Atomic-level Processes Research Articles

Low temperature surface preparation of GaN substrates for atomic layer epitaxial growth: Assessment of ex situ preparations

In situ and in vacuo surface studies of in situ and ex situ GaN substrate preparation were conducted to advance fundamental understanding of GaN surface preparation for low temperature atomic layer epitaxial growth. Grazing incidence small angle x-ray scattering (GISAXS) information is complemented with in vacuo x-ray photoelectron spectroscopy and ex situ atomic force microscopy studies to assess different ex situ sample preparation methods to produce the most suitable GaN surface for epitaxy. The authors have determined that a UV-ozone exposure followed by an HF dip produces the cleanest and smoothest GaN surface. They have further determined with GISAXS that subjecting the optimum surface to the established low temperature emulated gallium flash-off atomic level process (ALP) eliminates the need for any nitridation ALP. These ex situ and in situ cleaning preparations result in clean, highly-ordered surfaces that should provide an ideal substrate for high quality crystalline epitaxial films.

Effects of dislocation density, temperature and Cr concentration on helium behavior in α-Fe

Molecular dynamics (MD) simulation has been performed to study Helium (He) behavior and deformation feature of α-Fe with 0.1 at.% He injection. The effects of different pre-existing dislocation densities, temperatures and Cr concentrations were investigated in this study. Voronoi clusters (VCs) analysis and dislocation extraction algorithm (DXA) were performed to study the distribution of He clusters and the evolution of dislocations, respectively. The results show that He atoms are easier to segregate on pre-existing dislocations. Average size of He clusters is monotonically decreasing and total number is increasing with increasing dislocation density from 0 to 6.74 × 1011 cm−2. Furthermore, in tensile process, pre-existing dislocations accelerate the formation of larger He bubbles and these He bubbles enhance plasticity and promote dislocation multiplication. Higher dislocation density leads to higher yield stress, yield strain, and elongation stress. In addition, higher temperature (600 K and 800 K) can motivate He diffusion and improve dislocation mobility, whereas higher Cr concentration (9 at.% and 14 at.%) retards both of them. These increasing temperatures and decreasing Cr concentrations lead to decreasing yield stress, yield strain, but increasing dislocation density. All of these findings are helpful for understanding the atomic-level deformation process of He bubbles and dislocations evolutions in detail, which are elusive through experimental techniques. This will have profound significance for design and development of nuclear materials.

Chemical exchange at the ferroelectric phase transition of lead germanate revealed by solid state 207Pb nuclear magnetic resonance.

The nature of the dynamics and structural changes that take place at the ferroelectric phase transition in lead oxides is a rich field of study. Solid-state nuclear magnetic resonance (NMR) of 207Pb is well suited to study the local structure and disorder in lead oxide ferroelectric transitions at the atomic level. However, very large 207Pb shielding anisotropy results in poor resolution in 1D static and magic angle spinning (MAS) NMR spectra. We address this problem by using short high-power adiabatic pulses (SHAPs) with magic-angle-turning sequences to correlate the isotropic and anisotropic parts of the 207Pb chemical shift tensor in a 2D NMR experiment, yielding resolved 207Pb NMR spectra of the nine distinct lead sites in uniaxial ferroelectric lead germanate (Pb5Ge3O11). Using this technique we detect the magnetic environments of displaced Pb2+ ions and unambiguously identify the nature of the phase transition as mixed displacive and order-disorder. We also observe that the atomic-level process responsible for the phase transition in ferroelectric lead germanate is chemical exchange on the kilohertz timescale. We derive an activation energy of 103.4 ± 1.7 kJ mol-1 and compare it to dielectric spectroscopy studies on similar materials. These results show that this method can be used to characterize ferroelectric phase transitions of complex materials with high resolution using nuclei that are typically inaccessible due to their large shielding anisotropy.

