Influence of 20 MeV electron irradiation on the optical properties and phase composition of SiOx thin films

Abstract

Homogeneous films from SiO1.3 (250 nm thick) were deposited on crystalline Si substrates by thermal evaporation of silicon monoxide. A part of the films was further annealed at 700 °C to grow amorphous Si (a-Si) nanoclusters in an oxide matrix, thus producing composite a-Si-SiO1.8 films. Homogeneous as well as composite films were irradiated by 20-MeV electrons at fluences of 7.2 × 1014 and 1.44 × 1015 el/cm2. The film thicknesses and optical constants were explored by spectroscopic ellipsometry. The development of the phase composition of the films caused by the electron-beam irradiation was studied by transmission electron microscopy. The ellipsometric and electron microscopy results have shown that the SiOx films are optically homogeneous and the electron irradiation with a fluence of 7.2 × 1014 el/cm2 has led to small changes in the optical constants and the formation of very small a-Si nanoclusters. The irradiation of the a-Si-SiOx composite films caused a decrease in the effective refractive index and, at the same time, an increase in the refractive index of the oxide matrix. Irradiation induced increase in the optical band gap and decrease in the absorption coefficient of the thermally grown amorphous Si nanoclusters have also been observed. The obtained results are discussed in terms of the formation of small amorphous silicon nanoclusters in the homogeneous layers and electron irradiation induced reduction in the nanocluster size in the composite films. The conclusion for the nanoparticle size reduction is supported by infrared transmittance results.