BERKELEY, Calif., Aug. 4 -- Scientists with the US Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) at Berkeley have reported a "significant breakthrough" in the development of the highly prized semiconductor, gallium nitride, as a building block for nanotechnology. The researchers said they have been able to control the direction in which a gallium nitride nanowire grows, for the first time ever.
Growth direction is critical to determining the wire’s electrical and thermal conductivity and other important properties. 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.
Yang said, "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."
The wires the researchers have produced are just a few nanometers in diameter but stretch to several microns long. 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.
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," said Yang. "When we can do this, we will be able to answer some important fundamental questions, such as how the carrier mobility, light emission and thermoconductivity would vary 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 said he believes the group is within a few months of being able to produce a light-emission diode, a transistor or a hybrid nanowire-thin film laser.
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