For fast-pulse laser-crystallized thin-film Si on non-crystallinesubstrates, the average grain size exhibits a peak as a function excimerlaser energy density at a characteristic laserfluence FM. Theaverage grain size increases with increasing laser fluence and can reach amaximum value on the order of 10 pm or about 100 times the film thickness.The grain size then decreases with further increases in fluence. This peakin grain size is accompanied by a similar peak in the Hall electron mobilityand x-ray scattering intensity. Our experiments have investigatedas-deposited and ion-implanted samples, using a double-scan lasercrystallization process. Devices have also been fabricated and studied. Theresults are consistent with the increase in grain size occurring because ofthe destruction of nucleation sites with increasing laser fluence (i.e.,increased heating and complete Melting). But substrate damage occurs in thevicinity of FM, creating nucleation sites which give rise tosmall grain sizes in the solidified film. The disruption of the interfacecauses substantial current leakage through the dielectric of bottom-gatetransistors, implying that devices should be laser fabricated below Fm.