Simulation of Surface and Material Damage During Fast Ion Penetration

Citation

Hartman, John Walter (1997) Simulation of Surface and Material Damage During Fast Ion Penetration. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/kd7q-6k71. https://resolver.caltech.edu/CaltechTHESIS:07142025-203301767

Abstract

Simulation can be a powerful tool to investigate the interaction of materials with energetic ions. In this dissertation I describe the two most common methods used to simulate ion penetration and the subsequent induced damage: the Molecular Dynamics and Binary Collision techniques. After a discussion of their limitations and realms of applicability, they are applied to a collection of interesting physical systems. The topics of investigation were chosen because simulation could provide insights into their workings which neither theoretical nor experimental methods could provide; in such cases, simulations often suggest both promising new interpretations of experimental data and better experiments that would yield a deeper understanding of the processes involved. The topics were chosen to provide a variety of examples that demonstrate the utility of such simulations. Latter chapters of this dissertation are based on articles published and projects completed during my graduate career. Abstracts of these articles are as follows:

In Chapter 3, we match the predictions of molecular-dynamics simulations of 1.2 keV and 2.0 keV ⁷Li⁺ scattered from Al(100) to observed total Li atom spectra measured by time-of-flight spectroscopy. In doing so, the relevant parameters in a simple distance of closest approach model for the probability of production of single and double vacancies in the Li 1s shell during hard Li-Al collisions are determined. In the standard Fano-Lichten model of vacancy production, vacancies are produced with unit probability if the collision is hard enough to force the collision partners past some critical distance of closest approach. This assumption is insufficient to fit simulation results to experimental observations: a gradual turning-on of the vacancy production probability as the distance of closest approach decreases must be allowed. The resulting model will be useful in modeling atomic excitation effects in simulations of ion-impact processes in which inelastic losses to deep electronic orbitals are an important effect.

In Chapter 4, we present the results of simulations of cluster formation during Ar⁺ → In-Ga (liquid) sputtering events. This target has a natural segregation of atomic species: the surface is almost all indium while, just a few atomic layers down, the bulk is mostly gallium. The indium concentration in small k-atom clusters (k ≤ 4) is found to reflect the concentration in the target at the depth from which the clusters were sputtered. We find a strong correlation between the production of clusters and the size of the events responsible for their production. A simple model for the recombination of uncorrelated emissions into small clusters is developed and found to predict accurately the production of small k-clusters during events of size N: Y_k(N) ~ N^k. However, this uncorrelated recombination model does not predict the proper energy spectra for clusters nor does it predict the oft experimentally observed power law decay of the yield of clusters. The means by which a model for the recombination of correlated emissions may more readily explain these features is discussed.

In Chapter 5, we describe a new algorithm for simulating the penetration of crystalline or amorphous matter by fast atomic clusters which can integrate intra-cluster forces properly without resorting to a full Molecular Dynamics calculation. A simple numerical model is developed which describes the desorption of hydrogen from target surfaces as fast ions or clusters penetrate the surface. Experimental observations suggest that that ions of large charge q have a desorption yield proportional to q^2.7 (although the law fails for small q), and that clusters of n ions have a desorption yield proportional to n^2.7 when the cluster is tightly correlated and proportional ton when the cluster is weakly correlated. The model describes the transition from correlated to uncorrelated desorption yields during cluster penetration and suggests the origin of both the failure of the q^2.7 law at small ion charge and the lack of charge dependence in the cluster desorption yields. Simulations of the penetration of 0.42 and 1.0 MeV/C clusters of C₁₀ through thin carbon foils allow us to determine the range of the mechanism responsible for H⁺ desorption. We find that the range of the mechanism responsible for H⁺ desorption from a charged ion must scale as r_desorption ≃ 0.5Åq^2.7/2 with a lower limit on the desorption range set by the radius of the ion.

Thesis Files