Multiatomic Steps Research Articles

Effective removal of global tilt from atomically-resolved topography images of vicinal surfaces with narrow terraces

The main feature of vicinal surfaces of crystals characterized by the Miller indices (hhm) is rather small width (less than 10 nm) and substantially large length (more than 200 nm) of atomically-flat terraces. This makes difficult to apply standard methods of image processing and correct visualization of crystalline lattices at the terraces and multiatomic steps. Here we consider two procedures allowing us to minimize effects of both small-scale noise and global tilt of sample: (i) analysis of the difference of two Gaussian blurred images, and (ii) subtraction of the plane, whose parameters are determined by optimization of the histogram of the visible heights, from raw topography image. It is shown that both methods provide nondistorted images demonstrating atomic structures on vicinal Si(556) and Si(557) surfaces.

Achieving atomically flat copper surface: Formation of mono-atomic steps and associated strain energy mechanisms

Recent studies have demonstrated that achieving an atomically flat surface for metals can significantly enhance their oxidation resistance and further advance their electronic-optical applications. However, traditional heat treatments for polycrystalline metals tend to create numerous low-surface-energy steps and facets due to surface energy minimization. Moreover, surface diffusion is further limited by three-dimensional Ehrlich-Schwoebel barriers due to the formation of steps and facets. Therefore, achieving atomically flat surfaces is energetically unfavorable and kinetically unstable. Here, we covered graphene (Gr) on Copper (Cu) surface and performed systematic and statistical analysis of microstructures on three types of graphene-Cu (Gr/Cu) interfaces: annealed Cu, transferred and high-temperature deposited Gr/Cu interfaces. We found that mono-atomic steps formed at high-temperature deposited Gr/Cu interface, in comparison with multi-atomic steps at annealed Cu and transferred Gr/Cu interfaces. Molecular statics/dynamics simulations and thermodynamic analysis suggest that formation of mono-atomic steps can be ascribed to the minimization of Gr strain energy and high-temperature assisted surface diffusion. When a step height (h) is smaller than five atomic planes (h < 5), strain energy minimization of Gr will prevent step bunching (a non-uniform surface morphology), accelerate the formation of atomically flat surface. When h ≥ 5, Gr strain energy minimization will trigger step-bunching instability, decompose large steps and thus facilitate the surface diffusion to develop atomically flat surface. The present results may suggest a potential strategy to achieve atomically flat surface of bulk metals by high-temperature treatment.

Directly-coupled well-wire hybrid quantum confinement lasers with the enhanced high temperature performance

It is well known that the laser diode performance will inevitably deteriorate when the device is heated. It has been a difficult issue to solve to date. In this letter, we are reporting a new solution to improve high-temperature performance of the laser diodes. The device uses a kind of directly-coupled well-wire hybrid quantum confinement (HQC) structure of the active medium based on the InGaAs–GaAs–GaAsP material system. This special HQC structure is constructed based on the strain-driven indium (In)-segregation effect and the growth orientation-dependent on-GaAs multi-atomic step effect. The measurement and analysis for the HQC laser sample show that the carrier leakage loss, the Auger recombination and gain-peak shifting due to heating are reduced in the HQC structure. It therefore increases the optical gain for lasing at high temperature. The power conversion efficiency is enhanced by >57% and the threshold carrier density drops by >24% at T ⩾ 360 K, in comparison to the traditional quantum-well laser performance. A higher characteristic temperature of 240 K is obtained as well. It implies the better thermal stability of the HQC laser structure. These achievements show a significant prospect for developing high thermo-optic performance of laser diodes.

Super-gain nanostructure with self-assembled well-wire complex energy-band engineering for high performance of tunable laser diodes

Abstract Although traditional quantum-confined nanostructures e.g. regular quantum wells or quantum dots have achieved huge success in the field of semiconductor lasers for past decades, these traditional nanostructures are encountering the difficulty of enhancing device performance to a higher level due to their inherent gain bottleneck. In this paper, we are proposing a new super-gain nanostructure based on self-assembled well-wire complex energy-band engineering with InGaAs-based materials to break through the existing bottleneck. The nanostructure is constructed by utilizing the special strain-driven indium (In)-segregation and the growth orientation-dependent on-GaAs multi-atomic step effects to achieve the distinguished ultra-wide and uniform super-gain spectra. The structural details and its luminescence mechanism are investigated by multiple measurement means and theoretical modeling. The polarized gain spectra with the max fluctuation of &lt;3 cm−1 in 904 nm–998 nm for transverse electric (TE) mode and 904 nm–977 nm for transverse magnetic (TM) mode are simultaneously obtained with this nanostructure. It enables an ultra-low output power fluctuation of &lt;0.7 dB and a nearly-constant threshold power throughout an ultra-wide wavelength range under a fixed injection level. It was difficult to realize these in the past. Therefore, the described super-gain nanostructure brings a brand-new chance of developing high performance of tunable laser diodes.

