Strong Ultraviolet Photoluminescence in Polycrystalline ZnO Sputtered Films

Polycrystalline ZnO films both exhibiting strong ultraviolet (UV) photoluminescence (PL) and having smooth surfaces applicable to electronic devices have been obtained by rf-sputter deposition and subsequent heat treatment. The peak of UV PL was located at 3.05 eV and exhibited a linewidth as narrow as 70 meV, along with a strong intensity comparable to that observed for ZnO epitaxial film grown on sapphire under the same conditions. The UV PL intensity was found to be complementary to that of the red PL emerging at around 2 eV, which is caused by the crystal imperfection of the films along the c-axis. The increased film-growth temperature and subsequent heat treatment at low temperatures prominently reduced the red PL, which resulted in the above excellent features of the polycrystalline ZnO films.