Two surface roughening mechanisms have been distinguished in laser crystallized polycrystalline Si: one is related to rapid release of hydrogen from hydrogen‐rich plasma enhanced chemical vapor deposited amorphous Si and the other is independent of the hydrogen content of the material and is determined by the total number of pulses incident on the surface. At, or beyond, the melt threshold energy there is a positive feedback effect between a beam‐induced periodic surface roughness pattern and enhancement of this pattern by interference effects in subsequent pulses. ©1995 American Institute of Physics.