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ZnO technology could revolutionise LEDs and UV lasers

To make lasers and LEDs both n-type and p-type materials are used. Researchers have claimed that shedding excess energy at the p-n junction is what produces light in both these types of devices
Scientists from North Carolina State University say they have solved a long-standing materials science problem.

They claim that it is possible to create new semiconductor devices using zinc oxide (ZnO).

The development could pave the way for efficient ultraviolet (UV) lasers and LED devices for use in sensors and drinking water treatment, as well as new ferromagnetic devices.

“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,” says Lew Reynolds, co-author of a paper describing the research and a teaching associate professor of materials science and engineering at NC State.

“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,” continues Judith Reynolds, a research scientist at NC State and lead author of the paper.

To make laser and LED technologies, you need both “n-type” materials and “p-type” materials. 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 in the form of light as they travel from the n-type material to the p-type material.

The shedding of excess energy at the p-n junction is what produces light in lasers and LED devices.

Scientists have been interested in using ZnO to create these devices because ZnO produces UV light, and because it can be used to make devices with relatively fewer unwanted defects than other UV emitters. This means the resulting lasers or LEDs would be more energy efficient.

However, in the past, researchers have found it hard to consistently produce stable p-type materials out of ZnO.

Now the scientists at NC State say they have solved that problem by introducing a specific “defect complex,” using a unique set of growth and annealing procedures in the ZnO.

The table below shows conditions used for the research.

The defect complex looks different from a normal ZnO molecule. The zinc atom is missing and a nitrogen atom (attached to a hydrogen atom) substitutes for the oxygen atom. These defect complexes are dispersed throughout the ZnO material and serve as the “holes” that accept the electrons in p-type materials.

SIMS depth profiles of ZnO, H, and ZnN collected for N-doped ZnO films of (a) n type
sample 130A before (solid lines) and 130B after (dashed lines) an 800 C, 60s ex situ
anneal in N2 and (b) p-type sample 131C before (solid lines) and 131D after (dashed lines) a 30s ex situ anneal in N2

11.6K PL spectra of (a) 131C, 131E, and 144G, 144I before and (b) after the ex situ 8000C O2 anneal

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

More details of this work are published in the paper, “Shallow acceptor complexes in p-type ZnO,” by Judith Reynolds et al in Applied Physics Letters, 102, 152114 published online on 19th April 2013. http://dx.doi.org/10.1063/1.4802753

This research was supported by the Defence Advanced Research Projects Agency.

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