Luminescence from plasma deposited silicon films

We report the observation of room-temperature and low-temperature visible photoluminescence from nanocrystalline silicon (nc-Si) thin films produced by plasma-enhanced chemical vapor deposition (PECVD) through a gas discharge containing SiH₄ diluted in Ar and H₂. The nanocrystalline silicon films were characterized using transmission electron microscopy, spectroscopic ellipsometry, infrared and Raman spectroscopy, and were examined for photoluminescence. Luminescent films consisted of dense silicon nanocrystals that grew in a columnar structure with approximately 20%-30% void space dispersed inside the film. Aside from having small crystalline silicon regions, the structure of the nc-Si films is different than that of porous Si, another luminescent Si material generally produced by electrochemical anodization. Yet, the photoluminescence spectra of the thin nc-Si films were found to be similar to those observed from porous silicon. This similarity suggests that the same mechanism responsible for light emission from porous silicon may also be responsible for emission from nc-Si. The photoluminescence spectra are analyzed in terms of a simple quantum confinement model. Although the mechanism of visible luminescence from porous Si is still a point of controversy, our results support the hypothesis that some of the luminescence from porous silicon and nc-Si films is due to quantum confinement of electrons and holes in crystals with dimensions 2-15 nm.