AMORPHIZATION OF SILICON BY ELEVATED-TEMPERATURE ION IRRADIATION

Despite extensive study into the amorphization of silicon by ion irrad iation, a detailed understanding of the mechanisms associated with the process is still not available. This is especially true in the case o f elevated temperature irradiations where increased levels of dynamic annealing can balance defect production. By controlling the irradiatin g ion flux (defect production rate), the temperature at which an amorp hous 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 t wo orders of magnitude in ion flux and 320 degrees C in temperature. T he 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, t emperature. Previously interpreted as representing processes controlli ng the crystalline-to-amorphous phase transition, these values are dis cussed in terms of a two-stage nucleation-limited amorphization proces s. Results are presented which indicate, that, in the regime where dyn amic defect annealing is significant, the amorphization process is ini tially limited by the availability of nucleation sites, supplied by st able extended defect structures produced by defect accumulation and, s ubsequently, by the supply of simple defects to these sites.