(Invited) Origin of Weak Points in Surface Oxides at the Atomic Scale on Cr-Containing Alloys

Stainless steels (SS) are widely used in various industrial sectors because they are protected against corrosion by an oxide film formed at the surface, only a few nanometers thick and strongly enriched in Cr(III). Despite numerous studies of stainless steel passivity at the macroscopic scale, only recent surface analytical works have attempted to characterize the nature and elucidate the origin of sub-microscopic chemical and structural heterogeneities in surface oxide layers. In this work, the topographic, structural and chemical alterations of a (100)-oriented model 304 austenitic stainless steel single crystalline surface have been investigated at the nanometer/ atomic scales during early stage oxidation by Scanning Tunneling Microscopy (STM) combined with X-ray Photoelectron Spectroscopy. The experiments were performed by exposing an oxide-free Fe-18Cr-13Ni(100) surface to oxygen. The results presented first will be on the oxide-free alloy surface structure, with STM data revealing the presence of ordered vacancies under equilibrium conditions. Then the evidence of nucleation of chromium oxide at multi-atomic surface steps will be presented. A chromium pumping effect creates Cr enrichment in the oxide nuclei at the steps, and Cr-depleted zones at the border of the oxidized steps, allowing less protective iron oxide to be formed locally. Weak points in the surface oxide layers originate from this atomic scale surface process. The further discussion of the presented data will emphasize the importance of this observation for the understanding of the role of the oxidation mechanisms in the nanoscale heterogeneity of the growing surface oxide on Cr-containing alloys such as stainless steel, providing a new insight into the mechanisms of pit initiation at weak points of passive films. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement No. 741123, CIMNAS : Corrosion Initiation Mechanisms at Nanometric/Atomic Scale). Région Ïle-de- France is acknowledged for partial funding of the STM and XPS equipments. China Scholarship Council (CSC) is acknowledged for a scholarship to Li Ma (No. 201606380129).

Interplay between Single and Cooperative H2 Adsorption in the Saturation of Defect Sites at MgO Nanocubes

The configurations associated with reversible and irreversible adsorption of hydrogen on MgO escape consensus. Here, we report the dissociation of H2 on MgO nanocubes, which was examined by combining Fourier transform infrared spectroscopy and density functional theory (DFT)-based simulations. We found that the use of ultrahigh vacuum is essential for identifying the very first adsorption stages. Hydrogen pressure was varied from 10–8 mbar to near ambient, resulting in IR spectra of richer complexity than in current state of the art. Models with oxygen at regular corners (O3C) and Mg at inverse corners (MgIC) were identified to be the most reactive and to split H2 irreversibly already in the lowest pressure regime (PH2 < 10–3 mbar). The continuous increase in intensity of the corresponding IR bands (3712/1140 cm–1) in the intermediate range of pressures (10–3–1 mbar), along with the appearance of bands at 3605/1225 cm–1, was demonstrated to stem from cooperative adsorption mechanisms, which could be therefore considered as the main origin of irreversible hydrogen adsorption. At PH2 > 1 mbar, fully reversible adsorption was shown to occur at O4C (either on mono- or diatomic steps) and Mg3C sites. Another OH/MgH couple (3697/1030 cm–1) that became reinforced at high PH2 but remained stable upon pumping was correlated to O3C and MgIC in multiatomic steps. The difference in adsorption and desorption sequences confirmed the proposed cooperative adsorption of H2 molecules. Our study provides new insights into the mechanisms that can be beneficial for understanding the chemistry of H2 and other hydrogen-containing molecules, such as CH4, on oxide surfaces, but also for the advancement of hydrogen-storage technologies.

