It has been reported that there is a drastic increase of grain size in polycrystalline silicon because of secondary grain growth in ultrathin, heavilyn‐type doped films upon conventional furnace annealing. There has been very limited work on secondary grain growth during rapid thermal annealing (RTA). This letter presents for the first time extensive data on secondary grain growth in heavilyn‐type, P‐doped amorphous silicon‐on‐oxide films during RTA. Grains as large as 16 &mgr;m in diameter have been obtained in 160‐nm‐thick films which represent the largest secondary grains and largest grain size to film thickness reported in the literature. The role of charged silicon vacancies is invoked in a new way to explain the observed lower activation energy for grain boundary mobility during secondary grain growth than during normal grain growth.