Photoluminescence Mechanism Of Silicon Nanoparticles Embedded In Silicon Nitride Films

Silicon-rich hydrogenated amorphous silicon nitride thin films are deposited by helicon wave plasma-enhanced chemical vapor deposition (HWP-CVD) technique. The structural characterization and optical absorption properties of the films are analyzed by Raman scattering spectra, Fourier-transform infrared spectroscopy (FTIR), transmission electron micrograph (TEM) and optical absorption spectra. The luminescent characteristics of the films are investigated by means of photoluminescence (PL) and photoluminescence excitation (PLE) measurements and the luminescence mechanisms are discussed in detail.The results of microstructure characterization suggest that the films are in the structure of Si nanoparticles separately embedded in silicon nitride matrix. The average size of Si nanoparticles decreases with increasing nitrogen atomic concentration. The analysis of optical absorption properties indicates that the optical gap of the film increases with increasing nitrogen content due to the enhancement of quantum confinement effect for Si nanoparticles. The increasing of Urbach energy Eu indicates that the disorder degree and the width of band tail states increase with the decreasing of Si nanoparticle size.The PL spectra analysis shows that the PL spectra consists of three Gaussian bands including a dominant PL band blueshifted with the decreasing Si particle size, a green and a blue PL band fixed at 2.50 eV and 2.90 eV, respectively. The PLE measurements clarify the origination of the PL bands. The first band is related to the quantum confinement effect of Si nanoparticles. While the second and the third one originate from the recombination at silicon dangling bond defect centers and the localized defect states in amorphous silicon nitride. A detailed analysis of the PL band from Si nanoparticles indicates that the light emission can be interpreted by the model of quantum confinement effect excitation surface state luminescence. That is to say, the carriers are excited between the quantum states of Si nanoparticles and then relax to the surface states in which they radiatively recombine in the form of exciton, resulting in an energy shift between the luminescence and optical absorption.