ABSTRACT
Surface modification of single-crystal alkali halides [KBr(001),
RbI(001)] induced by low-energy ion beam irradiation has been
investigated in ultrahigh vacuum (UHV) by means of high resolution
atomic force microscopy (AFM) in contact and noncontact modes
of operation. Surface morphology development has been studied
as a function of ion beam parameters (ion mass: Ar and He; angle
of incidence: 0-80◦ off-normal; ion energy: 1-5 keV; ion fluence: 1013-1018 ions cm−2) and the target temperature (300 to 600 K). Several new features of the process have been found, directly
related to the ionic nature of halide surfaces, such as formation of
two-dimensional (2D) pits and rectangular 2D epitaxial adislands
on the initially atomically flat terraces. At low bombardment
fluence, the evolution of 2D pits proceeds along main surface
crystallographic directions. Such behavior is typical for the electron
stimulated desorption (ESD) process, well known from electron and
photon irradiation experiments. No epitaxial adisland formation by
ESD, or by ion impact, has been reported so far. For prolonged
ion bombardment, surface topography transforms into a regular
network of grooves and rims (a ripple structure) oriented parallel
to the incident beam. Such a structure has been observed for many
other materials (metals, semiconductors), with a major distinction,
however, that for KBr, the ripples are composed of small nano-size
crystallites with a persistent long range order on the (001) surface.
It is demonstrated that the fluence threshold for transition from
a random network of 2D pits and adislands into a well-oriented
crystalline nanoripple structure is directly related to the balance
between the electronic and the ballistic stopping of the impinging
ions. The interpretation of these new experimental observations
does require a new atomistic approach to the ion-solid interactions
at irradiated surfaces.
