deposited onto silicon substrates by EPD but also appeared to have their crystallographic axes to be oriented collinearly.  This discovery could have far reaching ramifications in the rapid assembly of nanomaterials for applications, such as magnetic recording media and catalysis, among others.  Further, recently developed statistical measurement tools, specifically Voronoi tessellation analysis, that can be used to quantify the degree of ordering in monolayers of nanoparticles have been applied to the cobalt ferrite and the iron oxide systems.  The evolution of order in cobalt ferrite NP monolayers are juxtaposed with that of iron oxide NPs to explore the underlying causes of the differences in ordering based and the physical properties of the NP films.
Construction of Asymmetric Graphene Sandwiches: Decoration Using Semiconductor and Metal Nanostructures
Authors: Toth Peter S , Dryfe Robert AW
Published: February 2020

The low-dimensional carbon materials, i.e. graphene-based metal and metal oxide nanocomposites fabrication procedure at the polarizable organic/water interface opens an alternative way to prepare catalyst materials.1-2 These graphene-based nanostructures could be useful substrates for catalytic processes, such as the hydrogen evolution or the oxidation of low molecular weight alkanes to liquid alcohol fuels. The polarizable organic/water interface offers the possibility of incorporating, as well as constructing in situ at the interface, highly complex catalytic centers and supports, including photoactive semiconductors, metallic nanostructures.3

Chemical vapor deposition (CVD) on copper foil was employed to grow macroscopic (cm scale) graphene and the high purity CVD GR monolayers assembled at an polarizable organic/water interface.1-2 The semiconductors, titanium oxide (TiO2) and molybdenum disulfide (MoS2) were exfoliated in an organic solvent, and assembled at the interface, then the top of CVD GR was decorated by fishing out either the TiO2 or MoS2 layer. The top-side of CVD GR was also decorated by metal and metal oxide nanoparticles (NPs), i.e. Au, Pd, Pt and PdO, PtO, respectively, using a solution chemistry-based galvanic displacement.1
In situ electrochemical and spontaneous, electroless metal deposition of Pd, Pt, Au and Ag NPs at the interface-assembled previously top-side decorated CVD GR was studied.1-2

We have fabricated a range of different dual-decorated CVD GR-based nanostructures, including semiconductors, metals and metal oxides on top, and metal NPs on bottom-side, respectively. Either the same nanostructures (symmetric decoration), or different ones (asymmetric decoration) were used for the preparation of bi-functionalized graphene sandwiches, which are adsorbed at the organic/water interface. The successful fabrication of such dual-decorated graphene-based metal nanocomposites was confirmed using various microscopic techniques (scanning electron and atomic force microscopies) and several spectroscopic methods (X-ray photoelectron, 3D energy dispersive X-ray tomography, mapping mode Raman spectroscopy and electron energy loss spectroscopy). Taken together it is inferred from these techniques that the location of deposited nanoparticles is on opposite sides of the graphene monolayer.

 

1. Toth, P. S.; Velický, M.; Ramasse, Q. M.; Kepaptsoglou, D. M.; Dryfe, R. A. W., Advanced Functional Materials, 2015, DOI: 10.1002/adfm.201500277.

2. Toth, P. S.; Ramasse, Q. M.; Velický, M.; Dryfe, R. A. W., Chemical Science, 2015, 6, 1316-1323.

3. Rodgers, A. N. J.; Booth, S. G.; Dryfe, R. A. W., Electrochemistry Communications, 2014, 47, 17-20.
Ion Exchange Membrane Based Ammonia Synthesis
Authors: Xu Hui , McCallum Tom , Kocha Shyam S
Published: February 2020

In the ammonia industry, Haber-Bosch process leads to produce 500 million tons of nitrogen fertilizer per year, mostly in the form of anhydrous ammonia, ammonium nitrate, and urea. These fertilizers are responsible for up to 50% of worldwide crop yields and sustain one-third of the Earth's population 1. However, the conventional Haber–Bosch process for ammonia synthesis requires high pressure and temperature (up to 300 atm and 500°C) and is extremely energy-intensive. Typically, 8,000-12,000 kWh of energy is required to produce one ton of ammonia (kWh/ton ammonia) 2.

