Optical and structural properties of SiOx films grown by molecular beam deposition: Effect of the Si concentration and annealing temperature

We study the properties of Si-rich silicon oxide SiOx (x<2) films grown on silica substrates by molecular beam deposition, in a wide range of Si content and annealing temperatures. The measured refractive index and absorption coefficient are successfully described using the effective medium approximation and the chemical compositions measured by x-ray photoelectron spectroscopy (XPS). The Si-SiO2 phase separation and the degree of Si crystallization increase with the annealing temperature; however, even after annealing at 1200 degrees C, the samples contain a large proportion of suboxides and partially disordered Si. The Si Raman signal and the absorption coefficient are nearly proportional to the amount of elemental Si provided by XPS. On the other hand, the Si Raman signal is much weaker than it is expected from the amount of elemental Si, which can be explained by the presence of ultra-small Si nanocrystals (diameters<2 nm) and/or by the difference in the properties of bulk and nanoscale Si. The 1.5-eV photoluminescence (PL) intensity is the highest for annealing at 1100-1150 degrees C and x = 1.8-1.9. In contrast, the PL quantum yield steadily increases when the intensity of the Si Raman signal decreases. This observation suggests that the Si nanocrystals observed in the Raman spectra are not direct light-emitting centers. The temperatures induced by laser light in these films are surprisingly high, especially at the highest Si content (x similar to 1.3). The laser-induced temperature (up to similar to 350 degrees C) substantially down-shifts the Raman band of Si nanocrystals (in our experiments from similar to 518 to similar to 512 cm(-1)) and increases the absorption coefficient (by a factor of similar to 1.4). (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4764893]