Principles of operation of a single electron memory cell using nanocrystals
incorporated in the gate oxide of a MOSFET were investigated and possibili
ties for the application to a multi-level flash memory cell with high relia
bility and low power using the Coulomb blockade effect at room temperature
were shown. Single electron memory cells using Si-0.73 Ge-0.27 nanocrystal
array as a storage electrode were fabricated and characterized. The nanocry
stals were deposited as amorphous islands in LPCVD and were crystallized by
rapid thermal annealing. The size of deposited nanocrystals was observed t
o be about 5 nm and the density was higher than 10(12) cm(-2). From the res
ult of the characterization of the fabricated devices, the threshold voltag
e shift corresponding to charging of single electron per nanocrystal was ob
served to be greater than 2 V, from which the density of the nanocrystal ar
ray was estimated to be about 2.6 x 10(12) cm(-2). Programming characterist
ics of the device with respect to time showed a multi-step tunneling phenom
enon, which seems to be due to an interaction among closely-spaced quantum
dots.