Investigations On The Growth Of ZnO Nanomaterials, Property And Its Field Emission Display Devices Prototype

ZnO and GaN, the two outstanding materials among the third generation semiconductors, are indispensable materials because of their wide band gaps of about 3.4 eV, which lead to emission in the ultraviolet spectral range.Zinc oxide (ZnO) is a semiconductor with a direct wide band gap of 3.37eV at room temperature. Its exciton binding energy is 60 meV, much larger than that of GaN (26 meV), another wide-gap semiconductor (Eg～3.40eV at room temperature) which is widely used for production of blue-ultraviolet and white light-emitting devices. The larger exciton binding energy makes ZnO more competitive in obtaining efficient lasing by excitonic emission compared to other wide-band-gap semiconductors. Because exciton-exciton scattering-induced stimulated emission occurs at a threshold lower than that for the electron-hole plasma recombination, ZnO is an ideal material for fabricating semiconductor laser devices operating at room temperature and higher temperature. On the other hand, nanostructured ZnO has a diverse group of growth morphologies, which is regarded as the richest family among all the nanomaterials. Many kinds of ZnO nanostructures such as nanowires, nanotubes, nanobelts and nanorings have been obtained so far, and have attracted increasing attention. ZnO nanostructures have promising potentials in extensive applications and are the fundamental building blocks for fabricating nano-optoelectronics and nano-electronics devices, nanosized gas sensors, transducers, and field emitters etc.In order to realize the application of ZnO nanostructures in nano-optoelectronic and nano-electronic devices, it is necessary to obtain high quality ZnO nanostructures by using non-metal catalyst or catalyst-free method. Since, the metal impurities may diffuse into the nanostructures and lead to deep level defects (these are fatal to the stability of devices.) Also, it is necessary to increase the conductivity of ZnO nanostructures for fabricating nano-optoelectronic and nano-electronic devices by intentional n-type doping. Recently, the field emission display technology—a new kind of self-emitting panel display technology has obtained great attentions becauseof its merits—cold cathode emission, self-emission, high-brightness, broad visual angle and quick responsibility. Although many efforts have been made because of its good field emission property, the research of ZnO nanostructures' field emission is still on the groping way. It still needs further studies on many issues such as further lowering the threshold field of ZnO nanostructures, enhancing the stability and fabricating the prototype of the field emission display device, etc. On the other hand, the ZnO nano-LEDs (light emitting diodes) have been reported. It is possible to realize the actual ZnO nano-LEDs in future. In order to widen the spectral range of emission from ZnO based materials and obtain higher luminescence efficiency, alloying ZnO with Mg or Cd is imperative for adjusting the bandgap.In the first section of this thesis, the thermal evaporation using non-metal catalyst was used to grow ZnO nanostructures. The growth mechanisms of the two kinds of nanostructures were also studied. On these basis, we have doped ZnO nanoarray with Al. The exact defect level of the Al impurity was also identified. In the next segment of the section, we prepared Al-doped quasi-alined ZnO nanorods using two kinds of buffer layer to optimize the field emission property. The prototype of the field emission display device was made and tested. In the last segment, two kinds of ZnMgO and ZnCdO nanostructures with peculiar shape were prepared. The main results are as follows:(1)Two kinds of ZnO nanostructures were grown at different initial temperature by using zinc acetate dihydrate (ZA) as self-catalysis materials. We suggest that two kinds of initial layer were formed because of different initial temperature, hence lead to the different morphology of ZnO nanostructures.(2)The quasi-aligned Al-doped ZnO submicro-rods were prepared and its conductivity were also characterized by C-AFM (conductance atomic force microscopy). It indicated that the conductivity of ZnO submicro-rods was enhanced evidently by Al-doping.Meanwhile, the investigation of the optical properties of the rods indicates: â‘ The exact donor-level of Al is about ～90meV. â‘¡At room temperature, an excitonicemission and its first LO-phonon replica dominates the NBE emission. Also, we suppose that the excitonic emission can be attributed to exciton bound to surfacedefects. â‘¢ The absorption edge of the Al-doped samples blueshifts, while the nearband edge emission redshifts.These new findings can help us to understand the defect levels of Al in ZnO.(3)Al-doped quasi-aligned ZnO nanorods were prepared on Si substrates with Au buffer layer. Field emission measurements were also conducted. The threshold field is 4.5V/μm. It is comparable to the results(the threshold field 4.3V/μm-19.1V/μm) reported in the literatures. Also, field emission display was obtained.(4)Dendritic ZnMgO nanostructures were grown on Si substrates. The investigations indicate that Mg exist in separate phase (MgO) in as-grown ZnMgO nanostructures. It can be transformed into ZnMgO (hexagonal phase) by annealing at 800°C. The photoluminesence of ZnMgO nanostructures have a blue-shift of about 6nm (0.05eV).The pagoda-like ZnCdO micro-needles were prepared on Si(111) substrates using Zn, ZA and Cadmium chloride 2.5-hydrate (CdCl₂·2.5H₂O) as the source materials without using metal catalysts. Alloying ZnO with 0.9at.% Cd have changed the effective bandgap to 3.23 eV, which is smaller than that of the pure ZnO.GaN is still a highlight in semiconductor research at present. It is one outstanding material of the third generation semiconductors for its success in applications in our lives. Great efforts have been made to grow high-quality GaN on Si substrates for the potential integration between optical and electrical devices by many researchers. The second section of this thesis focuses on this issue. Crack-free GaN layers on Si substrates were grown by MOCVD. Meanwhile, the structure and morphology of the layers were investigated. The dislocation density and the epitaxial relation between the substrate and the epilayer were investigated by high resolution XRD. Detailed results are as follows:(5)The nitrification of Si substrates can be avoided by suitable predeposition of the TMAl. AlN grown at 1050°C can be a good buffer layer for the growth of GaN. Afterthe growth of A1N, by adding a predeposition step of TMGa (it can increase the wetting-ablity of Ga on the buffer layer) and using AlN/GaN multi-buffers, early transformation from 3-D grwoth to 2-D growth can be induced. Crack-free GaN can be obtained by these processes.