Average Terrace Width Research Articles

From dissolution to controlled macrostepping of 4H-SiC during liquid Si-induced structuring in a sandwich configuration

The structuring of Si face 4°off 4H-SiC surfaces was investigated using Si melting in a SiC/Si/SiC sandwich configuration. The stacks were treated at 1550–1600 °C under H2 in a RF-heated cold-wall reactor. By fixing the liquid Si thickness to 30 µm, the vertical thermal gradient inside the stack generates carbon transport from the bottom to the top SiC wafer. The constant dissolution of the bottom SiC wafer (1.7 µm/h at 1550 °C) leads to surface structuring in macrosteps. The regularity of these macrosteps can be reproducibly controlled when performing the process on an epitaxial layer thanks to the pre-structuration in parallel microsteps of such kind of surfaces. The best regularity of the steps was obtained after a structuring process of 2 h, with an average terrace width of ∼3–5 µm.

Resolving mobility anisotropy in quasi-free-standing epitaxial graphene by terahertz optical Hall effect

In this work, we demonstrate the application of terahertz-optical Hall effect (THz-OHE) to determine directionally dependent free charge carrier properties of ambient-doped monolayer and quasi-free-standing-bilayer epitaxial graphene on 4H–SiC(0001). Directionally independent free hole mobility parameters are found for the monolayer graphene. In contrast, anisotropic hole mobility parameters with a lower mobility in direction perpendicular to the SiC surface steps and higher along the steps in quasi-free-standing-bilayer graphene are determined for the first time. A combination of THz-OHE, nanoscale microscopy and optical spectroscopy techniques are used to investigate the origin of the anisotropy. Different defect densities and different number of graphene layers on the step edges and terraces are ruled out as possible causes. Scattering mechanisms related to doping variations at the step edges and terraces as a result of different interaction with the substrate and environment are discussed and also excluded. It is suggested that the step edges introduce intrinsic scattering in quasi-free-standing-bilayer graphene, that is manifested as a result of the higher ratio between mean free path and average terrace width parameters. The suggested scenario allows to reconcile existing differences in the literature regarding the anisotropic electrical transport in epitaxial graphene.

Growth temperature dependence of cubic GaN step structures and cubic InN dot arrays grown on MgO (001) vicinal substrates

In this paper, we report on the detailed structural features of step-bunched surface formed on cubic (c-) GaN and self-assembled c-InN dot arrays grown on the c-GaN surface, particularly focusing on the growth temperature dependence. Samples were fabricated on MgO (001) vicinal substrates with off-cut angles of 2.0° and 3.5° toward [110] by RF-MBE. Multisteps and terraces with a certain periodicity in the vicinal direction were observed on c-GaN layers. The average terrace width narrowed with lowering the growth temperature and increasing the substrate off-cut angle. The formation of c-InN dots proceeded in the Stranski–Krastanov growth mode with critical thickness of 0.54–0.66 nm. The c-InN dot exhibited a variety of structural features depending on the growth conditions. Positional dot alignment along the multistep edges of the underlayer was observed under some conditions. The degree of alignment was found to be affected by the terrace width on the c-GaN underlayer.

Growth of Ag(1 1 1) on Si(1 1 1) with nearly flat band and abrupt interface

Growth of Ag films of up to 30 nm thickness on Si(1 1 1) 7 × 7 at room temperature is investigated by low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). LEED revealed the coexistence of Ag and Si spots starting with 1 monolayer (ML) of Ag deposited. The Ag lattice constant, starting with 25 ML, is slightly higher than for bulk Ag and increase linearly with Ag thickness, reaching about 4.2 nm for the thickest films. The average terrace widths detected from LEED spot profile analysis are about 30 nm for clean Si(1 1 1) 7 × 7 and about 5.5 nm for the thickest Ag(1 1 1) film, in agreement with STM observations. The intensity variation of core levels analyzed by XPS is taken into account by a model assuming the initial formation of Ag islands with linear variation of coverage vs. the amount of Ag deposited, followed by growth in a quasi layer-by-layer mode. The interface barrier is in the range of 0.4 eV, lower than all values reported previously. Ag deposited on Si(1 1 1) 7 × 7 at room temperature provides flat Ag(1 1 1) for synthesis of 2D materials, and may be used for low barrier Schottky diodes.

