Step-edge Structure Research Articles

Competitive Relaxation Mechanisms in Strained Epitaxial InGaSb on GaSb Substrate

Hetoroepitaxial growth of III-V compounds can produce both novel structures for fundamental research and electronic/photonic devices for practical applications. Antimony-based compounds are interested since they can interact with light in longer wavelengths ranging from near-infrared to terahertz frequency domains. In this work, we present an experimental observation of step-decorated surface and dislocation lines formed in an epitaxial In0.17Ga0.83Sb layer. The growth is performed by solid-source molecular beam epitaxy on (001) GaSb substrate at 400°C with the InGaSb deposition rate of 0.12 monolayer per second. The atomic force microscopy shows that the regular atomic steps are generally misaligned with the surface dislocation lines. The average distance between the steps is about 200 nm and the presence of dislocation lines affects the subsequent growth. Zigzag step-edge structure observed near the intersection of the step-edges and the dislocation lines can be explained by simple strain-considered models. Enhanced and depleted growth areas can be related to the degree of underneath strain. The understanding of this phenomena might lead to the simple and practical realization of regular arrays of nanostructures such as quantum wires and quantum dots.

One-step targeted treatment for Zn flatting and protection

Aqueous zinc metal batteries (AZMBs) have shown great promise as the advanced energy storage systems. Unfortunately, the common Zn foil anodes suffer from severe Zn dendrites growth and electrolyte corrosion, leading to limited application of AZMBs. Here, we prove that the Zn dendrites formation is bound up with the processing-induced step-edge structure on Zn foil anodes. Targeting to the high activity of the step-edge Zn, we design a solvent-controlled replacement reaction (Zn + SnF 2 → Sn + ZnF 2 ) to chemically flatten the step edges. Such treatment inhibits Zn nucleation along the step edges and guarantees uniform electric field distribution on Zn foil anode, thus enabling uniform Zn deposition without dendrites growth. In addition, the generated ZnF 2 is embedded in polyvinylidene fluoride (PVDF) to work as an anticorrosion protective layer on Zn foil surface. The as-designed Zn anodes (Zn-SP) maintain dendrite-free and corrosion-free electrochemical Zn plating/stripping over 1500 h at 2 mA cm −2 and 2 mA h cm −2 in the symmetric cells. The Zn-SP‖CNT@MnO 2 full cells also deliver high capacity retention of 98.6% at 1 A g −1 after 1600 cycles. This work provides a facile method as well as a new perspective of targeted surface treatment for developing stable metal anodes, such as Li, Na, K and Al.

CO organization at ambient pressure on stepped Pt surfaces: first principles modeling accelerated by neural networks.

Step and kink sites at Pt surfaces have crucial importance in catalysis. We employ a high dimensional neural network potential (HDNNP) trained using first-principles calculations to determine the adsorption structure of CO under ambient conditions (T = 300 K and P = 1 atm) on these surfaces. To thoroughly explore the potential energy surface (PES), we use a modified basin hopping method. We utilize the explored PES to identify the adsorbate structures and show that under the considered conditions several low free energy structures exist. Under the considered temperature and pressure conditions, the step edge (or kink) is totally occupied by on-top CO molecules. We show that the step structure and the structure of CO molecules on the step dictate the arrangement of CO molecules on the lower terrace. On surfaces with (111) steps, like Pt(553), CO forms quasi-hexagonal structures on the terrace with the top site preferred, with on average two top site CO for one multiply bonded CO, while in contrast surfaces with (100) steps, like Pt(557), present a majority of multiply bonded CO on their terrace. Short terraced surfaces, like Pt(643), with square (100) steps that are broken by kink sites constrain the CO arrangement parallel to the step edge. Overall, this effort provides detailed analysis on the influence of the step edge structure, kink sites, and terrace width on the organization of CO molecules on non-reconstructed stepped surfaces, yielding initial structures for understanding restructuring events driven by CO at high coverages and ambient pressure.

