A three charge-states model for silicon nanocrystals nonvolatile memories

In the field of nonvolatile memories, substantial improvement of reliability is obtained by replacing the continuous polysilicon floating gate by a planar distribution of silicon nanocrystals, each acting as a storage node. The test devices in the present paper are MOS capacitors containing a two-dimensional layer of nanocrystals located 2.5 nm away from the oxide/substrate interface, inside the SiO₂. This work presents various measurements of the charge current versus either bias voltage or time. On the other side, the charge and discharge dynamics of the nanocrystals had already been described by De Salvo using a model borrowed from the conventional floating-gate memory. We show this approach to be not completely suitable to explain the experimental observations. Thus, we describe and apply a so-called granular model, based on a mono-electronic principle limited by Coulomb blockade, in which electrons interact with the nanocrystals one by one. Omitting the reality of such a one-by-one principle may involve important mistakes in the interpretation of phenomena.