ZnO p-n junction light-emitting diodes (LEDs) were fabricated on c -plane Al2 O3 substrates by plasma-assisted molecular beam epitaxy. Gas mixture of N2 and O2 was used as the p -type dopant, by which the double-donor doping of N2 (O) can be avoided significantly. The fabricated p -type ZnO layers have a higher hole density and carrier mobility. The LEDs showed a very good rectification characteristic with a low threshold voltage of 4.0 V even at a temperature above 300 K. The LEDs can even emit intensive electroluminescence in the blue-violet region at the temperature of 350 K. The blue-violet emission was attributed to the donor-acceptor pair recombination at the p -type layer of the LED. © 2007 American Institute of Physics.