Ab initio study for adsorption and desorption behavior at step edges of AlN(0001) and GaN(0001) surfaces

The adsorption and desorption behavior of adatoms at step edges of AlN(0001) and GaN(0001) surfaces is investigated on the basis of ab initio calculations. Our calculations for single layer step edges along the [11̅00] direction reveal that the structure of the step edge depends on the growth condition. Furthermore, the adsorption behavior of adatoms close to the step edges is found to be dependent on the structure of step edges. The calculated results of adatom behavior suggests the possibility to control the morphology of GaN(0001) and AlN(0001) surfaces by growth parameters such as V/III ratio and temperature.

Single-crystalline boron-doped diamond superconducting quantum interference devices with regrowth-induced step edge structure

Superconducting quantum interference devices (SQUIDs) are currently used as magnetic flux detectors with ultra-high sensitivity for various applications such as medical diagnostics and magnetic material microstructure analysis. Single-crystalline superconducting boron-doped diamond is an excellent candidate for fabricating high-performance SQUIDs because of its robustness and high transition temperature, critical current density, and critical field. Here, we propose a fabrication process for a single-crystalline boron-doped diamond Josephson junction with regrowth-induced step edge structure and demonstrate the first operation of a single-crystalline boron-doped diamond SQUID above 2 K. We demonstrate that the step angle is a significant parameter for forming the Josephson junction and that the step angle can be controlled by adjusting the microwave plasma-enhanced chemical vapour deposition conditions of the regrowth layer. The fabricated junction exhibits superconductor–weak superconductor–superconductor-type behaviour without hysteresis and a high critical current density of 5800 A/cm2.

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...

A theoretical study of step edge geometry on sapphire(0001) and its effect on ZnO nucleation

Step-edge-induced nucleation plays a key role in controlling the growth of novel nanostructures and low-dimensional materials. However, it is difficult to experimentally determine the step edge structures of complex metal oxides. In this work, we present a detailed theoretical study of the stability of stoichiometric steps on sapphire(0001). Based on first-principles calculations and excess charge computation by Finnis’ approach, a pair of non-polar step edges are determined to be the most stable. By studying the adsorption characteristics of ZnO and combining previous works, we successfully explained how growth temperature and deposition rate affect the in-plane orientation of ZnO grown on sapphire(0001). The knowledge on the step edge structures and nucleation patterns would benefit the study on step-edge-guided nanostructure growth.

Step edge structures on the anatase TiO2 (001) surface studied by atomic-resolution TEM and STM.

Low-coordinate surface sites, such as those present on high-index step edges, often exhibit chemical reactivity that markedly differs from more close-packed facets. To understand the site-specific reactivity, insight into the three-dimensional atomic arrangement of step edges is needed. Here, we employ atomic-resolution transmission electron microscopy (TEM) of nanoparticles in combination with scanning tunneling microscopy (STM) of a single crystal surface to uncover the structure of prevalent step edges on the anatase TiO2 (001) surface.

Interlayer Resistance and Edge-Specific Charging in Layered Molecular Crystals Revealed by Kelvin-Probe Force Microscopy

Organic field-effect transistors (OFETs) having an active channel of solution-processed 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8–BTBT) were investigated by Kelvin-probe force microscopy (KFM). We found step-like potential distributions in a channel region, suggesting that the interlayer resistance between the conjugated BTBT core layers is quite high and each conjugated layer is electrically isolated from one another by insulating alkyl chain layers. We also found a noticeable positive charging in the channel region especially at the step edges after the device operation. The observed charging was explained by long-lived positive charges on the trap sites, and the trap density at the step edge was estimated to be on the order of 1011 cm–2. The KFM measurements suggest that the device performance of the staggered C8–BTBT OFETs could deteriorate due to the considerably high access resistance, which stems from the high interlayer resistance and/or by the site-specific charge trapping at the contact/semiconductor interface which originates from step edge structures.