Optical anisotropy of InGaAs quantum dot arrays aligned along multiatomic steps on vicinal GaAs(111)B

The optical anisotropy of InGaAs quantum dot (QD) arrays on vicinal GaAs(111)B is investigated, in which the QDs are aligned and their shape is elongated along the [-110] direction. Polarised photoluminescence (PL) studies showed that the PL is preferentially polarized in the [-110] direction, where the polarization degree ρ is about 16.9%. Electronic states in InGaAs QD arrays are also examined theoretically to clarify how the optical anisotropy is affected by (1) the adjacent QDs, (2) the multiatomic steps on the substrate surface, and (3) the strain including the piezoelectric effect. By assuming the QD shape as a semi-elliptic cylindrical form, we calculate the electron and hole wave functions and evaluate the polarization degree ρ. We find that each of the three factors only slightly affects the optical anisotropy; the adjacent QDs, the multiatomic steps, and the strain increase the polarization degree ρ by 0.5, 2.3, and 1%, respectively. In contrast, the polarization degree ρ drastically increases by 8.9%, when all the three factors are simultaneously taken into account. We also compare the calculated results to the experimental data and show that all the effects of the three factors are important to explain the optical anisotropy of the InGaAs QD arrays.

Hydration of MgO(100) Surface Promoted at ⟨011⟩ Steps

Dissolution of magnesium oxide (MgO)(100) surfaces in water was examined by an atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS) techniques. MgO(100) surfaces annealed at 1273 K in air for 288 h showed rectangular terraces surrounded by multiatomic height steps along the ⟨011⟩ directions. After immersion in water, bank-like structures appeared along the ⟨011⟩ steps: the end of the terraces remained smooth, while the inner regions of the terraces were roughened and subsided with respect to the end. Epitaxial growth of Mg(OH)2 from the ⟨011⟩ steps toward the inner regions of the terraces is proposed.

Self-assembled growth of GaSb type-II nanorods aligned along quasiperiodic multiatomic steps on vicinal (111) B GaAs

Self-assembled GaSb nanorods are formed on a vicinal (111) B substrate by molecular beam epitaxy. An atomic force microscope study shows that GaSb nanorods are elongated and aligned along quasiperiodic GaAs multiatomic steps, where the average period of the GaSb nanorod array is nearly twice as large as that of the underlying GaAs multisteps. On average, GaSb nanorods are about 30 nm in width, 84 nm in length, and 2.5 nm in height. In photoluminescence (PL) measurements, the peaks of the GaSb nanorods and wetting layer are observed at 1.07 and 1.29 eV. The PL peaks shift toward higher energies with increasing excitation power density, suggesting that the band lineup exhibit a type-ІІ staggered alignment. In addition, we investigate the temperature dependence of the integral PL intensity of the GaSb nanorods.

Mobility spectrum analysis of anisotropic electron transport in N-polar GaN/AlGaN heterostructures on vicinal sapphire substrates

In this work, we present results of a study of anisotropic two-dimensional electron gas (2DEG) transport in N-polar GaN/AlGaN heterostructures grown on slightly mis-oriented sapphire substrates. High-resolution mobility spectrum analysis of magnetic-field dependent Hall-effect and resistivity indicate an isotropic 2DEG sheet carrier density, yet significant anisotropy was observed in carrier mobility. A single electron species with a narrow mobility distribution was found to be responsible for conduction parallel to the multi-atomic steps resulting from growth on the vicinal substrates; whilst in the perpendicular direction two distinct electrons peaks are evident at T ⩽ 150 K, which merge near room temperature. The linewidth of the mobility distributions for transport in the perpendicular direction was found to be significantly broader than that of the single electron in the parallel direction. The broader mobility distribution and the lower average mobility for the 2DEG in the perpendicular direction are attributable to interface roughness scattering associated with the GaN/AlGaN interfacial steps.

