Substrate Terraces Research Articles

Evidence of anisotropic structures of free-standing porous silicon films

Optical and secondary electron beam images of free-standing porous silicon films reveal a pronounced layered structure. Films tend to split into sheets parallel to the 〈111〉 plane of the initial substrate and form steps and terraces at the broken edge of the film. We have investigated these layers by scanning and transmission electron as well as optical microscopy. We have not observed any significant differences in the microstructure of each layer. Film layering in the cross-section view, caused by these terraces, has a periodicity of about 1 μm. The origin of this effect is discussed and could arise from the electrochemical etching regime used, vertical inhomogeneities in the starting material or mechanical stress-related effect arising from electrolyte evaporation. © 1997 Elsevier Science S.A.

Substrate-mediated control of the microstructure of YBa 2Cu 3O 7−δ thin films

We have studied the surface of vicinal SrTiO 3(001) substrates and thereon deposited YBa 2Cu 3O 7−δ (YBCO) thin films by UHV-scanning tunneling microscopy (STM). We discuss the influence of the uniformity of substrate terraces on film growth. STM images indicate, that a combination of island and step-flow growth occurs on 1.2° miscut SrTiO 3. In particular, a regular nanoscale step structure of a 10° miscut SrTiO 3 surface generates an almost periodic surface structure of the YBCO films. Our findings indicate that a substrate-mediated control of the surface structure of YBCO films can be achieved.

Surface morphology and nucleation of infinite-layer (Sr,Ca)CuO 2 films on atomically flat SrTiO 3 (100) substrates studied by atomic force microscopy

The surface morphology and nucleation of molecular beam epitaxy-grown ‘infinite-layer’ structure, (Sr,Ca)CuO 2 films on atomically flat SrTiO 3 are studied by reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM). The SrTiO 3 (100) surfaces were prepared by an ultra-high vacuum (UHV) annealing and displayed regular arrays of 0.4 nm steps spaced by atomically flat terraces of about 400 nm. The terraces exhibited smooth edges which were parallel to the [110] substrate direction. AFM images of only a two unit cell thick (0.66 nm) (Sr,Ca)CuO 2 layers were recorded. Edge roughness of the substrate terraces was found to be affected by the growth process. Atomically flat (Sr,Ca)CuO 2 islands with the vertical steps of multiple heights of one unit cell were identified. The obtained AFM and RHEED data strongly suggested the island to step-growth mode of ‘infinite-layer’, (Sr,Ca)CuO 2 films grown on atomically smooth (100) SrTiO 3 surfaces. Different surface morphology of thicker (Sr,Ca)CuO 2 films grown with other Sr Ca nominal compositions were also investigated by AFM.

Finite size effects in nucleation processes

Abstract We have used Monte Carlo simulations to study the scaling relations describing submonolayer nucleation N ∝ F y D x θ z as a function of system size L (where N is the nucleated island density, D the diffusion coefficient, F the flux rate, θ the coverage). For island size distributions that obey scaling, the island density N can be easily related to the step density, S D . A simple model of critical size cluster i c = 1 is used. For a given system size L there is a minimum ( D F ) min ratio where scaling breaks down with x = y = 0. These considerations are relevant to systems with low terrace diffusion barrier or small substrate terraces.

RHEED studies of interlayer diffusion in the submonolayer growth regime on

Abstract We have used reflection high-energy electron diffraction (RHEED) to study the growth of Ag Ag (111) as a function of temperature T = 150–315 K and flux rate F = 1 125 – 1 4800 ML / s by monitoring the decay of the specular beam intensity. No oscillations are observed, which is consistent with a 3D growth mode and the existence of a step edge barrier. The intensity decay is independent of flux rate but depends on temperature. This can be understood in terms of the high diffusion rate on the finite substrate terraces ( ∼ 500 A ), which results in breakdown of the scaling conditions for nucleation and interlayer diffusion from higher to lower terraces.

