Smooth Steps Research Articles

Specific features of the growth of A II B VI films on (0001)Al2O3 substrates

The structure and orientation of CdTe and ZnO films on sapphire have been investigated for different techniques of pregrowth substrate treatment. Polycrystalline CdTe films are found to grow of substrates unannealed or annealed in vacuum at a residual pressure P < 0.13 Pa. Epitaxial CdTe films with the sphalerite cubic structure, oriented parallel to the substrate by the (111) plane, grow on substrates annealed in air at a temperature of 1000°C or more and having a system of smooth terraces and steps on the surface. For ZnO films with a wurtzite hexagonal structure obtained by magnetron sputtering, a similar correlation between the structural quality and the regime of treatment of sapphire substrates is observed. It is shown that thermal annealing of (0001) sapphire plates in air is the optimal way of substrate preparation for growing epitaxial ZnO films with the base orientation. The obtained epitaxial CdTe films contain a certain amount of structural defects (mosaicity and twins), while the epitaixal ZnO films treated in the same way are close to perfect.

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.

Structural contribution to the roughness of supersmooth crystal surface

Technological advances in processing crystals (Si, sapphire α-Al2O3, SiC, GaN, LiNbO3, SrTiO3, etc.) of substrate materials and X-ray optics elements make it possible to obtain supersmooth surfaces with a periodicity characteristic of the crystal structure. These periodic structures are formed by atomically smooth terraces and steps of nano- and subnanometer sizes, respectively. A model surface with such nanostructures is proposed, and the relations between its roughness parameters and the height of atomic steps are determined. The roughness parameters calculated from these relations almost coincide with the experimental atomic force microscopy (AFM) data obtained from 1 × 1 and 10 × 10 μm areas on the surface of sapphire plates with steps. The minimum roughness parameters for vicinal crystal surfaces, which are due to the structural contribution, are calculated based on the approach proposed. A comparative analysis of the relief and roughness parameters of sapphire plate surfaces with different degrees of polishing is performed. A size effect is established: the relief height distribution changes from stochastic to regular with a decrease in the surface roughness.

Turbulent boundary layers over smooth and rough forward-facing steps

The present work explores the impact of the roughness on the turbulent boundary layers over forward-facing steps. The roughness topography on the top surface of the rough step is replicated from a realistic turbine blade and embodies three-dimensional and highly irregular topographical features. High spatial resolution particle image velocimetry measurements are performed in the x−y planes at two different spanwise positions in turbulent boundary layers over both smooth and rough steps of the same mean heights at Reh=3450 and δ/h=8. Comparison of mean flow structures, Reynolds normal and shear stresses, quadrant analysis of instantaneous shear stress contributing events, and average spanwise vorticity reveals that the separated flow after the step is weakened by the surface roughness on top of the step while the flow ahead of the step is invariant to the surface conditions. The characteristics of the coherent spanwise vortices such as the numbers, size, and circulation distributions are also found to be significantly modified by the roughness topography.

Fabrication and Use of Atomically Smooth Steps on 6H-SiC for Calibration of z-Displacements in Scanning Probe Microscopy

Atomic-force microscopy and scanning tunnel electron microscopy have been used to study the surface of single-crystal 6H-SiC (0001) substrates subjected to step-by-step high-temperature annealing in vacuum. An annealing procedure leading to surface structuring by atomically smooth steps with heights of 0.75 and 1.5 nm has been found. It is suggested to use the structured surfaces as test objects for z-calibration of scanning probe microscopes.

Erste Ergebnisse mit einer neuen aberrationskorrigierten intraokularen Bifokallinse

