Probing the size and density of silicon nanocrystals in nanocrystal memory device applications

Structural characterization via transmission electron microscopy and atomic force microscopy of arrays of small Si nanocrystals embedded in SiO2, important to many device applications, is usually difficult and fails to correctly resolve nanocrystal size and density. We demonstrate that scanning tunneling microscopy (STM) imaging enables a much more accurate measurement of the ensemble size distribution and array density for small Si nanocrystals in SiO2, estimated to be 2-3 nm and 4 x 10^(12) - 3 x 10^(13) cm^(-2), respectively, in this study. The reflection high energy electron diffraction pattern further verifies the existence of nanocrystallites in SiO2. The present STM results enable nanocrystal charging characteristics to be more clearly understood: we find the nanocrystal charging measurements to be consistent with single charge storage on individual Si nanocrystals. Both electron tunneling and hole tunneling processes are suggested to explain the asymmetric charging/ discharging processes as a function of bias.