Spectroscopic ellipsometry investigation of the optical properties of nanostructured Si/SiNx films

In this paper we use spectroscopic ellipsometry to investigate the optical properties of silicon quantum dots (Si-QDs) embedded in silicon-rich silicon nitride (SRSN) films over the (1.5-5.9 eV) photon energy range. These films were elaborated by microwave plasma enhanced chemical vapor deposition for various gas flow ratio [R=(NH3)/(SiH4)]. We have analyzed the ellipsometric data using various modeling methods: Forouhi-Bloomer (FB) and Tauc-Lorentz (TL) dispersion formulas and wavelength-by-wavelength (lambda-by-lambda) numerical inversion. This comparative study allowed us to analyze in depth the influence of ammonia flow on the physical properties of SRSN films and Si-QDs. FB and TL models and lambda-by-lambda inversion have revealed that the imaginary dielectric function of the Si-QDs, is characterized by a single broad peak similar to that of amorphous silicon. The fitted band gap (E-g) and the other optical properties were found to depend on the ammonia flow. We also determined that Eg obtained by FB model seems to be less realistic than that obtained with TL dispersion formula. According to TL model, the obtained Eg values of Si-QDs varied between 1.58 and 2.07 eV with the change in R from 1.64 to 1.79. This band-gap extension is in good agreement with measurements observed by photoluminescence. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3331551]