Multi-peak photoluminescence of ultra-small Si quantum dots embedded in SiO2 films

• Multiple PL bands are obtained in ultra-small Si-QDs at room temperature.
• Optical phonon assisted optical emissions are obtained in high energy tail.
• Average decay times of 8.9–9.5 ns are obtained in all the narrow PL bands.

Si quantum dots (Si-QDs) embedded in SiO2 films have been prepared and multiple photoluminescence (PL) bands are obtained when the annealing temperature is below 900 °C. Microstructure analyses show that the Si-QDs are crystallized only when the annealing temperature is as high as 1000 °C, and the ultra-small Si-QDs are surrounded by SiOx compounds. Multiple peaks can be observed in the PL and PL excitation spectra. The multiple PL bands can be fitted to a broad band located at 2.2 eV together with seven narrow bands, and the energy separations for the adjacent narrow PL bands are between 140 meV and 158 meV, while the PL excitation spectrum can be fitted to eight optical excitation bands with energy separations about 160 meV. The broad PL band located at 2.2 eV is related to self-trapped exciton optical emission of ultra-small Si-QDs, while the narrow bands are caused by optical phonon assisted optical emissions, and both the PL excitation and emission processes can be explained in the configurational–coordinate model. Time-resolved PL spectra show that the average decay times of 8.9–9.5 ns are obtained for all the narrow PL peaks.