Shining new light on the motion of eutectic droplets across surfaces: A PEEM study of PtGe on Ge(110)

Abstract

Thermally stimulated motion of micron-sized eutectic PtGe droplets on Ge(110) has been studied in situ mainly by photoemission electron microscopy, low-energy electron microscopy, and spatially resolved low-energy electron diffraction. In line with earlier studies of eutectic AuSi, PtSi, AuGe, and PtGe clusters on, respectively, various Si and Ge substrates we find that the motion toward regions at higher temperature is driven by the entropy gain of substrate atoms which become constituents of the droplet during its journey. At \ensuremath{\sim}1100 K, i.e., well above the bulk eutectic temperature, the direction is governed solely by the local thermal gradient, irrespective of eventual crystalline preferences. Access to the diffusivity of the host material (in this case Ge) inside the eutectic droplets shows that this is well above one order of magnitude higher than expected if it was rate limiting for the velocity of the moving droplets. This excludes a significant gradient of the (Ge) concentration inside the droplet and disqualifies dissolution-diffusion-deposition flow as the driving force for motion of the droplets on the surface, as assumed widely hitherto, to explain surface diffusion of eutectic droplets on surfaces. In addition, the interface between the droplet and the surface appears flat and we find no indication for ``endotaxy.'' The droplets make direct contact with the flat Ge substrate through atomic steps, which are abundantly present at the interface. The droplets are surrounded by a $\mathrm{Pt}{\mathrm{Ge}}_{3}$ wetting layer with an ordered (2 \ifmmode\times\else\texttimes\fi{} 1) structure. Dissolution of the edges of the wetting layer at the leading edge of the droplet with an activation energy of 2.2 eV is identified as the rate limiting step for its motion.