Investigation of the direct electromigration term for Al nanodots within the depletion zone of a pn junction

We have investigated the effect of the large field produced within the depletion region of a pn junction on the shapes and positions of aluminum nanodots. Dots of 10–60 nm diameter were fabricated by the application of voltage pulses between an Al-coated scanning tunneling microscopy tip and the Si substrate. Reverse bias conditions produce an intense field (104–105 V/cm), but the current density is quite small (∼10−2 A/cm2). This electric field dominated configuration provides a unique model system for the separation of the two components of electromigration, direct and wind forces. We find that even after several tens of hours the observed motion of Al is confined to small changes in dot shape, with no evident motion of the dots as a whole. Our observations are consistent with a strong adhesion between the dots and the underlying substrate and a dominant direct force term.