Portland, Ore. - While nanotechnology promises devices with extremely high speed and simplified architectures, physical problems that make the structures unreliable and difficult to manufacture must be tackled. Recent developments in fabricating semiconducting nanowires and nanocrystals may move the industry closer to realizing new generations of ultrasensitive, high-frequency, high-density devices.
Northwestern University researchers have hit upon a reliable and efficient method for forming 5-nanometer gaps in nanowires that could be used to establish electrical contact to nanoscale devices such as nanocrystals and molecular transistors. Experimental physicists at Northwestern University, meanwhile, have applied theory developed at the Naval Research Laboratory to demonstrate a technique for doping nanocrystals. And work on ballistic electron devices at the University of Manchester has paid off with a nanowire-based diode that can operate at frequencies as high as 110 GHz.
One problem with nanoscale devices is the difficulty of fabricating and attaching electrodes to their inputs and outputs. Given the diminutive size of single-molecule devices, in particular, researchers may find it challenging enough just to pinpoint the device's location, much less attach an electrode.
The Northwestern researchers call their technique on-wire lithography (OWL). The resultant structures "will allow us to build all sorts of molecule-based devices that will allow us to uncover the fundamental secrets of electrical transport in molecules and develop a variety of new and powerful optical and biological sensors," said professor Chad Mirkin, director of the Institute for Nanotechnology at the university. "One-nanometer gaps are on the horizon."
The process starts with the creation of nanowires of about 5 microns in length and 360 nm in diameter using electroplating with a conventional lithographic template. Different metals are used during electroplating; by varying the current during electrodeposition, for example, a small nickel spacer could be created in a gold nanowire. The wires are built as an array on a substrate and are then removed using ultrasound.
Next, the wires are coated with silicon dioxide before being etched with nitric acid to remove the nickel, leaving small gaps in the gold nanowire. The team reports that it was consistently able to leave gaps ranging from 5 to 500 nm.
Dip-pen nanolithography is used to place the conductive polymers polyethylene oxide and polypyrrole into the gaps to enable the study of electron transport.
Nanocrystals consisting of only a few thousand atoms are of great interest to groups trying to build nanoscale devices. One generally desirable characteristic of crystal formation at that scale is the absolute purity of the material, which results from the energetics of crystal formation at that scale. Any impurities are simply expelled from the nanocrystal as it forms. But the intrinsic purity of the material can be a drawback for electronics, where it is desirable to introduce dopants to vary the properties of the nanocrystals.
Now a joint project between the Naval Research Laboratory (NRL) and the University of Minnesota has overturned the generally accepted idea that nanocrystals cannot be doped.
"We have shown that doping difficulties are not intrinsic and [that nanocrystals] indeed are amenable to systematic optimization using straightforward methods from physical chemistry," David Norris, a UMN associate professor of chemical engineering, said in a statement released by the NRL.
It turns out that the physics of nanocrystal formation is not as accurate as had been supposed by the research community. Steven Erwin, the lead theorist on the project, did a detailed study of surface energetics in which an impurity atom binds to the surface as the nanocrystal grows. At low binding energies, it will be kicked off as the crystal forms; but the analysis predicted that there is a range of binding energies where it will be incorporated into the nanocrystal.
Norris and his colleagues at UMN took Erwin's theoretical analysis as a guide and were able to show that dopants can be introduced into nanocrystals. They were able to demonstrate several methods of modifying the surface characteristics of the crystals as they grow so that dopants can be adsorbed-that is, incorporated into their internal lattice structure.
The researchers plan to create and study different doped nanocrystals that might enhance such technologies as solar cells and lasers and that might enable the creation of nanoscale spintronic devices.
Separately, a diverse research group in Europe demonstrated a 110-GHz nanoscale diode that operates at room temperature. The group consisted of members from the University of Manchester (U.K.), VTT Information Technology (Finland), the University of Wurzburg (Germany), the University of Salamanca (Spain) and Lund University (Sweden).
The concept originated from work on nanoscale ballistic devices performed by Amin Song, a lecturer in the Microelectronic Materials and Devices department of Manchester University. In a ballistic diode or transistor, electrons move through the device under their own momentum, rather than being controlled by an electrical field. Thus, by building channels and barriers somewhat along the lines of those in a pinball game, sensitive, ultrafast devices can be constructed.
The diode, called a self-switching device, is fabricated in a single, planar step. Traditional diodes, by contrast, require as many as 10 steps, in three dimensions.
The technique first writes insulating lines on a semiconductor layer to etch two back-to-back L-shaped trenches in a doped InP/InGaAs/InP quantum-well wafer. By defining 60- to 100-nanometer-wide, 1.2-micron-long trenches in the material, the researchers were able to fabricate 110-GHz diodes with threshold voltages that were tunable from 0 to about 10 volts.
The group is currently attempting to increase the frequency of the self-switching diode into the terahertz (1,000-GHz) range. -Chappell Brown contributed to this report.