To investigate the functional results with a new bifocal intraocular lens. The *Acri.LISA (*Acri.Tec) was implanted bilaterally in 20 patients after uneventful cataract surgery. The new bifocal IOL has a light distribution of 65% for distance and 35% for the near range. The diffractive optics of the lens are designed to be independent of pupil size. Smooth steps in diffractive structure should reduce glare. An aspheric design of the posterior optic surface is engineered to counteract the asphericity of the cornea (-0.26). Postoperative evaluation included 6 weeks after surgery: uncorrected (UCVA) and best corrected (BCVA) visual acuity for distance (ETDRS charts) and near (C.A.T. charts, Birkhäuser charts) monocular and binocular, defocus curve, contrast sensitivity under photopic and mesopic lighting conditions (F.A.C.T.), and subjective assessment of halos At the 6-week follow-up mean binocular UCVA and BCVA (LogMAR) were -0.02+/-0.10 and -0.07+/-0.09, respectively, for distance. Near UCVA (LogMAR) was 0.09+/-0.16; distance corrected near VA was 0.04+/-0.13. Visual acuity was significantly superior when tested binocularly compared to monocular testing (p<0.01). The depth of field showed an intermediate decimal VA of 0.6+/-0.21 at 70 cm and a pseudoaccommodation range of 5.5 D. Of 20 patients, 16 reported slight halos, but no patient was seriously impaired. The *Acri.LISA showed very good visual performance 6 weeks after bilateral surgery.

Structural and Optical Properties of Nonpolar InGaN/GaN Multiple Quantum Wells Grown on Planar and Lateral Epitaxially Overgrown m-Plane GaN Films

The structural and optical properties of nonpolar (1100) m-plane InGaN/GaN multiple quantum wells (MQWs), grown simultaneously on reduced-defect lateral epitaxially overgrown (LEO) m-plane GaN and high-defect-density planar m-plane GaN, were investigated. Atomic force microscopy images of the LEO m-plane MQW sample revealed the presence of smooth steps on the Ga-face wings. The presence of stacking-fault-related defects was observed in the window and the N-face wing regions. Transmission electron microscopy on the sample revealed abrupt quantum well interfaces. High-resolution X-ray diffraction analysis showed that the In incorporation was the same in the LEO sample and in the planar sample. Spot-excitation cathodoluminescence measurement revealed a similar emission wavelength for the nearly defect-free laterally overgrown regions and the defective window regions. However, the emission from the reduced-defect-density wings of the LEO m-plane GaN was more than six times stronger than the emission from the neighboring defective windows. Wide-area photoluminescence measurement showed a similar emission wavelength for the LEO sample and the planar MQW sample.

Survival of atomic monolayer steps during oxide desorption on GaAs (100)

Significant surface pitting and a degraded surface roughness are almost always observed on GaAs (100) surface after conventional thermal oxide desorption. Here we report on the use of a Ga-triggered low temperature oxide desorption method that can be used to preserve the atomic monolayer (ML) steps. By providing an external supply of atomic Ga at a relatively low substrate temperature of ∼450°C without an As4 overpressure, this technique resulted in an atomically smooth GaAs ML steps with a root mean square roughness of 0.25nm, nearly identical to as-grown GaAs surface (0.2nm). The demonstrated results show the potential for applications in optoelectronics such as regrowth and patterned substrate growth.

Bilayer-by-bilayer etching of 6H-GaN(0001) with Cl

Abstract Etching of Ga-polar GaN(0 0 0 1) surface with Cl has been studied using scanning tunneling microscopy. Three different processes in the thermally activated Cl reaction, i.e., the removal of Ga-fluid, the formation of alternative zigzag and smooth steps, and the initiation of triangle pits on the terrace that take place at different temperatures, are identified. The atom removal is found to proceed via a bilayer-by-bilayer mode on the GaN surface, and the underlying mechanism is discussed in terms of the surface atomic structure.

Improved nanotriode fabrication process for multiple gates and reduced leakage current

The nanotriode is a novel vacuum microelectronic device, in which electrons are emitted from a metal nanopillar with tip radius <1 nm in an integrated vacuum chamber, over a distance of about 100 nm. In this work, the nanotriode fabrication process has been revised, with the aim of creating nanotriode-type devices with additional gates. The resultant structure has smooth steps on the outer edges of the RF magnetron-sputtered tungsten–silicon dioxide multilayer stack, which aids the isolation of the electrode layers. This improved structure contributes reduced inter-electrode leakage currents. The form of the nanopillars has also been investigated by cross-sectional TEM.