Large Area Atomically Flat Surfaces via Exfoliation of Bulk Bi2Se3 Single Crystals

In this work, we present an exfoliation method that produces cm2-area atomically flat surfaces from bulk layered single crystals, with broad applications such as for the formation of lateral heterostructures and for use as substrates for van der Waals epitaxy. Single crystals of Bi2Se3 were grown using the Bridgman method and examined with X-ray reciprocal space maps, Auger spectroscopy, low-energy electron diffraction, and X-ray photoelectron spectroscopy. An indium-bonding exfoliation technique was developed that produces multiple ∼100 μm thick atomically flat, macroscopic (>1 cm2) slabs from each Bi2Se3 source crystal. Two-dimensional X-ray diffraction and reciprocal space maps confirm the high crystalline quality of the exfoliated surfaces. Atomic force microscopy reveals that the exfoliated surfaces have an average root-mean-square (RMS) roughness of ∼0.04 nm across 400 μm2 scans and an average terrace width of 70 μm between step edges. First-principles calculations reveal exfoliation energies of Bi2...

Charge carrier concentration and offset voltage in quasi-free-standing monolayer chemical vapor deposition graphene on SiC

Epitaxial Chemical Vapor Deposition growth of graphene on silicon carbide offers the maturity and reliability expected for large-scale fabrication of graphene-based devices. In particular, the ultimate challenge of graphene synthesis on SiC, i.e. quasi-free-standing monolayer graphene which comes through hydrogen atom intercalation of the sole buffer layer grown on the Si-face of SiC, offers high carrier mobility (as high as 6600 [cm2/Vs]) and electrical stability throughout the device processing cycle. In this report, we present extensive statistics of the electrical properties of QFS-monolayer graphene grown on 4H(0001) and 6H(0001) semi-insulating 10 mm × 10 mm substrates, being a result of 110 individual processes. The adopted explanation for the origin of the as-grown doping level in epitaxial graphene based on the spontaneous polarization of hexagonal SiC and its most up-to-date values is reaffirmed. We introduce the issue of the step-edge-induced offset voltage radial dependence and confront it with the morphological analysis of the average step edge height and terrace width, all related to the place of origin of a specific sample within a 4-inch SiC wafer. Finally, we conclude that within the step edge area QFS-monolayer graphene is statistically nearly half as resistive as the previously reported QFS-bilayer graphene.

Measurement of microscopic surface deformation due to low energy ion bombardment on Si(111)

A low energy electron diffraction spot profile analysis of the Si(111) surface, after argon ion bombardment at an elevated temperature, finds a continuous, low amplitude distribution of surface height, in addition to the atomic-step-and-terrace structure. With an amplitude of tenths of an Angstrom or less in height, correlated laterally over tens of Angstroms, this microscopic surface deformation is measured versus ion dose and for various sample temperatures during bombardment and annealing. For 230eV argon ion doses increasing in the range of 1015−1016cm−2 with the sample held at 580K and 800K, the amplitude of surface deformation, measured as the standard deviation wz of the continuous height distribution, increases steadily in the range 0.06−0.10Å. For higher ion doses in the range of 3×1016−1×1017cm−2, saturation-like behavior with wz≈0.14−0.15Å is measured at 800K. After an ion dose of 1017cm−2, the surface deformation shows a lateral correlation length of 20Å, implying an average lateral feature size of ≈40Å, slightly less than the average terrace width of ≈60Å. On the other hand, after the same ion dose at 300K followed by annealing at 800K, the surface deformation has a similar correlation length but a much smaller height amplitude of wz=0.06Å. It is notable that wz depends quite differently on sample preparation conditions overall, compared to the average terrace width and to the degree of (7×7) order indicated by diffraction intensities, both of which are found to evolve much more slowly with ion dose in conditions for which the height amplitude increased in the range of 0.06−0.14Å. The surface height deformation due to strain around buried, subsurface defects is estimated as a function of defect concentration using a simple, order-of-magnitude theoretical model, in which a distribution of subsurface defects is modeled as small inclusions in a continuous elastic medium. This approximate model is consistent with the measured growth of the deformation amplitude wz, for a range of possible parameter choices for small defect clusters located in a shallow layer below the surface. The surface deformation is partially removed by annealing above 870K.

Unprecedented Structural Sensitivity toward Average Terrace Width: Nafion Adsorption at Pt{hkl} Electrodes

Ultrathin Nafion overlayers ranging from 0 to 3.3 nm in thickness have been deposited on platinum single crystal electrodes in order to study their voltammetric behavior as a function of both Nafion surface coverage and platinum surface step density. Unusual structural sensitivity of Nafion-induced voltammetric peaks ascribable to Nafion interactions with step sites is observed as a function of average terrace width. For stepped surfaces containing both {111} and {100} terraces, average terrace widths of 2–3 atoms give rise to the most intense Nafion-step electrosorption peaks. Rapid quenching of the Nafion-step voltammetric peaks is observed as the average terrace width is gradually increased until for >5 atom wide terraces all such electrochemical features are completely attenuated. This rapid diminution of the step-Nafion voltammetric peak, within just three atoms of terrace width, is unprecedented, and a parallel with the “splitting” of hydrogen underpotential deposition (HUPD) step peaks in perchloric acid aqueous electrolyte for closely spaced steps is noted. On the basis of these measurements, it is suggested that Nafion is a molecular probe of adsorbed OH species on Pt electrodes.

