A Thermal Spike Analysis of Low Energy Ion Activated Surface Processes
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A THERMAL SPIKE ANALYSIS OF LOW ENERGY ION ACTIVATED SURFACE PROCESSES C.M. GILMORE*, A. HAERI* AND J.A. SPRAGUE** and Applied Science, The George Washington *School of Engineering University Washington, DC 20052 **Condensed Matter and Radiation Sciences Division, U.S. Naval Research Laboratory, Washington, DC 20375-5000
ABSTRACT A thermal spike analysis was utilized to predict the time evolution of energy propagation through a solid resulting from energetic particle impact. An analytical solution was developed that can predict the number of surface excitations such as desorption, diffusion or chemical reaction The analytical solution is limited to activated by an energetic particle. to materials with constant thermal substrates at zero Kelvin and diffusivities. These limitations were removed by developing a computer numerical integration of the propagation of the thermal spike through the solid and the subsequent activation of surface processes.
Introduction interest in the effect of There recently has been considerable energetic particles during deposition on the processes of thin film nucleation and growth with a number of papers reporting interesting The importance of the thermal spike in experimental observations [1,2,3]. explaining ion beam mixing (the displacement of substrate atoms from their has recently been discussed by original positions in bulk specimens) Averback with the conclusion that ion beam mixing at low temperature is it is Therefore, dominated by diffusion during the thermal spike [4]. reasonable to expect that displacement processes on the surface also will be significantly affected by surface diffusion during the thermal spike. It was the intent of this research to study the effect of the thermal spike caused by an energetic particle on the subsequent activation of surface processes such as desorption and diffusion. authors to provide an analysis of Seitz and Koehler were the first thermal spikes and to calculate the spike effects on thermally activated processes [5]. In their thermal spike analysis it was assumed that after a of the such as 10-13 seconds the energy (Qp) characteristic time (t) incoming particle that had not been lost to producing permanent structural It was assumed change in the crystal was transformed into a thermal spike. that the temperature of the crystal within the thermal spike was given by Seitz and Koehler assumed a the classical heat transfer equations. thermally activated process and calculated the number of times the process would occur. In Seitz and Koehler's analysis a constant temperature was assumed over a radius at the core of the thermal spike; also it was assumed that the substrate lattice was at absolute zero of temperature. Vineyard analyzed the thermal activation resulting from a point source inside an infinite solid and from a line source through an infinite solid [6]. Vineyard allowed the specific heat per unit volume (Cv) and the thermal conductivity (Kc) to be dependent upon temperature (T) by a power law relationship of the form: Kc - Kco
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