Silicon nanocrystals embedded in oxide films grown by magnetron sputtering

This paper presents a comparison of the results that we obtained and reported over the last few years on the structural, optical and light emitting properties of Si-SiO₂ and Si-Al₂O₃ films that were fabricated using a specific configuration of RF magnetron sputtering. In these films the Si volume fraction, x, varies along the film (which is typically 14 cm long) from a value of ~0.1 at one end to ~0.9 at the other end. For the films with x > 0.3, the formation of amorphous Si clusters was observed in as-deposited Si-SiO₂ and Si-Al₂O₃ films. Si nanocrystals (Si-ncs) were generated by high-temperature annealing of the films in nitrogen atmosphere. We found that two processes can contribute to the Si-ncs formation: (i) the crystallization of the existing amorphous Si inclusions in the as-deposited films, and (ii) the thermally stimulated phase separation. Process (i) can be responsible for the independence of Si-ncs mean sizes on x in annealed films with x > 0.5. At the same time, difference in the structural and the light emitting properties of the two types of films was observed. For the samples of the same x, the Si-ncs embedded in the Al₂O₃ host were found to be larger than the Si-ncs in the SiO₂ host. This phenomenon can be explained by the lower temperature required for phase separation in Si-Al₂O₃ or by the lower temperature of the crystallization of Si-ncs in alumina. The latter suggestion is supported by Raman scattering and electron paramagnetic resonance spectra. In contrast with the Si-SiO₂, the Si-ncs embedded in Si-Al₂O₃ films were found to be under tensile stress. This effect was explained by the strains at the interfaces between the film and silica substrate as well as between the Si inclusions and the Al₂O₃ host. It was also shown that exciton recombination in Si-ncs is the dominant radiative channel in Si-SiO₂ films, while the emission from the oxide defects dominates in Si-Al₂O₃ films. This can be due to the high number of non-radiative defects at Si-ncs/Al₂O₃ interface, which is confirmed by our electron paramagnetic resonance data and is consistent with our above suggestion of mechanical stresses in the Si-Al₂O₃ films.