Nanometer ripple formation and self-affine roughening of ion-beam-eroded graphite surfaces

The topography of (0001)-graphite (highly oriented pyrolytic graphite) surfaces eroded by a 5 keV Xe⁺ ion beam has been investigated using scanning tunneling microscopy. For tilted incidence of the ion beam and ion fluences of about 10¹⁷ cm^-2, a quasiperiodic ripple topography with characteristic wavelengths between 40 and 70 nm has been found. As predicted by continuum theory and Monte Carlo simulations, below a critical angle θC the ripples are oriented perpendicular to the ion beam projection onto the surface, while for angles above θC the ripple orientation is parallel to the ion beam projection. The critical angle θC lies between 60^∘ and 70^∘, in agreement with the predictions of the continuum theory. For rising ion fluences, large scale perturbations of the surface topography occur indicating a nonlinear behavior governed by the Kardar-Parisi-Zhang universality class.