Charging dynamics and strong localization of a two-dimensional electron cloud

The dynamics of charge injection in silicon nanocrystals embedded in a silicon dioxide matrix is studied using electrostatic force microscopy. We show that the presence of silicon nanocrystals with a density of 1011 cm-2 is essential for strong localization of charges, and results in exceptional charge retention properties compared to nanocrystal-free SiO2 samples. In both systems, a logarithmic dependence of the diameter of the charged area on the injection time is experimentally observed on a timescale between 0.1 and 10 s (voltage < =10 V). A field-emission injection, limited by Coulomb blockade and a lateral charge spreading due to a repulsive radial electric field are used to model the sample charging. Once the tip is retracted, the electron cloud is strongly confined in the nanocrystals and remains static.