The photoluminescence (PL) spectrum of crystalline Si (c-Si) shows sharp peaks at energies below the semiconductor energy gap due to radiative recombinations. These PL lines open the possibility for developing silicon-based optoelectronic devices. As an example, two particularly intense zero-phonon lines that appear in the PL spectra of damaged c-Si are the W (1.018 eV) and X (1.040 eV) lines. Both of them have been experimentally obtained in ion-implanted as well as in high energy electron- and proton-irradiated c-Si. Experiments have revealed that these centers have an interstitial nature although at these conditions a large variety of defects coexist. This makes the identification of defects responsible for these lines through experiments difficult. The association of PL lines with the particular defects that originate them is important to improve the non-destructive characterization capabilities of the PL technique for defect identification, and to explore the nature of these radiative defects, which is interesting from a technological point of view.
We found using kinetic Monte Carlo (kMC) simulations that at the experimental conditions at which the intensities of W and X lines are optimized, ∼98% of existing interstitial clusters has five or less interstitials. Taking this consideration into account, we used molecular dynamics simulations for exploring the configurational landscape of Si interstitial clusters containing from 2 up to 5 interstitials, which allowed us identifying more than 100 configurations. Among all of these defect configurations, we analyzed those with symmetry compatible with that of W and X centers were analyzed in detail by using ab initio simulations. This technique allowed us the evaluation of: (i) the donor level in the gap introduced by the defect, which can be related with the PL photon energy; (ii) the defect induced modifications of the electronic band structure, which shows whether a defect might favor radiative recombinations or not; (iii) their local vibrational modes, which can be directly compared to the peaks that appear in the phonon-side bands of zero-phonon lines in PL spectra; and (iv) the transition amplitudes at band edges, which permits evaluating which defect candidate has a stronger transition amplitude.
Atomic configuration of found defects with the symmetry compatible with that of W and X centers. Configurations are shown on suitable projections to highlight their symmetry, which is indicated in parentheses. Silicon lattice atoms are represented by white spheres, while Si atoms belonging to defect clusters are colored in blue (dark gray). From our ab initio simulations we concluded that I3-V is the most likely defect candidate to be associated to the W PL center in c-Si, whereas for the X PL center we cannot be conclusive about which one is the most likely defect candidate (I. Santos et al., J. Phys. D: Appl. Phys. 49, 075109 (2017) - Insights on the atomistic origin of X and W photoluminescence lines in c-Si from ab initio simulations).