Variable termination of MnSi/Si(111) × films and its effect on surface properties

Using scanning tunneling microscopy observations, we have found that, depending on the growth conditions, MnSi films on Si(111) can be formed with various terminating surfaces. In addition to the usual MnSi/Si(111) 3 × 3 surface which is known to be an Si-terminated quadruple layer of a B20 structure of bulk MnSi compound, a surface of a new type can be formed if the MnSi film growth proceeds under the condition of an Si deficit. The new surface also has a 3 × 3 periodicity but a different STM appearance and was ascribed to the quadruple layer in which the topmost Si layer is missing. Hence, it is assumed to be terminated by an Mn layer. The difference in structure leads to a great difference in surface properties, as has been revealed in experiments with the adsorption of selected species onto these surfaces. For example, the coefficient of surface diffusion for Au atoms differs for these surfaces by about four orders of magnitude.