Critical Atomic-level Processing Technologies: Remote Plasma-enhanced Atomic Layer Deposition and Atomic Layer Etching

As feature sizes of devices shrink every year, deposition and etching processes change to be very challenge, especially for sub-7 nm technology node. The acceptable variability of feature size is expected to be several atoms of silicon/germanium in the future. Therefore, Remote Plasma-Enhanced Atomic Layer Deposition (RPE-ALD) and Atomic Layer Etching (ALE) change to be more and more important in the semiconductor fabrication. Due to their self-limiting behavior, the atomic-scale fidelity could be realized for both of them in the processes. Compared with traditional Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD) methods, atomic-scale thickness controllability and good conformality can be achieved by RPE-ALD. Unlike conventional plasma etching, atomicscale precision and excellent depth uniformity can be achieved by ALE. The fundamentals and applications of RPE-ALD and ALE have been discussed in this paper. Using the combination of them, atomic-level deposition/etch-back method is also mentioned for achieving high quality ultra-thin films on three dimensional (3D) features.

Atomic-Level Processes of Shear Band Nucleation in Metallic Glasses.

The ability to control the plastic deformation of amorphous metals is based on the capacity to influence the percolation of the shear transformation zones (STZs). Despite the recent research progress, the mechanism of STZ self-assembly has so far remained elusive. Here, we identify the structural perturbation generated by an STZ in the surrounding material and show how such a perturbation triggers the activation of the neighboring STZ. The mechanism is based on the autocatalytic generation of successive strong strain and rotation fields, leading to STZ percolation and, ultimately, to the formation of a shear band.

Design of Reduction Process of SnO2 by CH4 for Efficient Sn Recovery

We design a novel method for the CH4 reduction of SnO2 for the efficient recovery of Sn from SnO2 through a study combining theory and experiment. The atomic-level process of CH4-SnO2 interaction and temperature-dependent reduction behavior of SnO2 were studied with a combination of a multi-scale computational method of thermodynamic simulations and density functional theory (DFT) calculations. We found that CH4 was a highly efficient and a versatile reducing agent, as the total reducing power of CH4 originates from the carbon and hydrogen of CH4, which sequentially reduce SnO2. Moreover, as a result of the CH4 reduction of SnO2, a mixture of CO and H2 was produced as a gas-phase product (syngas). The relative molar ratio of the produced gas-phase product was controllable by the reduction temperature and the amount of supplied CH4. The laboratory-scale experimental study confirmed that CH4 actively reduces SnO2, producing 99.34% high-purity Sn and H2 and CO. Our results present a novel method for an efficient, green, and economical recycling strategy for Sn with economic value added that is held by the co-produced clean energy source (syngas).

Effect of Alkali Metal Atom Doping on the CuInSe2-Based Solar Cell Absorber

The efficiency of Cu(In,Ga)Se2 (CIGS)-based solar cells can be markedly improved by controlled introduction of alkali metal (AM) atoms using post-deposition treatment (PDT) after CIGS growth. Previous studies have indicated that AM atoms may act as impurities or agglomerate into secondary phases. To enable further progress, understanding of atomic level processes responsible for these improvements is required. To this end, we have investigated theoretically the effects of the AM elements Li, Na, K, Rb, and Cs on the properties of the parent material CuInSe2. First, the effects of the AM impurities in CuInSe2 have been investigated in terms of formation energies, charge transition levels, and migration energy barriers. We found that AM atoms preferentially substitute for Cu atoms at the neutral charge state. Under In-poor conditions, AM atoms at the In site also show low formation energies and are acceptors. The migration energy barriers show that the interstitial diffusion mechanism may be relevant only f...

