Similar effects of the electric field and annealing on the near-band-edge photoluminescence in ZnO films

The response of the near-band-edge photoluminescence (PL) emission of ZnO thin films to annealing and the electric field’s action was investigated. These processes separately caused similar changes in the PL spectrum. The donor bound exciton emission at 3.36 eV, which is attributed to bulk defects, demonstrated invariance to any exposure, while the intensity of the 3.33-eV emission line decreased after annealing in nitrogen gas and was restored after annealing in an oxygen atmosphere. On the other hand, application of an electrical field during laser illumination resulted in the same change in the PL spectrum. The transition of defects related to the 3.33-eV emission from a radiative to a non-radiative state (and inversely) through the capture (release) of electrons was proposed as the mechanism responsible for the observed changes in the optical properties. The desorption of oxygen from the surface of the ZnO film during annealing in N2 or the motion of photogenerated electrons toward the surface during laser illumination were suggested to be the cause of this capture process.