Mean Terrace Research Articles

Study of 2D growth stability of SrRuO3 on SrTiO3

In order to control the surface roughness and chemical termination of an oxide film, a promising route is to use a vicinal substrate and to control the atomic step positions during growth in the so-called step flow mode. The stability of this mode is studied in the case of SrRuO3 heteroepitaxy on SrTiO3, focusing on the influence of deposition flux and substrate mean terrace width. Transitions from unstable step bunching to stable step flow and from step flow to island nucleation are observed and discussed in terms of the mean step velocity. The step bunching is traced back to a strain-based driving force in competition with a stabilizing asymmetry for incorporation of adatoms at steps. A thin film obtained after stable step flow gives a greater finite size effect than a step-bunched one, as seen by X-ray diffraction

Step velocity tuning of SrRuO3step flow growth on SrTiO3

Taking advantage of vicinal (001) SrTiO3 substrates with different mean terrace widths, the heteroepitaxial growth of SrRuO3 in the step flow mode has been mapped as a function of mean step velocity. Transition between stable and unstable step flow is shown to occur at a well-defined critical step velocity, with a step bunching instability observed below this threshold. The ability to pass from unstable to stable step flow during growth upon increasing the mean step velocity is demonstrated. This result is discussed in terms of a stress-based driving force for step bunching in competition with an effective step-up adatom current.

Dissolution–precipitation recrystallization of miscut crystal surfaces under stress

We studied the coarsening evolution of step patterns on sodium chlorate miscut surfaces due to recrystallization under compressive stress in a saturated solution. The coarsening rates and pattern geometries were measured by digital image analyses of optical microscopy recordings. The average distance between the macrosteps ( λ) initially grew according to λ∼ t 1/4, but changed to λ∼ t 1/2 later in the experiments. The relatively rapid transition in coarsening rate was not accompanied by a sudden transition in pattern geometry, characterized by the number of step intersection points and terrace aspect ratios. The mean terrace aspect ratio continuously decreased, due to a gradual transition from relatively straight (perpendicular to principal stress) to more undulating steps. Possible causes of the observed transition in pattern evolution are: (i) a reduction of step line tension; (ii) an increasing variation in the surface stress distribution; and/or (iii) a change in growth morphology of sodium chlorate due to a decreasing driving force (supersaturation) of recrystallization.

High-energy resolution photoemission microspectroscopy: (1 1 1) domains featured by the Shockley and the image-potential states at a polycrystalline Cu surface

A photoemission microspectroscopy of 30 meV energy resolution and 0.3 μm lateral resolution has been applied to observe crystalline domains formed on a polycrystalline copper plate. Surface images based on the Shockley state specific to the (1 1 1) surface clearly showed many crystalline islands composed of the (1 1 1) surface. Similarly, surface images based on the unoccupied image-potential state were measured by employing two-photon photoemission spectroscopy. The high-energy resolution spectroscopy provides insight on the nm-scale surface morphology. The Shockley state was observed even on a poorly crystallized area, while the image-potential state was observed only on an area where mean (1 1 1) terrace length is sufficiently long.

Surface Study by Energy-Filtered RHEED-AES.

RHEED is one of the powerful tools for surface studies and monitoring thin film growth. RHEED pattern provides information on surface structures, surface morphology and thin film crystallography. Further more information including inelastic scattering can be obtained by installation of energy-filter in RHEED apparatus. We present a newly constructed energy-loss measurement system for diffracted electrons and demonstrate the experimental data obtained from Si(111)7&times7 surface. For the morphology of regular step surface, it is shown that the mean terrace length can be evaluated by the energy-filtered RHEED pattern of the vicinal Si surface. Reflected electron beams from a specimen surface into vacuum are usually concerned in conventional RHEED. On the other hand, it is shown that the incident electron density distribution (wave field) formed in a crystal surface is useful to clarify the causes of the complicated peaks in the rocking curve. This approach is also effective for surface structural study using Auger excitation at selected atomic rows.

