Free charge carriers in mesoporous silicon

Abstract

Free charge carriers in mesoporous Si (meso-PS) consisting of Si nanocrystals of small dimensions of about 6--10 nm are investigated by the infrared-absorption technique. Adsorption of acceptor molecules or filling the pores with dielectric liquids are found to increase the concentration of free holes (p) in meso-PS up to the half of the doping level of the heavily boron-doped ${p}^{+}\ensuremath{-}\mathrm{Si}$ substrate $(p\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{18} {\mathrm{cm}}^{\ensuremath{-}3})$ from which the meso-PS was made. Considering the value of p and the dc electrical conductivity $\ensuremath{\sigma},$ the hole mobility is determined as about $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ and $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3} {\mathrm{cm}}^{2} {\mathrm{V}}^{\ensuremath{-}1} {\mathrm{s}}^{\ensuremath{-}1}$ for as-prepared meso-PS and meso-PS filled with a polar dielectric liquid, respectively. The activation energy is larger for $\ensuremath{\sigma}$ than for p giving evidence for thermal activation of the hole mobility. A model of the dielectric confinement for charge carriers and hydrogenic impurities is applied to explain the dependence of $\ensuremath{\sigma}$ and p on the dielectric constant of the ambience of the Si nanocrystals.