Interplay of adsorbate-adsorbate and adsorbate-substrate interactions in self-assembled molecular surface nanostructures

The adsorption of 2,6-naphthalenedicarboxylic acid (NDCA) molecules on the Ag(110), Cu(110), and Ag(111) surfaces at room temperature has been studied by means of scanning tunnelling microscopy (STM). Further supporting results were obtained using X-ray photoelectron spectroscopy (XPS) and soft X-ray absorption spectroscopy (XAS). On the Ag(110) support, which had an average terrace width of only 15 nm, the NDCA molecules form extended one-dimensional (1-D) assemblies, which are oriented perpendicular to the step edges and have lengths of several hundred nanometres. This shows that the assemblies have a large tolerance to monatomic surface steps on the Ag(110) surface. The observed behaviour is explained in terms of strong intermolecular hydrogen bonding and a strong surface-mediated directionality, assisted by a sufficient degree of molecular backbone flexibility. In contrast, the same kind of step-edge crossing is not observed when the molecules are adsorbed on the isotropic Ag(111) or more reactive Cu(110) surfaces. On Ag(111), similar 1-D assemblies are formed to those on Ag(110), but they are oriented along the step edges. On Cu(110), the carboxylic groups of NDCA are deprotonated and form covalent bonds to the surface, a situation which is also achieved on Ag(110) by annealing to 200 °C. These results show that the formation of particular self-assembled molecular nanostructures depends significantly on a subtle balance between the adsorbate-adsorbate and adsorbate-substrate interactions and that kinetic factors play an important role.

Data Analysis Technique of Atomic Force Microscopy for Atomically Flat Silicon Surfaces

A data analysis technology of atomic force microscopy for atomically flat silicon surfaces has been developed. Atomically flat silicon-surfaces composed of atomic terraces and steps are obtained on (100) orientation 200 mm diameter wafers by annealing in pure argon ambience at 1,200°C for 30 minutes. Atomically flat silicon surfaces are lead to improve the MOS inversion layer mobility and current drivability of MOSFETs and to decrease the fluctuations in electrical characteristics of MOSFETs. It is important to realize the technology that evaluates the flatness and the uniformity of atomically flat silicon surfaces. The off direction angle is calculated by using two straight edge lines selected from measurement data. And the off angle is calculated from average atomic terrace width under assumption that height difference between neighboring terraces is equal to the step height, 0.135 nm, of (100) silicon surface. The analyzing of flatness of each terrace can be realized by converting the measurement data using the off direction angle and the off angle. And, the average roughness of each terrace is about 0.017-0.023 nm. Therefore, the roughness and the uniformity of each terrace can be evaluated by this proposed technique.

Surface morphological evolution during annealing of epitaxial Cu(001) layers

Single crystal Cu(001) layers were grown on MgO(001) by ultrahigh vacuum magnetron sputtering at Ts=100 °C. Quantitative surface morphological analyses by in situ scanning tunneling microscopy show that the surfaces exhibit self-affine mound structures with a scaling exponent of 0.82±0.03 and a mound radius rc that increases from 31±8 to 39±6 nm for increasing layer thickness t=24–120 nm. In situ annealing at 200 and 300 °C leads to a thermodynamically driven mass transport that minimizes the surface step density, resulting in broader mounds and a smaller root mean square surface roughness σ. This effect is most pronounced for t=24 nm, for which rc increases from 31±8 to 70±20 nm and σ decreases from 1.3±0.1 to 0.74±0.08 nm, resulting in a decrease in the average surface slope from χ=7° to 2° and an increase in the average terrace width wT by more than a factor of 4. In contrast, wT increases by only 20% for t=120 nm. This remarkable difference between “thin” and “thick” layers is attributed to diverging surface morphological pathways during annealing: The strong smoothening for t=24 nm is due to a competitive coalescence process where some mounds grow laterally at the expense of their smaller neighbors, which die out. In contrast, the initially wider mounds of thicker layers (t=120 nm) combine to form a quasistable surface morphology that exhibits anisotropic mound structures, which limit mass transport and stabilize the surface step density.

