Formation of the thallium (T1) overlayers on the Si(1 0 0) 2 x I surface has been studied using scanning tunneling microscopy (STM). It has been found that adsorption of T1 atom involves a charge transfer leading to the development of a static dipole which is responsible for the field-assisted migration of the T1 adsorbate on the surface. When the STM tip bias voltage is positive, T1 atoms are repelled out from the region underneath the tip apex. In the case of the negative bias voltage, T1 is accumulated underneath the tip. Thus, the observed structure of the surface is controlled not only by T1 coverage, but also by the STM bias voltage and tunneling current. (C) 2004 Published by Elsevier B.V.