  The development of epitaxy techniques for localized growth of crystalline
silicon nanofilms and nanostructures has been crucial to recent advances in
electronics and photonics. A precise definition of the crystal growth location,
however, requires elaborate pre-epitaxy processes for substrate patterning. Our
molecular dynamics simulations reveal that homoepitaxial silicon nanofilms can
be directly deposited by a crystalline silicon tip rubbing against the
substrate, thus enabling geometrically controlled crystal growth with no need
for substrate pre-patterning. We name this solid-phase epitaxial growth
triboepitaxy as it solely relies on shear-induced amorphization and
recrystallization that occur even at low temperature at the sliding interface
between two silicon crystals. The interplay between the two concomitant,
shear-induced processes is responsible for the formation of an amorphous
sliding interface with constant nanometric thickness. If the two elastically
anisotropic crystals slide along different crystallographic orientations, the
amorphous layer can move unidirectionally perpendicular to the sliding plane,
causing the crystal with lowest elastic energy per atom to grow at the expenses
of the other crystal. As triboepitaxial growth is governed by the shear elastic
response of the two crystals along the sliding direction, it can be implemented
as a mechanical scanning-probe lithography method in which epitaxial growth is
controlled by tuning the crystallographic misorientation between tip and
substrate, the tip's size or the normal force. These results suggest a
radically new way to conceive nanofabrication techniques that are based on
tribologically induced materials transformations.
