EFFECT OF FAST SOLIDIFICATION ON IMPURITY TRAPPING AND AMORPHOUS FORMATION IN Si

Abstract

The main features of the thermal model for the semiconductor laser annealing are reviewed with the main emphasis to the parameters which affect the liquid solid interface velocity during the solidfication. The experimental conditions controlling this velocity are pointed out and several examples which cover a broad range of values, from few cm/s up to 100 m/s are shown. The velocity dependence of the impurity segregation and trapping is shown as experimentally determined by channeling backscattering measurements of Si implanted and irradiated samples. The trapping behaviour at the limiting condition of very high and very low speed is discussed also in connection with the cellular structure arising from interface instability. The role of the solid phase diffusivity on the segregation coefficient at very high speed is also evidenced by means of several examples including the comparison between trapping in Si and Ge of the same impurity. Results on the formation of amorphous silicon layers by very fast solidification after picosecond laser irradiation are also presented in comparison with calculation of the solid-liquid interface velocity. In this case a more complex model, which take into account the smearing out of the deposited energy distribution by free carrier generation and diffusion is required.