Influence of AlN interlayer on the anisotropic electron mobility and the device characteristics of N-polar AlGaN/GaN metal-insulator-semiconductor-high electron mobility transistors grown on vicinal substrates

This paper presents an experimental investigation of the influence of an AlN interlayer on the electron mobility and the device characteristics of N-polar AlGaN/GaN metal-insulator-semiconductor-high electron mobility transistors grown by metal-organic chemical vapor deposition on miscut sapphire substrates. Use of miscut substrates leads to the formation of multiatomic steps at the AlGaN/GaN interface and anisotropy in electron transport properties. A combination of van der Pauw Hall, gated transfer length measurements, and capacitance-voltage measurements has been used to study the desired properties in directions parallel and perpendicular to the multiatomic steps and qualitative explanations were provided for the observed trends. Similar to the Ga-polar devices, the introduction of AlN interlayer improved the device performance by increasing both the electron mobility and the two-dimensional electron gas charge density in the devices. Orienting the devices such that the conduction occurred parallel to the multiatomic steps was beneficial for better electron transport and device performance.

Atomic modification of stepped metal surfaces using vertical single-atom manipulation with a trimer-apex tip: First-principles and semiempirical simulations

We propose a controlled method to modify the atomic step of an Al(111) surface with atomic precision using the tip of the scanning probe microscope by first-principles molecular static simulations. Single atoms can be extracted vertically and repositioned at a mono-atomic step of an Al(111) surface using vertical single-atom manipulation with an Al trimer-apex tip. This reversible process uses only atomic tip–surface interactions and allows the precision modification of the metal surface at the atomic scale. The method is also applicable to multi-atomic steps and to the other stepped metal surfaces like Pt(111) according to our semiempirical simulations using embedded-atom-method potentials.

Sub-Angstrom oscillation amplitude non-contact atomic force microscopy for lateral force gradient measurement

We report the first results from novel sub-Angstrom oscillation amplitude non-contact atomic force microscopy developed for lateral force gradient measurements. Quantitative lateral force gradients between a tungsten tip and Si(1 1 1)–(7 × 7) surface can be measured using this microscope. Simultaneous lateral force gradient and scanning tunnelling microscope images of single and multi atomic steps are obtained. In our measurement, tunnel current is used as feedback. The lateral stiffness contrast has been observed to be 2.5 N/m at single atomic step, in contrast to 13 N/m at multi atomic step on Si(1 1 1) surface. We also carried out a series of lateral stiffness–distance spectroscopy. We observed lateral stiffness–distance curves exhibit sharp increase in the stiffness as the sample is approached towards the surface. We usually observed positive stiffness and sometimes going into slightly negative region.

Conductive atomic force microscopy study of InAs growth kinetics on vicinal GaAs (110)

Conductive atomic force microscopy has been used to investigate the effect of atomic hydrogen and step orientation on the growth behavior of InAs on GaAs (110) misoriented substrates. Samples grown by conventional molecular beam epitaxy exhibit higher conductivity on [11¯0]-multiatomic step edges, where preferential nucleation of InAs nanowires takes place by step decoration. On H-terminated substrates with triangular terraces bounded by [11¯5]-type steps, three-dimensional InAs clusters grow selectively at the terrace apices as a result of a kinetically driven enhancement in upward mass transport via AsHx intermediate species and a reduction in the surface free energy.

Noncontact lateral-force gradient measurement on Si(111)-7×7 surface with small-amplitude off-resonance atomic force microscopy

In this work, the authors report on a quantitative investigation of lateral-force gradient and lateral force between a tungsten tip and Si(111)-(7×7) surface using combined noncontact lateral-force microscopy and scanning tunneling microscopy. Simultaneous lateral-force gradient and scanning tunneling microscopy images of single and multiatomic step are obtained. In our measurement, tunnel current is used as feedback. The lateral-stiffness contrast has been observed to be 2.5N∕m at a single atomic step, in contrast to 13N∕m at a multiatomic step on Si (111) surface. They also carried out a series of lateral stiffness-distance spectroscopy, which show a sharp increase in tip-surface interaction stiffness as the sample is approached toward the surface.

Anomalous coarsening of self-assembled InAs quantum dots on vicinal GaAs (1 0 0) substrates

The formation process of InAs quantum dots (QDs) on vicinal GaAs (1 0 0) substrates is studied by atomic force microscopy (AFM). It is found that after 1.2 MLs of InAs deposition, while the QDs with diameters less than the width of the multi-atomic steps are shrinking, the larger QDs are growing. Photoluminescence measurements of the uncapped QDs correspond well to the AFM structure observations of the QDs. We propose that the QDs undergo an anomalous coarsening process with modified growth kinetics resulting from the restrictions of the finite terrace sizes. A comparison between the QDs on the vicinal GaAs (1 0 0) substrates and the QDs on the exact GaAs (1 0 0) further verifies the effect of the multi-atomic steps on the formation of QDs.

