We investigate the p-type doping in ZnO prepared by metal-organic vapor phase epitaxy, using dimethylhydrazine (DMHy) as a nitrogen dopant source. Results obtained from X-ray photoelectron spectroscopy show that DMHy exhibits a narrow temperature window from 500 to 550 degrees C for efficient nitrogen doping. Additional co-doping with Ga significantly enhances the nitrogen incorporation and the conductivity type of the co-doped ZnO film is critically influenced by N/Ga flux ratio in growth. The fabricated p-type ZnO film shows a hole concentration of about 2.41 x 10(18) cm(-3) and hole mobility of about 4.29 cm(2) V-1 s(-1). The corresponding nitrogen acceptor level is calculated to be about 160 meV from the PL spectrum. I-V characterization of a p-ZnO:(N,Ga)/n-ZnO homojunction shows clear rectifying behavior with a turn-on voltage of about 3.7 V. (C) 2007 Elsevier B.V. All rights reserved.