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Doping of the nanocrystalline-Si/SiO2 composite material with tungsten results in a quenching of the ‘‘red’’ photoluminescence (PL) and an appearance of an intense blue/violet one with a spectral maximum around 2.8 eV and decay time of ≤3 ns. Unlike the green/blue PL from silanol groups, this PL is stable upon annealing and does not show any polarization memory. A possible mechanism of this PL is suggested.

Plasma CVD allows one a good control of the crystallite size and dielectric tissue in the nc-Si/SiO2 compact films. It is shown that the green/blue PL originates from SiO2 (s) ≡Si-OH groups whereas the red PL is associated with states at the nc-Si/SiO2 interface. With ≥ 1 nm thick SiO2 tissue the efficiency of the red PL increases by more than three orders of magnitude when the crystallite size decreases from about 8 to 2 nm in agreement with the theoretical predictions. If, at a constant crystallite size, the thickness of the SiO2 tissue is decreased to 0.2-0.3 nm, the PL intensity decreases by two orders of magnitude. Possible reasons of this effect are discussed together with the question of optimizing the electroluminescence. Microwave absorption is used to distinguish between free and localized (emitting PL) photogenerated carriers.

We present evidence for the large increase of the band gap due to the quantum localization in nc-Si imbedded in a-SiO2 matrix, which is in agreement with the original theoretical calculations. This, together with additional experimental data explains the large red shift between the onset of the excitation spectra and the photoluminescence. This also provides strong support for the mechanism of the photoluminescence which originates from radiative centers either at the Si/SiO2 interface or within the SiO2 matrix. The strong decrease of the efficiency of the photoluminescence due to a decrease of the thickness of the a-SiO2 grain boundaries is shown and its origin discussed. Delocalization of the photogenerated charge carriers due to ultra thin a-SiO2 is excluded as the cause of this effect. Microwave absorption is used to study the effect of the grain boundaries on the localization and delocalization of photogenerated charge carriers in pure nc-Si together with concomitant phenomena observed in Raman scattering. Finally we show the strong decrease of the photoluminescence decay time to ≤ 500 ps due to molecular-like radiative centers which are formed in the nc-Si/SiO2 composites by appropriate doping.