In this work, we have found that the charging of nc-Si in a thin gate oxide can induce a reduction in the total gate oxide capacitance. The capacitance can approach zero value if all the nanocrystals are charged up. The reduction of the gate oxide capacitance is attributed to the premature breakdown in the gate oxide due to the charging up in the nanocrystals, as the reduction of the gate oxide capacitance corresponds to a large decrease in the gate oxide leakage current. Here the breakdown caused by the charging in the nanocrystals is somewhat similar to the soft or hard breakdown in pure SiO2 thin films that are related to the charge trapping in the oxide film. The breakdown caused by the charging in the nanocrystals is found to be fully recoverable under ultra-violet (UV) light illumination for 5 minutes and a thermal annealing at temperature of 100°C for 10 minutes. The reduction and recovery of the capacitance due to the charging and discharging in the nanocrystals is explained with an equivalent circuit model.
In this work, we have developed an approach to determination of optical constants of Si nanocrystals embedded in SiO2 matrix synthesized with Si ion implantation. The approach is based on the effective medium approximation, and appropriate models are developed to simulate the secondary ion mass spectroscopy and spectroscopic ellipsometry measurements on the material system. The energy gap expansion of the Si nanocrystals due to the nanocrystal size effect
has been obtained by modeling the optical properties with the single-oscillator model. From the energy gap expansion the nanocrystal size can be also obtained with the phenomenological model based on quantum confinement and the bond contraction model. In addition, a novel approach to quantitative determination of depth profiles of optical constants of Si nanocrystals embedded in SiO2 thin films, which is useful to the applications of light emitting and waveguiding of the
nanocrystals, is also developed in this work.
In this study, X-ray photoelectron spectroscopy (XPS) is used to study the annealing effects on the structure and chemical states of Si-rich SiOx (x<2) films. The analysis of the XPS Si 2p peaks shows the existence of the five chemical structures corresponding to the Si oxidation states Sin+ (n =0, 1, 2, 3, and 4) in the SiOx films. The XPS results clearly show the evolution of various chemical structures and the formation of Si nanocrystals (corresponding to the
oxidation state Si0) in the SiOx films as a function of annealing temperature and annealing time. The results are
explained in terms of the thermal decompositions of the suboxides Si2O, SiO and Si2O3 (corresponding to the oxidation states Si1+, Si2+ and Si3+, respectively). The thermal decompositions lead to the growth of SiO2 (corresponding to the oxidation state Si4+) as well as the formation of Si nanocrystals in the SiOx films. On the other hand, the depth profiling experiment was carried out with ion sputtering. The relative concentration of each oxidation state at various depths is determined quantitatively from the XPS analysis. In addition, annealing effects on both the oxidation states and their depth distributions are studied as well.