Benefits of atomic-level processing by quasi-ALE and ALD technique

A new technology has been developed using the atomic layer etching (ALE) and atomic layer deposition (ALD) concepts. It has been applied to self-aligned contacts (SAC) and patterning processes, for the sub 7 nm technology generation. In the SAC process, ultra-high selectivity of SiO2 etching towards SiN is required, for which we have developed quasi-ALE technique for SiO2 etching. We were able to significantly improve the trade-off between the etching ability of SiO2 on the micro slit portions and SiN selectivity. Quasi-ALE precisely controls the reaction layer thickness of the surface, by controlling the radical flux and ion flux independently, and hence enables etching at lower ion energies (Ei < 250 eV). On the other hand, in the patterning processes, the shrinking of critical dimensions (CD) without loading is mandatory. Therefore, we developed a new process flow that combines ALD technique and etching. With this method, we were able to achieve CD shrinking at atomic-layer level precision for various patterns, without causing CD loading. In addition, we were also able to uniformly control the CD shrinkage amount across the whole wafer. This is because this technique takes advantage of the deposition step which is independent of the pattern density and the location on the wafer by self-limited reactions.

Understanding the atomic-level process of CO-adsorption-driven surface segregation of Pd in (AuPd)147bimetallic nanoparticles

When the elements that compose bimetallic catalysts interact asymmetrically with reaction feedstock, the surface concentration of the bimetallic catalysts and the morphology of the reaction center evolve dynamically as a function of environmental factors such as the partial pressure of the triggering molecule. Relevant experimental and theoretical findings of the dynamic structural evolution of bimetallic catalysts under the reaction conditions are emerging, thus enabling the design of more consistent, reliable, and efficient bimetallic catalysts. In an initial attempt to provide an atomic-level understanding of the adsorption-induced structural evolution of bimetallic nanoparticles (NPs) under CO oxidation conditions, we used density functional theory to study the details of CO-adsorption-driven Pd surface segregation in (AuPd)147 bimetallic NPs. The strong CO affinity of Pd provides a driving force for Pd surface segregation. We found that the vertex site of the NP becomes a gateway for the initial Pd-Au swapping and the subsequent formation of an internal vacancy. This self-generated internal vacancy easily diffuses inside the NP and activates Pd-Au swapping pathways in the (100) NP facet. Our results reveal how the surface and internal concentrations of bimetallic NPs respond immediately to changes in the reaction conditions. Our findings should aid in the rational design of highly active and versatile bimetallic catalysts by considering the environmental factors that systematically affect the structure of bimetallic catalysts under the reaction conditions.

(Invited) Control of Internal Plasma Parameters Toward Atomic Level Processing

Plasma processes have been indispensable core technologies to manufacture ultra-large integrated circuits (ULSIs) and variety of devices, including MEMS and bio sensors. In order to develop the next generation devices, advancing of the plasma processes should be needed. However, the plasma processes have been developed based on the trial-and-error tuning of external parameters, such as electric power, gas pressure and gas flow ratio. Towards the surface processing with atomic level accuracy with extremely low damage, the precise control of internal parameters that actually define the surface reaction has become more important. Particularly, it is necessary to control the energies of species and eventually their chemical reactions on the surface directly. Furthermore, the spatiotemporally control of chemical species in gas phase in the processing is the key issue to obtain the high performance as well as establishment of the plasma science for manufacturing. We have developed an autonomous plasma manufacturing system, which is able to measure radical densities with a compact monitoring system and control them at optimum values in real time. The spatiotemporal fluctuation of species in the plasma affects much on the surface reactions, which is fatal to fabricate nano-size, atomic level structures. Then, we can make nano-scale etching with an ultrahigh precision by employing the autonomous plasma system. [1] The plasma induced damages on the organic material and the method to realize the precise etching were clarified. On the other hand, the wafer temperature has influenced considerably the process performances for the precise control of surface reactions. However, it was not dynamically controlled because the wafer temperature rises abruptly from the plasma ignition. Interactions between the plasma and the material surface at the beginning and the end of processing need to be considered as entirely different conditions. The temperature change should be minimized and kept in a specific range throughout the processing. We developed a feedback control system based on actual temperature monitoring in a real time by dynamical changes in plasma discharge on-off intervals. [2] This presentation will emphasize the importance of the internal parameter control of species together with a wafer temperature in the plasma processing for the atomic level control and discuss the advanced plasma processes for ULSIs and GaN semiconductors. [3] [1] H. Yamamoto, et al., J. Appl. Phys. 51 (2012) 016202. [2] T. Tsutsumi, et al., Appl. Phys. Lett. 103 (2013) 182102. [3] Z. Liu, et al., Jpn. J. Appl. Phys. 54 (2015) 06GB04.

