Robust low resistivity p-type ZnO:Na films after ultraviolet illumination: The elimination of grain boundaries

We propose negatively charged oxygen species at grain boundaries may be detrimental to the p-type behavior. After ultraviolet illumination to release oxygen species, the p-type behavior of moderately Na-doped ZnO films is strengthened in the subsequent several minutes. A robust p-type film with a hole mobility of 7.9 cm(2)/Vs, a hole concentration of 2.1 x 10(17) cm(-3), and a film resistivity of 3.8 Omega cm has been reproducibly achieved. Transformation from n-type to p-type conduction is observed for the lightly Na-doped ZnO after ultraviolet illumination. We believe that single crystalline p-type ZnO films are indispensable for ZnO light-emitting diodes. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4754003]