The surface of the epitaxial layer of AlP grown by atomic layer epitaxy (ALE) was observed by atomic force microscopy. It was discovered that the atomic scale surface flatness was not always retained during ALE growth even though the self-limiting growth of 1 monolayer per 1 alternative source gas supply was maintained. A reaction process model that focused on the methyl adsorbate lifetime on the Al surface was proposed to explain the experimental results. Based on this model, AlP surface morphology was improved by gas feeding control, and a flat surface with atomic steps was obtained even after 250 cycles of the ALE sequence. ©1997 American Institute of Physics.