(Invited) Control of Internal Plasma Parameters Toward Atomic Level Processing

Plasma processes have been indispensable core technologies to manufacture ultra-large scale integrated circuits and variety of devices. To develop the next generation devices, advancing of the plasma processes should be needed. Towards the surface processing with atomic level accuracy with low damage, the precise control of internal parameters that actually define the surface reaction has become more important. We have developed an autonomous plasma manufacturing system, which measures the densities of some atomic species with a compact monitoring system and controls them at optimum values in real time. The spatiotemporal fluctuation of species in the plasma affects much on the surface reactions, which is fatal to fabricate nano-size, atomic level structures. Then, we can make nano-scale etching with an ultrahigh precision by employing the autonomous plasma system.

Atomistic Mechanism of Pt Extraction at Oxidized Surfaces: Insights from DFT

In this article, we propose a novel mechanism for the atomic-level processes that lead to oxide formation and eventually Pt dissolution at an oxidized Pt(111) surface. The mechanism involves a Pt extraction step followed by the substitution of chemisorbed oxygen to the subsurface. The energy diagrams of these processes have been generated using density functional theory and were analyzed to determine the critical coverages of chemisorbed oxygen for the Pt extraction and Oads substitution steps. The Pt extraction process depends on two essential conditions: (1) the local coordination of a Pt surface atom by three chemisorbed oxygen atoms at nearest-neighboring fcc adsorption sites; (2) the interaction of the buckled Pt atom with surface water molecules. Results are discussed in terms of surface charging effects caused by oxygen coverage, surface strain effects, as well the contribution from electronic interaction effects. The utility of the proposed mechanism for the understanding of Pt stability at bimetallic surfaces will be demonstrated by evaluating the energy diagram of a CuML/Pt(111) near-surface alloy.

The Continuing Effort to Enhanced Learning of Mechanical Behavior of Materials via Combined Experiments and nanoHUB Simulations: Learning Modules for Sophomore MSE Students

AbstractThis paper outlines updates to and evaluation of a learning module that incorporates a molecular dynamics (MD) simulation tool with a physical lab to introduce students to the atomic-level processes that are responsible for plastic deformation. Sophomore materials science and engineering (MSE) students at Purdue University get experience with both a physical laboratory, where they perform and analyze tensile tests of macroscopic samples, and a simulation laboratory, where they run and analyze MD simulations of tensile tests of metal nanowires. By integrating these two activities and discussing reasons for the large difference in yield stress, students may build an intuitive feel for how plastic deformation is linked across length scales

