Tuning the emission wavelength of Si nanocrystals in SiO2 by oxidation

Si nanocrystals (diameter 2-5 nm) were formed by 35 keV Si⁺ implantation at a fluence of 6 x 10¹⁶ Si/cm² into a 100 nm thick thermally grown SiO₂ film on Si (100), followed by thermal annealing at 1100°C for 10 min. The nanocrystals show a broad photoluminescence spectrum, peaking at 880 nm, attributed to the recombination of quantum confined excitons. Rutherford backscattering spectrometry and transmission electron microscopy show that annealing these samples in flowing O₂ at 1000 °C for times up to 30 min results in oxidation of the Si nanocrystals, first close to the SiO₂ film surface and later at greater depths. Upon oxidation for 30 min the photoluminescence peak wavelength blueshifts by more than 200 nm. This blueshift is attributed to a quantum size effect in which a reduction of the average nanocrystal size leads to emission at shorter wavelengths. The room temperature luminescence lifetime measured at 700 nm increases from 12 µs for the unoxidized film to 43 µs for the film that was oxidized for 29 min.