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

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 T greater than or equal to 165 degrees 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 similar or equal to 250 degrees C. At T similar or equal to 270 degrees C the character of the surface morphology 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.