Wavelength tunability of ion-bombardment-induced ripples on sapphire

Abstract

A study of ripple formation on sapphire surfaces by $300--2000\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ ${\mathrm{Ar}}^{+}$ ion bombardment is presented. Surface characterization by in-situ synchrotron grazing incidence small angle x-ray scattering and ex-situ atomic force microscopy is performed in order to study the wavelength of ripples formed on sapphire (0001) surfaces. We find that the wavelength can be varied over a remarkably wide range---nearly two orders of magnitude---by changing the ion incidence angle. Within the linear theory regime, the ion induced viscous flow smoothing mechanism explains the general trends of the ripple wavelength at low temperature and incidence angles larger than $30\ifmmode^\circ\else\textdegree\fi{}$. In this model, relaxation is confined to a few nm thick damaged surface layer. The behavior at high temperature suggests relaxation by surface diffusion. However, strong smoothing is inferred from the observed ripple wavelength near normal incidence, which is not consistent with either surface diffusion or viscous flow relaxation.