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Inexpensive P-Type Zinc Oxide Nanowires

Photonics Spectra
Mar 2007
Michael J. Lander

LEDs and lasers used in solid-state lighting, display and other applications often incorporate gallium nitride, and the process employed for large-scale production of such diodes is costly. Although materials such as zinc oxide could form a viable, less expensive substitute for GaN, until recently researchers have produced mainly N-type ZnO nanostructures.

Engineers led by Deli Wang of the University of California, San Diego, have made the sought-after P-type ZnO nanowires with a simple tube furnace chemical vapor deposition method. Before the process, the researchers placed inside the reactor a mixture of Zn, ZnO, graphite and P2O5 on an aluminum-oxide plate. They heated the furnace to 945 °C for 30 minutes and used a blend of nitrogen and oxygen as the carrier gas. A wafer of A-plane sapphire acted as a collecting substrate.

On large areas of the wafer’s surface, the team noticed arrays of nanowires with an average length of about 2 μm. They ranged from 50 to 60 nm in diameter and displayed a slight taper near their tips.

LEDzinc_WangZnO.jpg

When researchers removed a sapphire substrate from the chemical vapor deposition furnace, they noticed arrays of P-type ZnO nanowires about 2 μm long and 50 to 60 nm in diameter on its surface, seen here in a low-magnification scanning electron micrograph.


When compared with their unintentionally N-doped counterparts, the significantly thinner and longer structure of the nanowires gave the researchers their first clue that they had incorporated phosphorous uniformly. When P atoms occupy the ZnO lattice, they exert a strain on it — one that relaxes with decreasing diameter. To ensure the synthesis of pure P-type ZnO nanowires, however, the scientists measured their photoluminescence and electrical transport.

For both N2-annealed and as-grown structures, the scientists observed no green luminescence. This indicated the absence of oxygen vacancies in them, which were the cause of the N-type conduction in the unintentionally doped ZnO nanowires. Presumably because of ionized P dopants, both P-type wires showed a red emission around 1.8 eV that disappeared at very low temperatures. Further low-temperature photoluminescence studies verified P-type doping only in the annealed wires.

Although researchers will have to make further refinements to this technology, devices that unite P-type ZnO nanowires such as these and existing N-type structures could allow economical mass production of LEDs, photon detectors and other optoelectronic devices.

Nano Letters, online Dec. 29, 2006, doi: 10.1021/nl062410c.


GLOSSARY
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
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