Ionoluminescence and photoluminescence study of annealing and ion irradiation effects on zinc oxide

Abstract

The effects of annealing at 473 K and 800 K and of 2 MeV H+ irradiation on the behaviour of defects in ZnO crystals were studied via ionoluminescence (IL) and photoluminescence (PL). The PL spectra of samples annealed in vacuum at 473 K and 800 K showed stronger visible emission and weaker UV emission than virgin samples. The stronger visible emission corresponded to an increase in the concentration of point defects in the samples. The PL spectra of the sample annealed in vacuum at 473 K revealed stronger visible emission than that of the sample annealed in vacuum at 800 K; hence, the concentration of point defects in the sample annealed at 473 K was higher. The IL spectrum of ZnO annealed in vacuum at 473 K showed a weaker intensity than the spectrum of the sample annealed at 800 K, which seemed inconsistent with the PL spectra results. This inconsistency was attributed to different mechanism of PL and IL. IL and PL in some way link the ZnO DBE to different point defect related electronic transfer processes. The PL spectra of samples showed that deep band emission (DBE) is a superposition of 2.1 eV band emission and 2.4 eV band emission and that near band emission (NBE) is a superposition of 3.1 eV band emission and 3.3 eV band emission. However, the DBE of IL is a superposition of 1.8 eV band emission and 2.1 eV band emission, and the NBE exhibited overall weak intensity caused by an increase in the number of defects induced during irradiation. Comparing the effects of annealing in an oxygen atmosphere and in vacuum, the enhanced peak area ratio of 2.1 eV band emission to 2.4 eV band emission in the PL spectra and the enhanced peak area ratio of 2.1 eV band emission to 1.8 eV band emission in the IL spectra of the sample annealed in an oxygen atmosphere demonstrate a correspondence between the ~1.8 eV band and Zn interstitials (Zni), between the ~2.1 eV band and O interstitials (Oi) and between the ~2.4 eV band and O vacancies (VO). The increase in the UV emission of the sample annealed in an oxygen atmosphere suggested that the NBE is a superposition of 3.1 eV band emission and 3.3 eV band emission, which are associated with Zn vacancies (VZn) and self-trapped excitons, respectively.