Activation energy of surface diffusion and terrace width dynamics during the growth of In(4×3) on Si(100)-(2×1) by femtosecond pulsed laser deposition

The nucleation and growth of indium on a vicinal Si(100)-(2×1) surface at high temperature by femtosecond pulsed laser deposition was investigated by in situ reflection high energy electron diffraction (RHEED). RHEED intensity relaxation was observed for the first ∼2 ML during the growth of In(4×3) by step flow. From the temperature dependence of the rate of relaxation, an activation energy of 1.4±0.2 eV of surface diffusion was determined. The results indicate that indium small clusters diffused to terrace step edges with a diffusion frequency constant of (1.0±0.1)×1011 s−1. The RHEED specular beam split peak spacing, which is characteristic of a vicinal surface, was analyzed with the growth temperature to obtain the average terrace width. Gradual reduction in the terrace width during growth of In(4×3) was observed with In coverage and is attributed to the detachment of In atoms from terrace edges. At a substrate temperature of 405 °C, the average terrace width decreased from 61±10 Å, which corresponds to the vicinal Si(100) surface, to an equilibrium value of 45±7 Å after deposition of ∼23 ML. Further In coverage showed a transition of the RHEED pattern from (4×3) to (1×1) and the growth of rounded In islands (average height of ∼1 nm and width of ∼25 nm), as examined by ex situ atomic force microscopy.

Extended One-Dimensional Supramolecular Assembly on a Stepped Surface

2,6-naphthalene-dicarboxylic acid was adsorbed on a Ag110 surface with an average terrace width of only some tens of a nm. Scanning tunneling microscopy shows that the adsorbates self-assemble into one-dimensional mesoscale length chains. These extend over several hundred nanometers and thus the structure exhibits an unprecedented tolerance to monatomic surface steps. Density functional theory and x-ray photoelectron spectroscopy explain the behavior by a strong intermolecular hydrogen bond plus a distinct template-mediated directionality and a high degree of molecular backbone flexibility.

Linear stability analysis for step meandering instabilities with elastic interactions and Ehrlich-Schwoebel barriers

Vicinal surfaces are known to exhibit morphological instabilities during step-flow growth. Through a linear stability analysis of step meandering instabilities, we investigate two effects that are important in many heteroepitaxial systems: elastic monopole-monopole interactions arising from bulk stress and the Ehrlich-Schwoebel (ES) barriers due to the asymmetric adatom incorporation rates. The analysis shows that the effects of the ES barriers increase as the average terrace width increases, whereas the effects of elastic monopole-monopole interactions decrease. The ES barriers favor an in-phase step pattern with a zero phase shift between consecutive steps, while elastic stress favors an out-of-phase pattern with a phase shift of pi. However, our analysis shows that the instability growth rate becomes nearly independent of the phase shift when either the ES-barrier effect or the stress effect is large. In particular, for ES-barrier-driven instability, the in-phase step pattern develops only within an intermediate range of terrace widths when bulk stress exists. Similarly, for the elastic-interaction-driven instability, an out-of-phase pattern only forms within a certain range of monopole strength; if the strength is too small, the ES barrier effect dominates, and if it is too large, the peak in the instability growth rate becomes delocalized in the phase shift and no patterns form. This transition between patterned and random step morphologies depends on the monopole strength, but is independent of the terrace width. A phase diagram that describes the regions of the ES-barrier-dominant instability and the elastic-interaction-dominant instability is established, along with the morphological phase diagrams that predict the step configurations as a function of the controlling parameters for the two types of instabilities.

Diffusion-limited kinetics of terrace growth on GaAs(110)

The kinetics of thermal annealing of surface roughness created by low-energy ion bombardment of GaAs(110) is characterized in situ by a low-energy electron diffraction (LEED) spot profile analysis to investigate the underlying diffusion mechanisms involved. The coarsening of the step-and-terrace structure on the partially annealed surface is observed, showing power-law growth of the average terrace width $l\ensuremath{\sim}{t}^{\ensuremath{\beta}}$ with an exponent $\ensuremath{\beta}=0.23\ensuremath{-}0.26$ over two orders of magnitude in annealing time $t$ in the temperature range 660--740 K. However, the terrace height distribution is shown to vary little with annealing, providing evidence for an Ehrlich-Schwoebel barrier at step edges. The LEED analysis also detects an additional, low amplitude height distribution contributing an interface width of less than 0.03 nm, which is interpreted as elastic deformation around subsurface defects and exhibits much slower relaxation kinetics during annealing than the terrace growth.

