We investigate doping effects in ZnO layers using Li3N as a doping source by plasma-assisted molecular-beam epitaxy. Li and N are simultaneously doped into ZnO layers through decomposition of Li3N into Li and N. Li- and N-doped ZnO layers grow three dimensionally at 400 degreesC, while undoped layers grow two dimensionally. The secondary ion mass spectroscopy of Li- and N-doped samples shows high incorporation of Li impurities at around the surface, which is due to surface segregation of Li impurities. A new emission at 3.321 eV is observed in PL spectrum of a heavily Li- and N-doped ZnO. The new emission can be assigned to exciton emission bound to neutral N acceptor. Li- and N-doped ZnO layers show n-type conductivity with high electron concentration of 6.8 x 10(19)/cm(3), while the electron concentration of an undoped ZnO layer is 1.0 x 10(17)/cm(3). (C) 2003 Elsevier Science B.V. All rights reserved.