Surface morphology of Ge(111) during etching by keV ions

Abstract

The surface morphology of Ge(111) during 5-keV Xe ion etching is characterized by in situ scanning tunneling microscopy and ex situ atomic force microscopy. Surface patterns of pits form at 250 and $275\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ due to asymmetric kinetics for the attachment of surface vacancies at step edges. The roughening and coarsening of Ge(111) surfaces etched at these temperatures agrees well with the Politi and Villain model of surface roughening; the roughening rate due to this mechanism is a factor of $\ensuremath{\sim}300$ larger than roughening calculated for curvature dependence of the sputtering yield. At $300\ifmmode^\circ\else\textdegree\fi{}\mathrm{C},$ a subtle pattern of ripples or step bunches appear at high ion fluence when the projected ion beam direction is aligned along the direction of the surface steps. At room temperature, pattern formation is not observed for ion fluences $<460\ifmmode\times\else\texttimes\fi{}{10}^{15}{\mathrm{ions}\mathrm{}\mathrm{cm}}^{\ensuremath{-}2};$ for amorphous surface layers, smoothing by ion-induced viscous flow is a factor of $\ensuremath{\sim}300$ stronger than smoothing by fourth-order terms in the expansion of the sputtering yield.