Effect of implanted Si concentration on the Si nanocrystal size and emitted PL spectrum

Implantation of Si+ in excess into SiO2 followed by annealing produces Si nanocrystals (Si-nc) embedded in the SiO2 layer, which can emit a strong photoluminescence (PL) signal. Several samples have been characterized by means of ellipsometry and transmission electron microscopy (TEM). For local Si concentrations in excess of ∼2.4 × 1022 Si+/cm3, the Si-nc diameter ranges from ∼2 to ∼22 nm in the whole sample, the Si-nc in the middle region of the implanted layer being bigger than those near the surface or the bottom of the layer. The depth distribution of the Si-nc agrees relatively well with the SRIM simulation as well as with the depth distribution of the n and k components of the complex refractive index. For SiO2 layers thermally grown on a Si wafer, the PL spectrum is modulated by optical interference of the pump laser and of the light emitted by the Si-nc in this layer. The good agreement between the results of the model calculations and experimental measurements indicates that for low and moderate Si concentration in excess (<8 × 1021 cm−3) the PL light emitters are localized in a layer situated at the same depth as the Si-nc depth distribution. However, for a Si concentration in excess of ∼2.3 × 1022 cm−3, the depth distribution of light emitters is narrow and situated mostly in the first half (relative to the surface) of the Si-nc depth distribution. This observation indicates that the recombination of the electron-hole pair at the interfaces could be responsible for the emitted PL spectrum.