MODEL FOR COMPOUND FORMATION DURING ION-BEAM MIXING

We propose an ion-beam-mixing model that accounts for compound formati on at a boundary between two materials during ion irradiation. It is b ased on Fick's law together with a chemical driving force in order to simulate the chemical reaction at the boundary. The behavior of the sq uared thickness of the mixed layer, X2, with the irradiation fluence, PHI, has been found in several mixing experiments to be either quadrat ic (X2alphaPHI2) or linear (X2alphaPHI), a result which is qualitative ly reproduced. Depending on the fluence range, compound formation or d iffusion is the limiting process of mixing kinetics. A criterion is es tablished in terms of the ratio of the diffusion coefficient D due to irradiation to the chemical reaction rate squared which allows us to p redict quadratic or linear behavior. When diffusion is the limiting pr ocess, D is enhanced by a factor which accounts for the formation of a compound in the mixed layer. Good agreement is found between the calc ulated mixing rates and the data taken from mixing experiments in meta l/Si bilayers.