Nanomelt-Induced Surface Patterning by Polyatomic Ions

It is the current understanding that no surface patterns form on elemental amorphous semiconductors by keV-ions if the beam hits the surface perpendicularly and if contamination with certain metals is avoided. This has been experimentally proven by many groups and is theoretically predicted by the two dominating theories of Bradley/Harper and Carter/Vishnyakov. In contrast to this we find under normal impact of heavy, polyatomic ions very pronounced, hexagonally ordered dot patterns [1]. Using monatomic ions of the same element, fluence and energy per atom, the surface remains flat. Consequently, the patterning must result from the collective action of several energetic atoms bombarding the surface in the same point at the same time. Recently we showed [1] that this collective action causes surface patterns only if in the collision cascade the mean energy posited per substrate atom exceeds the melting threshold. In Ge substrates heated to a sufficiently high temperature, the melting threshold can be also overcome by monatomic Bi+ ions [2]. The figure shows a MD simulation for a Bi_3^ ++ ion impact on Ge with an energy per Bi atom of 20keV. A melt pool forms at the surface, which is quenched after a few hundreds of ps into an amorphous phase[3]. In Si substrates, patterns form (i.e. the melting threshold is reached) even at elevated T only with polyatomic ions [4]. Here it will be shown that this pattern formation is driven by capillary forces: On the one hand, melting minimizes the surface locally which, by many ion impacts, leads to a global smoothing of the surface in accordance to (similar to laser polishing). On the other hand, the missing matter (due to sputtering) results in a melt pool meniscus (see figure), whose center is shifted with respect to the ion impact point for tilted surfaces. Thus, downhill from the impact more matter is missing, which is effectively an uphill current leading to a surface
destabilizing term . As usual, the competition of these two processes results in pattern selforganization