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The epitaxial crystallization rates of amorphous Si layers on crystalline Si substrates containing a considerable number of hydrogen atoms are markedly increased in the absence of oxygen atoms. This enhanced crystallization occurs both during 1-MeV Xe-ion-beam irradiation at 310 °C and during furnace annealing in vacuum at temperatures below 450 °C. Implantation-amorphized crystal Si layers epitaxially grown on the (100)-crystalline Si substrates by ultrahigh vacuum chemical vapor deposition (UHV-CVD) are epitaxially crystallized by furnace annealing in vacuum at temperatures below 450 °C. Implantation-amorphized bulk-crystal Si substrates, however, are not entirely crystallized by the same low-temperature annealing. Nanometer-scale microcrystallites, remaining at the near-surface region in the amorphous Si layer after 80-keV phosphorus implantation into the UHV-CVD epitaxial Si layer, grow three-dimensional during 1-MeV Xe-ion-beam irradiation at 310 °C, but not during furnace annealing at 600 °C in dry Ar ambient. This three-dimensional crystal growth does not occur in implantation-amorphized bulk-crystal Si substrates, even during 1-MeV Xe-ion-beam irradiation. Amorphous Si layers directly deposited by low-pressure CVD (LP-CVD) are crystallized epitaxially by 1-MeV Xe-ion-beam irradiation at 310 °C. The crystallization rate of the LP-CVD sample heated in the deposition furnace after evacuation is twice as high as that of the sample heated before evacuation. All these results are explained consistently by the presence of a considerable amount of hydrogen in polyhydride states in the amorphous Si layers in the absence of oxygen. It is suggested that the mechanism of low-temperature crystallization of amorphous Si containing hydrogen atoms in polyhydride states is closely related, in terms of vacant spaces and dangling-bonds, to the mechanism of ion-beam-induced epitaxial crystalliza- tion. © 1997 American Institute of Physics.