Fast and long retention-time nano-crystal memory

Abstract

A threshold-shifting, single transistor memory structure with fast read and write times and long retention time is described. The structure consists of a silicon field-effect transistor with nano-crystals of germanium or silicon placed in the gate oxide in close proximity of the inversion surface. Electron charge is stored in these isolated 2-5 nm size nano-crystals which are separated from each other by greater than 5 nm of SiO2 and from the inversion layer of the substrate surface by less than 5 nm of SiU2. Direct tunneling of charge from the inversion layer and its storage in the nano-crystal causes a shift in the threshold voltage which is detected via current sensing. The nano-crystals are formed using implantation and annealing or using direct deposition of the distributed floating gate region. Threshold shift of 0.3 V is obtained in Ge-implanted devices with 2 nm of SiO2 injection layer by a 4 V write pulse of 300 ns duration. The nanocrystal memories achieve improved programming characteristics as a nonvolatile memory as well as simplicity of the single pol v-Sigate process. The VT window is scarcely degraded after greater than 109 write/erase cycles or greater than 105 s retention time. Nano-crystal memories are promising for nonvolatile memory applications. © 1996 IEEE.