Computer Simulation of Electronic Excitation in Atomic Collision Cascades

The impact of an keV atomic particle onto a solid surface initiates a complex sequence of collisions among target atoms in a near-surface region. The temporal and spatial evolution of this atomic collision cascade leads to the emission of particles from the surface - a process usually called sputtering. In modern surface analysis the so called SIMS technology uses the flux of sputtered particles as a source of information on the microscopical stoichiometric structure in the proximity of the bombarded surface spots. By laterally varying the bombarding spot on the surface, the entire target can be scanned and chemically analyzed. However, the particle detection, which bases upon deflection in electric fields, is limited to those species that leave the surface in an \textit{ionized} state. Due to the fact that the ionized fraction of the total flux of sputtered atoms often only amounts to a few percent or even less, the detection is often hampered by rather low signals. Moreover, it is well known, that the ionization probability of emitted particles does not only depend on the elementary species, but also on the local environment from which a particle leaves the surface. Therefore, the measured signals for different sputtered species do not necessarily represent the stoichiometric composition of the sample. In the literature, this phenomenon is known as the Matrix Effect in SIMS. In order to circumvent this principal shortcoming of SIMS, the present thesis develops an alternative computer simulation concept, which treats the electronic energy losses of all moving atoms as excitation sources feeding energy into the electronic sub-system of the solid. The particle kinetics determining the excitation sources are delivered by classical molecular dynamics. The excitation energy calculations are combined with a diffusive transport model to describe the spread of excitation energy from the initial point of generation. Calculation results yield a space- and time-resolved excitation energy density profile E(r,t) within the volume affected by the atomic collision cascade.The distribution E(r,t) is then converted into an electron temperature, which in a further step can be utilized to calculate the ionization probabilities of sputtered atoms using published theory

Heat Transport in Nanoscale Heterosystems: A Numerical and Analytical Study

The numerical integration of the heat diffusion equation applied to the Bi/Si-heterosystem is presented for times larger than the characteristic time of electron-phonon coupling. By comparing the numerical results to experimental data, it is shown that the thermal boundary resistance of the interface can be directly determined from the characteristic decay time of the observed surface cooling, and an elaborate simulation of the temporal surface temperature evolution can be omitted. Additionally, the numerical solution shows that the substrate temperature only negligibly varies with time and can be considered constant. In this case, an analytical solution can be found. A thorough examination of the analytical solution shows that the surface cooling behavior strongly depends on the initial temperature distribution which can be used to study energy transport properties at short delays after the excitation

Creation of multiple nanodots by single ions

In the challenging search for tools that are able to modify surfaces on the nanometer scale, heavy ions with energies of several 10 MeV are becoming more and more attractive. In contrast to slow ions where nuclear stopping is important and the energy is dissipated into a large volume in the crystal, in the high energy regime the stopping is due to electronic excitations only. Because of the extremely local (< 1 nm) energy deposition with densities of up to 10E19 W/cm^2, nanoscaled hillocks can be created under normal incidence. Usually, each nanodot is due to the impact of a single ion and the dots are randomly distributed. We demonstrate that multiple periodically spaced dots separated by a few 10 nanometers can be created by a single ion if the sample is irradiated under grazing angles of incidence. By varying this angle the number of dots can be controlled.Comment: 12 pages, 6 figure