Nuclear energy and cascade effects on the ion-assisted crystal nucleation in amorphous silicon

We have investigated the kinetics of the ion-assisted nucleation of crystalline grains in amorphous silicon layers as a function of the ion mass and the incident energy, by using C, Si, Ge, and Pt ion beams accelerated at energies ranging between 80 keV and 7 MeV. By transmission electron microscopy analyses we have characterized the morphology of the irradiated layers and we estimated the nucleation and the crystal growth rates for each irradiation condition. While the crystal growth velocity increases linearly by increasing the number N1 of displaced atoms generated per unit length by the impinging ions, the nucleation rate is constant at low values of N1 and then decreases. Moreover, it does not depend on the density of the collision cascade. The data are explained in terms of a description according to which the ion-assisted amorphous to crystal transition in silicon is controlled by the balance of a prompt amorphization process and a crystallization kinetics enhanced by long living defects generated by the ion beam.