Fluence Effects in C60 Cluster Bombardment of Silicon†

In this theoretical investigation, we combine the results of molecular dynamics (MD) simulations with a simple statistical sputtering model (SSM) in order to understand the factors limiting the optimum depth resolution achievable in sputter depth profiling experiments. The advantage of the SSM model is that it can be used to extrapolate the MD simulations toward the regime of high projectile fluence. First, a recently developed scheme [Russo, M. F., Jr.; Postawa, Z.; Garrison, B. J., J. Phys. Chem. C 2009, 113, 3270] is adapted to calculate 200 cumulative impacts of 20-keV C60 bombardment on a Si substrate, which corresponds to a projectile fluence of 3.5 × 1013 cm-2. The following results are studied as a function of fluence: the development of surface topography, the amount and depth of origin of sputtered material, and the relocation of substrate particles, which produces interlayer mixing. Data from the MD simulations are used as input parameters in the SSM, which is able to reproduce results consistent with the MD simulations. A major finding from these studies is that the statistical nature of the sputtering process has a significant effect on the achievable depth resolution. The optimum delta layer response width for Si is estimated to be about 3 nm, while corresponding values of the order of 10 nm are predicted for molecular systems.