"Our results will come as a surprise to those who have said that growth direction can't be controlled, that you get what you get when you grow semiconductor nanowires," says Peidong Yang, a chemist with Berkeley Lab's Materials Sciences Division and a professor with UC Berkeley's Chemistry Department who led this research.
A report discussing these research results first appears in the August, 2004 issue of the journal Nature Materials. Co-authors of the report, Crystallographic Alignment of High-density Gallium Nitride Nanowire Arrays, in addition to Yang, were Yanfeng Zhang, Donald Sirbuly and Jonathan Denlinger of Berkeley Lab, and Tevye Kuykendall, Peter Pauzauskie and Joshua Goldberger, of UC Berkeley.
Nanotechnologists are eager to tap into the enormous potential of gallium nitride for use in high power, high performance optoelectronic devices. Already, single-crystalline gallium nitride nanowires and nanotubes have shown promise in blue light-emitting diodes, short-wavelength ultraviolet nanolasers, and nanofluidic biochemical sensors.
"Control over nanowire growth direction is extremely desirable, in that anisotropic parameters such as thermal and electrical conductivity, index of refraction, piezoelectric polarization, and bandgap may be used to tune the physical properties of nanowires made from a given material," Yang says.
Yang and his research group have been pioneers in the fabrication of semiconductor nanowires, especially gallium nitride, zinc oxide and silicon/germanium. The wires they've produced measure only a few nanometers in diameter but stretch out to several microns in length. For this experimental work, they grew single-crystal gallium nitride nanowires using a metal–organic chemical vapor deposition (MOCVD) technique that was similar to an earlier technique they used to produce nanowire lasers. In their earlier work, Yang and his group demonstrated the ability to control the size, aspect ratio, position and composition of their nanowires. Now they've added the ability to control crystallographic growth direction. The key to this new capability is the selection of a choice substrate.
Explains Yang, "In nanowires made from the exact same gallium nitride material but grown on different substrates, the light-emission of these wires was blue-shifted by 100 meV (milli-electron volts). We believe the emission difference is a clear manifestation of the different crystal growth directions."
For this study, Yang and his group used substrates of lithium aluminum oxide and magnesium oxide. The crystals of both materials are geometrically compatible with gallium nitride crystals but the lithium aluminum oxide features a two-fold symmetry that matches the symmetry along one plane of the gallium nitride crystals, whereas the magnesium oxide has a three-fold symmetry that matches gallium nitride symmetry along a different plane.
As a result, when a vapor of gallium nitride condenses on either of these substrates, the resulting nanowires grow perpendicular to the substrate, but aligned in a direction unique to each substrate. Because of the different growth direction, cross-sections of the gallium nitride nanowires grown on lithium aluminum oxide form an isosceles triangle, while the cross-sections of those grown on magnesium oxide are hexagonal.
"Our goal is to put together a generic scheme for controlling the directional growth of all semiconductor nanowires," says Yang. "When we can do this, we will be able to answer some important fundamental questions such as how would the carrier mobility, light emission and thermoconductivity differ along different crystallographic directions for nanowires with same compositions and crystal structures. The use of MOCVD for gallium nitride nanowire growth will also allow us to integrate nanowires and thin films of various compositions so we can start making real devices."
Yang believes that he and his group are within a few months of being able to produce a light emission diode, a transistor, or a hybrid nanowire-thin film laser.
This research effort was funded by the U.S. Department of Energy's Office of Science, the Camille and Henry Dreyfus Foundation, the Beckman Foundation, and the National Science Foundation.
Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California. Visit our Website at http://www.lbl.gov/.
Journal
Nature Materials