Feb 5, 2004
Scientists embed nanotubes in hybrid semiconductors
Scientists from the University of Copenhagen, Denmark, claim to have made the first electronic hybrid nanotube-semiconductor devices. They encapsulated single-walled carbon nanotubes in epitaxially grown semiconductor heterostructures such as GaAs/AlAs and (Ga,Mn)As.
“Contacting single-walled carbon nanotubes by epitaxially grown semiconductor material is a totally new approach,” researcher Ane Jensen told nanotechweb.org. “It may be a way around the difficulties of attaining good electrical contacts to the tubes.”
Jensen and colleagues grew the devices on n-doped GaAs substrates prepared by molecular-beam epitaxy (MBE). They deposited a 100 period superlattice of 2 nm GaAs plus 2 nm AlAs ended by 20 nm GaAs as an insulating layer, before coating the wafer with a layer of amorphous As. This layer of As was key to the epitaxial growth process. Carbon nanotubes were then deposited on the surface from a dichloroethane suspension, and the As layer was removed in a MBE chamber to leave a clean, smooth GaAs surface before epitaxial growth of the ferromagnetic semiconductor Ga0.95Mn0.05As. The team grew semiconductor layers between 20 and 50 nm thick and capped them with 3 nm of GaAs to prevent oxidation.
Finally, the researchers used lithography to make three-terminal devices from the structures. Islands of (Ga,Mn)As acted as the source and drain to the nanotube, while the n-doped GaAs base served as a back gate.
“Presumably the technique will work with other types of semiconductors since the tubes are mechanically robust and can withstand elevated temperatures,” said Jensen. “We believe that the incorporation of single-walled carbon nanotubes in the traditional Si and GaAs semiconductor electronic and optic circuits […] opens up possibilities for designing hybrid nanotube/semiconductor devices, where nanotubes act as interconnects in traditional semiconductor integrated circuits or as active devices.”
Examples of such devices could include: a tube field-effect transistor (FET) made from a semiconducting single-walled carbon nanotube, with the source, drain and gate electrodes constructed from semiconductor material grown epitaxially on top of the tube; a two-dimensional electron gas FET, where the source and drain are traditional diffused contacts but the gate is a nanotube; and a nanotube laser, where a semiconductor cavity contains a semiconducting carbon nanotube doped to form a p-n junction.
“Within the scope of the rapidly growing field of spintronics, we have applied the ferromagnetic semiconductor contacts to nanotubes with the aim of studying spin-polarized electron transport in the tubes,” said Jensen. “We hope to present the results from this in the near future. We also wish to explore new semiconductor hybrid devices such as single-walled carbon nanotubes contacted by a two-dimensional electron gas.”
The scientists reported their work in Nano Letters.
About the author
Liz Kalaugher is editor of nanotechweb.org.