Together with researchers at the US Army Aviation & Missile Research, Development & Engineering Center (AMRDEC) and Duke University, a team led by North Carolina State University (NCSU) claims to have solved a long-standing materials science problem, making it possible to create devices using zinc oxide (ZnO) – including efficient ultraviolet (UV) lasers and LED devices for use in sensors and drinking water treatment, as well as new ferromagnetic devices (J.G. Reynolds et al, ‘Shallow acceptor complexes in p-type ZnO’, Applied Physics Letters 102, 152114 (2013)).
“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 co-author Dr Lew Reynolds, a teaching associate professor of materials science and engineering at NCSU. “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,” adds the lead author, NCSU research scientist Dr Judith Reynolds.
There is interest in using ZnO to create lasers and LEDs because ZnO produces UV light, and because ZnO can be used to make devices with fewer unwanted defects than other UV emitters – which means that the resulting lasers or LEDs can be more energy efficient.
However, researchers had been unable to consistently produce stable p-type materials from ZnO. Now, the NCSU-led team claims to have solved this problem by introducing a specific defect complex, via a unique set of growth and annealing procedures, into the ZnO. Compared with a normal ZnO molecule, in the defect complex the zinc atom is missing and a nitrogen (N) atom (attached to a hydrogen atom) substitutes for the oxygen atom. These defect complexes are dispersed throughout the ZnO material and serve as the electron-accepting holes in p-type materials.
Specifically, N-doped ZnO films grown by metal-organic vapor phase epitaxy (MOVPE) on a sapphire substrate have been shown to exhibit significant room-temperature p-type behavior (an acceptor concentration of ~1018cm–3) when sufficient nitrogen is incorporated and the material is annealed appropriately. Substitutional N on the oxygen (O) sublattice is a deep acceptor; however, shallow acceptor complexes involve N, H and zinc vacancies (VZn). By combining the use of secondary-ion mass spectrometry, Raman scattering, photoluminescence and Hall-effect data, the researchers have established the evolution of N from its initial incorporation on a Zn site to a final shallow acceptor complex VZn-NO-H+ with an ionization energy of about 130meV, responsible for the observed p-type behavior.
Not only does the research demonstrate how to create p-type materials from ZnO, but the defect complex also allows the ZnO p-n junction to function efficiently – and produce UV light – at room temperature, say the researchers.
The research work was supported by the US Defense Advanced Research Projects Agency (DARPA).
