The amorphous Si layers deposited by low-pressure chemical vapor deposition on (100)-crystal-Si substrates and subjected to Xe-ion-beam irradiation are crystallized epitaxially in a layer-by-layer fashion to the surface during 600 °C furnace annealing. Layer-by-layer crystallization can be accomplished by irradiating the layers with a 1-MeV Xe-ion-beam for a 2×1015/cm2 dose at 310 °C prior to furnace annealing. In all cases during furnace annealing that amorphous Si layers are polycrystallized or are grown vertically in isolated epitaxial-columnar-structures and then grown laterally into the amorphous region surrounding each column, the ion-beam-induced epitaxial crystallization (IBIEC) method epitaxially crystallizes them in a layer-by-layer fashion. This is because O atoms that were at the initial interface and that prevented layer-by-layer crystallization or columnar-epitaxial-growth diffuse remarkably because of irradiation. This diffusion decreases the peak concentration and facilitates layer-by-layer crystallization. O atoms at the interface are also diffused by irradiation with 80-keV P, 100-keV As, and 150-keV As ions. This diffusion results in the columnar growth during 600–800 °C furnace annealing. Whether layer-by-layer growth or columnar growth occurs during the furnace annealing depends on the peak concentration of oxygen at the interface. Direct evidence is shown that O diffusion is enhanced by the amount of inelastic electronic scattering of incident ion beam under the same elastic nuclear scattering conditions. The rates of IBIEC and of epitaxial crystallization during furnace annealing after 1-MeV Xe-ion-beam irradiation for a 2×1015/cm2 dose are affected by the amount of oxygen in the amorphous layer. The rate of layer-by-layer IBIEC using a 1-MeV - Xe-ion-beam is nearly twice as high for a sample heated in the deposition furnace after evacuation as it is for a sample heated before evacuation. This difference is due to the smaller amount of oxygen in the amorphous Si layer of the former sample. © 1997 American Institute of Physics.