Zinc Oxide Lasers and LEDs on Horizon

By adding three elements to zinc oxide, researchers have solved a longstanding materials science puzzle: whether it is possible to create semiconductor devices from the compound. Finding the answer could lead to zinc-oxide-based ferromagnetic and semiconductor devices, including ultraviolet lasers and LEDs for use in sensors and drinking water treatment.

Laser and LED technologies need both n-type and p-type materials to work. N-type materials contain an abundance of free electrons; p-type materials have “holes” that attract those free electrons. But the holes in the p-type materials have a lower energy state, which means that electrons release their excess energy as light when traveling from the n-type to the p-type material, at what is known as the p-n junction.

Raman spectra of n-doped zinc oxide films for selected frequency ranges (a) 1200-1700 cm⁻¹, (b) 3000-3200 cm⁻¹, and (c) 3200–3650 cm⁻¹, for p-type sample 131 °C before ex situ anneal (i, black); n-type sample 131E after 30 s ex situ 800 ºC O₂ anneal (ii, red); and n-type sample 144G before (iii, blue); and 144I after (iv, magenta) ex situ O₂ anneal. Courtesy of Reynolds et al, Applied Physics Letters, Vol. 102, 152114 (2013).

Researchers have been interested in using zinc oxide (ZnO) to create lasers and LEDs because the compound generates UV light and eliminates many of the unwanted defects associated with other UV emitters, resulting in more energy-efficient devices. However, researchers have not been able to consistently produce stable p-type materials using ZnO.

Now scientists at North Carolina State University have found the solution: introducing a specific “defect complex” into ZnO using a unique set of growth and annealing procedures. Unlike a normal ZnO molecule, in a defect complex the zinc atom is missing, and a nitrogen atom (attached to a hydrogen atom) substitutes for the oxygen atom. These complexes are dispersed throughout the ZnO material and serve as the “holes” that accept the electrons in p-type materials.

“The challenge of using ZnO to make these devices has stumped researchers for a long time, and we’ve developed a solution that uses some very common elements: nitrogen, hydrogen and oxygen,” said Dr. Lew Reynolds, an associate professor of materials science and engineering at NC State.

The research illustrates how to create p-type materials from ZnO and also how the defect complex allows the ZnO p-n junction to function efficiently — and produce UV light — at room temperature.

“We’ve shown that it can be done, and how it can be done — and that opens the door to a suite of new UV laser and LED technologies,” said Dr. Judith Reynolds, a research scientist at NC State.

The research, supported by the Defense Advanced Research Projects Agency, appeared in Applied Physics Letters (doi: 10.1063/1.4802753).

For more information, visit: www.ncsu.edu