Step edge selection during ion erosion of Cu(001)

Upon Ar+ ion erosion of a Cu(001) surface, we observed with HR-LEED faceted structures on the surface oriented preferentially along <100> at 200 K and along <110> at 330 K. These results are evaluated with a geometric rule, relating under kinetic conditions the orientation of the surviving steps to their advance rates. Application of this generic rule demonstrates that the <100> steps advance slowest. This reveals enhanced interlayer mass transport across these steps. We also show that a transition from kinetically favored step orientation into the thermodynamically favored one occurs at very short time scales even near room temperature.     

D(A) steps and 2D islands of double layer height in the SiGe(001) system

The surfaces of step graded, partially relaxed Si(1-x)Ge(x)/Si(001) buffers were studied by scanning tunneling microscopy. The surface slips along <110> forming the crosshatch pattern, consisting of bunches of D(A) steps of double layer height. The D(A) steps are present in regions of large surface gradients close to the slips, as well as in planar regions between the slips. These regions are also characterized by the appearance of 2D islands of double layer height. The observations can be explained by assuming the strain due to the misfit dislocations to be locally anisotropic. Anisotropic misfit strain and efficient strain relaxation by the ( 2x8) Ge reconstruction were identified as the main factors causing the unusual step structure.     

The structure and properties of the stepped surfaces of MgO and NiO

The static lattice computer simulation method has been used to study the structure and properties of high index faces of MgO and NiO. The (10_n) series of faces can be considered as stepped (001) surfaces and have been studied for both materials. In addition, the (403) and (302) faces which can be considered as stepped (101) surfaces have been studied for NiO. The calculated energies for steps on the (001) face are 3.72×10^?10 and 3.62×10^?10 J m^?1 for MgO and NiO respectively. The NiO energy also requires correction for the crystal field splitting. The energy of steps on the (101) surface is at least an order of magnitude lower. The interaction between the steps is repulsive but of short range. The large variation of surface energy with angle indicates that torque terms cannot be neglected in the analysis of thermal grooving experiments. The step structure is modified by substantial ionic displacements leading to an obtuse step angle. The structure is qualitatively similar for both NiO and MgO. The large distortions are likely to modify the step properties from those deduced by consideration of only the ideal geometry.     

Electronic and structural properties of steps on cleaved semiconductor surfaces

The work function of clean, cleaved p-Si(111) surfaces was measured in dependence of the density of cleavage steps. The contact potential difference CPD was observed first to increase then to decrease linearly with increasing step density. This dependence is caused by an increase in band bending, which saturates at a density of approximately 1 × 10⁶ steps per cm, and a decrease of the surface dipole which is proportional to the density of steps. Since the 2 × 1 reconstruction of the cleaved Si surface is ionic and buckled it increases the surface dipole moment compared with the 1 × 1 structure. Thus, a decrease of the surface dipole may be caused by removal of the 2 × 1 reconstruction on part of the step terraces. The experimental data give a width of about 28 Å for the unreconstructed stripe along the step edge.     

Site-specific reactivity of oxygen at Cu(110) step defects: an STM study of ammonia dehydrogenation

Defects on surfaces, such as steps, are known to influence catalytic activity considerably, but few microscopic observations have been made of such site-specific reactivity. Using atomic-resolution scanning tunneling microscopy (STM), we have observed the reactivity of oxygen adsorbed in the (2 × 1) structure at step sites on the Cu(110) surface with ammonia. It is found that the reactivity is high at both the top and bottom of a [11¯0] step and at the bottom of a [001] step, whereas almost no reaction occurs at the sites bordering the top of a [001] step. Reaction at the bottom of [001] steps appears to lead to deposition of copper atoms at the step edge.     

Hydrogen-induced reconstruction on a high step density W(001) surface

The hydrogen-induced reconstruction on a high step density W(001) crystal, ($\text{2}\text{×}\text{2}$)R45°-H, with steps oriented parallel to the [110] and ～ 28 Å average terrace width has been investigated using LEED symmetry, beam shape analyses, and EELS. The symmetry of the LEED pattern is observed to change from p2mg for the ($\text{2}\text{×}\text{2}$)R45° clean surface reconstruction to c2mm for the commensurate phase ($\text{2}\text{×}\text{2}$)R45°-H reconstruction. Correspondingly, the shapes of the half-order beams indicate that the hydrogen-induced reconstruction domains are much less elongated than the clean surface domains. A splitting of each half-order beam into four beams at higher exposures indicates the existence of two domains of the incommensurate phase. A commensurate phase v₁ vibrational loss peak centered at 160 meV in the EELS spectrum broadens on the low-energy side during the incommensurate phase and then shifts toward 130 meV and narrows as the (1×1)-H saturation structure develops. These observations imply that there is no long-range inhibition ( ～ 20 Å) to the formation of either commensurate or incommensurate phase; hydrogen induces a switching of the atomic displacements from 〈110〉 directions on a clean surface to 〈100〉 directions, even with steps oriented parallel to the [110]; and in the incommensurate phase there is a distribution of hydrogen site geometries with the most probable geometry more like the commensurate phase geometry than the saturation phase geometry.     

