Step instabilities: A new kinetic route to 3D growth

Atomic force microscopy studies of Ge /Si(001) molecular beam epitaxy growth reveal a crucial new role of surface steps in the 2D to 3D transition. At or near step flow we show that SA steps undergo a stress-driven triangular step instability. The resulting spatial variation of surface strain, although small, can dramatically influence the activation barrier for 3D island nucleation. This provides a surprising kinetic route for the onset of 3D growth associated with the apex regions of triangular steps.