First-principles prediction of optical absorption enhancement for Si native defect clusters under biaxial strain

We use density-functional theory calculations to qualitatively explore the effects of fourfold-coordinated vacancy (V₄) and interstitial (I ₄) clusters on optical absorption spectra in crystalline Si (c-Si) under selected conditions of biaxial strain (ε=-3, 0, and 3%). While both native defect clusters enhance c-Si absorption by redshifting the absorption edge, we observe additional enhancement from biaxial strain. Increased strain magnitude tends to increase the absorption enhancement effect, but the optimal sign of strain exhibits a complementary relationship: compressive strain most effectively enhances V₄ absorption, while tensile strain most effectively enhances I₄ absorption. The absorption redshift as a function of strain correlates well with effective bandgap reduction, including the appearance of an intermediate band under certain conditions (ε=-3 and 0%) for V₄. Our results suggest that manipulation of native defect distributions and their strain fields can be used to engineer the Si absorption spectra.