Optical transition in nanocrystalline Si doped SiO₂ thin films formed by cosputtering

Optical transition properties of nanocrystalline silicon (nc-Si) doped SiO₂ thin films prepared by cosputtering with SiO₂ target and Si-chips followed by high temperature annealing were investigated as a function of ratio of target area (Si/SiO₂) in the range of 0 to 0.19. Optical transmission measurements revealed that the indirect band-gap of nc-Si decreased from 4.87 to 2.32 eV with increasing Si/SiO₂ from 0.04 to 0.19, whereas no noticeable absorption was observed for the samples with Si/SiO₂ = 0.00. Photoluminescence (PL) measurements showed that considerably weak emissions due to oxygen deficient center (ODC) in SiO₂ films were observed at 514-539 nm for samples with lower Si/SiO₂ of 0.0-0.09, while the strong emissions due to an interfacial layer between the nc-Si core and the SiO₂ surface layer were observed at 746-808 nm for samples with higher Si/SiO₂ of 0.13-0.19. PL excitation spectrum monitored at 800 nm showed that the PL at 800 nm was excited through nc-Si. On the other hand, cathodoluminesncence peaks were observed at 455 and 465 nm for the samples with 0.04 and 0.13, respectively, in which both peaks were assigned to originate from oxygen vacancies created during electron beam irradiation. The mechanisms of three types of luminescence, due to the ODC in SiO₂, band-to-band transition of nc-Si, and the interfacial layer between the nc-Si core and the SiO₂ surface layer, are discussed. (author)