Changes in composite nc-Si-SiO2 thin films caused by 20 MeV electron irradiation

Homogeneous films from SiO_x (x = 1.2, 1.3) were deposited on crystalline Si substrates by thermal evaporation of silicon monoxide in vacuum. A part of the films was further annealed at 1000 °C to grow Si nanocrystals in a silicon dioxide matrix. Homogeneous and composite films with initial x = 1.2 were irradiated by 20-MeV electrons at a fluence of 3.6 × 10¹⁵ electrons/cm² and the effect was compared with that caused by irradiation of films with x = 1.3 at much lower fluence of 2.4 × 10¹⁴ electrons/cm². Transmission electron microscopy, infrared transmission and Raman spectroscopies and spectroscopic ellipsometry were used to get information about the SiO_x films composition, nanoparticle crystallinity and space distribution as well as for electron-beam induced changes in the nanocrystal size. The infrared data have indicated that electron irradiation with 3.6 × 10¹⁵ electrons/cm² induced phase separation in the homogeneous films with x = 1.2. Not only silicon nanocrystals but also a small amount of amorphous silicon phase have been detected in the composite films before and after electron irradiation. Nanocrystallite size decrease induced by the electron beam irradiation at a fluence of 3.6 × 10¹⁵ electrons/cm² has been assumed on the basis of the observed changes in the effective refractive index, extinction coefficient and nanocrystallite volume fraction in the composite films.