Ion beam induced epitaxy of amorphous Si layers onto ⟨100⟩ substrates has been investigated by varying the As concentration. At As concentrations below 4×1018/cm3no rate effect is observed. In the intermediate regime, between 4×1018/cm3and 2×1021/cm3, the growth rate increases linearly with the logarithm of As concentration and reaches a value about a factor of 2 higher than that of intrinsic Si. At concentrations above 2×1021/cm3, the epitaxy experiences a sudden, severe retardation. Finally, at a concentration of ∼6×1021/cm3, twins are observed to form.