In this work, we have developed an energy-efficient electrochemical approach for ammonia synthesis that utilizes an ion-exchange membrane (IEM) as the electrolyte and selected metal nanomaterials as the catalysts.  At the anode the hydrogen oxidation reaction (HOR) takes place releasing protons and electrons. Protons transport through the PEM and reach the cathode where nitrogen is reduced to ammonia with the participation of protons and electrons. At the cathode, highly active and selective nanostructured catalysts are incorporated for enhanced nitrogen reduction reaction (NRR).

Our preliminary experiments demonstrate that more than 10-3 mmol/h-cm² (active membrane area) NH3 production rate was achieved at ambient pressure and 80 °C, which is one order of magnitude higher than typically reported 3. The calculated highest hydrogen conversion is 18%, comparable with typical conversion 14% for the single stage Haber–Bosch process. Since the conversion via the proposed approach is achieved at ambient pressure and 80°C, its overall energy consumption (kWh/ton ammonia) is much lower than that for the Haber–Bosch process. The application of nanostructured high-efficiency NRR catalysts in this work will further enhance the hydrogen conversion and energy efficiency for electrochemical ammonia synthesis.

References:

1. Appl, M., Ammonia, in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH (2006).

2. Smil, V., Enriching the Earth: Fritz Haber, Carl Bosch, and the Transformation of World Food Production, The MIT Press, Cambridge, MA, 2001, ISBN 0-262-19449-X.

3.Amar, A, R. Lan, C.T.G. Petit, and S. Tao, “Solid-state electrochemical synthesis of ammonia: a review,” J. Solid State Electrochem., 15, 1845-1860 (2011).

Acknowledgement: The financial support is from the USDA under the Contract Grant No. 2014-33610-21933.
Selective Removal of Native SiO2 Using XeF2
Authors: Hinckley Adam , Mancheno-Posso Pablo , Lai C. Steven , Muscat Anthony J.
Published: February 2020

Selective removal of native silicon dioxide (NOx) relative to the underlying Si substrate and to other films present in a device structure is important in forming low resistance electrical contacts. Xenon difluoride (XeF2) was chosen as the F-source because of the weak molecular bonds and the absence of precursors that could promote the uncontrolled formation of water on the surface. Etching of SiO2 with XeF2 has been shown to require the etching byproducts of elemental Si in close proximity.1 Both NOx and thermal SiO2 (TOx) films were etched with XeF2, titanium tetrachloride (TiCl4), and water vapor at different temperatures, pressures and process sequences. Samples were analyzed in situ with x-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). The O 1s XPS spectrum in Fig. 1 after static XeF2 exposure at room temperature and 1 Torr showed a decrease in the O coverage to near the detection limit. The surface was visibly roughened by this process because the Si substrate was etched spontaneously by F atoms produced by the reaction of XeF2. Flowing XeF2 at a pressure below 10 mTorr, NOx was fluorinated without removing it and without roughening the underlying substrate. The fluorinated NOx was desorbed by heating above 677 °C. The TPD spectra in Fig. 2 show that F disrupted the oxide causing it to desorb beginning at about 190 °C. Si desorbed at this temperature likely as SiF4 based on the SiF3 daughter species observed. Control experiments showed that the onset of native oxide desorption was about 770 °C without XeF2. Atomic force microscopy (AFM) images showed that the surface roughness was unchanged by low pressure XeF2 and heating to high temperature.

In order to understand the reaction kinetics and reduce the high temperature anneal required for NOx removal, XeF2 was dosed at 120 °C and TiCl4 was used as a co-reactant because of the facile reaction that it undergoes with silanol groups. Three treatments of XeF2 at 120 °C removed all oxide as shown by the O 1s XPS spectra in Fig. 3. More controlled NOx removal was achieved via the introduction of TiCl4 prior to XeF2. The Ti 2p XPS spectra in Fig. 4 shows that TiO2 was formed after TiCl4 pulses at 250 °C, and the TiO2 was removed by XeF2 under continuous flow conditions. In addition, peak area ratios between O 1s and Si 2p states decreased by 20% after XeF2 exposure. Based on an initial thickness measured via ellipsometry of 15.8±0.2 Å, the amount of NOx etched was about 3 Å or approximately one monolayer. The surface was fluorinated after XeF2 exposure and unreactive to further TiCl4 pulses. Two process sequences were demonstrated for the controlled removal of NOx, one involving XeF2 and high temperature annealing and the other involving sequential doses of TiCl4 and XeF2.

