Optical properties of Si+-ion implanted sol–gel derived SiO2 films

Optical properties of commonly investigated Si+-implanted SiO2 layers (thin SiO2 films prepared by thermal growth on c-Si substrate) are compared with those of non-conventional SiO2 layers fabricated by sol–gel process. The sol–gel films, deposited on different substrates, were implanted and annealed in a similar way as the thermal SiO2 films on c-Si. Striking differences between these two types of samples were found. The conventional Si+-implanted SiO2/c-Si layers contain Si nanocrystals and their photoluminescence (PL) properties were found very similar to porous silicon (PL in the red/NIR spectral regions, PL decay time of the order of 10 μs, PL temperature dependence well described by the exciton singlet–triplet splitting model). On the other hand, the Si+-implanted sol–gel SiO2 films exhibit room temperature PL spectra peaked in the blue–green region, PL decay is considerably faster (of the order of 1 ns) and also PL temperature dependence differs substantially from the samples of the first type. Possible influence of different substrates (silica, c-Si) is also investigated and it is shown that the observed PL is an inherent property of the implanted sol–gel SiO2 layers. The slow red PL, in agreement with other authors, is ascribed to radiative recombination of excitons in Si nanocrystals, while the fast blue PL characteristic of ion-implanted sol–gel derived SiO2 films is obviously of defect origin.