The nucleation and growth of the Si0.5Ge0.5 alloy layer on Si (100) substrate during ion‐beam‐assisted deposition (IBAD) have been investigated by atomic force microscopy, reflection high‐energy electron diffraction, transmission electron microscopy, and double‐crystal rocking diffraction. We confirmed that Si0.5Ge0.5 nucleates on Si (100) via the Stranski–Krastanov (SK) mechanism by IBAD, and Ar‐ion bombardment suppressed SK growth mode as well as improved crystalline perfection. The epitaxial temperature was observed at 200 °C, and it was much lower than the growth temperature (550–600 °C) in molecular‐beam epitaxy (MBE). The χmin value (the ratio of channeling to random backscattering yields) was 10.5% lower than the obtained MBE value. The effect of ion bombardment on nucleation was explained as the result of ion‐bombardment‐induced dissociation of three‐dimensional islands and enhanced surface diffusion, and appeared only at low deposition temperatures where the dissociation of three‐dimensional islands is more favorable than the formation of those islands. © 1995 American Institute of Physics.