An Atom-Level View of Silicon Nanowire Doping

Researchers use Northwestern University instrumentation to get a close-up look at a promising nanotech material

Semiconductor nanowires — tiny wires just a few billionths of a meter in diameter — have sparked great interest as a powerful and versatile nanotechnological building block. They could find applications as new transistors and circuits for next-generation electronics, as well as in photonics, solar cells, biosensing, and neuro-engineering technology.

David SeidmanResearchers at Northwestern University’s McCormick School of Engineering, working with collaborators from École Polytechnique de Montréal in Canada and Max Planck Institute of Microstructure Physics in Germany, have recently used atom-probe tomography to reveal details about the doped silicon nanowires on an atomic level. The research pinpointed the three-dimensional positions and elemental identities of the atoms in the nanowires.Scientists have learned that nanowires’ properties can be customized by doping, or adding trace amounts of impurities. Great precision is required in order to maximize the effects, but because of the nanowires’ extremely small size, an atomic-level understanding of the doping process remains elusive.

A paper about the research, “Colossal Injection of Catalyst Atoms into Silicon Nanowires,” was published April 4 by the journal Nature

Dieter IsheimThe research began at the Max Planck Institute, where researchers grew nanowires using an uncommon vapor-solid-liquid technique, a phase transition in which a gaseous material transforms into the solid phase after being absorbed in a liquid nano-droplet. Aluminum was used as a nucleating agent instead of gold, as aluminum enhances silicon’s electrical conductivity, especially when the aluminum atoms are uniformly distributed throughout the silicon.

Lacking the instrumentation to fully characterize the nanowires, the German researchers turned to their partners at Northwestern, home of the Northwestern University Center for Atom-Probe Tomography (NUCAPT). (Atom-probe tomography uses a position-sensitive detector to produce an atom-by-atom three-dimensional reconstruction of a sample with atomic resolution.) At NUCAPT, researchers used a highly focused picosecond ultraviolet laser to generate atom-specific tomographs of the nanowires.

Oussama Moutanabbir“Using the characterization methods available at NUCAPT, 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,” said David Seidman, Walter P. Murphy Professor of Materials Science and Engineering and NUCAPT’s director. 

The detected amounts of aluminium exceeded by several orders of magnitude the concentrations anticipated from thermodynamics. To describe this phenomenon, the authors developed theoretical models that include kinetic effects. The authors also demonstrated that this phenomenon of colossal injection of aluminum atoms influences a nanowire’s morphology, thus providing the possibility of tailoring the shape of nanowires in addition to their chemical and electrical properties.

Other authors were Dieter Isheim, research assistant professor of materials science and engineering and manager of NUCAPT at Northwestern; Oussama Moutanabbir from the École Polytechnique de Montréal, the paper’s lead author; and Horst Blumtritt, Stephan Senz, and Eckhard Pippel, all from the Max Planck Institute of Microstructure Physics.

Structure and 3D map of Al-catalyzed Si nanowire. (a) A high-resolution cross-sectional transmission electron microscopy (TEM) image displays the interface between the catalyst particle and the nanowire. (b) 3D atom-probe tomography (APT) atom-by-atom map of a nanowire grown at 410 degrees Celsius. A cross-sectional TEM image of an identical silicon nanowire is displayed in the inset. (c) Si-50 at percent isoconcentration surfaces of a 80 nm-long segment of a nanowire determined by analyzing a three-dimensional atom-probe tomographic reconstruction.