Enhancement of Nonvolatile-memory Performance by Using Multiply-stacked Ge Nanodots Prepared at Room Temperature

One-to-three-period multilayers (MLs) of Ge nanodots (NDs) for nonvolatile memories (NVMs) have been self-assembled at room temperature by using ion beam sputtering deposition of 5monolayer Ge between SiO2 layers. Using the structure of 4-nm tunnel oxide/Ge-ND MLs (middle oxide: 2 nm)/15-nm control oxide, were fabricated NVM metal-oxide-semiconductor (MOS) fieldeffect-transistor devices based on the 0.6-μm CMOS standard processes. The size and the distribution of the Ge NDs remained almost unchanged after the device-fabrication processes. The memory window and the program speed increased from 1.1 to 1.7 V and from 10 ms to 500 μs at +18 V, respectively, with increasing number of periods from 1 to 3. The programmed threshold voltages in the cycling behaviors were almost constant within about 0.05 V up to ∼10 program/erase cycles for the twoand the three-period devices. In contrast, the erased threshold voltages showed a drift-up for all devices, with the drift-up being decelerated in larger-period devices. The charge-loss rate was also reduced for larger-period devices.