Silicon-rich SiO$_x$ layers were fabricated in an ion beam sputtering deposition system by varying the thickness and the oxygen content ($x$) from 6 to 20 nm and from 1.0 to 1.8, respectively. The samples were subsequently annealed at 1100 $^circ$C for 20 min to form Si nanocrystals (Si NCs) and were hydrogenated at 650 $^circ$C for 1 h under a hydrogen gas flow to passivate the Si dangling bonds. High-resolution transmission electron microscopy (HRTEM) images demonstrated the existence of Si NCs. For $x leq 1.2$, the photoluminescence (PL) peaks showed small blueshifts with increasing layer thickness for thinner samples, but the peak shifts became almost saturated with further increases in the layer thickness. For $x geq 1.4$, the PL showed monotonic redshifts as the layer thickness was increased, resulting from the dominance of PL from the larger NCs in the bulk region of SiO$_x$. The SiO$_2$-capped samples were blue-shifted compared to the uncapped ones, which is consistent with the effect of the increase in x due to the mixing of the SiO$_x$ and the SiO$_2$ layers. Possible growth and luminescence mechanisms for the Si NCs are proposed to explain the experimental results.
Keywords: Si nanocrystals, Photoluminescence, Silicon-rich oxide, Oxygen content, Thickness, Cap layer