A recent study of large-scale simulations of silicon nanowires has yielded information concerning the future performance of nano-meter-scale devices in Electronic Design. The results of this study, jointly conducted by the Georgia Institute of Technology and the IBM T.J. Watson Research Center and sponsored by the U.S. Department of Energy, suggest that quantum mechanical effects will dominate the behavior of materials at the nanometer scale. Issues investigated within the study included atomic structure, electronic properties, and electrical transport in silicon nanowires.
As engineers are forced to reduce the size of electronic devices and fit more of them on a single chip, research into the electrical behavior of nanometer-scale technology becomes pertinent. Experts predict that once electronics reach the nanometer level, device operation will be dominated by quantum mechanical effects. To test this theory, researchers simulated silicon nanowires etched from bulk silicon as well as nanowires self-assembled from clusters containing 24 atoms of silicon (see the figure). In both cases, the attachment of hydrogen atoms to unused bonds passivated the silicon. The researchers then connected the wires to aluminum leads.
Data On Aluminum Doping
Conducted on an IBM SP-2 computer at Georgia Tech, the theoretical simulations procured data concerning the nanowires' electrical conductance, the influence of the silicon-metal interface, and the role that doping with aluminum atoms may play in altering material properties. A secondary investigation yielded new ways of doping ultra-small transistor channels that could circumvent current technological difficulties.
Large-scale simulations showed that electronic states formed by a combination of orbitals from the aluminum leads and silicon wire atoms penetrate completely through nano-wires of less than about 1 nm in length. This penetration creates silicon bridges with finite conductance. Yet researchers discovered that such complete penetration does not occur in longer structures. In the latter structures, silicon retains its semiconducting properties.
A second finding of the nanowire simulations is the similarity of Schottky barrier heights in nanoscale and familiar scale devices. The transfer of electrons creates a localized dipole at the junction between the silicon and aluminum leads. The height of the barrier formed depends upon the nature of the bonding and the atomic arrangement at the contact itself. While the Schottky barrier heights were 40% to 90% larger than those found in macroscale contacts, the variation of height for various configurations of nanowires was consistent with those at familiar scales.
Researchers feared that variation of dopant concentration in the manufacturing of nanoscale silicon devices could be detrimental to their reliability. While doping of semiconductors is routinely used to optimize device characteristics at the familiar scale, experts were concerned that device-to-device statistical variations in dopant concentration at the nanoscale level would lead to inconsistent and unpredictable device performance.
The simulations conducted within this study found that the method of building nanowires from silicon clusters may reduce variation and increase consistency in dopant concentration. Since the silicon clusters form hollow cages, they may be fabricated around a dopant atom. This configuration provides low device-to-device variations in dopant concentration.
One concern in the effectiveness of nanoscale device configurations is the production of interference resonances at this scale. In silicon nanowires used as current channels, the wave-like nature of the electrons may cause interference effects in the electric conductance. Electrons penetrating the silicon nanowire from one of the aluminum leads may, when voltage is applied to open the channel, rebound from the contact to the other lead and flow back toward the source contact.
In this manner, interference resonances may cause the channel to appear transparent, leading to the occurrence of spikes in the current flow through the nanoscale channel. This behavior of silicon nanowires has been identified as a design issue pertinent in the development of nanoscale silicon electronic devices.
The large-scale simulation of silicon nanowires has identified key issues in the future development of devices at this level. Future research will be aimed at the actual fabrication and testing of devices at the nanoscale level.