Ion Assisted Nucleation in Amorphous Silicon: Mechanism and Theoretical Description
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ABSTRACT We present new results on the ion-assisted amorphous to polycrystal transition in silicon. The grain growth velocity Vg and the nucleation rate r exhibit quite different behaviours: i) ion irradiation enhances r by a factor which is several orders of magnitude larger than that observed for vg; ii) irradiation with lighter ions produces a decrease of Vg and a strong increase of r; iii) the increase of dose rate produces a decrease of both Vg and r, but is particulary severe for r. These results have been explained by assuming that ion irradiation produces three fundamental effects: i) generation of long living defects which increase the free energy of the amorphous phase to the value of the fully unrelaxed amorphous silicon, causing the decrease of the thermodynamic barrier to nucleation; ii) generation of defects promoting the transition kinetics at concentrations well above the thermal equilibrium value; iii) prompt amorphization of a small volume at the surface of each crystalline grain.
INTRODUCTION Ion bombardment of pure amorphous silicon (a-Si) (in the absence of any pre-existing amorphous-crystal interface) produces several remarkable effects. Indeed, defects are introduced within the amorphous network and can be monitored by Raman spectroscopy [1], calorimetry [2], and conductivity measurements [3,4]. The annealing kinetics of these defects shows that different defect structures exist in a-Si. However, the bimolecular dynamics of annihilation, the long living character, and the dose rate effects have all been confirmed and measured in situ directly during ion bombardment. The thermodynamical parameters of a-Si are also strongly affected both by irradiation and by annealing. For instance, it has been shown that the difference in free energy, Ag, between the amorphous and the crystal phase is - 0.15 eV/atom in asimplanted a-Si (unrelaxed state) while reduces to - 0.11 eV/atom in annealed a-Si (relaxed state) [1,5]. This difference has been shown to produce dramatic effects in the crystal grain nucleation in a-Si during ion bombardment [6]. In these experiments the nucleation is observed at substrate temperatures around 500 'C, well below the pure thermal regime. In fact, since during irradiation Ag, is increased, the thermodynamical barrier for nucleation is decreased and the kinetics of the nucleation process increases by several orders of magnitude. The rate of transformation is controlled by beam parameters such as dose rate and average energy deposited in the layer by nuclear elastic collisions. However, when comparing in detail the effects due to ions characterized by rather large differences in the average nuclear elastic energy loss, the modeling does not account for all the details. In the present paper we will show results obtained during ion assisted nucleation in a-Si using a wide range of irradiation conditions. The experimental results are quantitatively explained and it is demonstrated that a crucial role in the nucleation process is played by the balance between amorphization and crys
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