Single-Electron Charging and Discharging Analyses in Ge-Nanocrystal Memories

The transient charging/discharging of electrons in Ge-nanocrystal (NC) memories are measured by a pump-and-probe method that allows keeping track of the number of electrons per NC. The experiments are simulated with a quantum kinetic mechanical model based on the density-functional theory, which can describe the NCs´ charging state. In the transient charging, electrons are captured faster than predicted by simulations. This was attributed to the presence of defects in the NC surface, action of which is twofold: 1) The incoming electrons are captured by NC states and are quickly thermalized down to the surface traps. 2) Those traps enlarge the spatial distribution of the confined wave functions, increasing their penetration in the tunneling oxide and the incoming transition rates. As for the discharging, the calculations and experiments agree until there are only few electrons left per NC. Then, the out tunneling becomes slower than predicted by calculations. The remaining electrons are confined in trap states with energies located in the NC bandgap, and they have to be thermally excited to NC states and to tunnel out to the substrate.