Surface segregation of Ge at SiGe(001) by concerted exchange pathways

The segregation of Ge during growth on SiGe(001) surfaces was investigated by ab initio calculations. Four processes involving adatoms rather than ad-dimers were considered. The two most efficient channels proceed by the concerted exchange mechanism and involve a swap between an incorporated Ge and a Si adatom, or between Si and Ge in the first and the second surface layers, respectively. The calculated activation energies of approximately 1.5 eV explain well the high-temperature experimental data. Segregation mechanisms involving step edges are much less efficient.     

Structural phase transition on Si(001) and Ge(001) surfaces

Among a variety of solid surfaces, Si(001) and Ge(001) have been most extensively studied. Although they seem to be rather simple systems, there have been many conflicting arguments about the atomic structure on these surfaces. We first present experimental evidence indicating that the buckled dimer is the basic building block and that the structural phase transition between the low-temperature c(4x2) structure and the high-temperature (2x1) structure is of the order-disorder type. We then review recent theoretical work on this phase transition. The real system is mapped onto a model Ising-spin system and the interaction parameters are derived from total-energy calculations for different arrangements of buckled dimers. The calculated critical temperature agrees reasonably well with the experimental one. It is pointed out that the nature of the phase transition is crucially affected by a small amount of defects on the real surfaces.     

Embedded Co islands on Ge(001)

The adsorption of Cobalt (Co) on Germanium (Ge) (001) surfaces has been studied using scanning tunneling microscopy. Upon annealing at temperatures of 500–550 K well-ordered rectangular shaped embedded islands are formed. Based on our scanning tunneling microscopy data we propose that the elementary building block of these embedded islands consist of six Co atoms arranged in a hexagonal pattern. A statistical analysis reveals that the embedded Co islands exhibit an attractive interaction in a direction perpendicular to the substrate dimer rows and a repulsive interaction in a direction along the substrate dimer rows. The embedded Co islands eventually convert to perfectly straight and micrometers long nanowires upon annealing at temperatures that exceed 700–750 K.     

Cobalt induced nanocrystals on Ge(001)

The deposition of several monolayers of cobalt on germanium (001) substrates results in the formation of two types of clusters: flat-topped and peaked nanocrystals. Scanning tunneling spectroscopy and helium ion microscopy measurements reveal that these nanocrystals contain cobalt. The shape evolution of the flat-topped and peaked nanocrystals as a function of their size is investigated with scanning tunneling microscopy. For small sizes the nanocrystals are compact. Beyond a critical size, however, the peaked nanocrystals exhibit an elongated shape, whilst the flat-topped nanocrystals remain compact. The shape transition of the peaked nanocrystals is driven by a competition between boundary and strain energies. For small sizes the boundary energy is the dominant term leading to a minimization of the peaked nanocrystal's perimeter, whereas at larger sizes the strain energy wins resulting in a maximization of the perimeter. On the top facet of the flat-topped nanocrystals one-dimensional structures are observed that are comprised of small square shaped units of about 1 nm². Time-resolved scanning tunneling microscopy measurements reveal that these square shaped units are dynamic at room temperature.     

Second-harmonic spectroscopy of Ge/Si(001) and Si1-xGex(001)/Si(001)

0.9 Ge_0.1(001)/Si(001) films with SH photon energies 3.1<2hν<3.5 eV near the bulk E₁ critical point of Si(001) or Si_0.9Ge_0.1(001). Ge was deposited on Si(001) by using atomic layer epitaxy cycles with GeH₄ or Ge₂H₆ deposition at 410 K followed by hydrogen desorption. As Ge coverage increased from 0 to 2 monolayers the SH signal increased uniformly by a factor of seven with no detectable shift in the silicon E₁ resonant peak position. SH signals from Si_0.9Ge_0.1(001)/Si(001) were also stronger than those from intrinsic Si(001). Hydrogen termination of the Si_0.9Ge_0.1(001) and Ge/Si(001) surfaces strongly quenched the SH signals, which is similar to the reported trend on H/Si(001). We attribute the stronger signals from Ge-containingsurfaces to the stronger SH polarizability of asymmetric Ge-Si and Ge-Ge dimers compared to Si-Si dimers. Hydrogen termination symmetrizes all dimers, thus quenching the SH polarizability of all of the surfaces investigated. Received: 13 October 1998 / Revised version: 18 January 1999     

Dynamics of Au-induced nanowires on Ge(001)

We have studied the dynamics of Au-induced nanowires on Ge(001) using scanning tunnelling microscopy. The ridges of these nanowires consist of buckled dimers that have their dimer bond aligned in a direction perpendicular to the nanowire direction. Neighbouring dimers have a preference to buckle in an opposite direction, leading to rows with a zigzag appearance. Dimers located at anti-phase boundaries continuously flip back and forth between their two buckled configurations. This process is thermally induced and occurs at an average frequency of 25 Hz at room temperature.     

