The mechanisms of photoluminescence from Si nanocrystals formed by pulsed-laser deposition

We have investigated the different mechanisms of photoluminescence (PL) of Si nanocrystals (NCs) due to the quantum confinement effect (QCE) and interface states. Si NCs were formed by pulsed-laser deposition (PLD) in Ar or O₂ gas ambient. The collisions between the ejected species greatly influence the morphology of Si NCs and cause a transition from film structure to porous cauliflower-like structure, as the ambient gas pressure increases from 1 mTorr to 1 Torr. The optical absorption of the Si NCs shows an indirect band transition. Broad PL spectra are observed from Si NCs. The peak position and intensity of the PL band at 1.8–2.1 eV are dependent on excitation laser intensity, while intensity changes and blue shifts are observed after oxidation and thermal annealing. The PL band at 2.55 eV displays vibronic structures with periodic spacing of 97 ± 9 meV, while no peak shift is found before and after oxidation and thermal annealing. Combined with the PL of Si NCs obtained by crumbling electrochemical-etched porous Si layer, the results give strong evidence that the PL band at 1.8–2.1 eV is due to the QCE in Si NC core while the PL band at 2.55 eV is related to the localized surface states at SiO_x/Si interfaces. Laser annealing of Si NC films was studied. Laser annealing improves the crystallinity and enhances the PL intensity, while high laser fluence causes damages in the films. After laser annealing, ripple structures consisting of nanoparticles are formed around the droplets on the film surface. The ripple period depends on the laser incident angle, which can be explained by the surface scattered wave theory.