Inexpensive P-Type Zinc Oxide Nanowires

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.

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.