Gallium nitride nanowires take direction from substrates
Researchers at Lawrence Berkeley National Laboratory (LBNL; Berkeley, CA) and the University of California at Berkeley (UC Berkeley) have reported success in controlling the growth direction of gallium nitride (GaN) nanowires...
Researchers at Lawrence Berkeley National Laboratory (LBNL; Berkeley, CA) and the University of California at Berkeley (UC Berkeley) have reported success in controlling the growth direction of gallium nitride (GaN) nanowires, based on controlling the crystalline orientation of the substrate upon which the nanowires were grown.1
Single-crystalline GaN nanowires and nanotubes have already shown promise in blue light-emitting diodes, short-wavelength ultraviolet nanolasers, and nanofluidic biochemical sensors. And nanotechnologists are eager to tap into the potential of GaN for use in high-power, high-performance optoelectronic devices, according to Peidong Yang, a chemist with LBNL's Materials Sciences Division and a professor with UC Berkeley's Chemistry Department, which led the research.
"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," said Yang, who believes that he and his group are within months of producing a light-emitting diode, a transistor, or a hybrid, nanowire–thin-film laser.
For the current experimental work, they grew single-crystal GaN nanowires using metal-organic chemical vapor deposition (MOCVD) in a similar manner to previous work in which they were able to control the size, aspect ratio, position, and composition of their nanowires. This time, however, they added the ability to control crystallographic growth direction by selecting different substrates.
Both lithium aluminum oxide and magnesium oxide have crystalline structures that are geometrically compatible with GaN crystals, but the lithium aluminum oxide features a twofold symmetry that matches the symmetry along one plane of the GaN crystals, whereas the magnesium oxide has a threefold symmetry that matches GaN symmetry along a different plane. When a vapor of GaN condenses on either of these substrates, the resulting nanowires grow perpendicular to the substrate but aligned in a direction unique to each substrate (see figure).
The orientation-induced effect was measured using temperature-dependent photoluminescence studies, Yang said. "In nanowires made from the exact same GaN material but grown on different substrates, the light-emission of these wires was blue-shifted by 100 meV. We believe the emission difference is a clear manifestation of the different crystal-growth directions."
Yang and his research group have worked with other semiconductor nanowire materials, such as zinc oxide and silicon/germanium. "Our goal is to put together a generic scheme for controlling the directional growth of all semiconductor nanowires," he said. "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 the same compositions and crystal structures. The use of MOCVD for GaN-nanowire growth will also allow us to integrate nanowires and thin films of various compositions so we can start making real devices."
Yang said that he is already preparing publications on the device work, but preferred not to discuss it until after publication.
- T. Kuykendall et al., Nature Materials advanced online edition (July 25, 2004).