Shape stability of InAs self-assembled islands on vicinal GaAs(0 0 1) substrates

We have grown InAs self-assembled islands on vicinal GaAs(0 0 1) substrates. Atomic force microscopy and photoluminescence studies show that the islands have a clear bimodal size distribution. While most of the small islands whose growth is limited by the width of one multi-atomic step have compact symmetric shapes, a large fraction of the large islands limited by the width of one step plus one terrace have asymmetric shapes which are elongated along the multi-atomic step lines. These results can be attributed to the shape-related energy of the islands at different states of their growth.

Influence of the substrate misorientation on the properties of N-polar InGaN/GaN and AlGaN/GaN heterostructures

Smooth N-polar InGaN/GaN and AlGaN/GaN multiquantum wells (MQWs) and heterostructures were grown by metal organic chemical vapor deposition on (0001) sapphire substrates with misorientation angles of 2°–5° toward the a-sapphire plane. For all investigated structures the tendency toward formation of multiatomic steps at the film surface and at interfaces increased with increasing misorientation angle. Thereby the crystal misorientation led to a stronger degradation of the interface quality and periodicity of InGaN/GaN in comparison to the AlGaN/GaN MQWs. While the alloy composition of AlGaN films was unaffected by the misorientation, the indium mole fraction in the InGaN layers and the wavelength of the MQW related luminescence decreased with increasing misorientation angle. The properties of the two dimensional electron gas (2DEG), which formed at the upper interface of semi-insulating GaN/AlGaN/GaN heterostructures, were strongly anisotropic. Whereas the resistivity of the 2DEG measured perpendicular to the surface steps/undulations decreased with increasing misorientation angle, the resistivity measured in the parallel direction was significantly lower and unaffected by the crystal misorientation. Electron mobility values as high as 1800 cm2/V s were determined for conduction parallel to the surface steps/undulations.

Properties of N-polar AlGaN/GaN heterostructures and field effect transistors grown by metalorganic chemical vapor deposition

Smooth N-polar GaN/AlxGa1−xN/GaN heterostructures with a different Al mole fraction were grown by metalorganic chemical vapor deposition on (0001) sapphire substrates with a misorientation angle of 4° toward the a-sapphire plane. The sheet electron density of the two-dimensional electron gas (2DEG), which formed at the upper GaN/AlxGa1−xN interface increased with an increasing Al-mole fraction in the AlxGa1−xN layer and increasing silicon modulation doping, similar to the observations for Ga-polar heterostructures. The transport properties of the 2DEG, however, were anisotropic. The growth on vicinal substrates led to the formation of well ordered multiatomic steps during AlxGa1−xN growth and the sheet resistance of the 2DEG parallel to the steps was about 25% lower than the resistance measured in the perpendicular direction. The fabricated devices exhibited a drain-source current, IDS, of 0.9 A/mm at a gate-source voltage +1 V. At a drain-source voltage of 10 V and IDS=300 mA/mm, current-gain and maximum oscillation frequencies of 15 and 38 GHz, respectively, were measured.

Formula omitted] nano- and microtubes fabricated on (1 1 0)-oriented GaAs- and GaP-substrate using the metal-organic vapor-phase epitaxy

We fabricated III–V nano- and microtubes by using the bending of ultra-thin strained films by releasing them due to selective wet-chemical etching of a sacrificial layer from the substrate. The curvature is obtained by a new mechanical equilibrium shape for which the elastic strain energy of the layer-system is minimal. The resulting diameter of these tubes depends on the layer composition, layer thickness, substrate orientation and rolling direction. We studied the influence of thickness variations on the surface due to multiatomic steps on the structural properties of BGaAs/InGaAs-tubes grown on (1 1 0)-oriented GaAs-substrates. Cross-sectional transmission electron microscopy (TEM)-images on the rolled structures showed, that the large difference of the outer and inner diameter depends on the multiatomic steps at the surface of the layer-system. We also present a new material-system for the fabrication of rolled-up nanotubes (RUNTs): BGaP/InGaP with AlGaP sacrificial layer on (1 1 0)-oriented GaP-substrate. The tube diameter and the internal strain in the rolled and unrolled multilayer systems were modeled using continuum strain theory.