Phase transformation and impurity redistribution during pulsed laser irradiation of amorphous silicon layers

Microstructural changes and impurity redistribution have been studied in 30Si+-, 75As+-, 63Cu+-, and 115In+-implanted, laser-annealed amorphous silicon layers for various pulse energy densities. Electron microscopy showed the formation of two distinct polycrystalline regions after irradiation with pulse energy densities below the threshold for ‘‘defect-free’’ annealing. A large polycrystalline (LP) region is found at the surface; a fine polycrystalline (FP) region lies below the LP and any remaining amorphous material. The extent of impurity redistribution, determined by Rutherford backscattering measurements, was much greater in the LP region than in the FP region, indicating a very short melt lifetime in the latter. Accumulation of Cu and In at the free surface was found, indicating motion of an interface towards the free surface during growth of the large polycrystals. Similarly, accumulation was found at the FP-amorphous Si boundary, indicating motion of an interface away from the free surface during growth of the fine polycrystals. The LP thickness was found to correspond to the maximum depth of melting calculated by a heat-flow program. These results indicate that the FP layer forms by explosive crystallization.