Improved nanotriode fabrication process for multiple gates and reduced leakage current

The nanotriode is a novel vacuum microelectronic device, in which electrons are emitted from a metal nanopillar with tip radius <1 nm in an integrated vacuum chamber, over a distance of about 100 nm. In this work, the nanotriode fabrication process has been revised, with the aim of creating nanotriode-type devices with additional gates. The resultant structure has smooth steps on the outer edges of the RF magnetron-sputtered tungsten–silicon dioxide multilayer stack, which aids the isolation of the electrode layers. This improved structure contributes reduced inter-electrode leakage currents. The form of the nanopillars has also been investigated by cross-sectional TEM.

SPM Characterization of Substrate Surfaces Prepared for Carbon-Related-Film Deposition

AbstractsVariation of surface steps on sapphire (0001) and (1120) substrates processed with thermal annealing in air or a reducing environment at 1000 to 1400°C for 1 to 10 hours were investigated with an atomic force microscope (AFM). The annealed (0001) surfaces consist of atomically smooth and large terraces and atomic-height steps, whose configurations strongly depend on annealing conditions. On the (1120) surfaces, where crystallographic misorientation is almost an order of magnitude larger than that of the (0001) surfaces, step height and terraces increase in size with the longer annealing time and higher annealing temperature. Characteristic step figures due to the symmetry of atomic arrangement were observed on the (0001) surface.

Ag-mediated step-bunching instability on vicinal Si(100)

Silver-mediated multistep formation on vicinal Si(100)-(2 × 1) is investigated by spot profile-analysing-low-energy electron diffraction and scanning tunnelling microscopy. The employed 4°-miscut of the sample towards [110] leads to a single domain substrate with smooth and equidistant double steps. Surface reconstruction and step structure of the vicinal are altered drastically upon submonolayer adsorption of Ag at ∼700K: a (3 × 2) superstructure is formed which is characterised by an anisotropic interaction with steps, thus preserving the single domain character of the surface. Furthermore, pronounced step-bunching leads to the formation of a regular array of multiple double steps with preference for sixfold step height. It is supposed that the attempt to lower surface stress is the driving force for this step-bunching instability. As a general feature of the restructured step topography, the long-range order of lateral step positions is not affected, i.e. the vicinal remains flat with respect to the macroscopic surface orientation. Finally, the experimental results are verified by comparative numerical simulations of the diffracted intensity I(k ⊥, k |) .

Ion milling damage and regrowth of oxide substrates studied by ion channeling and atomic force microscopy

We have studied the effect of ion milling on the surface crystallinity of the metal oxide substrates LaAlO3, SrTiO3, and NdGaO3 which are used for fabrication of high-Tc Josephson junctions and circuits. Ion channeling of the milled substrates reveals a damage-induced peak corresponding to a disordered layer of ≈60 Å at the surface. Annealing the substrates in oxygen ambient at various temperatures ranging from 600 to 1100 °C resulted in the regrowth of the damaged layer at the surface of the substrates as was indicated by the reduction in size of the surface peak observed in the channeled spectrum, as well as by formation of lattice steps as seen by atomic force microscopy. A significant reduction in the damage peak size and the formation of smooth completed lattice steps are seen only after annealing at temperatures ⩾950 °C.

Liquid Phase Epitaxy (LPE) of YB2Cu3O7‐δ High Tc Superconductors

AbstractYBa2Cu3O7‐δ films were grown by liquid phase epitaxy in Y‐ZrO2 crucibles on (110)NdGaO3 substrates. A change in the preferred crystallographic growth direction was observed reproducibly. The growth mode changes from pure c‐axis oriented over mixed c/a‐axis oriented films to pure a‐axis oriented films with increasing undercooling. X‐ray diffraction measurements and atomic force microscopy investigations with c‐axis oriented as well as a‐axis oriented films are presented. It will be shown that atomically smooth steps with 0.2‐0.8 μM width can be achieved by LPE, which are free from melt remnants. Furthermore the values for the enthalpy of dissolution from literature, which are varying in a wide range, will be combined and a new value will be calculated in terms of a multi particle model.

