Using planar view transmission electron microscope (TEM) and transient reflectance (TR) analyses, we have investigated the excimer laser crystallization of amorphous silicon (a-Si) films on SiO2. Emphasis was placed on characterizing the microstructures of the single-shot irradiated materials, as a function of the energy density of the laser pulse and the temperature of the substrate. The dependence of the grain size and melt duration as a function of energy density revealed two major crystallization regimes. In the low energy density regime, the average grain size first increases gradually with increases in the laser energy density. In the high energy density regime, on the other hand, a very fine grained microstructure, which is relatively insensitive to variations in the laser energy density, is obtained. In addition, we have discovered that at the transition between these two regimes an extremely small experimental window exists, within which an exceedingly large grain-sized polycrystalline film is obtained. We suggest a liquid phase growth model for this phenomenon, which is based on the regrowth of crystals from the residual solid islands at the oxide interface.