Atomic-scale Steps Research Articles

Density functional theory study on the effect of NO annealing for SiC(0001) surface with atomic-scale steps

The effect of NO annealing on the electronic structures of the 4H-SiC(0001)/SiO2 interface with atomic-scale steps is investigated. The characteristic behavior of conduction band edge (CBE) states is strongly affected by the atomic configurations in the SiO2 and the step structure, resulting in the discontinuity of the CBE states at the step edges, which prevents electrons from penetrating from the source to drain and decreases the mobile free-electron density. We found that the behavior of the CBE states becomes independent from the atomic configuration of the SiO2 and the density of the discontinuities is reduced after NO annealing.

Tunable Atomic-Scale Steps and Cavities Break Both Stability and Activity Limits of CoO x Nanosheets to Catalyze Oxygen Evolution

Tunable Atomic-Scale Steps and Cavities Break Both Stability and Activity Limits of CoO _x Nanosheets to Catalyze Oxygen Evolution

Slip-Line-Guided Growth of Graphene.

Manipulating the crystal orientation of emerging 2D materials via chemical vapor deposition (CVD) is a key premise for obtaining single-crystalline films and designing specific grain-boundary (GB) structures. Herein, the controllable crystal orientation of graphene during the CVD process is demonstrated on a single-crystal metal surface with preexisting atomic-scale stair steps resulting from dislocation slip lines. The slip-line-guided growth principle is established to explain and predict the crystal orientation distribution of graphene on a variety of metal facets, especially for the multidirectional growth cases on Cu(hk0) and Cu(hkl) substrates. Not only large-area single-crystal graphene, but also bicrystal graphene with controllable GB misorientations, are successfully synthesized by rationally employing tailored metal substrate facets. As a demonstration, bicrystal graphenes with misorientations of ≈21° and ≈11° are constructed on Cu(410) and Cu(430) foils, respectively. This guideline builds a bridge linking the crystal orientation of graphene and the substrate facet, thereby opening a new avenue for constructing bicrystals with the desired GB structures or manipulating 2D superlattice twist angles in a bottom-up manner.

Mesoscopic Structures and Coexisting Phases in Silica Films

Silica films represent a unique two-dimensional film system, exhibiting both crystalline and vitreous forms. While much scientific work has focused on the atomic-scale features of this film system, mesoscale structures can play an important role for understanding confined space reactions and other applications of silica films. Here, we report on mesoscale structures in silica films grown under ultrahigh vacuum and examined with scanning tunneling microscopy (STM). Silica films can exhibit coexisting phases of monolayer, zigzag, and bilayer structures. Both holes in the film structure and atomic-scale substrate steps are observed to influence these coexisting phases. In particular, film regions bordering holes in silica bilayer films exhibit vitreous character, even in regions where the majority film structure is crystalline. At high coverages mixed zigzag and bilayer phases are observed at step edges, while at lower coverages silica phases with lower silicon densities are observed more prevalently near step edges. The STM images reveal that silica films exhibit rich structural diversity at the mesoscale.

(Invited) Isotopically Engineered Silicon Testbeds for Advanced CMOS and Quantum Information

