Nanometer-scale surface modification of KBr (001) single crystal by Ar ion bombardment

Morphological development on alkali halide crystal surfaces, under ion irradiation, has been investigated by means of Atomic Force Microscopy in UHV conditions. The KBr (001) surface has been exposed to Ar+ beam with varying incident angles (from 5° to 75° off-normal) and energies (from 1.0 to 5.0 keV). Different kinds of nanostructures, depending on ion incident angle, have been observed on the irradiated surfaces such as nanocavities, or nanorings and nanodots, preferentially for close to normal incidence, and ripples for the oblique incidence. The formation of ripple structures on KBr surfaces as a function of the incidence angle and energy has also been observed. The orientation of the ripples has been found to be incident angle dependent and it is perpendicular, or parallel to the ion beam projection on the irradiated surface for angles of 30° and 75°, respectively. Moreover, it has been observed that the wavelength of the ripples also depends on energy of the projectile ions. The AFM images acquired with atomic resolution reveal well ordered, crystalline structure of the ripples even after bombardment with high fluence. The results have been discussed in terms of electronic and ballistic processes of sputtering and existing theories for surface modifications.