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Transition of a Si-As solution to the amorphous phase induced by pulsed-laser quenching.

Si-As supersaturated solid solutions obtained by ion implantation on 〈100〉- and 〈111〉-oriented Si substrates and laser annealing have been made amorphous by pulsed-laser irradiation. The transition to the amorphous phase has been detected in situ by time-resolved reflectivity measurements, which demonstrated that it takes place via a liquid phase as in the case of pure Si. Heat-flow computations have been employed to correlate irradiation parameters with the solidification velocity. The critical liquid-solid interface velocity to grow amorphous layers is much lower for the Si-As solution than for pure silicon. It decreases from 5 to 3 m/s with increasing As concentration from 8 to 13 at. %. Moreover, it is a factor 1.5 smaller for the 〈111〉 substrates than for the 〈100〉 ones. We explain this effect in terms of the difference between the crystalline and the amorphous melting temperature decreasing with increasing As concentration. The results are consistent with an evaluation of the free energy of Si-As solutions performed by assuming a heat of solution in the amorphous phase lower than in the crystal phase. Both experimental data and thermodynamic evaluations indicate that when the As concentration approaches an upper limit of about 15 at. % only the liquid-amorphous transition is possible since the free energy of the crystal and the amorphous phase tend to coincide.