There are many obstacles in the way to turning carbon nanotubes into something useful. Materials engineers at the University of Wisconsin-Madison have now brought carbon nanotubes (CNTs) one step closer to becoming practically applicable for semiconductor electronics. In particular, the team managed to assemble arrays of carbon nanotube transistors that outperform their silicon-based predecessors.
One obstacle the researchers had to overcome were metallic impurities, which are present in arc-discharge-generated carbon nanotubes and practically short-circuit the materials semiconducting properties. To get rid of these impurities, the researchers developed a sorting mechanism based on a solution of polymers which wrap around the CNTs and, under special conditions, select the good, semiconducting carbon nanotubes from the bad ones with a purity of 99.98 %.
Another challenge was the alignment and spacing of the carbon nanotubes on a wafer, where the production of performant transistors requires the tubes to be aligned in parallel and with a certain pitch. The team’s scalable and fast “floating evaporative self-assembly” (FESA) process achieves this by slowly pulling a substrate vertically out of a water-bath with the previously generated solution of high-purity nanotubes floating on the surface. This causes tubes to self-align on the substrate at the air-water interface while the solvent evaporates. Baking the resulting wafers afterward removes the non-conductive polymer wrapper from the sorting step.
To turn the deposited carbon nanotubes into transistors, the wafers are first coated with a PMMA resist and then patterned using electron-beam lithography. The unwanted regions are then etched away before the resulting transistor structures are freed from the remaining PMMA with acetone. Finally, palladium contacts are deposited to interface with the transistors.
This new process chain results in carbon nanotube field effect transistor (FET) arrays at a density of 50 transistors per micrometer and with quasi-ballistic conduction, a fast-lane conduction state where electrons travel almost unhindered through the nanotubes, thus obeying Newton’s second law of motion. Under test, these transistors show a conductance of 1.7 mS/μm and a saturation current of 900 μA/μm, which the researchers describe as 1.9 times the current in state-of-the-art silicon transistors of the same size and architecture.
The research paper describing the process and its results marks an important milestone in carbon nanotube research. For the first time, carbon nanotube transistors are manufactured in a scalable way and with properties that can at least compete with silicon and gallium arsenide. Since carbon nanotube research breakthroughs keep happening at regular intervals, the return of tube-logic is at least foreseeable. Enjoy the video below, where the researchers explain the significance of this development.