Structural investigation of keV Ar-ion-induced surface ripples in Si by cross-sectional transmission electron microscopy

Using cross-section transmission electron microscopy (XTEM) we have studied the surface and subsurface structure of individual ripples having submicron scale wavelength and nanometer scale amplitude, generated by obliquely incident (50--120 keV) Ar ion bombardment of Si. The XTEM results reveal that the front slopes of ion-induced ripples have amorphous layers containing bubbles with sizes ranging from about 3 to 15 nm facing the ion beam direction. A thinner amorphous layer without bubbles, on the other hand, persists on the rear slope of ripples. We also observe an irregular interface between a-Si and c-Si, which is due to the direct impact amorphization mechanism prevalent near the end-of-range during heavy ion irradiation.