To realize advanced fully-depleted transistors, high-quality ultrathin Si films on insulators are required. In conventional growth techniques, crystal quality of Si films deteriorates with decreasing thickness. To solve this problem, we investigate effects of Sn-doping on solid-phase crystallization of Si thin films (thickness: <40 nm). For Sn-doped (1%) Si films, circular domain structures appear after annealing at low-temperatures (<600^oC), which are not observed in pure Si films. Growth velocities of the domains decrease with decreasing thickness. Detailed analyses reveal that grown films (thickness: 30 nm) dominantly consist of large single crystal grains (> 10 um).