Phase separation of thin SiO layers in amorphous SiO/SiO2 superlattices during annealing

The preparation of ordered and arranged Si quantum dots using a SiO/SiO2 superlattice approach is presented. The different processes of phase separation and crystallization are studied in detail by infrared (IR) absorption and photoluminescence (PL) spectroscopy for different annealing temperatures from 300 to 1100 degreesC. IR spectra show a continuous shift of the Si-O-Si asymmetric stretching mode to higher energies with increasing annealing temperature, which is a sign of phase separation to Si and SiO2. Three PL bands are distinguished and correspond to the three processes of phase separation. A band centred at 2.2 eV is present in as-prepared samples and vanishes for annealing above 800 degreesC which is clearly correlated with defects. The second band shifting from 1.7 to 1.4 eV is detected for annealing temperatures between 300 and 900 degreesC. A strong red luminescence due to quantum confinement is observed for annealing above 900 degreesC. Our results indicate that the different and seemingly contradictory PL observations in the literature could originate from different states of network reorganization during the phase separation and crystallization processes. The origins of the different IR and PL bands are discussed in comparison with those of bulk crystalline SiO and SiO2.