Stranski-Krastanov growth of Sn on a polycrystalline Al film surface initiated by the wetting of Al by Sn

The deposition of tin (Sn) layers on polycrystalline aluminum surfaces exhibits pronounced islanding. The island formation slightly depends on the deposition rate, substrate temperature and on the surface topography of the Al underlayer. The present study clarifies this islanding mechanism by combined SEM, atomic force microscopy and scanning Auger microscopic (SAM) investigations. The detection of a continuous Sn coverage ("wetting layer") between the islands indicates a Stranski-Krastanov growth mode of the Sn islands despite the polycrystallinity of the underlying Al surface. The wetting of the Al surface by Sn was proved by in situ sputter cleaning of the sample surface in the SAM chamber. It was possible to completely remove the wetting layer by the sputter cleaning process. Surprisingly, the clean Al surface was covered again by Sn after several minutes as observed by a re-appearance of the Sn-AES signal. The Sn coverage of the surface developed at room temperature within some minutes after removing the wetting layer. Spatially resolved SAM measurements proved that the Sn emerged from the Sn islands (and not from the Al bulk by surface segregation) and spread over the Al surface uniformly. This process led to the complete wetting of the Al surface by Sn despite the thermodynamic immiscibility of this binary system. (C) 2001 Elsevier Science B.V. All rights reserved.