Amorphization of silicon by elevated temperature ion irradiation

Despite extensive study into the amorphization of silicon by ion irradiation, a detailed understanding of the mechanisms associated with the process is still not available. This is especially true in the case of elevated temperature irradiations where increased levels of dynamic annealing can balance defect production. By controlling the irradiating ion flux (defect production rate), the temperature at which an amorphous layer first appears has been determined for a specific fluence of ions, ranging in mass from 12 (C) to 132 amu (Xe), over a matrix of two orders of magnitude in ion flux and 320°C in temperature. The apparent activation energies determined from the data span from 0.7 to 1.7 eV and increase as a function of ion mass and, consequently, temperature. Previously interpreted as representing processes controlling the crystalline-to-amorphous phase transition, these values are discussed in terms of a two-stage nucleation-limited amorphization process. Results are presented which indicate, that, in the regime where dynamic defect annealing is significant, the amorphization process is initially limited by the availability of nucleation sites, supplied by stable extended defect structures produced by defect accumulation and, subsequently, by the supply of simple defects to these sites.