The theory of crystal growth for diffuse and for non-singular surfaces is re-examined. It is found that if a critical driving force is exceeded the surface will be able to advance normal to itself without needing steps; if this driving force is not exceeded lateral step motion is necessary. For extremely diffuse interfaces this critical driving force will be so small that any measurable driving force will exceed it. For sharp interfaces the critical driving force will be very large, and most growth will occur by lateral step motion. For most systems however the critical driving force should be accessible experimentally.

In addition the nature of a step in a diffuse interface is discussed and its energy calculated. The conditions for interface motion by classical nucleation or screw dislocation mechanisms are derived.

This article originally appeared in Acta Metallurgica 8(8), 1960, Pages 554–562.