Georg Rosenfeld

New phenomena in homoepitaxial growth of metals

The growth of Pt(111) by Pt vapour deposition is studied by He diffraction as a function of substrate temperature and deposition rate. At a deposition rate of about 2.5×10−2 monolayers/second several growth modes are observed: layer-by-layer (2D-) growth at 450 K≲Ts≲800 K, multilayer (3D-) growth at 340 K≲Ts≲450 K and reentrant layer-by-layer (2D-) growth at Ts≲340 K. The observed growth modes and in particular the reentrant 2D-growth are shown to be characteristic of growing Pt(111) under clean conditions, i.e. not influenced by contaminants. The influence of the intra- and interlayer mass transport on the growth mode is discussed in the light of experimental and simulation results. The 3D-growth mode is attributed to the existence of an activation barrier which suppresses the descent of adatoms from the top of the growing adatom islands onto the lower terraces. The barrier can be overcome by thermal adatoms at Ts≳450 K enabling interlayer mass transport which leads to 2D-growth. The reentrant 2D-growth occurs due to a break down of this barrier for small, irregularly shaped islands.

New concepts for controlled homoepitaxy

On the basis of a kinetic growth model we discuss new methods to grow atomically flat homoepitaxial layers in a controlled way. The underlying principle of these methods is to change the growth parameters during growth of an atomic layer in such a way that nucleation on top of a growing layer is suppressed, and thus, layer-by-layer growth is achieved. Experimentally, this can be realized by changing the substrate temperature or deposition rate during monolayer growth in a well-defined way. The same can be achieved at constant temperature and deposition rate by simultaneous ion bombardment during the early stages of growth of a monolayer, or by adding suitable surfactants to the system. Model experiments on Ag(111) and on Cu(111) using thermal energy atom scattering and scanning tunneling microscopy demonstrate the success of these methods.

Conventional and manipulated growth of Cu-Cu(111)

Molecular beam epitaxy of Cu on Cu(111) was studied using thermal energy He scattering, in the temperature range between 100 and 450 K. Three-dimensional growth was observed in the whole temperature range. To determine the onset of various diffusion processes, submonolayer films formed by deposition at low temperature were annealed. Annealing proceeds in two steps. The first step is interpreted as a change in island shape, the second as Ostwald-ripening. A comparison with homoepitaxy on Pt(111) and Ag(111) is made. Growth manipulation was carried out by artificially increasing the island number density via intervention in the nucleation stage of each layer. The procedures applied were temperature reduction during nucleation as well as pulsed ion bombardment. These techniques enabled the convenient growth of good quality films consisting of a large number of monolayers. Finally, the use of oxygen as a surfactant modifying the growth mode was investigated. Under some growth conditions, pre-exposure of the surface to oxygen was found to induce weak He-intensity oscillations during deposition. The quality of the films grown in this way was, however, low.

Local correlation during Ostwald ripening of two-dimensional islands on Ag(111)

Using two-dimensional Ag adatom islands on Ag(111) as a model system, we study the importance of local correlations in diffusion-limited Ostwald ripening. For the coverages studied (0.08, 0.21, and 0.3 ML), we find that the ripening can be surprisingly well described in a nearest neighbour model based on pairwise atom exchange between neighbouring islands. The results reveal the local nature of diffusion-limited Ostwald ripening and demonstrate the limits of the mean-field theories in describing experimental data.

The concept of two mobilities in homoepitaxial growth

A general kinetic concept is introduced which can be used to control growth modes in homoepitaxy. Its basic idea is that during growth of a layer, the characteristics length scale associated with nucleation is deliberately varied. The power of this concept lies in the fact that it can be realized experimentally in a variety of ways and is not restricted to special systems. It helps to understand various effects reported in the literature and may serve as a guideline for future methods of growth manipulation.

Kinetics of island diffusion on Cu(111) and Ag(111) studied with variable-temperature STM

The diffusion of vacancy islands on Cu(111) and Ag(111) and of adatom islands on Ag(111) has been studied using fast scanning STM. Diffusion of atoms along island edges (periphery diffusion) is much more effective in contributing to the diffusion of the islands than diffusion of atoms via terrace sites, under the conditions considered in this work. Assuming power-law scaling of the diffusion coefficient with the size of the islands, effective exponents, β, between 1.33 and 1.63 were determined. The effective diffusion barrier Ed was found to be 0.49 eV on Cu and between 0.51 and 0.53 eV on Ag, respectively. Activation energies for the diffusion of atoms along the island edges and for the breaking up of closed island edges (core breakup) were calculated using potentials derived from effective medium theory. Qualitative and quantitative comparisons between the measured and calculated activation energies for diffusion show that one cannot explain the island diffusion as being governed by one single rate-limiting process. The results are consistent with a mechanism for which both periphery diffusion and core breakup are rate-limiting for island diffusion.

On the shape of the in-phase TEAS oscillations during epitaxial growth of Pt(111)

The growth of Pt(111) from its vapour phase is investigated by means of TEAS (thermal energy atom scattering) in a wide range of substrate temperatures: 100-800 K. The evolution of the in-phase He specular peak height during Pt deposition is studied in particular. At higher substrate temperatures (T s >500 K), the in-phase peak height exhibits longlived temporal oscillations with a clearly asymmetric shape. This asymmetry, which increases with temperature, reveals substantial coarsening of the adatom islands during monolayer deposition, most likely due to Ostwald ripening processes.

Electron density contour smoothening for epitaxial Ag islands on Ag(100)

Rocking curves acquired with thermal energy He-atom scattering indicate that for small 2D Ag islands grown on Ag(100) at 200 K, the apparent step height normal to the surface is reduced by about 0.6 A compared to the step height calculated on the basis of bulk separation. For higher growth temperatures this effect is not observed. SPA-LEED measurements indicate that this behavior cannot be explained with a corresponding relaxation of island atoms. Rather, it may be attributed to a smoothening of the electronic corrugation (Smoluchowski effect), which is probed by the He atoms and which can be expected to occur for small and/or closely spaced islands. The dependence on the growth temperature indicates that this effect is, indeed, related to the size and proximity of the islands, being most pronounced for closely spaced islands consisting of a few tens of atoms only. These findings suggest that the difference between the morphology of the electron density contours, probed by methods which are sensitive to the density of valence electrons, such as atom scattering or scanning tunneling microscopy, and the morphology at the atomic core level is significant for small metal structures on metal surfaces.

Growth and Morphology of Ni Films on Cu(111)

Conventional and manipulated molecular beam epitaxy (MBE) of Ni on Cu(111) was studied by helium atom scattering and low-energy electron diffraction over a wide temperature range. During conventional MBE at low temperatures, three-dimensional rough growth is observed. The films consists of pyramids with {445} microfacets. At room temperature, growth leads to smoother films which consist of hexagonal structures. At high temperatures, the films partially grow via stepflow. To improve the film quality, growth manipulation methods, based on the concept of two mobilities, were applied. By temperature reduction or rate enhancement during nucleation and pulsed ion beam assisted deposition, the island number density was artificially increased and smoother growth of the first layers was promoted in most cases. However, temperature reduction during nucleation failed to promote better growth, which is explained by an unusual annealing behaviour of the nuclei. The morphology of films grown by manipulated and conventional MBE are compared.

STM-imaging of nanostructure dynamics on Ag(111)-experimental challenges and solutions

We describe experimental problems arising with the continuous observation of nanostructure dynamics by STM. We discuss the necessity to use a high-speed STM, possibilities to deal with the thermal drift, and tests to rule out the influence of the scanning process on the observation.