Study Of Structral, Electrical And Optical Properties In N Ion Implanted Zinc Oxide Thin Films

The ZnO thin films deposited on glass substrate were implanted at room temperature with 140 keV N ions at a dose of 2×1016/cm2. Techniques, such as x-ray diffraction (XRD), scanning electron microscopy (SEM) and ultraviolet-visible spectrophotometer (UV-Vis), fluorescence spectrophotometer, physical properties measurement system (PPMS), have been used to study film structures, optical, electrical properties and their thermal evolutions.The results from XRD measurements show that high dose N ion implantation can induce deterioration of the crystallinity and sharp decrease of (002)-peak. The damaged structures have been found to be partially recovered upon annealing, which leads to growth of grain size. Images from the SEM reveal that the grain size is about 30~ 50 nm and the annealing at 600℃induce lots of pores with size of 50 nm,which may be suggest that the nitrogen oxygen compound has been evaporated at the 600℃.The optical transmission and absorption in N-implanted ZnO films and their thermal evolutions have been investigated. Optical band gap (Eg) and the parameters of band tail (Eo) have been used to analyze the optical absorption edge and near-absorption edge characteristics. Our results show that optical band edge has shifted to higher energy and effect of band tail can be clearly seen after N ion implantation. It indicates that the Burstein-Moss shift has occurred. Subsequently, with the increase of annealing temperature, optical band gap is widened while effect of band tail is weakened. Electrostatic potentials in the grain boundary have been discussed to explain the band-gap widening. Moreover, impurity and structural disorder of the film are responsible for band tail.The three luminescence band at 2.65, 2.8 and 3.0 eV has been observed by photoluminescence spectra. The luminescence band at 2.65 eV is related to the oxygen vacancies, and the 2.8 and 3.0 eV band is attributed to the zinc interstitial. N ion implantation induces the decease of luminescence bands. However, subsequent annealing gives rise to the increase of luminescence. At 300℃, the luminescence band at 2.8 and 3.0 eV can be clearly observed, which can be ascribed to activation of zinc interstitial. Owing to the lots of oxygen vacancies at 600℃, the band at 2.65 eV is notable. Therefore, the results suggest that the green luminescence band can be attributed to the oxygen vacancies. N-implanted ZnO films annealed at 600℃are still n-type. Strain mismatch induced by thermal expansion and the high doping levels are responsible for the difficulty in fabrication of p-type ZnO thin film by N ion implantation. Meanwhile, our results show that ZnO thin film deposited on GaN substrate with low dose N ion implantation may be promising for development of p-type ZnO. Howerer, the theoretical calculations conceive that the acceptor level of nitrogen doping is relatively high and it is not suitable to develop reproductive p-type ZnO. The calculated results suggest that the N and Mg co-doping may be effective methods to fabricate the p-type ZnO. The further experimental results are eager to be made to confirm our conclusions.