1. Veyan, J.F.; Halls, M.D.; Rangan, S.; Aureau, D.; Yan, X.M.; Chabal, Y.J. J. Appl. Phys.
108, 114914 (2010).

Figure 1
(Invited) Divide Et Impera: Towards New Frontiers with Atomic Layer Etching
Authors: Lill Thorsten , Kanarik Keren J. , Tan Samantha S.H. , Shen Meihua , Pan Yang , Marks Jeffrey , Vahedi Vahid , Gottscho Richard A.
Published: February 2020

Atomic layer etching (ALE) is rapidly emerging as a solution for advanced etch applications which require atomic scale fidelity /1/. In analogy to Atomic Layer Deposition (ALD), the ALE process is separated into two individual steps, one for surface modification and one for removal /2,3,4/. When these steps are saturated in time, intrinsic uniformity across the wafer and feature-to-feature as well as atomic level surface smoothness can be obtained. Separation of these steps also allows to reduce the ion energies in directional ALE which provides surface damage and selectivity benefits /4/.

Separation of complex continuous reactive ion etch (RIE) processes into simple, individual steps has profound implications. It is interesting to note the parallels to advanced mathematical algorithm development where so called “Divide and Conquer” algorithms break a complex problem into two or more sub-problems, until they can be solved and combined to give a solution to the original problem. In this talk we will discuss ALE in the context of the fundamentals of RIE, show the common roots and where ALE goes beyond RIE to allow new levels of performance and reduced complexity. The latter drives the need and opens the opportunity for better understanding of the surface reactions such as adsorption and sputtering. Latest experimental results will be shown for how dividing RIE processes into individual steps allows to conquer new heights of etch performance.

K.J. Kanarik, T. Lill, E. Hudson, S. Tan, S. Sriraman, J. Marks, V. Vahedi, R.A. Gottscho; J. Vac. Sci. Technol. A 33, 020802 (2015)

G.S. Oehrlein, D. Metzler, C. Li; ESC J. Solid State Technol. 4, N5041 (2015)

T. Faraz, F. Roozeboom, H.C.M. Knoops, W.M.M. Kessels; ESC J. Solid State Technol. 4, N5032 (2015)

S. Tan, W. Yang, K.J. Kanarik, T. Lill, V. Vahedi, J. Marks, R.A. Gottscho; ESC J. Solid State Technol. 4, N5010 (2015)
Spatial Atmospheric ALD of Functional Layers for CIGS Solar Cells
Authors: Illiberi A. , Frijters Corne , Balder Ellis , Poodt Paul , Roozeboom Fred
Published: February 2020

Copper Indium Gallium di-Selenide (CIGS) solar cells are a promising approach in photovoltaic technology, having low production costs, high conversion efficiencies (> 20 %), as well as the possibility to manufacture them on flexible substrates. State-of-the-art in CIGS solar cells manufacturing is to use a stack of CdS, intrinsic ZnO (i-ZnO) and an Al-doped ZnO TCO on top of the CIGS film. Replacement of CdS by a non-toxic Cd-free layer with wider band gap (> 2.4 eV) would a) decrease the production cost by avoiding the expensive treatment of toxic wastes and b) increase the overall cell efficiency by enhancing the quantum efficiency in the blue range. Moreover, the use of  a “soft” and highly conformal deposition technique is preferred to improve the electrical properties of the buffer layer/CIGS interface.

