The nucleation of pits is shown to be an additional mechanism by which films roughen to relieve strain. We discuss the morphological evolution of moderately strained InGaAs/GaAs (lattice mismatch f approximate to 2%) as a function of growth conditions, We show that the surface progresses through several morphological regimes. including layer-by-layer growth, island nucleation followed by pit nucleation, and finally ripple formation. The morphological features call be greatly altered by changing the growth conditions. For example, the critical thickness for roughening can be significantly increased by increasing the arsenic overpressure. We suggest that pit nucleation is a kinetically driven process strongly dependent on the supersaturation of adatoms. (C) 2002 Published by Elsevier Science B.V.