Abstract:
Dielectric properties of thin films precipitated on solid substrates from colloidal solutions containing silicon nanoparticles (average diameter is 10 nm) are studied by optical ellipsometry and impedance-spectroscopy. In the optical region, the values of real $\varepsilon'$ and imaginary $\varepsilon''$ components of the complex permittivity $\varepsilon$ vary within 2.1–1.1 and 0.25–0.75, respectively. These values are significantly lower than those of crystalline silicon. Using numerical simulation within the Bruggeman effective medium approximation, we show that the experimental $\varepsilon'$ and $\varepsilon''$ spectra can be explained with good accuracy, assuming that the silicon film is a porous medium consisting of silicon monoxide (SiO) and air voids at a void ratio of 0.5. Such behavior of films is mainly caused by the effect of outer shells of silicon nanoparticles interacting with atmospheric oxygen on their dielectric properties. In the frequency range of 10–10$^6$ Hz, the experimentally measured $\varepsilon'$ and $\varepsilon''$ spectra of thin nanoscale silicon films are well approximated by the semi-empirical Cole–Cole dielectric dispersion law with the term related to free electric charges. The experimentally determined power-law frequency dependence of the ac conductivity means that the electrical transport in films is controlled by electric charge hopping through localized states in the unordered medium of outer shells of silicon nanoparticles composing films. It is found that the film conductivity at frequencies of $\le$ 2 $\times$ 10$^2$ Hz is controlled by proton transport through Si–OH groups on the silicon nanoparticle surface.
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