Scaling of Metal-Nanotube Contacts |
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| Jerry Tersoff, Vasili Perebeinos |
| T.J. Watson Research Center, IBM, Yorktown Heights, NY, United States |
| tersoff@us.ibm.com |
| For carbon nanotube transistors to compete with silicon, they must be integrated with very high device density. This route is being actively pursued, but there are challenges in scaling devices to the required size and spacing. Perhaps the most important, and least understood, is the transfer length over which electrons transfer from the metal to the nanotube. Devices with contacts much shorter than this transfer length have proportionally increased contact resistance. Unfortunately, the measured length is surprisingly long, roughly 100nm, much larger than the desired contact lengths. We predict that the transfer length under realistic device conditions is actually much shorter than the measured value, because the transfer length shrinks with decreasing tube diameter and increasing bias voltage. The physical mechanisms will be discussed. Device integration also requires closely-spaced arrays of tubes, with several tubes per device, to give adequate drive current. We find that with scaling of the tube spacing toward smaller pitch, there is a structural transition to a geometry in which the metal poorly wets the nanotube and substrate. This gives a sudden decrease in contact adhesion, and probably also an increase in contact resistance. Several interesting aspects of the wetting geometry will be discussed. |