XPS and STM Study of Passive Films Formed on Fe‐22Cr(110) Single‐Crystal Surfaces

X‐ray photoelectron spectroscopy and ex situ scanning tunneling microscopy measurements have been combined to investigate the thickness, the chemical composition, and the structure of passive films formed in 0.5 M on Fe‐22Cr(110). Aging under polarization at +500 mV/SHE causes a dehydration (anodic) reaction of the outer chromium hydroxide layer of the passive film. This anodic reaction results in a thickening of the inner mixed Cr(III) and Fe(III) oxide layer enriched in . It also causes a coalescence of the oxide nuclei of the passive film and a crystallization of the inner oxide layer in epitaxy with the substrate. The epitaxial relationship is with three different azimuthal orientations. Aging under polarization is beneficial to the stability of the passive film in air and prevents the dehydration reaction of the hydroxide coupled to an oxidation reaction of iron which are observed on freshly passivated surfaces. Polarization at +700 mV/SHE activates the dissolution of substrate terrace atoms whereas polarization at +300 or +500 mV/SHE activates the dissolution of substrate atoms at step edges only.

In-situ STM investigation of Tl and Pb underpotential deposition on chemically polished Ag(111) electrodes

In-situ STM was used to investigate the adlayer formation and structure of Tl adsorbates at chemically polished Ag(111) electrodes in perchlorate electrolyte, and to study the effect of extended polarization of incomplete Tl and Pb adlayers at high initial adsorbate coverage. In the Tl+/Ag(111) system, the full first monolayer adsorbate has a hexagonally close packed structure with interatomic distances that are compressed in comparison with the bulk Tl phase. The subsequently formed second monolayer has also a hexagonally close packed (hcp) structure, but shows more pronounced disordered domains. Incomplete first Tl monolayers of high coverage have the same hcp structure as a complete monolayer, but do not extend to the very boundary of the substrate terrace. Extended polarization of such monolayers leads to a conversion of the original hcp layer into a coverage with [√3 × √3]R30 °—symmetry which is interpreted as a mixed coverage, where every third Ag atom is replaced by a Tl atom. Similar phenomena are observed during long-time polarization of an incompletely formed Pb-coverage. The results are discussed with regard to the role of the atomic steps and reveal the importance of surface singularities for the understanding of the adsorption behaviour at real electrodes.

An STM study of C 60 adsorption on Si(100)-(2 × 1) surfaces: from physisorption to chemisorption

The adsorption of C 60 molecules on Si(100)-(2 × 1) surfaces before and after annealing has been studied using scanning tunneling microscopy (STM). For room temperature deposition, the STM images reveal that the C 60 molecules adsorb predominantly at the four-dimer sites in the troughs between dimer rows. C 60 molecules bonded to two-dimer sites are also observed in small molecular clusters forming even at a low coverage. The nature of the interaction between the adsorbates and the Si(100)-(2 × 1) surface is explained in terms of a dipole-induced dipole interaction. Subsequent annealing of the samples to 600°C changes the adsorption characteristics of the C 60 molecules. First, the annealing causes a strong covalent bonding between the carbon atoms of the C 60 molecules and the silicon atoms of the substrate, and modifies the bonding sites of the adsorbates from locations in the troughs to locations on the dimer rows. Second, the annealing causes some initial surface diffusion and clustering of the C 60 molecules, which now tend to stick to the ends, rather than the sides, of the dimer rows. Furthermore, after the annealing process, it is observed that small silicon islands form on the substrate terraces along with isolated and clustered adsorbates.

Antiphase boundaries as nucleation centers in low-temperature silicon epitaxial growth.

Abstract : The epitaxial growth of silicon on Si(001) from disilane at 720 K has been investigated using scanning tunneling microscopy. Epitaxy occurs by island nucleation and growth, with islands nucleating preferentially at antiphase boundaries between different regions on the substrate terraces. These islands begin as a single dimer string which grows to cover the entire antiphase boundary before lateral growth begins. The islands increase in size until they meet on the surface, forming new antiphase boundaries (50% of the time) for nucleation of the next layer. We find that islands nucleated at such boundaries account for approximately 94% of the area of a growing layer, indicating that essentially all epitaxial growth at this temperature occurs by this mechanism.

