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 $S_A$ 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.