Effect of location of Si or Ge nanocrystals on the memory behavior of MNOS structures

• Memory behavior of MNOS structures with Si or Ge nanocrystals is presented.
• Memory window, memory hysteresis and retention behavior are simulated.
• Effects of the oxide thickness and of the size and location of nanocrystals are studied.
• The experimental results are explained on the basis of the results of simulations.

Charge injection and retention behaviors of metal-nitride-oxide-silicon (MNOS) memory structures with Si or Ge nanocrystals embedded at a depth of 3 nm in the nitride layer were studied. The effect of Si nanocrystals on these properties was opposite in comparison with that of Ge nanocrystals. To understand the origin of these opposite effects, the influence of the oxide thickness and of the depth, size and location of semiconductor nanocrystals has been studied on the charging behavior of MNOS non-volatile memory structures by the calculation of electron and hole tunneling probabilities, and by the simulation of memory window, memory hysteresis and retention behavior.For MNOS structures it is obtained that the presence of nanocrystals enhances the charge injection resulting in better performance, but only for structures with thin tunnel oxide layer (below 3 nm), and if the nanocrystals are located close to the oxide/nitride interface. In the case of very high tunneling probability, i.e., of high tunneling currents the system approaches equilibrium and the memory behavior collapses. There is a narrow range of oxide thickness or depth of nanocrystals, where the charging properties change very fast. Retention exhibits a very sharp dependence on the oxide thickness and on depth of nanocrystals as well. Most part of the experimental results can be explained on the basis of the results of simulations.