Atomic force microscopy has been used to investigate the influence of controlled oxygen incorporation on the surface morphology of GaAs films grown by metalorganic vapor phase epitaxy (MOVPE). Oxygen influences the periodic morphology observed in GaAs surfaces, with concentrations about 1018cm−3leading to a breakup of the periodicity. To account for these observations, we propose a model in which oxygen preferentially attaches at steps with a subsequent reduction in step mobility and a concomitant increase in the surface roughness. ©1996 American Institute of Physics.