Layer-by-layer growth of GaAs( [formula omitted]) studied by in situ synchrotron X-ray diffraction

We investigate the time-dependent surface evolution during molecular beam epitaxy of GaAs(0 0 1) using synchrotron X-ray diffraction at a newly-built dedicated beamline at the synchrotron BESSY II. The crystal truncation rods analyzed at growth temperature agree with the room-temperature β(2×4) reconstruction published in the literature. The layer coverage evolution during growth is analyzed by fitting the oscillating intensity along crystal truncation rods. Our results show that the structure of the reconstructed surface unit cell does not change during growth. Using numerical simulations, we determine the terrace size distribution on the surface at growth temperature and verify the validity of our analysis for multi-level initial surfaces, as long as the mean terrace size is larger than the nucleation distance during growth.

Growth of thin Co films on W( [formula omitted]) and Au( [formula omitted]) layers prepared on Al 2O 3( [formula omitted

Thin epitaxial W(1 1 0)/Co and Au(1 1 1)/Co films were prepared by molecular beam epitaxy on single crystal sapphire Al 2O 3(1 1 2 ̄ 0) substrates and investigated by means of low energy electron diffraction and Auger electron spectroscopy. The growth of W on Al 2O 3(1 1 2 ̄ 0) was found to depend strongly on substrate temperature and film thickness. W films deposited at a substrate temperature T p=1200 K with a film thickness d W⩾100 Å revealed a bcc(1 1 0) structure with a mean terrace size of 40 Å. Epitaxial growth of Au was achieved at T p=300 K with a layer thickness d Au=30 Å for a deposition onto the W(1 1 0) substrate layers. The structural properties of thin Co films grown at T p=300 K on the W(1 1 0) and Au(1 1 1) substrate layers were studied as a function of Co film thickness for wedge-shaped layers. The Co films were found to grow epitaxially in both cases with distinct growth modes and lattice strains.

Hexagonal growth spirals on GaN grown by molecular-beam epitaxy: Kinetics versus thermodynamics

GaN grown by molecular-beam epitaxy on Ga-polar GaN templates prepared by metal organic chemical vapor deposition shows a variety of morphologies that depend on defects and growth conditions. We measured the mean terrace widths of hexagonal growth spirals or hillocks versus ammonia and Ga fluxes and substrate temperature. The measurements were compared to a near equilibrium model of the growth. The results indicate that under excess Ga growth conditions, Ga-polar GaN(0001) has a mean step-edge energy of 0.27 eV/Å.

Surface Step Information of Vicinal Si(001) in EF-RHEED Pattern.

For a vicinal Si(001) 2×1 surface, a row of extra spots has been clearly observed along the reciprocal lattice rod in energy-filtered reflection high-energy electron diffraction (ER-RHEED) pattern. The glancing angle dependence of these extra spots along (0 0), (1/2 0) and (1 0) rods was simulated by kinematical analysis based on a monatomic regular step surface. Behavior of these extra spots was reproduced well by the calculations and the mean terrace length along the [110] direction was estimated. It has been found that this method can evaluate very small vicinal angle near 0.1o, which is superior to low energy electron diffraction (LEED) method in this point.

On the mechanism of rapid mound decay

The observation of rapid mound decay events on Ag(111) is reported. As in the case of Cu(111), the critical terrace width for the onset of the rapid decay corresponds to about six atom rows. For Ag(111), this distance is incompatible with the surface state model proposed earlier for Cu(111). A new mechanism for the rapid decay events is considered, which involves steps in close proximity. It is shown that the observed mean terrace width in the final, rapid decay of a mound is well described by a combination of the random walk and shape fluctuations of the islands with the proposed local decay mechanism. Approximate activation energies for the new process are determined for Cu(111) and Ag(111).

