K.R. Roos

Electromigration in self-organized single-crystalline silver nanowires

We present electromigration experiments on single-crystalline silver nanowires. The wires were grown on 4° vicinal silicon (100) substrates by self-organization and were contacted by electron beam lithography. The electromigration experiments were performed in situ in a scanning electron microscope at room temperature with constant dc conditions. In contrast to other experiments we observe void formation at the anode side of the wires. If the current is reversed, the electromigration behavior is also reversed.

High temperature self-assembly of Ag nanowires on vicinal Si(001)

We have used low energy electron microscopy (LEEM) and photo emission electron microscopy (PEEM) to study the high temperature (620 °C) self-assembly of Ag nanowires on vicinal Si(001), miscut 4° in the [110] direction. After formation of an initial wetting layer, growth of wire-like structures proceeds with subsequent deposition. Simultaneously, compact islands form and the nanowires comprise only a minority of the total Ag deposit. The wires display quasi-one-dimensional behaviour as their length is observed to increase while their width remains constant. The lengths of the wires can be controlled and we have routinely grown wires longer than 100 µm. A kinetically limiting process, wherein mass transport is suppressed in the direction normal to the direction of elongation, is identified as a contributor to the growth of the nanowires.

An Incremental Approach to Computational Physics Education

In Bradley Universitys physics department, the author has developed an incremental approach to computational physics instruction. It interweaves modern computational techniques with traditional aspects of upper-level classical mechanics and thermodynamics courses, building from simple to more complex concepts and assignments. By directly programming and controlling all aspects of their computer applications, students can best grasp computational principles

The influence of anisotropic diffusion on Ag nanowire formation

Photoemission electron microscopy is used to study the growth of single-crystalline silver nanowires on flat and vicinal Si(001) substrates. The growth experiments were performed at various temperatures and showed a temperature dependence of nanowire formation. The nanowires on Si(001) are evenly distributed in the [110] and [Formula: see text] directions on the surface, whereas on a 4° vicinal surface the wires grow only along the steps, in the [Formula: see text] direction. This change in wire distribution is attributed to the increasing diffusion anisotropy as the vicinality of the substrate increases.

Absence of surface stress change during pentacene thin film growth on the Si(111)-(7 ◊ 7) surface: a buried reconstruction interface

We use high-resolution surface stress measurements to monitor the surface stress during the growth of pentacene (C22H14) on the (7◊ 7) reconstructed silicon (111) surface. No significant change in the surface stress is observed during the pentacene growth. Compared to the changes in the surface stress observed for Si and Ge deposition on the Si(111)-(7 ◊ 7) surface, the insignificant change in the surface stress observed for the pentacene growth suggests that the pentacene molecules of the first adsorbate layer, although forming strong covalent bonds with the Si adatoms, do not alter the structure of the (7◊ 7) reconstruction. The (7 ◊ 7) reconstruction remains intact and, with subsequent deposition of pentacene, eventually becomes buried under the growing film. This failure of the pentacene to affect the structure of the reconstruction may represent a fundamental difference between the growth of organic thin films and that of inorganic thin films on semiconductor surfaces.

Imaging diffusion fields on a surface with multiple reconstructions: Ag/Si(111)

Photoemission electron microscopy is used for studying the thermal decay of Ag islands grown epitaxially on a Si(111) surface. During the decay, the islands feed adatoms to the surrounding surface. The adatoms diffuse and eventually desorb, resulting in a radial coverage gradient that induces the formation of two concentric reconstructed zones, namely and (3×1), around each island. We have developed a diffusion model to describe this multizone formation and demonstrate how diffusion constants can be determined for different reconstructed phases in a simple experiment.

Finite size effects in nucleation processes

Abstract We have used Monte Carlo simulations to study the scaling relations describing submonolayer nucleation N ∝ F y D x θ z as a function of system size L (where N is the nucleated island density, D the diffusion coefficient, F the flux rate, θ the coverage). For island size distributions that obey scaling, the island density N can be easily related to the step density, S D . A simple model of critical size cluster i c = 1 is used. For a given system size L there is a minimum ( D F ) min ratio where scaling breaks down with x = y = 0. These considerations are relevant to systems with low terrace diffusion barrier or small substrate terraces.

RHEED studies of interlayer diffusion in the submonolayer growth regime on AgAg(111)

Abstract We have used reflection high-energy electron diffraction (RHEED) to study the growth of Ag Ag (111) as a function of temperature T = 150–315 K and flux rate F = 1 125 – 1 4800 ML / s by monitoring the decay of the specular beam intensity. No oscillations are observed, which is consistent with a 3D growth mode and the existence of a step edge barrier. The intensity decay is independent of flux rate but depends on temperature. This can be understood in terms of the high diffusion rate on the finite substrate terraces ( ∼ 500 A ), which results in breakdown of the scaling conditions for nucleation and interlayer diffusion from higher to lower terraces.

Inter-layer mass transport in a low-coverage, low island-density regime

Abstract We use Monte Carlo simulations to study inter-layer mass transport during submonolayer epitaxial growth in systems where the ratio of the diffusion coefficient for intra-layer mobility to the deposition flux D F is very high, such that the average diffusion length is comparable to the average terrace length. Under such growth conditions, there exists a low island-density regime where the scaling relation between the island density N and the ratio D F breaks down. We employ realistic terrace boundaries in our simulations (i.e. atom capture at ascending steps and a step edge barrier Es between adjacent substrate terraces at descending steps) to investigate how inter-layer diffusion depends on the D F ratio in this low island-density regime. Information about the magnitude of Es can be extracted by analyzing the distribution of deposited atoms among the three possible capture sites: ascending steps, descending steps and nucleated islands. We apply the results of these simulations to the growth of Ag Ag (111) .

Decay of isolated hills and saddles on Si(001)

We discuss the high temperature decay of isolated hills and saddle points on Si(001). Using in situ dark-field imaging in low energy electron microscopy, we track the movement of individual steps during high temperature annealing. We find different temperature dependent decay rates for the top of the hill compared to a saddle point with low step density that is present in the vicinity of the hill. The decay rate of the hill is always higher than the decay rate at the saddle. The two rates converge with increasing temperature and become equal at temperatures above 1060 °C. We also report an alternating fast and low decay rate for the layer-by-layer decay of the hills. This surprising finding is independent of temperature and is explained by macroscopic strain in the sample.