In this paper we present spatial atmospheric atomic layer deposition of a Zn(O,S) buffer layer as CdS replacement for CIGS solar cells. Spatial ALD is emerging as an industrially scalable deposition technique at atmospheric pressure which combines the advantages of temporal ALD, i.e. excellent control of film composition and uniformity on large area substrates, with high growth rates (up to nm/s). Films are grown by sequentially exposing the substrate to oxygen and sulfur precursors (H2O, H2S) and the zinc metal precursor (i.e., DEZn). By controlling the kinetics of surface reactions between evaporated precursors and reactive sites at the film surface, the composition of Zn(O,S) can be precisely tuned. The incorporation of S into ZnO results in a bowing of the band gap in the range from 3.3 eV (ZnO) to 2.7 (S/O+S ~ 0.5) and 3.4 eV (ZnS), as measured by spectrophotometry. The morphology of the Zn(Ox-1,Sx) films varies from polycrystalline (for 0<x<30 and 70<x<100) to amorphous (30<x<70), as measured by X-ray diffraction. CIGS solar cells with a Zn(O,S) buffer layer show an increased spectral response around 400 nm compared to solar cells with a CdS buffer layer. The solar cells with the Zn(O,S) buffer layer had an efficiency of  15.9 %, compared to 15.5 % for the reference solar cells with a CdS buffer layer.
(Invited) Engineered Combinations of Inorganic ALD/CVD Layers and Monomolecular Organic Films
Authors: Armini Silvia
Published: February 2020

Capacitance scaling in damascene interconnects is achieved by introducing ultra-porous low dielectric constant  materials. Nevertheless, one of the key issues in the integration of such porous materials is their inability to prevent gaseous penetration, e.g. during the metallization.

Several strategies are being followed to minimize the processing damage to the porous films during etching and ashing. The subsequent important step is to seal the pores at the surface, especially at the sidewalls after the trench and via etching. This is to prevent metal or precursor penetration during the metallization process, especially by advanced conformal deposition techniques such as chemical vapor deposition (CVD) and atomic layer deposition (ALD).

There are several routes to sealing: i) by thin film deposition; ii) by plasma-surface interaction; iii) by surface cross-linking/reconstruction. In this contribution, we report about the achievement of a complete pore sealing by combining a surface treatment of the low-k material together with self-assembled monolayers (SAMs) deposition and a low temperature ALD and CVD processes.  In particular, CVD-based MnSixOy films are serious candidates for sub-25 nm-wide trench and via metallization technology because of their conformal nature and their ability to act as effective Cu diffusion barriers in the range of 1-2 nm thickness.

Self-assembled molecular nanolayers composed of sub-1 nm organic chains and terminated with desired functional groups are attractive for modifying surface properties for a variety of applications, such as lubricants, in nanolithography, corrosion protection and in the crystallization of biominerals. Monolayers can help engineering thin-film interfaces, both as active components in molecular devices, and as passive layers, inhibiting interfacial diffusion, promoting adhesion and toughening brittle nanoporous structures.

In this contribution, we report  about  the interaction between CVD and ALD films and organic thin films. Special attention is paid to selective ALD/CVD growth on functionalized surfaces having modulated surface energies.
Synthesis of Transition Metal Dichalcogenide WSe2thin Films By Atomic Layer Deposition
Authors: Zhang Kai , Chen Xin , Lin Pengtao , Rice Quinton , Abdel-Fattah Mahmoud , Seo Felix J. , Li Qiliang , Baumgart Helmut
Published: February 2020

Two-dimensional layered transition metal dichalcogenides, such as MoS2, WSe2 and MoSe2  have received a lot of attention recently due to their promising unique thermal, mechanical, electrical, optical properties. Tungsten Diselenide (WSe2), one of  the transition metal dichalcogenides of interest for 2-D electronic systems, exhibits not only good thermal stability and high melting point, but also excellent optical and electrical properties. WSe2  is a   semiconductor with a bandgap  which could be used for the fabrication of solar cells and  LEDs of various colors [1, 2].

Many techniques have been developed for synthesis of WSe2 films on different substrates in recent years, such as sold state reaction, elctrodepositon, RF-magnetron sputtering, electrodepostion, pulsed laser depostion, chemical vapor depositon. However, there are very few references that report  atomic layer deposition (ALD) results of WSe2 films. This technique exhibits self-limiting surface reactions, which means accidental overdosing of precursors does not result in increased film deposition on the substrate surface. This self-terminating character is able to accurately control thickness and composition of ultra thin films and achieves uniformity, conformality and even monolayers of transition metal dichalcogenides.

