Shallow Li-acceptor levels in ZnO films codoped with Li and F atoms

Li,F-codoped ZnO films have been prepared by simultaneous evaporation of LiF at 780 °C during the ZnO growth. The codoped ZnO film has the resistivity of 79 Ωcm and the electron density of 6.8 ×1016 cm–3. Photoluminescence spectra of the codoped ZnO film in the range of 14 to 120 K show new emissions at 3.3136 and 3.3270 eV, besides bound excitonic emissions. The emission at 3.3136 eV disappears at a temperature higher than 40 K, and is assigned to donor-acceptor pair emission. The emission at 3.3270 eV is ascribed to free-electron to acceptor transition. The Li-acceptor levels are estimated to be 0.11 eV from free-electron to acceptor transition. As excitation intensity increases, the photon energy of the donor-acceptor pair emission remains unchanged. These results lead to the interpretation that Li and F atoms are located at nearest neighbour sites in the ZnO lattice and that the presence of Li-F complexes suppresses the formation of interstitial Li. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)