Symmetry and stability of solitary dimer rows on Si(100).

Metastable, monolayer-high structures as narrow as one dimer row and as long as the substrate's terrace width decorate antiphase boundaries on Si(100) following 225 eV Xe bombardment. On the basis of tunneling microscope STM observations and first-principles calculations, we propose that the bonding toplogy of a buckled dimer row on an antiphase boundary is responsible for the metastability of this ``zipper'' structure, and in turn for the preferential nucleation of dimer rows on antiphase boundaries during growth.

Epitaxy of Germanium on SI(001) Grating Templates

ABSTRACTEpitaxial Ge films (< 3 ML) have been grown at elevated temperatures on Si (001) grating substrates (repeat spacing of 2.0 μm) and imaged using room temperature scanning tunneling Microscopy (STM). The Ge films exhibit the 2×n reconstruction associated with missing dimer rows. The value of n and the growth morphology are influenced by the deposition rate and by annealing. At substrate temperatures of 600° C and deposition rates >0.5 ML/Min., islands elongated along the the dimer row direction nucleate at steps and on terraces. With sufficient annealing at 800° C, the islands coarsen and are eventually eliminated. The roughness of the A-type step becomes greater than that of the B-type step, which is the reverse of the situation with pure Si (001). The separation between missing dimer rows and hence the value of n are increased by annealing. Differences in substrate terrace widths due to the periodically varying step density of thegratings affect the growth Modes: two-dimensional islands occur near the extrema of the gratings whereas step flow occurs when steps are separated by ∼150 Å or less.

Scanning tunneling microscopy and scanning electron microscopy observations of the early stage of silver deposition on graphite single crystal electrodes

The early stages of Ag overpotential deposition on highly oriented pyrolytic graphite (HOPG) from Ag+-ion-containing acid solutions have been studied by ex situ scanning tunneling microscopy and scanning electron microscopy imaging complemented with electrochemical, energy dispersive X-ray analysis, and Auger electron spectroscopy data. Nucleation and 3D Ag growth initiate at surface defects. Unstructured 3D Ag nuclei decorating HOW steps and flat geometric Ag islands are formed around the steps. The island structure is compatible with a local layer-by-layer growth. The entire morphology of the Ag deposit is consistent with a growth mechanism involving Ag atom diffusion from 3D nuclei at step edges toward Ag flat domains. Images with atomic resolution reveal large uncovered HOPG areas and Ag submonolayer domains with the nearest-neighbor distance d = 0.33 f 0.02 nm, whereas flat Ag islands exhibit d = 0.29 f 0.02 nm as expected for the nearest-neighbor distance in the Ag lattice. A model for these structures is discussed. The interest in the very early stages of metal electrodeposition both in the underpotential (upd) and overpotential (opd) regions encourages further investigations to demonstrate the validity of growth models and phase transitions operating at different stages of the process.' The rapid development in the applioation of Scanning tunneling microscopy (STM) to electrochemical systems involving well-defined substrates under different operating conditions allows the examination of growth processes from the early formation of a single monolayer to deposition equivalent to several atomic layers. Despite the fact that there are still no universally accepted criteria concerning the optimum STM operating mode in electrochemistry, a number of very interesting results and conclusions concerning the el~~tallization of metals have been reported in recent Investigations of the upd of metals with atomic resolutionH and the early stages of Cu opd on Au single-crystal electrodes have been made using STM.7 For upd of metals, ordered overlayers have been reported from STM and AFM imaging.3 These overlayers exhibit open structures with packaging densities which depend on the anion prtsent in the electrolyte solution. A partial charge transfer from anions to metal adatoms has been proposed to explain the experimental results. It should be noted that due to the difference in electronegativities between substrate and metal adatoms, the latter remains with a small positive chargeas STM observations indicate that the initial opd of metals occurs at step edges, dislocations, and defects in the substrate surface. Subsequently, the deposition of metals takes place on flat substrate terraces, the rate of growth of the deposit being far faster in the x- and y-directions than it is in the z-directi~n.~ For Cu electrodeposition on Pt, it has been concluded that a 2D layer-by-layer growth mechanism does not account for the process, although in this case the shape of Cu crystallites is not well-defined at the early stages of growth, it changes into a better defined morphology at later stages.

