Study On The Nanostructured ZnO Thin Films Grown By MOCVD And The Light Emitting Devices

In this work, basing on the existing research works, we have successively tried to explore the possibility of realizing the practical use of ZnO based light emitting devices. Considering the unique properties of ZnO nanostructure materials and the p-type doping problem of ZnO, we have focused our attention on ZnO nanostructure materials and unipolar light emitting device. Large part of this work studies the material properties of ZnO nanostructures and unrevealed working mechanisms behind the ZnO based metal-insulator-semiconductor（MIS） device. The goal is to utilize the advantages of ZnO nanostructure materials to enhance the functionality of normal ZnO based MIS devices. The detailed works and related results are summarized as follows:1. We employed metal-organic chemical vapor deposition（MOCVD） to grow ZnO thin films featured with nanorods morphology on c-Al₂O₃. Such thin films have a double layer structure with nanorods on a（2D） film structure. The dependence of the crystallinity and morphology of such double-layer thin films on the growth temperature has been investigated and systematically discussed from aspects of thermodynamics and MOCVD growth kinetics. Photoluminescence（PL） has been used to characterize the double-layer thin films. It has been revealed that the nanorods and bottom film structure exhibit different characteristic of carrier radiative recombination. The dependence of radiative recombination characteristics of nanorods on growth temperature and substrate materials have also been investigated. We have utilized the Hall effect to characterize the electrical properties for the bottom film layers, and revealed the mechanisms behind the poor electrical property limited by strong grain boundary effect for low temperature grown samples. The improvement of electrical property resulted from increase of temperature has also been fundamentally explained.2. we have obtained thin films with nanowalls and nanopyramids morphology. For the nanowalls featured thin films, we have compared morphology and crystallinity between samples grown on c-Al₂O₃ and GaN substrates. Related discussion has been provided mainly based on thermodynamics. The optical properties of nanowalls structure have been characterized by PL. The results show that nanowalls on GaN have stronger exciton-phonon coupling and higher fluorescent quantum efficiency. Comparing with the PL spectra for nanorods, nanowalls structures have strong DLE emission, but have higher fluorescent quantum efficiency. The thin films featured with nanopyramids can only be grown on GaN substrates. We have demonstrated that with increase of temperature, the nanorods gradually evolve to nanopyramids morphology, meanwhile the crystallinity of the grown ZnO epi-layers also get improved. The nanopyramids have also been characterized by PL. Compared with nanorods and nanowalls, the nanopyramids show strongest exciton-phonon coupling and highest fluorescent quantum efficiency.3. ZnO nanorods with good optical quality have been grown on ITO glass substrates, based on which Au-MgO-ZnO MIS devices have been fabricated. The effect of the thickness of MgO on the device performance, both for forward and reverse bias, have been systematically discussed from several aspects, such as injection and recombination of holes, charge transport in the device, intensity distribution of electric field in materials, and the situation of energy band bending. Combining the discussion results based on ideal device of planar structure with a proposed device model for practical situation, the experiment results can be appropriately explained. In order to improve the performance of the reverse biased devices, a new type of device of MIPN structure has been designed and fabricated. The expected performance and working mechanism of the MIPN device has been thoroughly explained. The device shows very pure ZnO NBE emission in EL spectra, and higher light output efficiency compared with normal MIS device, which is consistent with our expectation.4. Based on the research works about ZnO nanorods structure, we have explored the possibility of growth of ZnO nanorods with good optical quality on metal substrates by MOCVD. Zn O nanorods like structures have successfully been grown on Cu, Al and stainless steels. Particularly for stainless steels, we have investigated the effect of growth temperature on the morphology, crystallinity and optical properties of the grown ZnO. The obtained ZnO nanorods sample with optimized fluorescent quantum efficiency and radiative recombination property has been used to construct organic-inorganic hybrid MIS type light emitting device. The device is of ITO/PEDOT:PSS/PMMA/ZnO structure, which shows a certain extent of flexibility. The device shows strong NBE emission and very weak DLE from ZnO under reverse bias. The working mechanism of the device has been discussed in detail.