Spectroscopic ellipsometry analyses of sputtered Si/SiO2 nanostructures

A spectroscopic ellipsometry technique is used to attempt a quantitative analysis of thin ${\mathrm{Si/SiO}}_{\mathrm{2}}$ nanocomposite films obtained by magnetron co-sputtering. The layers are first fabricated with varying values of deposition temperature and sputtered silicon area, before being annealed at different temperatures. Using an effective medium model, the ellipsometry tool allowed the estimate of the thickness and the volume fraction of silicon agglomerates or grains, in addition to the optical parameters of the layer through a tentative determination of the dielectric function of the Si nanostructures. The Si content obtained by the ellipsometry approach agrees with those previously determined by direct measurements. Besides the obvious increase of incorporated Si with the sputtered area of this material, we noticed a maximum excess of Si for a deposition temperature in the $\text{400–500\hspace{0.05em}°}\mathrm{C}$ range. In this respect, the peak position of the photoluminescence (PL) detected only in the annealed samples is found to be closely connected with the amount of excess Si, and in consequence, with the mean size of the thermally grown nanocrystallites. The systematic red shift of maximum PL with the rise of Si content, and implicitly with the increase of the particle size, regardless of the deposition conditions used for the fabrication of the original layer, is liable to allow conclusive argument as for the attribution of the PL in the 1.30–1.65 eV range to the quantum confinement effect. Besides the main contribution of the nanoparticle density, the quality of the ${\mathrm{Si/SiO}}_{\mathrm{2}}$ interface, as examined by infrared absorption spectroscopy, appears determining for the PL efficiency, and is increasingly improved towards stoichiometric ${\mathrm{SiO}}_{\mathrm{2}}$ when the annealing temperature is increased up to 1100 °C.