In this paper, the luminescence properties of thin, thermally grown SiO2 layers implanted with silicon and carbon ions are explored. The doses and energies were chosen in such a way that the resulting peak concentration of excess Si and C amounts to 5-10% in a depth region of 60-180 nm below the surface. The microstructure was investigated by Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). Amorphous nanostructures with a size between 2 and 3.5 nm were found in depth region between 80 and 150 nm below the oxide surface. Strong photoluminescence (PL) around 2.1 and 2.7 eV has been observed after excitation at 4.77 eV. SiyC1-yOx complexes with x < 2 are assumed to cause the observed PL in the blue spectral region.