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.     

Pt-chain induced formation of Ge nanowires on the Ge(001) surface

A new reconstructed Pt/Ge(001)–4 × 2 surface structure of 0.25 ML Pt deposition is suggested based on density functional theory. The Ge dimers form nanowire arrays on a Pt-chain modified Ge(001) surface in which the chain is located between the two quasi-dimer rows and below the Ge nanowire. The simulated scanning tunneling microscope (STM) images of the surface are in excellent agreement with the previously observed STM features and sample bias dependence. It is the nanowire Ge dimers and not the Pt atoms that contribute to the STM images for occupied states at high sample biases, contrary to what has always been assumed in experiments. The surface bands of the Pt chain and quasi-dimer rows exhibit quasi-one-dimensional metallic behavior in the direction of the nanowire. When changing from the 4 × 2 to the 4 × 4 structure, there are likely pseudogaps opened at the new surface Brillouin zone boundary, which simultaneously reduce the metallicity. This may be related to the Peierls instability. The interaction between the Pt chain and the quasi-dimer row, as well as the inter-quasi-dimer row interaction, is of essential importance for stabilization.     

Valence surface electronic states on Ge(001)

The optical, electrical, and chemical properties of semiconductor surfaces are largely determined by their electronic states close to the Fermi level (E{F}). We use scanning tunneling microscopy and density functional theory to clarify the fundamental nature of the ground state Ge(001) electronic structure near E{F}, and resolve previously contradictory photoemission and tunneling spectroscopy data. The highest energy occupied surface states were found to be exclusively back bond states, in contrast to the Si(001) surface, where dangling bond states also lie at the top of the valence band.     

Surface vibrations of Na(001) and K(001) surfaces

We present the results of a theoretical study of the dynamics of the atom motion of Na(001) and K(001) surfaces. The total electronic energy is calculated using a pseudopotential approach with a confined electron gas as unperturbed system. With this theory the dynamical matrix can he derived without resorting to empirical parametrizations. Surface phonon dispersion curves are reported for the high symmetry directions of the two-dimensional Brillouin zone for ideal and relaxed configurations. The calculated spectra are compared with the results of semi-empirical force constant calculations. The effects of single and multilayer relaxations on the location and the nature of the main surface bands are examined.     

Optical anisotropy of cyclopentene terminated GaAs(001) surfaces

Up to now most of the experimental work regarding the adsorption of organic molecules has been concerned with silicon. Here we study the interface formation on a III–V-semiconductor, GaAs(001). We show that reflectance anisotropy spectroscopy (RAS) is a sensitive technique for investigating the interface formation between organic molecules and semiconductor surfaces. With RAS it is possible to determine the surface reconstruction and the structural changes at the interface during the deposition of organic molecules. These changes and the underlying adsorption process are discussed here for the adsorption of cyclopentene on GaAs(001)c(4×4), (2×4) and (4×2). PACS 61.66.Hq; 72.80.Le; 34.50.Dy; 68.47.Fg     

Diffusion driven concerted motion of surface atoms: Ge on Ge(001)

The diffusion of Ge dimers on the Ge(001) surface has been studied with scanning tunneling microscopy. We have identified three different diffusion pathways for the dimers: diffusion of on-top dimers over the substrate rows, diffusion across the substrate rows, and diffusion of dimers in the trough. We report on a heretofore unknown phenomenon, namely, diffusion driven concerted motion of substrate atoms. This concerted motion is a direct consequence of the rearrangement of substrate atoms in the proximity of the trough dimer adsorption site.     

Instability of one-dimensional dangling-bond wires on H-passivated C(001), Si(001), and Ge(001) surfaces

We investigate the instability of one-dimensional dangling-bond (DB) wires fabricated on the H-terminated C(001), Si(001), and Ge(001) surfaces by using density-functional theory calculations. The three DB wires are found to show drastically different couplings between charge, spin, and lattice degrees of freedom, resulting in an insulating ground state. The C DB wire has an antiferromagnetic spin coupling between unpaired DB electrons, caused by strong electron–electron interactions, whereas the Ge DB wire has a strong charge-lattice coupling, yielding a Peierls-like lattice distortion. For the Si DB wire, the antiferromagnetic spin ordering and the Peierls instability are highly competing with each other. The physical origin of such disparate features in the three DB wires can be traced to the different degree of localization of 2p, 3p, and 4p DB orbitals.