Field emission microscopy was used to study the effect of high electric fields upon the activation energy for the surface diffusion of Ge in the (121)  (100) direction on W, at various adsorbate concentrations. It was found that the activation energy was a linear function of the external field for all of the investigated concentrations. The slope of the activation energy versus field strength curve changed sharply when the adsorbate concentration approached 1 monolayer (figure 2). This behavior could be explained by taking account of the double layer capacity. At low concentrations, the Ge atoms exhibited an elective dipole moment of positive sign. This was considered to be the result of the electron density concentration on the bond between adatom and substrate.

V.G.Flisyuk, T.P.Nagorskaya, G.G.Vladimirov, B.B.Bolotov: Physics of Low-Dimensional Structures, 1997, 7, 11-20