Supersonic molecular beam deposition of pentacene thin films on two Ag(111) surfaces with different step densities

The structure of pentacene thin films grown by supersonic molecular beam deposition on two Ag(111) single-crystal surfaces with different average step distances ( $\ensuremath{\sim}400\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ and $>2000\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$) have been studied by low-energy atom diffraction. While the initial stage of the growth is similar on the two different surfaces, regardless of the pentacene kinetic energy and substrate temperature, thicker films show different structural and thermal properties. The ultrathin-film phase has the same structure on both surfaces, showing, however, much larger domain sizes on the substrate with larger average terrace width. In spite of the inferior quality of the ultrathin layer grown on the narrower terraces, upon continuing the growth, a well-ordered multilayer structure is obtained. This, however, occurs in a narrow range of deposition conditions, i.e., at a relatively high kinetic energy of the molecules $(\ensuremath{\sim}5\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ and a low substrate temperature $(200\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. On the other, almost ideally flat surface, the order of the pentacene multilayers is, surprisingly, much poorer in spite of the facts that for this surface we have measured an unprecedented 90% of He atom reflectivity and that the measured width of the diffraction peaks of the first monolayer was the narrowest we have ever measured for an organic thin film.

Surface features of self-organized SrTiO 3 (001) substrates inclined in [100] and [110] directions

The dependence of self-organized surface features in inclined SrTiO 3 (001) substrates such as step edge, step height, and terrace width on the inclined angle from 0° to 6° and inclined direction of [100] and [110] were investigated. Wavy step edges, straight step edges, and triangular step edges are formed in the flat, the [100], and the [110] inclined substrates, respectively. The average step height increases with the inclined angle, while the average terrace width decreases. The steps on the inclined substrates after self-organization increase, but the same ratio of step height to terrace width is conserved when the inclined angle is constant. These behaviors in surface features of the self-organized substrate with inclined angles are closely related to the step interaction energy required to form the step height and terrace width. The inclined angle and the inclined direction should be important factors for controlling nanostructure morphology.

Evolution of yttria-stabilized zirconia (100) surface morphology with temperature

Abstract The evolution with temperature of the morphology of yttria-stabilized zirconia (100) stepped surfaces has been studied by AFM in a non-contact dynamic mode in air. The characteristics of the surface structure as the average step heights and terrace widths are determined as a function of the annealing temperature. Step formation begins around 0.43 T M (1000 °C). Step coalescence is observed above 0.50 T M (1200 °C) and leads to a stable stepped structure at 0.60 T M (1500 °C). The processes responsible for the structural evolution observed on the sample surfaces are discussed and a comparison with results obtained on Al 2 O 3 (0001) single crystal surfaces under similar experimental conditions is presented.

Reflection high-energy electron diffraction study of surface morphology of vicinal Au(1 1 0) throughout the (2 × 1) to (1 × 1) phase transition

The vicinal Au(1 1 0) surface morphology throughout the (2 × 1) to (1 × 1) phase transition is studied by quantitative reflection high-energy electron diffraction. As the surface is heated from 300 K, the average terrace width remains unchanged up to 635 ± 10 K. Above that temperature, the average terrace width increases and at 683 K has a value (34 ± 10)% more than at 300 K. At higher temperatures, the average terrace width decreases. The average string length at step terraces on Au(1 1 0) remains basically unchanged up to ∼650 K and decreases sharply at higher temperatures indicative of a step-induced roughening transition. Thermal-induced adatom/vacancy generation on terraces is shown to increase significantly above ∼680 K. At room temperature, the spacing between the topmost two layers of the Au(1 1 0) surface is contracted by 0.31 ± 0.03 Å or ∼22% of the bulk-terminated structure.

Morphology of the rutile (110) surface after low sputter dose and annealing

The kinetics of diffusion-mediated smoothing of a ${\mathrm{TiO}}_{2}$ rutile (110) surface is studied over atomic length scales, by using a spot profile analysis of low-energy electron-diffraction data to characterize the morphology of the surface during thermal annealing. After the random removal of less than 1 ML of atoms by sputtering with an argon ion beam, the interface width and the distribution of terrace heights were found to stay nearly constant during annealing at 800 K, with terraces at just two heights making up \ensuremath{\approx}90% of the surface area over lateral distances of \ensuremath{\approx}400 \AA{}. Meanwhile, the coarsening of this nearly two-dimensional island structure is characterized by the growth of the average terrace width l from 20 to 60 \AA{} with annealing time, following $l\ensuremath{\sim}{t}^{\ensuremath{\beta}}$ with exponent $\ensuremath{\beta}=0.24\ifmmode\pm\else\textpm\fi{}0.04.$ In addition, the stability of an 8% occupation of a third terrace height indicates negligible diffusion flux between layers during annealing. These results are compared with existing models for the microscopic dynamics involved.