Thermal behavior ofc(2×2)layers in the Pb/Cu(110) system and the influence of surface defects on the order-disorder transition

The thermal behavior of a $c(2\ifmmode\times\else\texttimes\fi{}2)$ Pb overlayer on Cu(110) surface is studied by thermal energy atom scattering. Diffraction analysis reveals a continuous reversible order-disorder transition with temperature at $437\ifmmode\pm\else\textpm\fi{}2.5$ K. Two of the extracted critical exponents, $\ensuremath{\gamma}$ and $\ensuremath{\nu},$ are in good agreement with those of the two-dimensional Ising universality class. The third, $\ensuremath{\beta},$ associated with the disappearance of long-range order in the layer, is, however, smaller than the theoretical expectations. The observed transition is discussed in a framework of model, where the deviation of one of the exponents $\ensuremath{\beta},$ $\ensuremath{\gamma},$ and $\ensuremath{\nu}$ from its exact value depends in a different way on the kind of ``imperfection'' (mean terrace width, domain boundaries, ``frozen-in'' surface defects, local distortions, and surface alloying) in the real physical system. The annihilation of domain walls, created during the simultaneous growth of $c(2\ifmmode\times\else\texttimes\fi{}2)$ phase on two equivalent sublattices of ${$110$}$ fcc surface, is found at 310 K. The experimental findings indicate also a noticeable change in the surface corrugation during the observed order-disorder transition.

Structural changes of a Ru(0001) surface under the influence of electrochemical reactions

The structural properties of Ru(0001) surfaces after various electrochemical treatments were investigated by ex-situ electron diffraction [low-energy electron diffraction (LEED)/reflection high-energy electron diffraction (RHEED)] and Auger spectroscopy experiments in conjunction with cyclic voltammetry. At potentials of 0, 0.3 and 0.9 V s. Ag/AgCl in HClO4 solution, (2 × 2), (3 × 1) and (1 × 1)-O phases, respectively, are formed whereby the Ru surface remains smooth. On the other hand, an electrode previously roughened by argon ion bombardment becomes smoother (as reflected by a substantial increase of the mean terrace widths) by repeated cycles of CO adsorption and stripping. This effect of “electrochemical annealing” is attributed to the enhanced mobility of Ru surface atoms under the influence of the strong interaction with the CO adsorbate. Longer polarization (>1 min) at potentials above 1.1 V in HClO4 leads to the epitaxial growth of RuO2 islands on the Ru electrode surface. Oxidation/reduction cycles between −0.2 and 0.85 V in HClO4, on the other hand, lead to the formation of rough surfaces exhibiting the (1 × 1)-O phase, while the same procedure in the presence of CO causes transformation into the (2 × 2)-phase (with smaller O concentration) and the above-mentioned smoothing effect.

STEP–STEP INTERACTION ENERGY ON Cu(111) VICINAL SURFACES

We have measured the terrace width distribution on copper (111) vicinal surfaces with (100) steps. For all surfaces, the distribution is well fitted by a Gaussian. The width of the Gaussian scales with the mean terrace width L, which is indicative of a repulsive 1/L2 interaction potential between steps. We determine an interaction strength of the potential of 3.2± 0.2 meV per atom length. From the temperature-dependent analysis of the step–step distance distribution, we find a kink formation energy for (100) steps of 0.12± 0.03 eV, which is consistent with previous measurements of the kink energy on copper (100) vicinal surfaces.

Self-assembly of multilayer arrays from Ag nanoclusters delivered to Ag(111) by soft landing

Three-dimensional Ag nanostructures containing 150–600 000 atoms were created on Xe multilayers at 50 K. They were delivered to Ag(111), strained Ag(111), and Ag/Si(111)-( 3 × 3 )R30° following Xe desorption, and they were imaged with a scanning tunneling microscope after warming to room temperature. On Ag(111), they wetted the surface and produced hexagonal multilayer epitaxial islands that decayed via step–step contact while maintaining a constant mean terrace width of ≈11 Å. On strained Ag(111), there was wetting and an irregular hexagonal footprint but strain stabilized the multilayers by changing the energetics of atom exchange and step fluctuation. On Ag/Si(111)-( 3 × 3 )R30°, contacts were established but decay was not observed.

Analysis of terrace size fluctuations of B-induced stripe structures on the Si (001) surface

Terrace size fluctuations of B-induced stripe structures on Si (001) were analyzed by using scanning tunneling microscopy. The terrace size was varied independently of the substrate temperature by supplying diborane (B2H6). Histograms of the terrace size were fitted by Gaussian distributions and the standard deviation was proportional to the square root of the mean terrace size. These results indicate that the B-induced stripe structures arise from the competition between the stress anisotropy and step energy on Si (001), and mean that the surface anisotropy is almost independent of the B concentration.

