Structural and chemical features of Si nanocrystallites in nc-Si:H films

초록

Nanocrystalline hydrogenated silicon (nc-Si:H) thin films have attracted much attention due to their potential application in various devices. In particular, it is expected that these materials can be used to fabricate micro- and opto-electronic devices compatible with Si integrated circuit technology. Recently, it has been known that quantum size effects of Si nanocrystallites are the main source for the unique and useful optical features of the nc-Si:H thin films. In this study, we investigated the hydrogen effects on the nanostuctural features of nc-Si:H thin films, based on the structural and chemical analysis; the films were prepared by plasma enhanced chemical vapor deposition (PECVD) techniques. The nanostructural and chemical features of nc-Si:H thin films were investigated as a function of deposition conditions. SiH4 gas flow rate was varied to control the amount as well as the type of hydrogen-related bonds in the films. It was found that the crystallite size varied with the relative fraction of Si-H3 bonds in the films, / n=integer, which was sensitively related with the flow rate of SiH4 reaction gas. The Si nanocrystallites in the films enlarged from ~2.0 to ~8.0 nm in their size with increasing the SiH4 gas flow rate from 8 to 20 sccm, while the PL emission energy varied from 2.5 to 1.8 eV; the relative fractions of the Si-H3, Si-H2, and Si-H bonds in the amorphous matrix were also varied sensitively with the SiH4 gas flow rate. A model for the nanostructure of the nc-Si:H films was suggested to feature the variation in the size and chemical bonds of the nanocrystallites as well as the amorphous matrix depending on the deposition conditions.