  We report a phenomenon of strain-driven shape transition in the growth of
nanoscale self-organized endotaxial CoSi2 islands on Si (100) substrates. Small
square shaped islands as small as 15\times15 nm2 have been observed. Islands
grow in the square shape following the four fold symmetry of the Si (100)
substrate, up to a critical size of 67 \times 67 nm2. A shape transition takes
place at this critical size. Larger islands adopt a rectangular shape with ever
increasing length and the width decreasing to an asymptotic value of ~25 nm.
This produces long wires of nearly constant width.We have observed nanowire
islands with aspect ratios as large as ~ 20:1. The long nanowire
heterostructures grow partly above (~ 3 nm) the surface, but mostly into (~17
nm) the Si substrate. These self-organized nanostructures behave as nanoscale
Schottky diodes. They may be useful in Si-nanofabrication and find potential
application in constructing nano devices.
