An ultrahigh vacuum atomic force microscope, operated in the noncontact mode, is used to characterize n-doped Si(111) surfaces. Ionized impurities are observed with electrostatic forces, demonstrating contrast reversal by changing the polarity of the voltage between probing tip and sample. The impurities form one-dimensional domains on the silicon surface. Steps and lines, connecting kink sites, are preferentially occupied by these impurities.