Multiple scattering calculations of step contrast in REM images of the Si(001) surface

Simulations of reflection electron microscopy (REM) images of both monolayer and bilayer steps on the bulk-terminated Si(001) surface, for the case when the primary electron beam azimuth is directed parallel to the line of the step, are presented. The simulations employ our previously reported theory of REM image formation which uses a 2D Bloch wave formulation of dynamical (multiple scattering) elastic RHEED theory to calculate diffracted amplitudes propagating from the surface. The only step contrast mechanism considered here is phase contrast and this is sufficient to produce the characteristic “black-white” appearance observed experimentally. Defocusing of the simulated images is also discussed.     

The effect of adsorbate-induced surface reconstruction on the apparent arrhenius parameters for desorption

The surface reconstruction described in terms of the second-order phase transition theory, is shown to result in a decrease in the apparent pre-exponential factor for desorption in comparison with the value predicted on the basis of the standard transition state theory provided that adsorbed particles suppress the phase transition. If adsorbed particles facilitate the phase transition, the reconstruction leads to an increase in the pre-exponential factor. The effect may be strong (of the order of 10^±4. The pre-exponential factors for CO desorption from the Ni(111), Ru(001) and Ir(110) surfaces are compared and discussed. The low value of the desorption pre-exponential factor for the latter surface is explained by the surface reconstruction.     

Surface diffusion of Pd and Au on W single crystal planes: II. Anisotropy of Pd surface diffusion due to the influence of substrate structure

Large anisotropy effects have been observed for the spreading of Pd on W single crystal planes exhibiting regular step structures. The spreading rate was studied as function of step density and step direction at temperatures around 1000 K. The terraces had always (110) orientation. Step densities as low as 3.3 × 10⁴cm^?1 corresponding to an average terrace width of 3000 Å already cause enhanced spreading rates parallel to the step direction. Steps parallel to the [001] direction showed the largest promoting action. The rate perpendicular to steps does not depend markedly on step density. The diffusion behaviour on the “step free” (110) plane turned out to be isotropic. A model is proposed which assumes the mass transport to occur in the second Pd layer. The increase of the diffusion rate along steps is correlated with the much larger density of diffusing atoms in step sites as compared to terrace sites due to the difference in binding energy.     

Pathway for the strain-driven two-dimensional to three-dimensional transition during growth of Ge on Si(001)

The two-dimensional (2D) to three-dimensional (3D) morphological transition in strained Ge layers grown on Si(001) is investigated using scanning tunneling microscopy. The initial step takes place via the formation of 2D islands which evolve into small ( approximately 180 A) 3D islands with a height to base diameter ratio of approximately 0.04, much smaller than the 0.1 aspect ratio of 105-faceted pyramids which had previously been assumed to be the initial 3D islands. The "prepyramid" Ge islands have rounded bases with steps oriented along <110> and exist only over a narrow range of Ge coverages, 3.5-3.9 monolayers.     

Self-organized in-plane incorporation of Si atoms in GaAs by molecular beam epitaxy

The preferential attachment of Si atoms at misorientation steps on vicinal GaAs(001) surfaces has been studied by RHEED. By analysing the time evolution of the specular beam intensity and the change in surface reconstruction during Si deposition we show that a self-organized Si incorporation along the step edges takes place. The observed (3×2) structure is due to an ordered array of dimerized Si atoms with missing dimer rows. Taking into account the structure of the (3×2) unit mesh and its orientation with respect to the As-terminated or Ga-terminated steps, a characteristic minimum in the RHEED intensity recording corresponds to the number of Ga step-edge sites. Since the preferential path for Ga as well as for Si adatom diffusion is along the [110] direction, the critical terrace width for wirelike Si attachment is much larger for a misorientation toward (111)As than for a misorientation toward (111)Ga. Despite the high local impurity concentration, the Si-modified surface can be overgrown with GaAs without adverse effects on the growth front. This is promising for the fabrication of doping wires.     

