Surface morphology of Ge(001) during etching by low-energy ions.

Abstract

The roughening of a crystalline semiconductor during etching by low-energy ions is characterized using in situ scanning tunneling microscopy. Ge(001) surfaces are bombarded by 240-eV Xe ions using a wide range of exposure times and temperatures. Ge dimers are resolved for surfaces etched at Tg+165 \ifmmode^\circ\else\textdegree\fi{}C and imaged at room temperature. For fixed ion exposure, the roughness increases with increasing temperature; a maximum surface roughness is reached for etching at \ensuremath{\simeq}250 \ifmmode^\circ\else\textdegree\fi{}C. At T\ensuremath{\simeq}270 \ifmmode^\circ\else\textdegree\fi{}C the character of the surface morophology changes from a relatively disordered arrangement of mounds to a more regular pattern of pits. The isotropy of this pattern formation and the dependence of the in-plane length of the roughness on exposure time suggest that asymmetric kinetics for the attachment of dimer vacancies at ascending versus descending steps drives roughening during etching.