Acceptors in ZnO nanocrystals

While zinc oxide (ZnO) has potential for optoelectronic applications, the lack of reliable p -type doping remains a major challenge. We provide evidence that ZnO nanocrystals contain uncompensated acceptors. IR absorption peaks at liquid-helium temperatures suggest a hydrogenic acceptor with a hole binding energy of 0.4-0.5 eV. Electron paramagnetic resonance (EPR) measurements in the dark showed a resonance at g = 2.003 , characteristic of acceptors that involve a zinc vacancy. An EPR resonance due to vacancy hydrogen complexes was observed after exposure to light. Given the lack of alternatives, vacancy complexes may provide a feasible route toward p -type conductivity.