FI-STM study of hydrogen adsorption on Si(100) surface

Chemisorption of atomic hydrogen on the Si(100)2 × 1 surface has been investigated in detail by using a field ion-scanning tunneling microscope (FI-STM). The results showed that the adsorption geometry changed from the 2 × 1 monohydride phase to the 1 × 1 dihydride phase with increasing exposure of hydrogen. The data of desorption of the hydrogen-saturated Si surface showed that on annealing at 670 K the surface becomes highly disordered: the 1 × 1 dihydride structure is eliminated and the 2 × 1 reconstructed monohydride is also hardly to identify. When the temperature rises to as high as 730 K, the surface is dominated by the 2 × 1 structure with missing dimer rows, and some adatom chains occur on the Si substrate terraces. We attribute the formation of these atomic chains to an epitaxial growth of Si atoms which are formed by the dissociation of SiHx (x = 1, 2, 3 or 4) compounds on the Si surface.

In situ scanning tunneling microscopy as a probe of adsorbate-induced reconstruction at ordered monocrystalline electrodes: carbon monoxide on platinum(100)

Abstract : The atomic level structure of ordered Pt(100) electrodes prepared by airhydrogen flame annealing followed by iodine dosing and solution-phase replacement by CO has been examined by in-situ scanning tunneling microscopy. While large (1 x 1) substrate terraces are discernable in the presence of iodine adlayers, subsequent CO adsorption in 0.1 M HC104 yields dense arrays of smaller, 20-40 angstrons (1 x 1) islands. These domains coalesce upon positive alterations in the electrode potential. Parallel in-situ infrared spectroscopic measurements shows that these structural changes are accompanied by an attenuation of bridge-bound CO in the close packed adlayer structure. The bridging CO is speculated to be present at the edges of the (1 x 1) domains, contained within which are predominantly atop coordinated CO.

Double Crystal X-Ray Diffraction Measurement of a Triclinicly Distorted and Tilted AlxGa1-xAs Unit Cell Produced by Growth on Offcut GaAs Substrates

ABSTRACTCorrections are required for double crystal X-ray diffraction characterization of epitaxial AlxGa1-xAs layers grown on offcut GaAs (100) substrates. Double crystal X-ray diffraction measurements show that the cubic film unit cell defined by Vegard's law is triclinicly distorted and tilted with respect to the substrate unit cell. The distortion and tilt angles oppose each other defining a crystal geometry where the substrate and film <100= axes remain approximately coplanar with the surface normal. This film/substrate crystal geometry leads to formulation of a model describing heteroepitaxy on offcut (100) substrates. When film atoms are bonded to an offcut substrate, the already tetragonaly distorted film unit cell is subjected to additional cell distortions. The magnitude of this additional strain depends on where the film atoms are positioned on a substrate terrace. The first few layers of film atoms establish swain grades across individual substrate terraces. Constrained by the geometry of this interface region and driven by strain relaxation in the net growth direction, subsequent heteroepitaxy forms the measured film/substrate crystal geometry.

Monte Carlo simulation of the growth of a Cu(100) surface from its own vapor; island nucleation and step propagation growth modes

The temporal oscillations in the specularly scattered intensity of thermal energy helium scattering experiments while a Cu(100) surface is growing from its vapor have been reproduced by Monte Carlo simulations involving only parameters directly obtained from experiments for a variety of surface temperatures. The results allow us to gain insight into the different growth modes of the film (three-dimensional, formation of two-dimensional islands on the substrate terraces and step propagation) and on the temporal variations that they cause in the scattered intensity.