In situ observation of step-edge in-plane growth of graphene in a STEM

It is extremely difficult to control the growth orientation of the graphene layer in comparison to Si or III–V semiconductors. Here we report a direct observation of graphene growth and domain boundary formation in a scanning transmission electron microscope, with residual hydrocarbon in the microscope chamber being used as the carbon source for in-plane graphene growth at the step-edge of bilayer graphene substrate. We show that the orientation of the growth is strongly influenced by the step-edge structure and areas grown from a reconstructed 5–7 edge are rotated by 30° with respect to the mother layer. Furthermore, single heteroatoms like Si may act as catalytic active sites for the step-edge growth. The findings provide an insight into the mechanism of graphene growth and defect reconstruction that can be used to tailor carbon nanostructures with desired properties.

Current-dependent periodicities of Si(553)-Au

We investigate quasi-one-dimensional atomic chains on Si(553)-Au with a scanning tunneling microscope (STM). The observed periodicity at the Si step edge can be altered by the STM and depends on the magnitude of the tunneling current. In a recent report this reversible structural transition was attributed to transient doping with a characteristic time scale of a few milliseconds [S. Polei et al., Phys. Rev. Lett. 111, 156801 (2013)]. Here we explore the evolution of the STM topography as a function of the magnitude of the tunneling current for a wide temperature range. Based on a decomposition of topographic line profiles and a detailed Fourier analysis we conclude that all observed current-dependent STM topographies can be explained by a time-averaged linear combination of two fluctuating step-edge structures. These data also reveal the precise relative alignment of the characteristic STM features for both phases along the step edges. A simple diagram is developed, presenting the relative contribution of these phases to the STM topography as a function of tunneling current and temperature. Time- and current-dependent measurements of fluctuations in the tunneling current reveal two different transition regimes that are related to two specific current injection locations within the surface unit cell. A method based on spatially resolved $I$($z$) curves is presented that enables a quantitative analysis of contributing phases.

Atomically flat planarization of Ge(100), (110), and (111) surfaces in H2 annealing

We demonstrate that Ge(100), (110), and (111) Ge surfaces are planarized with atomic level step and terrace structures in H2 annealing. The temperature required for such planarization is different among the three orientations. The step edge structure on the Ge(100) surface is composed of alternate smooth and rough steps (Sa + Sb steps) owing to the (2 × 1) reconstruction on that surface. It is also shown that the terrace widths on the Ge(110) and (111) surfaces are on average controlled by adjusting the off-angle from the respective surface orientation.

Synthesis and characterization of CeO2 thin film with well-ordered step edges

A simple hydrothermal precipitation method is created to synthesize CeO2 thin film by self-organization at the normal temperature and atmospheric pressure. Nano-thick CeO2 films with well-ordered step edges covering the surface are fabricated in alkali solution without any substrate and surfactant. Owing to the one dimensional step edge structure, the CeO2 films have a higher fault probability and more active zone, which are of great potential as functional materials.

Surface Morphologies and Optical Properties of Si Doped InGaN Multi-Quantum-Well Grown on Vicinal Bulk GaN(0001) Substrates

Morphological and optical properties of Si doped In0.07Ga0.93N multi-quantum-well (MQW) were studied on a vicinal bulk GaN(0001) substrate with low dislocation density. Surface morphology of InGaN MQW was sensitive to the misorientation direction due to the anisotropic step edge structure peculiar to a hexagonal crystal. Appropriate Si doping was useful to suppress instability of the step front and a well-aligned straight step structure was demonstrated for the misorientation direction of [11̄00] with Si doping of 5×1018 cm-3. Low temperature photoluminescence (PL) indicated that good luminescence properties were maintained under the wide range of doping concentration, while PL degradation was observed for heavily doped MQW's. The luminescence properties were discussed based on a self-consistent calculation of the electronic structure of Si-doped MQW's.

