ION-ASSISTED NUCLEATION IN AMORPHOUS-SILICON - ION MASS AND DOSE-RATE EFFECTS

The nucleation of crystal grains in amorphous silicon induced by 600 keV Kr or Ar irradiation is investigated. It is shown that at 500-degrees-C the nucleation rate is greatly enhanced with respect to the pure thermal value. An unexpected behaviour is observed by changing ion mass and/or dose rate in contrast with the trend measured for the growth rate. An increase in dose rate results in a decrease in the nucleation rate without any appreciable variation of the growth velocity. These data, that are successfully described by classical models of nucleation and growth, are discussed and qualitatively explained by assuming that beam-generated defects modify the free-energy content of both amorphous and crystalline phases.