Mobility and carrier density in nanoporous indium tin oxide films

The charge-carrier concentration and mobility of various nanoporous indium tin oxide films have been studied by Hall measurements and optical spectroscopy. The carrier density obtained by Hall measurements is found to be significantly lower than that derived from optical spectroscopy. The results of these complementary measurement techniques thus support recent theoretical work on granular conducting layers by Kharitonov and Efetov [Phys. Rev. B 77, 045116 (2008)], which predicts that the Hall carrier concentration $n$ of such a system is reduced by a geometrical factor. As expected, the mobility inside the particles, derived from optical transmission spectra (22 -- 35 $\frac{{\mathrm{cm}}^{2}}{\mathrm{Vs}}$), is much higher than the macroscopic Hall mobility ($0.12\ensuremath{-}0.7\frac{{\mathrm{cm}}^{2}}{\mathrm{Vs}}$).