Here we report on a large-area synthesis of WSe2 films on native oxide covered and hydroxyl terminated Si substrates by using the Savannah 100 ALD system from Ultratech/Cambridge Nanotech. Tungsten hexacarbonyl (W(CO)6) and (trimethylsilyl) selenide ((Me3Si)2Se) are employed as the chemical ALD precursors for Tungsten and selenium, respectively. Generally 20 sccm N2 was used as a carrier gas for the precursors. The growth temperature was set at 275, 300 and 325oC. The crystal structure and chemical composition of WSe2 films were identified by using X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). The film morphology was characterized by field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The optical property was analyzed by Raman spectroscopy and photoluminescence (PL) spectroscopy. The strong Raman scattering of in-plane E1
2g and out-of-plane A1g vibration modes, and the dominant PL of direct transition from Kc to Kv and indirect transition from Λc to Γv with absence of Kc to Γv are typical optical spectra of layered WSe2. The reduction of out-of-plane vibration mode and the absence of PL from indirect transition provide the evidence of fewer layer WSe2.

References:

H. Wang, D. Kong, P. Johanes, J. J. Cha, G. Zheng, K. Yan, N. Liu, and Y. Cui, Nano Lett. 13, 3426 (2013).

C. Huang, S. Wu, A. M. Sanchez, J. J. P. Peters, R. Beanland, J. S. Ross, P. Rivera, W. Yao, D. H. Cobden and X. Xu, Nature materials 13, 1096 (2014).

Figure 1
On the Growth of Silver Thin Films By Atmospheric-Plasma Spatial ALD
Authors: Mameli Alfredo , van den Bruele Fieke , Kessels W.M.M. , Roozeboom Fred
Published: February 2020

Silver nanoparticles and thin-films are gaining attention for plasmonics applications in photovoltaics[1] and optical lithography[2], amongst others. However, the growth of ultra-thin noble metals films is generally challenging due to their high surface energy that hinders the wetting on the substrate. In this work we investigated the influence of substrates with different surface energy on the nucleation and growth evolution of silver thin films deposited by atmospheric-pressure plasma enhanced spatial Atomic Layer Deposition (ALD) in a rotary reactor(Fig.1)[3].

The ultra-thin Ag films were deposited using Ag(fod)(PEt3), triethylphosphine (6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate silver(I), as the precursor and an H2/N2 plasma generated by a surface dielectric barrier discharge (SDBD) source as the co-reactant. Silver thin films were deposited at 120 °C with a rotation speed of 40 rpm. Ag(fod)(PEt3) was evaporated into the deposition chamber using Ar as carrier gas flowing at 100 sccm, at the same temperature as that of the reactor. The H2/N2 co-reactant gas mixture was composed from 800 sccm H2 and 600 sccm N2. Two substrates were used: crystalline Si (100) and borosilicate glass, with and without sputter-deposited molybdenum as a wetting layer.

Molybdenum films are known to have high surface energy (3 Jm-2)[4]. Thus Ag is expected to wet a Mo film better than a bare Si surface. To verify this, the differences in the growth mode of Ag Mo wetting layers were investigated on the basis of electrical, optical and morphological properties as analyzed by four-point probe (4PP), spectroscopic ellipsometry (SE) and SEM imaging (Fig. 2), respectively. Preliminary results show that the coalescence of the Ag film is more facile on 80 nm Mo/Si than on bare Si. In the case of Ag thin film growth on an 80 nm Mo wetting layer on top of Si, the Ag sheet resistance derived from 4PP measurements shows that percolation is readily achieved. Going from bare Si substrate to Mo thin film substrate, the sheet resistance drops from ~10 Ω/□ to ~3.5 Ω/□. In the case of a bare Si surface for different exposure times the measured sheet resistance, the increasing contribution of the Drude tail in the SE analysis and the SEM inspection all indicate that bare Si delays the onset of layer-by-layer growth of closed Ag thin films, leading to a Volmer-Weber island growth mode. Furthermore, the effect of the wetting layer has been investigated in greater detail, by varying its thickness from a 10 nm non-coalesced Mo film to an 80 nm closed one.

Figure 1
Contamination Specifications, an Overall Perspective
Authors: Mertens Paul W
Published: February 2020

Specifications on residual contamination have played a determining role in the development of cleaning processes. The intention of this contribution is to reflect on contamination specifications and show a potential route of how to proceed. One major platform for specifications is the ITRS [1], where a set of different contamination specifications are posted.

Currently, particle specifications are as follows. For 20 nm half pitch technology nodes the critical particle diameter is 10 nm. It should be noticed that a Si sphere with a diameter of 10 nm consist of only 25000 Si atoms.

