Most nanowires are grown standing up, rising vertically from a substrate to reach heights in the range of tens of micrometers. They typically require post-fabrication processing to form aligned arrays of nanowires suitable for use in an electronic or optical device. Attempts to grow nanowires horizontally on a surface have had some success, but the resulting nanowires were still in the micrometer-length range, with limited control over their crystallographic orientation. Now, researchers at the Weizmann Institute of Science in Israel, led by Ernesto Joselevich, have reported in the August 19 issue of Science (DOI: 10.1126/science.1208455; p. 1003) the development of a process for producing millimeter-long GaN nanowires by guided growth on various crystallographic planes of a sapphire surface. The process allows the researchers to grow “very long and perfectly aligned horizontal nanowires with exquisite control of their crystallographic orientation,” according to Joselevich.
The research team, which included graduate student David Tsivion, postdoctoral fellow Mark Schvartzman, and staff scientists Ronit Popovitz-Biro and Palle von Huth, used chemical vapor deposition of GaN on eight different sapphire planes seeded with Ni catalysts to achieve these results.
Analysis of the nanowires produced on these various planes revealed that those formed on surface steps and grooves were better aligned than those formed on a smooth plane. For instance, on a well-cut, smooth sapphire c-plane, nanowires grew in random triangular patterns following six isomorphic directions. However, on a 2° miscut c-plane, the nanowires grew along only two directions, forming parallel arrays.
“We found that when the substrate is cut in a slightly tilted or unstable plane,” Joselevich said, “the surface wrinkled up upon heating, and the tiny steps and grooves that formed on it made the alignment of the nanowires much better than on a smooth surface.” Or, as the researchers reported in their article, “graphoepitaxy overrules epitaxy.”
The researchers report that their GaN nanowires have few defects and that they exhibit excellent optical and electronic properties, which makes them potential candidates for nanoscale high-power circuits, light-emitting diodes, lasers, photovoltaic cells, photodetectors, and radio-frequency, photonic, and nonlinear optical devices. The relative absence of defects is atypical for semiconductors grown on a substrate, because stresses usually develop that produce defects.
“We think this is because, unlike a two-dimensional film, which usually gets stressed, a nanowire can relax by shrinking or swelling sidewise, making the system much more tolerant to mismatch than one is used to seeing in continuous two-dimensional films,” Joselevich said. “This is a new one-dimensional nanoscale effect, which, together with the effect of graphoepitaxy, somehow changes the paradigm not only in the new field of nanowires, but also in the well-established fields of epitaxy and thin films.”
(a) Illustration of nanowires growing along nanogrooves; (b) top view of the nanowires by scanning electron microscopy; and (c) cross-section view of a nanowire along a V-shaped nanogroove by transmission electron microscopy. Image credit: Ernesto Joselevich.
