Nitrogen is a deep acceptor in ZnO

Zinc oxide is a promising material for blue and UV solid-state lighting devices, among other applications. Nitrogen has been regarded as a potential p-type dopant for ZnO. However, recent calculations [Lyons, Janotti, and Van de Walle, Appl. Phys. Lett. 95, 252105 (2009)] indicate that nitrogen is a deep acceptor. This paper presents experimental evidence that nitrogen is, in fact, a deep acceptor and therefore cannot produce p-type ZnO. A broad photoluminescence (PL) emission band near 1.7 eV, with an excitation onset of ～2.2 eV, was observed, in agreement with the deep-acceptor model of the nitrogen defect. The deep-acceptor behavior can be explained by the low energy of the ZnO valence band relative to the vacuum level.