Adatom formation and atomic layer growth on Al(1 1 1) by ion bombardment: experiments and molecular dynamics simulations

Rare gas ion bombardment of the Al(1 1 1) surface is found to produce adatoms with high efficiency, while at the same time stable vacancy clusters in the bulk are produced. As a result, scanning tunneling microscopy experiments show an initial bombardment-induced growth of several atomic layers instead of the expected erosion. This phenomenon is observed for Ne+, Ar+, and Xe+ for temperatures between 100 and 400 K and for ion energies between 0.3 and 8 keV. On the basis of molecular dynamics simulations insight into the atomic mechanism of bombardment-induced growth is gained. Ion bombardment causes local melting of the Al crystal near the surface. The energy is rapidly transported out of the molten volume, and after a few picoseconds a highly undercooled melt results, which has swollen above the initial surface layer. During recrystallization of the amorphous zone from the crystal inner towards the surface, vacancy clusters are left at the phase boundary, which stabilize part of the swollen material as adatoms, thus leading to the observed bombardment-induced growth.