Since single-walled carbon nanotubes (SWNTs) have small diameter of ~1 nm, high-electric field concentration is easily generated around SWNTs. Therefore, the carbon nanotube field-effect transistors (CNTFETs) using SWNTs as a channel are expected for low power consumption nonvolatile memory. The conventional CNTFETs, however, whose channels are exposed to the atmosphere, exhibit large hysteresis due to charge trapping by impurities, such as water molecules, around SWNT channels. In the present paper, we have fabricated nonvolatile memory based on top-gated CNTFETs with double gate insulator layers after removal of the impurities around SWNT channels.

A double layer thin SiNX /SiO2 films were deposited on SWNT channels using catalytic chemical vapor deposition. After the double layers deposition, a top gate electrode was fabricated. The interface between SiNX and SiO2 films in the device is expected as a charge storage node of nonvolatile memory. The transfer characteristics of the CNTFET as a function of back- and top-gated voltages at 300 K in vacuum were measured. The negligible hysteresis is observed for sweeping the back-gated voltage, indicating that impurities around the SWNT channel are completely removed. In contrast, hysteresis increases with increasing sweep range for top-gated voltage. The counterclockwise hysteresis loops are due to the charging and discharging processes of holes in the top-gated CNTFETs. These results mean that the charge was trapped at the interface of SiNX /SiO2 films by the applied top gate bias. Due to the small diameter of the CNT, the electric field concentration occurs, and only 2V application of the top gate bias produces the hysteresis. This gate bias is 10 times smaller value than that of the conventional planer type memory. The single-charge effects are observed using the CNT-based memory devices. When top-gated voltage was swept forth and backward for the ranging of ± 0.7 V, the abrupt peaks on the currents are observed for both directions. The abrupt drop or increase in drain currents corresponds to single-hole charging and discharging phenomena in CNT-based memory devices, respectively. The same measurements were carried out twenty times, and four discrete Id-Vg curves are clearly observed, which are attributed from the effect of single-holes traps.

We have fabricated the CNTFETs nonvolatile memory, which operate 10 times smaller bias owing to the electric field concentration effects. Single charge detection was also demonstrated in the CNT memory device.