Silicon isotope engineering invokes a new paradigm into materials science for development of advanced CMOS technology and practical quantum computers [1]. Current development of planer-type CMOS devices confronts an immediate issue for downscaling limit, demanding a basal changeover to gate-all-around transistors employing three-dimensional (3D) nanostructures, such as vertical nanowires or stacked nanosheets. Recently, nanoscale metrology of 3D electronic devices is enabled by means of atom probe tomography (APT) having nearly atomic resolution [2]. We extended this APT technique to verify Si self-diffusion in isotopically modulated nanopillars, which allowed spatial visualization of individual isotope atoms crossing the heterointerface [3]. As a result, Si self-diffusivity in gate-oxidized nanopillars having a diameter of ~200 nm was found to remain the same as the one measured for standard planar oxidation. More recently, process tracking technique based on transmission electron microscope (TEM) was established for identical Si nanopillars, which led to unveil a gate-oxidation overview for various conditions [4]. In addition, the silicon isotope engineering provides a crucial solution to eliminate unwanted decoherence for Si-based spin qubits. We adopted the isotope engineering to tailor high-quality CVD-grown 28Si epi-wafers [1], where state-of-the-art Si-MOS nanoelectronic devices were implemented to demonstrate two-qubits operations encoded by single donor electron and nuclear spins [5]. In parallel, the isotopically enriched wafers were employed to demonstrate single and two spin-qubit operations in 28Si-MOS quantum dots (QDs) that were defined by standard CMOS lithography [6]. Meanwhile, we accomplished isotope enrichment of strained-Si/SiGe quantum-well (QW) heterostructures, where the QDs were placed away from gate-oxide interface traps. Thanks to efficient suppression of magnetic nuclear-spin noise, the isotopically enriched 28Si/SiGe QDs prolonged electron-spin dephasing time, consequently recording the highest single-spin gate fidelities of 99.93% in benchmark test [7]. However, it was revealed that inherent charge noise induces temporal fluctuation of qubit frequency. Moreover, the low-energy excited states responsible for Si valley degeneracy should be well-lifted for the higher-fidelity operations, including spin-qubit readout and initialization. Our recent 28Si/SiGe heterostructures exhibit large valley splitting, demonstrating a well-defined QW interface having fewer atomic-scale steps [8]. And also, major impurities of carbon and oxygen can be suppressed below 1016 cm-3. In order to address the remaining source of step generation, we developed direct imaging scheme for crystallographic tilting effects on the basis of the TEM diffraction [9]. It has been identified that the local lattice rotation is preferentially caused in the proximity of the dislocations having threading components. This fact provides experimental signature for the global step generation at the QW interface.This work has been supported by MEXT Quantum Leap Flagship Program (MEXT Q-LEAP) Grant Number JPMXS0118069228, JST-ACCEL Grant Number JPMJAC1301, and the Center for Spintronics Research Network, Keio University. The authors acknowledge fruitful collaborations with Yasuyoshi Nagai, Yasuo Shimizu, Tetsuo Endoh, Seigo Tarucha, Mark Eriksson, Andrew Dzurak and Andrea Morello.[1] K. M. Itoh and H. Watanabe, MRS Communications 4, 143 (2014).[2] D. J. Larson et al., MRS Bulletin 41, 30 (2016).[3] R. Kiga et al., J. Appl. Phys. 127, 045704 (2020).[4] T. Wako et al., in preparation.[5] J.T. Mohonen et al., Nature Nanotechnol. 9, 986 (2014).[6] M. Veldhorst et al., Nature Nanotechnol. 9, 981 (2014); M. Veldhorst et al., Nature 526, 410 (2015).[7] J. Yoneda et al., Nature Nanotechnol. 13, 102 (2018).[8] S. Miyamoto et al., IEEE IEDM – Technical Digest, pp.6.4.1 - 6.4.4 (2018).[9] K. Takeuchi et al., in preparation.

Electric-Carrying Nanofriction Properties of Atomic-Scale Steps on Graphene

The electric-carrying nanofriction properties of graphene are vitally important in the reliability and lifetime of graphene-based micro-/nanoelectromechanical systems. However, ubiquitous atomic-scale steps on graphene have an impact on their friction properties. In this work, electric-carrying nanofriction properties of atomic-scale steps on graphene were studied using the conductive atomic force microscope. Lateral force at uncovered step increased noticeably with the increasing voltages. Electrostatic interaction between the tip and dangling bonds along the step edges significantly increases the lateral force. In contrast, the lateral force of covered steps has only a slight increase as the voltage increases. In addition, oxidation of graphene is more likely to occur at uncovered step due to water molecules adsorbed by dangling bonds. The observations provide a deeper insight into the frictional behavior of atomic-scale steps on graphene.

