Photoluminescence characterization of dependence of Si-nanocrystals formation in Si-rich SiOx on thickness, oxygen content, and the existence of a SiO2 cap layer

Silicon-rich SiOx 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 degrees C for 20 min to form Si nanocrystals (Si NCs) and were hydrogenated at 650 degrees 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 <= 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 >= 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 SiOx. The SiO2-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 SiOx and the SiO2 layers. Possible growth and luminescence mechanisms for the Si NCs are proposed to explain the experimental results.