Nanocrystal memory cell using high-density Si _0.73Ge _0.27 quantum dot array

Principles of operation of a single electron memory cell using nanocrystals incorporated in the gate oxide of a MOSFET were investigated and possibilities for the application to a multi-level flash memory cell with high reliability and low power using the Coulomb blockade effect at room temperature were shown. Single electron memory cells using Si _0.73Ge _0.27 nanocrystal array as a storage electrode were fabricated and characterized. The nanocrystals were deposited as amorphous islands in LPCVD and were crystallized by rapid thermal annealing. The size of deposited nanocrystals was observed to be about 5 nm and the density was higher than 10 ¹² cm ^-2. From the result of the characterization of the fabricated devices, the threshold voltage shift corresponding to charging of single electron per nanocrystal was observed to be greater than 2 V, from which the density of the nanocrystal array was estimated to be about 2.6×10 ¹² cm ^-2. Programming characteristics of the device with respect to time showed a multi-step tunneling phenomenon, which seems to be due to an interaction among closely-spaced quantum dots.