The maximum surface concentration , is derived from the assumption that it should correspond to a yield number of 99.9 for an active area of 0.1 cm2. If one assumes that each particle with a size equal or larger than the critical value leads to one defect. This should leads to the following specification: particle density < 0.01/cm2. This specification is represented in Fig. 1 (as the a corner of the red-area). It should be noted that both specifications for diameter and particle density are (orthogonal in the sense) obtained in totally independent ways.

When specifying particles, one should realise that a particular size (and concentration) may only be a temporary embodiment of an amount of contamination.. Small particles can agglomerate to larger ones or alternatively large particles (e.g. flakes) can fragment into many smaller pieces. Through various processes.

If one assumes the total volume is maintained, it implies that contamination could evolve along the slanted lines (isochores) as indicated in the diagram of Fig. 1 This would mean that contamination situations that are within specification (green) could evolve to out-of specification conditions (red)even when the total amount (volume) of contamination remains constant.

Another specification that is used is the surface concentration of molecular dispersed O and C on the surface of 1e12/cm2. This specification has been added to the diagram, by using the Van der Waals diameter of the atoms for the horizontal coordinate.

One can see that the total contamination volume represented by the O- and C-specification is several orders of magnitude higher than that for the small particles. In other terms the amount of contamination allowed in particles is much smaller than the amount that can be tolerated as molecular contaminant on the surface.

Finally, fig 2 shows a possible alternative specification of the total amount of impurities on the surface (irrespective of its morphology).

References:

[1] ITRS 2013 edition, http://www.itrs.net/.

Figure 1
Submicron Particle Removal during FPD Oxide TFT Process
Authors: Lee Jun , Kim Min-Su , Kim Hyun-Tae , Choi In-Chan , Kim Woo-Young , Lim Dae-Seung , Park Jin-Goo
Published: February 2020

Recently, panel size has become increasingly large reaching the 8th generation (2200mm × 2500mm) and there is a trend that is changing from a-Si TFT (amorphous-Si Thin Film Transistor) to oxide TFT because of a demand for high resolution display and an expansion of OLED (Organic Light Emitting Diodes) industry. In previous panel manufacturing processes, cleaning process was not so important. However, nowadays pixel size is becoming smaller and the number of pixels per unit area is gradually increasing due to the high resolution. Consequently, an influence of submicron sized contaminated particles is increasing, so the importance of cleaning process has increased. In the oxide TFT, to achieve the high resolution, Cu based gate structure having a low resistance characteristic is necessary and in order to prevent a leakage current through the active layer, SiO2 deposition is needed on a gate insulator layer and ITO (Indium Tin Oxide) layer is used as a transparent electrode. For a large area and high resolution panel with a good production yield, a research on an effective cleaning process which can remove the submicron contaminated particles deposited on the Cu, SiO2and ITO surfaces during a TFT manufacturing process, is required. Generally, physical cleaning processes are used to remove the submicron particles in FPD (Flat Panel Display) process. Among them, a research on an eco-friendly physical cleaning processes such as DI water megasonic, mixing-jet, brush and electrolyzed ionized water (EIW) is actively being carried out.

In this study, we have optimized these cleaning processes to remove the submicron particles from the substrate surface. The primary objective is to establish an optimal cleaning process by applying above processes for the Cu, SiO2 and ITO substrates. And a mechanism of cleaning process and a theoretical understanding of interaction force such as van der Waals force, zeta potential and electrostatic force between the particle and substrate for the optimization of particle cleaning are studied. In the course of the experiment, the particles (glass and aluminum) which were produced in TFT manufacturing process were intentionally deposited on the Cu, SiO2 and ITO substrate surface. And then, the cleaning test using megasonic (1 MHz and 250 W), mixing-jet (570 lpm CDA and 4 lpm DI water), brush (diamond shape) and EIW (pH 10.2 and -790 mV ORP) processes was conducted. After that, we evaluated the cleaning efficiency by counting the number of particles on the surface before and after the cleaning test using an optical microscope. As a result, we achieved a high cleaning efficiency of above 95% in the electrolyzed ionized water cleaning process by applying the megasonic cleaning on all substrates.

