HETEROGENEOUS AMORPHIZATION OF SI DURING ION IRRADIATION - DEPENDENCE OF AMORPHOUS SI NUCLEATION KINETICS ON DEFECT ENERGY AND STRUCTURE

During irradiation of polycrystalline Si thin films by 1.5 MeV Xe+ ions, amorphous Si is nucleated heterogeneously at internal interfaces, such as grain boundaries and stacking faults in the temperature range of 150-225-degrees-C. The heterogeneous nucleation kinetics of amorphous Si are strongly dependent on grain boundary structure and energy. Nucleation kinetics of amorphous Si and grain boundary structure have been characterized experimentally using high-resolution electron microscopy. The strong dependence of the heterogeneous nucleation rate of amorphous Si on defect structure suggests that diffusive kinetic processes after the "prompt" part of the cascade are important in determining the rate of amorphization. Comparison of the heterogeneous nucleation kinetics of amorphous Si with tight binding, semiempirical and continuum theory estimates of grain boundary energies for particular boundary structures suggests that grain boundary-mediated amorphization is a thermodynamically limited process, and is analogous to grain boundary melting of the solid at the equilibrium temperature than to mechanical deformation.