High-resolution angle-resolved photoemission from Cu(610)

Cu(610) has a mean terrace width of six atoms. Although the regular arrangement of steps was verified by SPALEED, no step-induced surface umklapp process could be identified in UPS spectra taken with HeI radiation. This fact allows an easy comparison with results from Cu(100). In particular, we demonstrate that the final state wave vector can be determined by triangulation using exclusively the Cu(610) surface, since equivalent k-space points contribute at different electron emission angles. However, spectra from equivalent points exhibit differences in the UPS linewidth and this gives additional information on the final state lifetime. Finally we discuss the modification of a Tamm state [its analogue is observed at M ̄ on the (100) surface] due to confinement by the steps.

Measurement of Terrace Width Distribution on an Si(110) Surface Using High-Temperature Scanning Tunneling Microscopy

High-temperature scanning tunneling microscopy is used to study the statistical distribution of terrace width on an Si(110) surface at 720° C. It is confirmed that the terrace width is fitted by the Gaussian distribution, standard deviations (σ) of which are proportional to the mean terrace width. It is considered that the step-step interaction potential energy is Ax-2 (x: normal distance to the step edge and A: proportionality constant). As the mean value of the main peak is 19.27 nm and the step height 0.19 nm, it is confirmed that the surface is oriented within 1° from the (110) surface.

Competing effects of current and strain on step structures on Si(001)2 × 1 studied by REM

Competing effects of current and strain on the step structures on Si(001)2 × 1 surfaces have been studied by reflection electron microscopy (REM). A REM specimen holder which can apply surface strain of about 0.1% to surfaces of Si specimens of 1 × 5.5 × 0.15 mm 3 in size and can heat them by DC current fed through the specimens was newly constructed. A tensile strain along the 〈110〉 direction favors growth of terraces with dimers perpendicular to the strain direction irrespective of terrace width, while a compressive strain favors those with dimers parallel to the strain direction. Thus strain vs domain population (defined as relative occupancy of the two domain terraces) curves show asymmetry as have been reported by Webb et al. (Surf. Sci. 242 (1991) 23). On the other hand, a step-up current parallel to the 〈110〉 direction expands terraces with dimers parallel to the current if mean terrace width is wider than 0.1 μm. Thus, the competing two effects are expected on surfaces with wide terraces. It was found that the current apparently shifts the strain-domain population curves parallel to the strain axis depending on the direction of the current direction and the mean terrace width.

In-plane strained superlattices grown on slightly misoriented (1 1 0) InP substrates by molecular beam epitaxy

InAs/GaAs in-plane strained superlattices (IPSSLs) were grown by molecular beam epitaxy. The substrates used were misoriented (1 1 0) InP tilted in the [0 0 1] direction. We grew alternately half monolayers of InAs and GaAs while keeping the periodic single monolayer steps. The obtained structures were evaluated by transmission electron microscopy (TEM) and photoluminescence (PL) measurements. The TEM images revealed that the laterally periodic structures were formed almost in the [0 0 1] direction. The lateral period corresponded to the mean terrace width of the substrate. The PL peak energy obtained at 77 K was 0.77 eV, which was 40 meV lower than those of homogeneous InGaAs alloys. From the estimation of the transition energy dependence of In 1-x Ga x As/In x Ga 1-x As strained superlattices on the mixing ratio x, we deduced that the obtained IPSSL was composed of In 0.74 Ga 0.26 As/In 0.26 Ga 0.74 As. This composition was consistent with the measurement by energy-dispersive X-ray microanalysis.

Asymmetric peak broadening in photoemission of surface states as a consequence of lateral electron confinement

In addition to the inverse lifetime, the photoemission peak width of a surface state depends on the quality of the surface. For a stepped surface, in particular, there is an energy shift of the surface state owing to the confinement of electrons. A random distribution of steps causes an asymmetric peak broadening, which is examined as a function of the shift parameter, the inverse lifetime and the mean terrace width. The model is discussed for the Cu(111) sp-surface state for which an influence of steps on the peak shape is obtained particularly for mean terrace widths less than 100 Å.