Theoretical Characterization of a Nanocrystal Layer for Nonvolatile Memory Applications

Abstract

On the road to miniaturization, nanocrystal layers are promising as floating gate in nonvolatile flash memories. Although much experimental work has been devoted to the study of these new memory devices, only few theoretical models exist to help the experimentalists to understand the physical phenomena encountered and explain the behavior of the device.

We have developed a model based on the geometrical and physical properties of the elementary structure of a nanocrystal flash memory, i.e. one nanocrystal embedded in an oxide between the channel and the gate electrodes. To obtain a fine analysis of the observed phenomena, several specific hypotheses have been taken into account. Concerning the channel, the contribution of the subbands is explicitly included. In the case of an electrode with a quasi-continuum of energy levels, we replace the continuum by equivalent sets of 2D subbands in order to be able to isolate the energy range that really contributes to the charging/discharging of the nanocrystal. The properties of the materials (bulk band structure, dielectric permittivity, …) can be easily set as well as the geometrical specifications of the elementary structure (nanocrystal radius, tunnel and control oxyde thicknesses, …).

The behavior of a layer of nanocrystals is described according to a statistical approach starting from single nanocrystal results. This method allows us to take into account the fluctuations of geometrical parameters. Thus we are able to simulate various types of materials for the nanocrystals (Si, Ge, …), the oxide layer (SiO₂, HfO₂, …) and the electrodes, for both a single nanocrystal and layers of nanocrystals.