Strong enhancement of surface diffusion by nonlinear surface acoustic waves

The phenomenon of acoustic activation of surface diffusion is investigated in a combined computational and experimental study. The ability of pulsed laser-generated surface acoustic waves (SAWs) to enhance the mobility of small atomic clusters is demonstrated by directly tracking, with fluorescence microscopy, individual $\mathrm{A}{\mathrm{u}}_{8}$ clusters moving on a (111) silicon substrate. A 19-fold increase in the effective diffusion coefficient is measured in room temperature experiments in the presence of SAWs generated by nanosecond pulse laser irradiation at a 100 Hz repetition rate. A strong enhancement of cluster mobility by SAWs is also observed in large-scale molecular dynamics simulations of surface diffusion of small atomic clusters. The analysis of the computational results demonstrates that the nonlinear sharpening of SAWs and the corresponding enrichment of the SAW spectra by high frequency harmonics which are capable of dynamic coupling to the cluster vibrations are responsible for the efficient acoustic activation of surface mobility in the simulations. The increase in the effective diffusion coefficient is proportional to the number of the SAW pulses passing through the diffusion region per unit time and a dramatic 4500-fold diffusion enhancement (corresponds to an equivalent temperature increase by $\ensuremath{\sim}{10}^{3}\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) is predicted in the simulations for $15\phantom{\rule{0.16em}{0ex}}\mathrm{GHz}$ SAWs. The ability of SAWs to affect atomic-level surface processes has far-reaching implications for the design of new techniques where the acoustic energy serves as an effective substitution for thermal activation in applications where heating must be avoided or rapid switching of surface conditions is required.

Effect of Temperature on the Friction and Wear of PTFE by Atomic-Level Simulation

The tribological behavior of oriented, self-mated poly(tetrafluoroethylene) (PTFE) sliding surfaces is examined as a function of temperature from 25 to 300 K. Three distinct sliding configurations are considered: sliding perpendicular to the chain alignment in both PTFE surfaces, sliding parallel to chain alignment in both PTFE surfaces and sliding parallel to the chain alignment on one surface but perpendicular to chain alignment on the other. Our simulations reveal a general trend of increasing friction coefficient with decreasing temperature for configurations where the chains are aligned in the direction of sliding or crossed. However, for conditions where the chains are aligned but the sliding motion is perpendicular to this alignment, gross deformation and athermal friction behaviors are observed. The atomic-level processes associated with these increases in friction at low temperatures are characterized.

Catalytic reaction processes revealed by scanning probe microscopy. [corrected

Heterogeneous catalysis is of great importance for modern society. About 80% of the chemicals are produced by catalytic reactions. Green energy production and utilization as well as environmental protection also need efficient catalysts. Understanding the reaction mechanisms is crucial to improve the existing catalysts and develop new ones with better activity, selectivity, and stability. Three components are involved in one catalytic reaction: reactant, product, and catalyst. The catalytic reaction process consists of a series of elementary steps: adsorption, diffusion, reaction, and desorption. During reaction, the catalyst surface can change at the atomic level, with roughening, sintering, and segregation processes occurring dynamically in response to the reaction conditions. Therefore, it is imperative to obtain atomic-scale information for understanding catalytic reactions. Scanning probe microscopy (SPM) is a very appropriate tool for catalytic research at the atomic scale because of its unique atomic-resolution capability. A distinguishing feature of SPM, compared to other surface characterization techniques, such as X-ray photoelectron spectroscopy, is that there is no intrinsic limitation for SPM to work under realistic reaction conditions (usually high temperature and high pressure). Therefore, since it was introduced in 1981, scanning tunneling microscopy (STM) has been widely used to investigate the adsorption, diffusion, reaction, and desorption processes on solid catalyst surfaces at the atomic level. STM can also monitor dynamic changes of catalyst surfaces during reactions. These invaluable microscopic insights have not only deepened the understanding of catalytic processes, but also provided important guidance for the development of new catalysts. This Account will focus on elementary reaction processes revealed by SPM. First, we will demonstrate the power of SPM to investigate the adsorption and diffusion process of reactants on catalyst surfaces at the atomic level. Then the dynamic processes, including surface reconstruction, roughening, sintering, and phase separation, studied by SPM will be discussed. Furthermore, SPM provides valuable insights toward identifying the active sites and understanding the reaction mechanisms. We also illustrate here how both ultrahigh vacuum STM and high pressure STM provide valuable information, expanding the understanding provided by traditional surface science. We conclude with highlighting remarkable recent progress in noncontact atomic force microscopy (NC-AFM) and inelastic electron tunneling spectroscopy (IETS), and their impact on single-chemical-bond level characterization for catalytic reaction processes in the future.