Nanoimprinted Step-Edge Vertical-Channel Organic Transistors

Flexible and low-cost organic field-effect transistors (OFETs) are desired for a variety of organic electronics. In this paper, we describe step-edge vertical-channel OFETs (SVC-OFETs) having excellent device performance fabricated by nanoimprint lithography and a self-aligned process. SVC-OFETs can be used to fabricate a submicron channel by forming the channel region around the step edge. The carriers flow in the vertical direction in the short channel along the step-edge structure. Both n- and p-channel FETs are also realized by a solution process.

Nanoimprinted Step-Edge Vertical-Channel Organic Transistors

Flexible and low-cost organic field-effect transistors (OFETs) are desired for a variety of organic electronics. In this paper, we describe step-edge vertical-channel OFETs (SVC-OFETs) having excellent device performance fabricated by nanoimprint lithography and a self-aligned process. SVC-OFETs can be used to fabricate a submicron channel by forming the channel region around the step edge. The carriers flow in the vertical direction in the short channel along the step-edge structure. Both n- and p-channel FETs are also realized by a solution process.

Characterization of atomic step structures on CaF2(111) by their electric potential

The structure and polarity of step edges on cleaved CaF2(111) are investigated by non-contact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy. Ledges produced by cleaving the crystal appear with two distinctly different polarities denoted as type I and type II arising from the sectioning of ledges with steps having different polarities. With respect to the stoichiometric terrace, the surface potential is slightly reduced at ledges predominately composed of type I steps, while the potential of ledges predominantly composed of type II steps is significantly higher (typically 100 mV). We propose that the positive potential of type II steps stems from low coordinated Ca2+ ions inducing a dipole at step edges and confirm this by atomically resolved NC-AFM images revealing the Ca2+ ion sub-lattice with repulsive-mode imaging contrast.

Structure and electronic properties of step edges in the aluminum oxide film on NiAl(110)

Thin film aluminum oxide has been investigated at step edges of the NiAl(110) substrate by means of low temperature scanning tunneling microscopy and atomic force microscopy. A preference of the step edges for certain orientations and a restructuring of the substrate step edges during the oxidation were shown. The surface unit cell of the oxide film at the step edge is similarly extended as at the antiphase domain boundaries (APDBs), which are oxygen deficient defects with ${\text{F}}^{2+}$-like centers. The local electronic structure and the local work function at the step edges resemble that of the APDBs. Therefore, an oxide film structure at the step edge which is similar to the APDB as well as a carpetlike coverage of the substrate steps is concluded. This means there are ${\text{F}}^{2+}$-like centers at step edges.

High-Speed Operation of Step-Edge Vertical-Channel Organic Transistors with Pentacene and 6,13-Bis(triisopropyl-silylethynyl) Pentacene

Organic field-effect transistors (OFETs) with a novel structure, step-edge vertical-channel OFETs (SVC-OFETs), have been developed to obtain a short channel by a self-aligned process. SVC-OFETs enable the fabrication of a submicron channel by forming the channel region around the step-edge structure. Carriers flow in the vertical direction in the short channel of the step-edge structure. SVC-OFETs show excellent device performance characteristics with a high current and a high cutoff frequency of approximately 1.5 MHz, which is a very high value for organic transistors.

A hierarchical research by large-scale and ab initio electronic structure theories—Si and Ge cleavage and stepped [formula omitted] surfaces

The ab initio calculation with the density functional theory and plane-wave bases is carried out for stepped Si ( 1 1 1 ) − 2 × 1 surfaces that were predicted in a cleavage simulation by the large-scale (order- N) electronic structure theory (T. Hoshi, Y. Iguchi and T. Fujiwara, Phys. Rev. B 72 (2005) 075323). The present ab initio calculation confirms the predicted stepped structure and its bias-dependent STM image. Moreover, two (meta)stable step-edge structures are found and compared. The investigation is carried out also for Ge ( 1 1 1 ) − 2 × 1 surfaces, so as to construct a common understanding among elements. The present study demonstrates the general importance of the hierarchical research between large-scale and ab initio electronic structure theories.