Reconstruction, disordering and roughening of metal surfaces

Until recently, surface reconstruction and roughening have been studied as separate surface phenomena and have been described theoretically by means of separate models. This approach is reasonable when the deconstruction transition has a displacive character as occurs for example on Mo(001) and W(001) surfaces. In these cases only in-plane degrees of freedom are involved in the reconstruction process. Conversely, a unified approach to reconstruction and roughening seems more appropriate when the reconstructed phase is produced by the low-temperature ordering of a large concentration of point defects (vacancies, adatoms) or extended defects (steps). The reconstruction in these cases involves off-plane degrees of freedom. Since roughening derives from the proliferation of thermally excited steps, the interplay between steps and deconstructive defects may provide the connection between the two transitions. A notable example of these latter systems is provided by the (110) surface of noble and near-noble metals; the (110) surface of the heavier metals (Au, Pt) reconstructs at low temperature in the 2 x 1 missing-row structure. In the present article we review some statistical mechanics models able to display both deconstruction and roughening of the fcc(110) missing-row phase, chosen as a paradigmatic example of the interplay between the two transitions. The properties of the phase between deconstruction and roughening, produced by the different models are analysed, and compared with the results of molecular-dynamics simulations with continous potentials. The fingerprint of the different phases in scattering measurements are discussed and compared with the experimental data available.     

Williamson–Hall study on synthesized nanocrystalline tungsten carbide (WC)

Ag clusters obtained by the gas-aggregation technique were deposited on the cleavage face (001)NaCl. The average size of the nanoparticles was varied in the range from 0.2 to 10 nm and the temperature was varied from room temperature to 470 K. Owing to the mobility of the clusters on the substrate, they coalesced with formation of larger particles, decorating atomic cleavage steps on (001)NaCl already at room temperature. It was shown that the step decoration is possible only when the average size of the deposited clusters is commensurable with the height of the cleavage steps.     

Surface structure of bismuth terminated GaAs surfaces grown with molecular beam epitaxy

We determine the atomic surface structure of the Bi-terminated GaAs(001) (1 × 3) reconstruction for the first time using scanning probe microscopies, photoemission spectroscopy, and ab initio calculations. The proposed kinked-dimer (4 × 3) model is consistent with experimental characterization and can accommodate a variety of species configurations due to an availability of low-energy sites for Bi substitution, accounting for the significant observed local disorder. In addition, experiments show that stability of this reconstruction coincides with a dramatic change in surface step morphology, giving rise to strong up/down step interaction and a counterintuitive smoothing effect on the micrometer length scale.     

The adsorption and reconstruction of strong electron acceptor tetracyanoethylene (TCNE) on Si(001)-(2 × 1): A density functional theory investigation

The adsorption and reconstruction of strong electron-acceptor TCNE on the Si(001)-(2 × 1) surface are investigated by density functional theory (DFT). The results show that TCNE prefers to adsorb on the trench between two adjacent dimer rows and the CC double bond is parallel to dimer rows. Charge density difference calculation and Bader charge analysis indicate that abundant negative charge transfers from dimer Si to TCNE. Strong interaction makes it difficult for TCNE to move on Si(001), which is confirmed by nudged elastic band (NEB) analysis. In addition, the correlations between simulated STM images and molecular orbitals are discussed and two surface reconstructions of (2 × 1) and (4 × 2) are predicted at different TCNE coverages.     

Kinetic step bunching instability during surface growth

We study the step bunching kinetic instability in a growing crystal surface characterized by anisotropic diffusion. The instability is due to the interplay between the elastic interactions and the alternation of step parameters. This instability is predicted to occur on a vicinal semiconductor surface Si(001) or Ge(001) during epitaxial growth. The maximal growth rate of the step bunching increases like F4, where F is the deposition flux. Our results are complemented with numerical simulations which reveal a coarsening behavior in the long time evolution for the nonlinear step dynamics.     

Els and leed study of CO adsorption on Pd(001) and Pd(001) 6° [110]

LEED studies indicate that both CO and carbon adsorb differently on Pd(001) surfaces with and without steps. However, electron energy loss spectra for Pd(001) samples with and without ordered steps show no detectible surface structural sensitivity to CO adsorption. For both samples an additional loss feature at 13.5 eV appears upon CO adsorption. This feature is identified as a (1π?5σ) to 2π^★ type intramolecular electronic excitation, and it appears to be a universal feature of molecular CO adsorption on metals. A second loss expected in the 6–7 eV range associated with a d to 2π^★ type charge-transfer excitation was not detected. Either the transition probability is low for this excitation or hybridization with surface plasmons may obscure its identification.