Role of structural disorder in optical absorption in silicon

Using density-functional theory calculations, we explore the effects of structural disorder on optical absorption in crystalline Si. We incorporate neutral, ground-state vacancy clusters (V-n, n <= 6, n = 12 and 32) to incrementally introduce disorder and compare results to the limiting cases exemplified by amorphous and crystalline Si. In particular, we compute optical spectra for the dielectric function, epsilon(omega), and absorption coefficient, alpha(omega), and observe substantial absorption enhancement for all fourfold-coordinated vacancy clusters, similar to the expanded alpha(omega) envelope of amorphous Si over crystalline Si. We isolate structural contributions to absorption using constant-density vacancy cluster distributions to show that increased structural disorder predictably enhances absorption. In addition, we observe diminished absorption of V-1 as a function of supercell size, suggesting dilute Vn concentrations may be difficult to physically verify. Furthermore, we found the density effect on absorption insignificant in amorphous Si at the subnanometer scale, so we discuss the expected contribution of density from a multiscale perspective.