Ultrafast Generation of Unconventional $\{001\}$ Loops in Si

Ultrafast laser annealing of ion implanted Si has led to thermodynamically unexpected large $\{001\}$ self-interstitial loops, and the failure of Ostwald ripening models for describing self-interstitial cluster growth. We have carried out molecular dynamics simulations in combination with focused experiments in order to demonstrate that at temperatures close to the melting point, self-interstitial rich Si is driven into dense liquidlike droplets that are highly mobile within the solid crystalline Si matrix. These liquid droplets grow by a coalescence mechanism and eventually transform into $\{001\}$ loops through a liquid-to-solid phase transition in the nanosecond time scale.

Figure 1

Dependence of the $\mathcal{I}$-cluster mean energy per DA with size at several annealing temperatures. Energies are measured with respect to the perfect crystal. At each temperature, energy values have been obtained by averaging the energy per DA for all $\mathcal{I}$ clusters with the same size, no matter their particular atomic configuration. The different $\mathcal{I}$ cluster morphologies at low and high temperatures are illustrated in the snapshots, corresponding to ZY projections of DAs within the simulation cell taken after 10 ns annealing at (a) $T^{*}=0.50$, showing tetra-$\mathcal{I}$’s, small $⟨011⟩$ chains, and $\{111\}$ rodlike defects; and (b) $T^{*}=0.79$, showing tetra-$\mathcal{I}$’s, large disordered $\mathcal{I}$ clusters and $\{001\}$ loops.

Figure 2

Inset: ZY view of a disordered $\mathcal{I}$ cluster formed during annealing at $T^{*}=0.88$. Main plot: PDFs of this $\mathcal{I}$ cluster and two liquids at different temperature and pressure conditions.

Figure 3

Dependence with $\mathcal{I}$-cluster size of (a) the ratio of DAs per $\mathcal{I}$ atom, and (b) the cluster hydrostatic pressure. Values have been obtained by averaging for all $\mathcal{I}$ clusters with the same size at each temperature. In (c) pressure values are represented as a function of $T^{*}$ along with the solid-liquid equilibrium line of the Si phase diagram. Circles correspond to liquid $\mathcal{I}$ clusters and squares to $\{001\}$ loops.

Figure 4

Cross-section TEM micrographs from the SOI substrate annealed at $0.9\mathrm{J}/{\mathrm{cm}}^2$, and the bulk Si substrate annealed at $2.3\mathrm{J}/{\mathrm{cm}}^2$. Superimposed are the corresponding local maximum temperature profiles calculated by phase-field simulations. Green dashed line indicates the melt depth.