The fracture behavior of twinned Cu nanowires: A molecular dynamics simulation

The molecular dynamics simulations are performed to explore the fracture behavior and the ductility of the twinned Cu nanowires containing orthogonally oriented growth CTBs due to the uniaxial tensile deformation. The results reveal that, the fracture behavior and the ductility of the twinned nanowires are not related to the length of the nanowires but also intensively related to the twin boundary spacing. When the twin boundary space is changed, the twinned nanowires undergo three distinct failure modes which include ductile fracture, brittle fracture and ductile-to-brittle transition depending on the length of the nanowires. We also find a reduction in the ductility of the twinned nanowires, which is ascribed to the deformation localization induced by the Lomer dislocation and the rapid necking resulted from the twinning partial slipping. Finally, the atomic-level process that occurs during deformation until final fracture are examined in detail, and a new formation mechanism of the Lomer dislocation is observed when a 90° partial dislocation transmits across a coherent twin boundary.

Enhanced Learning of Mechanical Behavior of Materials via Combined Experiments and nanoHUB Simulations: Learning Modules for Sophomore MSE Students

ABSTRACTUndergraduate materials engineering students have difficulty conceptualizing the atomic-level processes responsible for plastic deformation. To aid in developing this conceptual understanding, interactive molecular dynamics (MD) simulations were introduced into the sophomore-level materials curriculum, integrating simulation with the traditional tensile testing laboratory. Students perform a tensile test using MD simulations on nanowire samples, and then compare these results with those from the physical tensile tests to develop a visual and more intuitive picture of plastic deformation of crystalline materials.

Atomic-Scale Wear of Amorphous Hydrogenated Carbon during Intermittent Contact: A Combined Study Using Experiment, Simulation, and Theory

In this study, we explore the wear behavior of amplitude modulation atomic force microscopy (AM-AFM, an intermittent-contact AFM mode) tips coated with a common type of diamond-like carbon, amorphous hydrogenated carbon (a-C:H), when scanned against an ultra-nanocrystalline diamond (UNCD) sample both experimentally and through molecular dynamics (MD) simulations. Finite element analysis is utilized in a unique way to create a representative geometry of the tip to be simulated in MD. To conduct consistent and quantitative experiments, we apply a protocol that involves determining the tip-sample interaction geometry, calculating the tip-sample force and normal contact stress over the course of the wear test, and precisely quantifying the wear volume using high-resolution transmission electron microscopy imaging. The results reveal gradual wear of a-C:H with no sign of fracture or plastic deformation. The wear rate of a-C:H is consistent with a reaction-rate-based wear theory, which predicts an exponential dependence of the rate of atom removal on the average normal contact stress. From this, kinetic parameters governing the wear process are estimated. MD simulations of an a-C:H tip, whose radius is comparable to the tip radii used in experiments, making contact with a UNCD sample multiple times exhibit an atomic-level removal process. The atomistic wear events observed in the simulations are correlated with under-coordinated atomic species at the contacting surfaces.

How Dynamic Visualization Technology can Support Molecular Reasoning

This paper reports the results of a study aimed at exploring the advantages of dynamic visualization for the development of better understanding of molecular processes. We designed a technology-enhanced curriculum module in which high school chemistry students conduct virtual experiments with dynamic molecular visualizations of solid, liquid, and gas. They interact with the visualizations and carry out inquiry activities to make and refine connections between observable phenomena and atomic level processes related to phase change. The explanations proposed by 300 pairs of students in response to pre/post-assessment items have been analyzed using a scale for measuring the level of molecular reasoning. Results indicate that from pretest to posttest, students make progress in their level of molecular reasoning and are better able to connect intermolecular forces and phase change in their explanations. The paper presents the results through the lens of improvement patterns and the metaphor of the “ladder of molecular reasoning,” and discusses how this adds to our understanding of the benefits of interacting with dynamic molecular visualizations.