The ultraviolet and green luminescence centers in undoped zinc oxide films

Undoped ZnO films were deposited on Si substrates by DC reactive sputtering. The samples were annealed in air, pure O-2 (1atm) and pure N-2 (1atm) at 850 degreesC, 950 degreesC, 1000 degreesC for 1h. The X-ray diffraction patterns of the as-deposited and annealed samples all have a widened diffraction peak of ZnO (002). It is evident that all the films we used are ZnO films with [001] orientation. The photoluminescence (PL) spectra of these samples include two emission peaks, centered at 3.18eV (ultraviolet, UV) and 2.38eV (green). It is fount that the intensities. of these peaks vary with annealing temperature and atmosphere. At same annealing temperature, the intensity of green peak increases evidently with the increase of oxygen partial pressure, and the intensity of UV peak is also increased somewhat. In the same atmosphere, the intensity of green peak enhances sharply with the increase of annealing temperature, but the intensity of UV peak increases slowly and approaches a maximum value at 950 degreesC, then decreases as the annealing temperature increases. We investigated the dependence of the crystallization and also the intrinsic defects in ZnO film, such as zinc vacancy V-Zn, oxygen vacancy V-0, interstitial zinc Zn-i, interstitial oxygen O-i and anti-sited oxygen O-Zn on the annealing conditions. Compared with the calculated data of these intrinsic defects, it is suggested that the UV radiation of ZnO is due to band-edge exciton transition, and the green emission corresponds to the transition from the bottom of the conduction band to the local level composed of oxide misplaced defects.