  The use and the study of semi-insulating layers on metals and semiconductors
surfaces have found continuous interest in the past decades. So far, the
control of the sizes and growth location of the insulating islands on the
substrate is either ill-defined or usually constrained to the use of
evaporation masks which size can easily exceed tenth of nanometers. Here, we
show that it is possible to grow self-organized periodically spaced thin
ribbons of semi-insulating stripes on the bare Si(100) surface. The epitaxial
growth of these structures is obtained by the evaporation of CaF$_2$ molecules
on the silicon surface with a coverage of 1.2 monolayers. They are investigated
via scanning tunneling techniques at low temperature (9K). The obtained ribbons
exhibit a surface bandgap of ~3.2 eV as well as a resonant state at the central
part of the ribbons at ~2.0 eV below the Fermi level energy. The use of the
density functional theory allows suggesting a model structure of the observed
ribbons and reproducing the experimental STM topographies. The formation of the
thin ribbons is discussed and we point out the influence of the mechanical
forces inside and between the structures that may influence their periodicity.
