Semiconductor nanowires represent versatile nanoscale building blocks, finding applications as new transistors and circuits for next-generation electronics, as well as in photonics, solar cells, biosensing, and neuro-engineering technology. Silicon nanowires have attracted particular interest, where their properties are controlled by doping with foreign ions. It has recently been demonstrated that aluminum provides an effective growth catalyst for silicon nanowires, where the aluminum also provides an effective p-type dopant, being homogeneously distributed throughout the silicon.
As reported in the April 4 issue of Nature (DOI: 10.1038/nature11999; p. 78), an international team of researchers from École Polytechnique de Montréal in Canada, Max Planck Institute of Microstructure Physics in Germany, and Northwestern University in Illinois have now gained an atomic-level understanding of this process. Silicon nanowires were grown by heating a silicon substrate supporting aluminum islands to a temperature where only the aluminum melts, and not the silicon. The substrate was then exposed to a vapor-phase silane reactant. Part (a) in the figure shows how the surface of an aluminum drop adsorbs silicon from the silane, which then migrates to the bottom of the drop where it deposits in layers. Significantly more aluminum is thus embedded in the silicon wire than was to be expected theoretically.
“The silicon here takes up as much as 10,000 times more aluminum than the laws of thermodynamics allow,” said Eckhard Pippel, one of the participating researchers from the Max Planck Institute of Microstructure Physics.
Theoretically, fewer than one in a million atoms should be replaced by aluminum in a silicon crystal. However, the aluminum content of the silicon wires is actually around 4%. Co-researcher David Seidman, the Walter P. Murphy Professor of Materials Science and Engineering at Northwestern, and Dieter Isheim, Research Assistant Professor, said, “We could see that aluminum triggers a self-doping process that results in an unexpectedly high concentration of aluminum atoms uniformly distributed throughout the nanowires.” The researchers made their discovery with the aid of an ultraviolet laser-assisted atom probe tomography, which reveals the type and position of each individual atom in nanoscopic samples—see part (b) in the figure.
Oussama Moutanabbir, a professor at École Polytechnique in Montréal, said, “The data surprised us because of the high concentration, on the one hand, and also because the aluminum atoms do not form clusters in the silicon.” The number of charge carriers in silicon increases only when the aluminum atoms are distributed uniformly. This increase is important for electronic applications.
In order to understand why more aluminum ends up in the silicon wire than is actually allowed, the researchers developed a model of how quickly the process proceeds on the atomic level. If this time is long, the atoms would arrange themselves until the chemical equilibrium is achieved. However, the time is not long enough for this, and atomic exchange stops as soon as one row of silicon atoms has been completed. “An aluminum atom that has previously been embedded remains permanently trapped,” said Moutanabbir. “Until now, it has been assumed that the atoms can be exchanged between the metal drop and silicon until the whole silicon layer is complete.” As the researchers have now clarified the process, it should be possible to apply it to the targeted doping of nanowires.
(a) The high-resolution transmission electron microscope image shows the interface between aluminum and silicon (© Nature/MPI of Microstructure Physics). (b) Atom probe tomography reveals the atomic structure of the material: red shows the representative positions of the aluminum atoms and blue those of the silicon atoms. An analysis of the data shows the very high concentration of aluminum in the silicon and its uniform distribution. For experimental reasons, the nanowire was coated with a protective layer of nickel (green) for the analysis. Inset: size marker is 40 nm. (© Nature/Northwestern University, Illinois)