Figure 1
Removing Organic Residues Using Backside Brush Scrubber Clean
Authors: Chauhan Kripa Nidhan , Sih Vincent , Bhat Talapady , Kang Min Hyo , Kabutoya Eiji , Cheng Gordon
Published: February 2020

Cleaning is an important part of advanced semiconductor manufacturing process. As the technology node advances, it has become more and more challenging. In 2X nm devices, excursions of organic residues are detected on the product wafers. These clusters of organic residues are found to be die killers and hence, reduce the yield of the product.

        In this work, we will focus on root cause finding for the organic residue clusters and the proposed solution. Defectivity, electrical, and yield data are presented to show the effectiveness of the proposed solution. Figure 1 shows defectivity improvement of 11X as compared to the current integration scheme. Figure 2 is typical example of organic residues and cluster on wafers. Figure 3 shows the comparison of chemistries applied. Figure 4 demonstrates the product yield improvement of 3.1% after the introduction of the proposed solution.

                                                                               References

H.-S. Sohn et.al
Removal of Backside Particles by Single Wafer Magasonic Systems, ECS transactions, 11(2) 95-100 (2007)

Antoine Pacco et.al
Focus Spot Reduction by Brush Scrubber Cleaning, Solid State Pheonomena Vol 219, 276-279 (2015)

Figure 1
Surface Cleaning of SiGe(100) and Passivation of Ge(100) with Aqeuous Ammonium Sulfide
Authors: Heslop Stacy Lynn , Engesser Philipp , Okorn-Schmidt Harald F. , Muscat Anthony J.
Published: February 2020

As scaling of microelectronic devices continues, germanium and silicon-germanium are promising materials for integration into silicon-based technology. Germanium and silicon-germanium based field-effect transistors can reduce leakage current and consume less power by means of improved carrier mobility and a narrower band gap, as opposed to traditional electronics. However, silicon-germanium surfaces are more challenging to work with because the topmost surface layer is comprised of germanium oxides that complicate cleaning and passivation processes. Here we seek to obtain a smooth, oxide free SiGe surface and hope to garner an understanding of the best conditions to effectively remove surface oxides. Additionally, we hope to further our understanding of aqueous ammonium sulfide passivation processes on Ge surfaces that will prevent re-oxidation of the surface for applications towards modern field-effect transistors (FETs).

            The first portion of this work focuses on the removal of oxides from the SiGe surface with 50 and 75% Ge molar ratios using a variety of aqueous processes. These processes include SC-1, HCl, HF, and HCl/HF mixtures. The SiGe surface was characterized using X-ray photoelectron Spectroscopy (XPS) after each chemical treatment. Film thicknesses were measured using spectroscopic ellipsometry. Immersion in HCl showed similar results to as shipped samples and was ineffective in removing surface oxides. Aqueous HF, however, removed both surface oxides and carbon contamination.  SC-1 treatments selectively reduced the Ge in the SiGe film and left the surface enriched in Si, which oxidized as confirmed by XPS.  If the Ge could be completely removed from the top-most layer, the SiO2 layer that forms could be used to passivate the SiGe surface and avoid problematic Ge oxides. Ellipsometry results showed that SC-1 solutions (NH4OH:H2O2:H2O, 1:1:100 v/v) etched 1 nm of the SiGe film, which was then oxidized. Ge 2p to Si 2p XPS peak ratios confirmed that increasing concentrations of SC-1 depleted the Ge content on the surface from 0.8 to 0.4, confirming the enrichment of the surface in Si. Ge to Si peak area ratios as a function of the SC-1 dilution factor showed a non-linear trend for SiGe 50% surfaces.

            Passivation of Ge with aqueous (NH4)2S was also studied. Because Ge oxides are unstable and water soluble, it is advantageous to remove them and deposit a sulfur layer to prevent further oxidation when exposed to air. Surface passivation was studied as a function of deposition time, solution concentration and the addition of H2O2. Passivation of the surface was ineffective at times below 20 min.  Changing the concentration of (NH4)2S also had no noticeable effect on the surface based on XPS. Furthermore, at high ammonium sulfide concentrations the addition of H2O2 did not affect the passivation process. The addition of  H2O2 to dilute (NH4)2S showed poor passivation due to oxidation of the surface by H2O2.