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The charge state of individual silicon atoms can be selectively controlled via nanoscale contacts, according to researchers at Canada's National Institute for Nanotechnology (NINT) in Edmonton, Alberta, who presented their work Dec. 16 at Pacifichem, the meeting of Pacific Rim chemical societies that is held in Hawaii once every five years. The results show that a sometimes-worrisome silicon surface phenomenon that can hamper the performance of electronic devices can instead be exploited to drive the development of atomic-scale circuit components.
Reporting on the study at a symposium devoted to electronic applications of nanomaterials, NINT's Jason L. Pitters explained that unsatisfied electron valencies—so-called dangling bonds—on silicon atoms can dictate the chemical and electronic properties of silicon—based nanostructures. For that reason, researchers would like to be able to control the dangling-bond charge state.
Depending on conditions, a dangling bond can be occupied by a single electron, which would make the atom neutral. Alternatively, that bond can be unoccupied, resulting in a positively charged atom. Or the bond can hold two electrons, which would render the atom negatively charged.
One way to select the charge associated with silicon dangling bonds is by choosing the manner in which a silicon crystal is doped. But doping a crystal with a high concentration of negative-charge-carrying dopants to impart a negative charge, for example, does not provide active or selective control over the charge state of dangling bonds, Pitters explained. So he and his coworkers devised an alternative method.
Working with Iana A. Dogel and Robert A. Wolkow, Pitters deposited a minuscule amount of titanium on silicon and subjected the crystal to a heat treatment that yielded nanometer-sized titanium disilicide islands. The group's aim was to decorate the silicon surface with TiSi2 features that would function as nanoscale versions of Schottky contacts. In microelectronic devices, these contacts generally take the form of metal pads that are deposited on a semiconductor and patterned via photolithography in a way that customizes the material's electronic properties. The nanoscale versions of the contacts work well, Pitters reported.
Specifically, on the basis of scanning tunneling microscopy experiments, he concluded that the TiSi2 islands alter the silicon crystal's surface potential and cause a depletion of electrons in the nearby silicon atoms. That condition changes the occupancy of the dangling bonds on atoms immediately adjacent to the islands.
The effect manifests itself in STM images as differences in the appearance of otherwise identical silicon atoms. As a result of the way the team's crystal was doped, all atoms should appear as dark spots (topographic depressions) signifying negatively charged dangling bonds. But they don't. The atoms closest to the islands appear as bright protrusions, indicating that those atoms' dangling bonds are uncharged. Now the team is planning follow-up experiments to learn to further control the charge state by applying a voltage to the islands.
"This work represents an important step forward in learning to control the charge state of silicon dangling bonds, which in turn, could enable development of atomic scale devices," says Gregory P. Lopinski, a surface imaging specialist at the Canadian National Research Council's Steacie Institute for Molecular Sciences in Ottawa. Because charged and neutral dangling bonds will no doubt exhibit distinct chemical reactivities, this development also suggests a novel approach to exquisite control of chemical reactions on silicon surfaces, he adds.
A paper describing the work has recently been submitted to the journal ACS Nano.
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