Ring Oscillator Built in Single Carbon Nanotube

IBM (Yorktown Heights, N.Y.) researchers built the first complete electronic IC around a single carbon nanotube (CNT) molecule: a ring oscillator with speeds nearly a million times faster than previously demonstrated circuits with multiple nanotubes. While this is still slower than the speeds obtained by today's silicon chips, the IBM team believes that new nanofabrication processes will eventually unlock the superior performance potential of CNT electronics. "Carbon nanotube transistors have the potential to outperform state-of-the-art silicon devices," said T.C. Chen, vice president of science and technology for IBM Research. "However, scientists have focused so far on fabricating and optimizing individual carbon nanotube transistors. Now, we can evaluate the potential of carbon nanotube electronics in complete circuits — a critical step toward the integration of the technology with existing chipmaking techniques."

The IBM researchers noted in the journal Science that single-walled CNTs (SWCNTs) have been shown to exhibit excellent electrical properties, such as ballistic transport over several hundred nanometers at room temperature. Field-effect transistors (FETs) made from individual tubes show DC performance specifications rivaling those of state-of-the-art silicon devices. The researchers built a five-stage ring oscillator that comprises, in total, 12 FETs side by side along the length of an individual 18 µm CNT.

To obtain both p- and n-type FETs on the same SWCNT, the polarities of the FETs were controlled by using metals with different work functions as the gates. Palladium was chosen as the metal gate for the p-FET and aluminum for the n-FET.

By arranging five inverters (10 FETs) side by side on one SWCNT, the researchers successfully applied the CMOS architecture onto a single molecule to realize a ring oscillator circuit (Figure ). An extra inverter stage (two FETs), which eliminates interference between the measurement setup and oscillator operation, follows the ring oscillator portion to allow for the signal to be read by a spectrum analyzer.

A close-up view of the five-stage CMOS type nanotube ring oscillator. The upper right inset shows the nanotube itself with a diameter of ~2 nm.

The researchers plan to used the intramolecular ring oscillator as a tool to characterize the suitability of SWCNTs for AC applications. The central goal is to ultimately benefit from the expected intrinsic switching speed in SWCNT FETs, which is predicted to allow for terahertz applications.

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