Atomic scale step structure and orientation of a curved surface ZnO single crystal

We have investigated the surface structure of a curved ZnO-crystal, going from the (0001)-facet at 0° miscut to the (101¯4)-facet at a miscut of 24.8° using scanning tunneling microscopy and low energy electron diffraction. We find that the surface separates locally into (0001)-terraces and (101¯4)-facets, where the ratio between the facets depends on the miscut angle. In X-ray photoemission spectroscopy (XPS) the intensity of an O 1s component scaling with the step density of the surface is observed. No other facets were observed and the surface maintains a high degree of order over all angles. Such a curved ZnO crystal can be used for systematic studies relating the step density to the chemical reactivity using XPS to probe the curved surface at different positions.

Atomic-Scale Identification of the Electrochemical Roughening of Platinum.

Electrode degradation under oxidizing conditions is a major drawback for large-scale applications of platinum electrocatalysts. Subjecting Pt(111) to oxidation–reduction cycles is known to lead to the growth of nanoislands. We study this phenomenon using a combination of simultaneous in situ electrochemical scanning tunneling microscopy and cyclic voltammetry. Here, we present a detailed analysis of the formed islands, deriving the (evolution of the) average island growth shape. From the island shapes, we determine the densities of atomic-scale defect sites, e.g., steps and facets, which show an excellent correlation with the different voltammetric hydrogen adsorption peaks. Based on this combination of electrochemical scanning tunneling microscopy (EC-STM) and CV data, we derive a detailed atomistic picture of the nanoisland evolution during potential cycling, delivering new insights into the initial stages of platinum electrode degradation.

Variations in Atomic-Scale Step Edge Structures and Dynamics of Dissolving Calcite in Water Revealed by High-Speed Frequency Modulation Atomic Force Microscopy

Calcite dissolution plays critical roles in the global carbon cycle in the Earth, biomineralizations, and weathering of buildings. However, in spite of the importance, the atomistic events at the step edges during the dissolution have remained elusive because of the difficulties in their direct visualization. In this study, we used high-speed frequency modulation atomic force microscopy (FM-AFM) for visualizing various atomic-scale step edge structures and dynamics during the calcite dissolution in water. The obtained images reveal the coexistence of the steps with and without a transition region (TR); a Ca(OH)2 monolayer formed along the step edge as an intermediate state in the dissolution. The TRs are more frequently observed along the slowly dissolving acute steps than along the rapidly dissolving obtuse steps. This finding and our imaging of the TR formation process suggest that the free energy to form a TR is comparable to that for removing ions directly from the step edge and an increase in the loc...

Effect of relative humidity on the frictional properties of graphene at atomic-scale steps

The atomic-scale steps are ubiquitous on graphene. The frictional behavior of graphene at atomic-scale steps under different relative humidity (RH) is very important for application of graphene as solid lubricant in complicated environment. Frictional properties at atomic-scale graphene steps were investigated using calibrated atomic force microscopy (AFM) in various RH. Lateral force at uncovered atomic step decreased with the increase of RH, while lateral force at covered step were independent of RH. Uncovered steps possessed higher work function than graphene plane from the scanning Kelvin probe microscopy (SKPM) measurement due to the existence of dangling bonds and oxygen-containing functional groups. Then the adsorption of water molecule on hydrophilic oxygen containing functional groups at uncovered step decreased the work function and reduced the lateral force under high RH. Also, lateral force at steps increased even fast than plane after plasma treatment, because dangling bonds is easier to be oxide functionalization. The studies provide a further understanding of frictional properties at atomic scale steps and could be helpful in the applications of graphene as solid lubricant in humid environment.

Impact of atomic-scale surface morphology on the size-dependent yield stress of gold nanoparticles

Size-dependent mechanical properties have been revealed for nanowires, nanopillars and nanoparticles. On the surfaces of these nanosized elements, discrete atomic-scale steps will be naturally generated, however their impact on the mechanical properties and deformation has seldom been a concern. In this paper, large-scale molecular dynamics simulations are conducted to calculate the yield stress of gold nanoparticles under compression. In addition to absolute particle size, atomic-scale surface morphology induces significant fluctuation of the yield stress. An analytical relation is advanced to predict the yield stress of nanoparticles accounting for the influence of both size and surface morphology, which agrees well with atomic simulations. This study illuminates an important mechanism in nanosized elements, atomic-scale surface steps.