Atomic force microscopy study of (001) SrTiO 3 surfaces

By means of atomic force microscopy, we have investigated the surfaces of SrTiO 3(001) substrates (used for the growth of high- T c superconducting films) after different heat treatments. These treatments were (i) annealing in O 3, (ii) annealing in O 2, (iii) annealing in O 3 followed by deposition of SrTiO 3, and (iv) annealing in UHV. Our results suggest that, independently of the type of annealing, the surfaces of the polished SrTiO 3 substrates rearrange, either by diffusion or desorption of atoms, until reaching the energetically most favourable surface structure. For vicinal (001) substrates, this equilibrium structure corresponds to atomically flat terraces separated by smooth steps of one unit-cell in height, with the orientation and width of the terraces being determined by the miscut of the substrate. However, the kinetics involved in the reconstruction of the SrTiO 3 surface were observed to be strongly dependent on the annealing conditions. In particular, the use of O 3 instead of O 2 or UHV was found to accelerate the reconstruction of the SrTiO 3 surface. Finally, this study provides an accurate characterisation of the defects present on SrTiO 3 substrates.

A novel configuration of atomic steps observed on vicinal silicon (100) surface annealed in a hydrogen atmosphere

Abstract The vicinal silicon (100) surface with a misorientation of 0.05° towards [011] and [001] has been investigated with atomic force microscopy after annealing at 1200°C in argon and hydrogen. For the samples tilted towards [011], while the surfaces annealed in argon are featured with the alternate smooth and rough steps of monolayer height (S steps), which unambiguously correlate themselves with the so-called (Sa + Sb) structure, the hydrogen annealing, in contrast, generates a configuration of orthogonal S steps. Through an analysis of the surface tilted towards [001], we show that a miscut toward an arbitrary direction can be accommodated by two identical S steps developed independently along two perpendicular {011} axes if the crystal is annealed in hydrogen. On the contrary, a surface annealed in argon shows no conformability with the misorientation towards an axis other than {011}. It is believed that the formation of the orthogonal S steps is a chemically driven process resulting from the relaxation of the stress field associated with atomic dimers and we propose that a hydrogen-induced transformation of Sb into Sa is responsible for the surface structure dominated by Sa steps.

The growth of cobalt films on vicinal copper surfaces

We have studied the morphology of thin cobalt films on Cu(1 1 17) using scanning tunneling microscopy. Submonolayer quantities of cobalt deposited at room temperature lead to a dramatic roughening of the steps involving a substantial restructuring of the copper template. It is argued that this restructuring is caused by the cobalt-induced misfit strain. Films of 2–3 monolayer thickness display a rather rugged structure with many steps oriented perpendicular to the original step direction. With further deposition of cobalt, the steps become smoother and eventually above about 5 to 8 monolayers one has step flow growth with smooth steps. In agreement with earlier observations we find that pinholes remain in the film during growth. Their density decreases with the film thickness and disappears above five monolayers. Upon annealing to 380 K, the pinholes in the thin layers grow to large, rectangular shaped pits with a mean size of about 10 nm. Eventually copper begins to migrate onto the cobalt film. The growth of the pits is attributed to the release of misfit strain in the cobalt layer. The relation between the film morphology and the magnetic anisotropy is discussed.

Discontinuities in finite-potential and gate-induced electron waveguides

Ballistic electron wave propagation through discontinuous finite-potential electron waveguides is analyzed. The wave function in each uniform waveguide section surrounding a discontinuity is represented as a sum of the local eigenmodes of that section, and both propagating and evanescent modes arising from bound and discretized continuum states are included in the expansion. The local eigenmodes are found using a transfer-matrix technique that allows the mode-matching across discontinuities to be calculated efficiently. A scattering matrix formalism is used to cascade multiple discontinuities and extract the current reflection and transmission characteristics. The method is shown to be accurate and efficient. The discontinuity analysis is applied to waveguides induced by split gates over two-dimensional electron gases. An analytical method is developed to approximate the potential energy profile produced by arbitrary-shaped gates as a function of gate voltage and material parameters. Simulation of a gate-induced constriction (quantum point contact) shows that the conductance as a function of gate voltage exhibits smooth quantized steps without oscillations in agreement with published experimental results.

Effect of suction on the stability of subsonic flows over smooth backward-facing steps

Mach numbers up to 0.8 are considered.As expected, suction considerably reduces the separation region.