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

Optical anisotropy induced by ion bombardment of Ag(001)

Grazing incidence ion bombardment results in the formation of nanoripples that induce an anisotropic optical reflection The evolution of the reflectance anisotropy has been monitored in situ with reflectance anisotropy spectroscopy. The Rayleigh-Rice theory (RRT) has been used to analyze the optical spectra quantitatively and provides the evolution of the average ripple period and root-mean-squared surface roughness. After an incipient phase, both the increase in the periodicity and the roughness vary roughly with the square root of the sputter time. Additional high-resolution low-energy electron diffraction (HR-LEED) measurements have been performed to characterize details of the average structure created by ion bombardment

Hydrogen adsorption configurations on Ge(001) probed with STM

The adsorption of hydrogen on Ge(001) has been studied with scanning tunneling microscopy at 77 K. For low doses (100 L) a variety of adsorption structures has been found. We have found two different atomic configurations for the Ge-Ge-H hemihydride and a third configuration that is most likely induced by the dissociative adsorption of molecular hydrogen. The Ge-Ge-H hemihydride is either buckled antiparallel or parallel to the neighboring Ge-Ge dimers. The latter configuration has recently been predicted by M. W. Radny et al. [J. Chem. Phys. 128, 244707 (2008)], but not observed experimentally yet. Due to the presence of phasons some dimer rows appear highly dynamic

Direct determination of the step-edge formation energies of the energetically stable and unstable double-layer step edges of Si(001)

Scanning tunneling microscopy images of 4.5° misoriented double B stepped Si(001) have been analyzed to determine the double-layer step-edge formation energies of the energetically stable double step (B-type) as well as the energetically unstable double step (A-type). The ordering of the various single- and double-layer step-edge formation energies is in accordance with semiempirical tight-binding-based total-energy calculations performed by Chadi [Phys. Rev. Lett. 59, 1691 (1987)]. Finally, the miscut angle at which the transition between the single- and double-layer stepped surface occurs as calculated using the experimentally obtained step-edge formation energies is in agreement with the experiment

He Scattering from Compact Clusters and from Diffusion-Limited Aggregates on Surfaces: Observable Signatures of Structure

The angular intensity distribution of He beams scattered from compact clusters and from diffusion limited aggregates, epitaxially grown on metal surfaces, is investigated theoretically. The purpose is twofold: to distinguish compact cluster structures from diffusion limited aggregates, and to find observable {\em signatures} that can characterize the compact clusters at the atomic level of detail. To simplify the collision dynamics, the study is carried out in the framework of the sudden approximation, which assumes that momentum changes perpendicular to the surface are large compared with momentum transfer due to surface corrugation. The diffusion limited aggregates on which the scattering calculations were done, were generated by kinetic Monte Carlo simulations. It is demonstrated, by focusing on the example of compact Pt Heptamers, that signatures of structure of compact clusters may indeed be extracted from the scattering distribution. These signatures enable both an experimental distinction between diffusion limited aggregates and compact clusters, and a determination of the cluster structure. The characteristics comprising the signatures are, to varying degrees, the Rainbow, Fraunhofer, specular and constructive interference peaks, all seen in the intensity distribution. It is also shown, how the distribution of adsorbate heights above the metal surface can be obtained by an analysis of the specular peak attenuation. The results contribute to establishing He scattering as a powerful tool in the investigation of surface disorder and epitaxial growth on surfaces, alongside with STM.Comment: 41 pages, 16 postscript figures. For more details see http://www.fh.huji.ac.il/~dan

Influence of dimer buckling on dimer diffusion: A scanning tunneling microscopy study

The diffusion of Ge dimers along the substrate dimer rows of Ge(001) has been investigated with scanning tunneling microscopy. The jump frequency of on-top Ge dimers along symmetric dimer rows at room temperature is found to be eight times higher than the diffusion along asymmetric dimer rows (0.36 s–1 versus 0.044 s–1). We ascribe this difference to limitations associated with the rocking motion that a dimer has to perform while diffusing along asymmetric dimer rows

Anomalous Decay of Electronically Stabilized Lead Mesas on Ni(111)\ud

With their low surface free energy, lead films tend to wet surfaces. However, quantum size effects (QSE) often lead to islands with distinct preferred heights. We study thin lead films on Ni(111) using low energy electron microscopy and selected area low energy electron diffraction. Indeed, the grown lead mesas show distinct evidence for QSE’s. At about 526 K metastable mesas reshape into hemispheres within milliseconds, driven by a huge reduction in interfacial free energy. The underlying diffusion rate is many orders of magnitude faster than expected for lead on bulk lea

Dynamics and Energetics of Ge(001) Dimers

The dynamic behavior of surface dimers on Ge(001) has been studied by positioning the tip of a scanning tunneling microscope over single flip-flopping dimers and measuring the tunneling current as a function of time. We observe that not just symmetric, but also asymmetric appearing dimers exhibit flip-flop motion. The dynamics of flip-flopping dimers can be used to sensitively gauge the local potential landscape of the surface. Through a spatial and time-resolved measurement of the flip-flop frequency of the dimers, local strain fields near surface defects can be accurately probed

Channeling in helium ion microscopy: Mapping of crystal orientation

Background: The unique surface sensitivity and the high resolution that can be achieved with helium ion microscopy make it a\ud competitive technique for modern materials characterization. As in other techniques that make use of a charged particle beam, channeling\ud through the crystal structure of the bulk of the material can occur.\ud Results: Here, we demonstrate how this bulk phenomenon affects secondary electron images that predominantly contain surface\ud information. In addition, we will show how it can be used to obtain crystallographic information. We will discuss the origin of\ud channeling contrast in secondary electron images, illustrate this with experiments, and develop a simple geometric model to predict\ud channeling maxima.\ud Conclusion: Channeling plays an important role in helium ion microscopy and has to be taken into account when trying to achieve\ud maximum image quality in backscattered helium images as well as secondary electron images. Secondary electron images can be\ud used to extract crystallographic information from bulk samples as well as from thin surface layers, in a straightforward manner