Effect of interlayer bonding strength and bending stiffness on 2-dimensional materials’ frictional properties at atomic-scale steps

Atomic-scale steps generally presented in 2-dimensional materials have important influence on the overall nanotribological properties of surface. Frictional properties at atomic-scale steps of two types of 2-dimensional materials are studied using calibrated atomic force microscopy (AFM) tip sliding against the steps. The lateral force at uncovered step is larger than covered step due to the bending of step edge. The lateral force at monolayer uncovered step edge of h-BN is lower than graphene because h-BN possesses higher interlayer bonding strength than graphene and the bending of h-BN step edge is suppressed to some extent. The high uncovered step exhibits much larger lateral force than low uncovered step, which could be mainly induced by increased bending stiffness of step edge rather than increased step height. The results revealed that interlayer bonding strength and bending stiffness have great influence on the lateral force at atomic-scale steps. The studies can provide a further understanding of frictional properties at atomic scale steps and could be helpful for the applications of 2-dimensional materials as lubricant coating.

Defect interactions with stepped CeO₂/SrTiO₃ interfaces: implications for radiation damage evolution and fast ion conduction.

Due to reduced dimensions and increased interfacial content, nanocomposite oxides offer improved functionalities in a wide variety of advanced technological applications, including their potential use as radiation tolerant materials. To better understand the role of interface structures in influencing the radiation damage tolerance of oxides, we have conducted atomistic calculations to elucidate the behavior of radiation-induced point defects (vacancies and interstitials) at interface steps in a model CeO2/SrTiO3 system. We find that atomic-scale steps at the interface have substantial influence on the defect behavior, which ultimately dictate the material performance in hostile irradiation environments. Distinctive steps react dissimilarly to cation and anion defects, effectively becoming biased sinks for different types of defects. Steps also attract cation interstitials, leaving behind an excess of immobile vacancies. Further, defects introduce significant structural and chemical distortions primarily at the steps. These two factors are plausible origins for the enhanced amorphization at steps seen in our recent experiments. The present work indicates that comprehensive examination of the interaction of radiation-induced point defects with the atomic-scale topology and defect structure of heterointerfaces is essential to evaluate the radiation tolerance of nanocomposites. Finally, our results have implications for other applications, such as fast ion conduction.

Transport and STM studies of hyperbolic surface states of topological insulators

Motivated by the transmission of topological surface states through atomic scale steps, we study the transport of gapless Dirac fermions on hyperbolic surfaces. We show that the density of states of the electrons shows a dip at concave step edges which can be measured by a scanning tunneling microscope tip. We also show that the tunneling conductance measured by a polarized scanning tunneling probe exhibits an unconventional dependence on the polar and azimuthal angles of the magnetization of the tip as a function of the curvature of the surface and the sharpness of the edge. However, no backward scattering takes place and transmission of the topological surface states remains completely independent of the geometrical shape (within the hyperbolic model) of the surface.

Friction, adhesion, and elasticity of graphene edges

Frictional, adhesive, and elastic characteristics of graphene edges are determined through lateral force microscopy. Measurements reveal a significant local frictional increase at exposed graphene edges, whereas a single overlapping layer of graphene removes this local frictional increase. Comparison of lateral force and atomic force microscopy measurements shows that local forces on the probe are successfully modeled with a vertical adhesion in the vicinity of the atomic-scale graphene steps which also provides a new low-load calibration method. Lateral force microscopy performed with carefully maintained low-adhesion probes shows evidence of elastic straining of graphene edges. Estimates of the energy stored of this observed elastic response is consistent with out-of-plane bending of the graphene edge.

Conductance quantization in graphene nanoconstrictions with mesoscopically smooth but atomically stepped boundaries

We present the results of million atom electronic quantum transport calculations for graphene nanoconstrictions with edges that are smooth apart from atomic scale steps. We find conductances quantized in integer multiples of 2e2/h and a plateau at ~0.5*2e2/h as in recent experiments [Tombros et al., Nature Physics 7, 697 (2011)]. We demonstrate that, surprisingly, conductances quantized in integer multiples of 2e2/h occur even for strongly non-adiabatic electron backscattering at the stepped edges that lowers the conductance by one or more conductance quanta below the adiabatic value. We also show that conductance plateaus near 0.5*2e2/h can occur as a result of electron backscattering at stepped edges even in the absence of electron-electron interactions.

Threading dislocation behavior in AlN nucleation layers for GaN growth on 4H-SiC

Threading dislocations in thin (<200nm) AlN nucleation layers (NLs) grown by metal-organic chemical vapor deposition on top of 4H-SiC on-axis mesas with atomic-scale steps were analyzed by transmission electron microscopy. The AlN NL controlled threading dislocations in an overlying ∼2μm GaN layer through two identified mechanisms: threading half-loop formation and dislocation bending at V-shaped pits. Threading dislocations in the AlN film could be traced directly to bilayer 4H-SiC steps at the substrate/film interface. These observations reveal several approaches to extended defect reduction in GaN films grown on 4H-SiC.

CVD Growth of 3C‐SiC on 4H/6H Mesas

AbstractThis article describes growth and characterization of the highest quality reproducible 3C‐SiC heteroepitaxial films ever reported. By properly nucleating 3C‐SiC growth on top of perfectly on‐axis (0001) 4H‐SiC mesa surfaces completely free of atomic scale steps and extended defects, growth of 3C‐SiC mesa heterofilms completely free of extended crystal defects can be achieved. In contrast, nucleation and growth of 3C‐SiC mesa heterofilms on top of 4H‐SiC mesas with atomic‐scale steps always results in numerous observable dislocations threading through the 3C‐SiC epilayer. High‐resolution X‐ray diffraction (HRXRD) and high resolution cross‐sectional transmission electron microscopy (HRXTEM) measurements indicate non‐trivial, in‐plane, lattice mismatch between the 3C and 4H layers. This mismatch is somewhat relieved in the step‐free mesa case via misfit dislocations confined to the 3C/4H interfacial region without dislocations threading into the overlying 3C‐SiC layer. These results indicate that the presence or absence of steps at the 3C/4H heteroepitaxial interface critically impacts the quality, defect structure, and relaxation mechanisms of single‐crystal heteroepitaxial 3C‐SiC films.

Critical conditions for dislocation nucleation at surface steps

Dislocation nucleation at a surface step is analyzed based on a general variational boundary integral formulation of the Peierls–Nabarro dislocation model. By modelling the surface step as part of the surface of a three-dimensional crack, the free surface effect is taken into account by transferring the half space problem into an equivalent one in the infinite medium. The profiles of embryonic dislocations, corresponding to the relative displacements between the two adjacent atomic layers along the slip planes, are then rigorously solved through the variational boundary integral method. The critical conditions for dislocation nucleation are determined by solving the stress-dependent activation energies required to activate the embryonic dislocations from their stable to unstable saddle-point configurations. In particular, the effect of the step geometry, such as the height of the step, the dip angle of the slip plane and the inclined angle of the step surface on dislocation nucleation, is quantitatively ascertained. The results show that the atomic-scale surface step can rapidly reduce the critical stress required for dislocation nucleation from the surface by nearly an order of magnitude. The decrease in critical stress as a function of the height of the step is more significant for slip planes with smaller dip angles and surface steps with smaller inclined angles.

Improved ultraviolet emission from reduced defect gallium nitride homojunctions grown on step-free 4H-SiC mesas

We previously reported 100-fold reductions in III-N heterofilm threading dislocation density achieved via growth on top of (0001) 4H-SiC mesas completely free of atomic scale steps. This letter compares the electroluminescent (EL) output of GaN pn junctions grown on top of 4H-SiC mesas with and without such steps. An average of 49% enhancement of the ultraviolet luminescence (380nm) was observed in step-free mesas over comparable “stepped” counterparts. Despite the intense EL from the step-free devices, significant leakage was observed through the periphery of the device, possibly due to the lack of GaN junction isolation processing.