| Zinc oxide (ZnO) is a new-type Ⅱ-Ⅵ semiconductor with direct wide-band-gap of3.37eV at room temperature (RT). It has a high exciton binding energy of60meV and hexagonal wurtzite structure at the air condition. It has been investigated extensively because of its low cost as well as its excellent electrical, optoelectronic, luminescent properties and absence of toxicity making suitable for many applications such as light emitting diodes, solar cell, photodetectors, electroluminescence, transparent conductive film, surface acoustic waves device and so on.Magnetron sputtering is convenient for the deposition of semiconductor thin films and has the advantage that it can be scaled for large area and uniform deposition. Usually, as-deposited films require a thermal treatment to improve stability and reduce the possible undesirable influence of the substrate. For semiconductors, thermal annealing is also used to activate dopants and to alloy ohmic contacts. Thermal annealing is a widely used method to improve crystal quality and to study structural defects in materials. During an annealing process, dislocations and other structural defects will move in the material and adsorption/decomposition will occur on the surface, thus the structure and the stoichiometric ratio of the material will change. Such phenomena can have major effects on semiconductor device properties, light emitting devices being particularly effected. However, direct growth of ZnO films on substrate is extremely difficult and often results in amorphous or polycrystalline films due to the mismatch of large lattice and thermal expansion coefficients between ZnO and substrate. Films deposited on a buffer layer are better than those grown directly on the substrate.In this thesis, ZnO films are prepared by radio frequency reactive magnetron sputtering technique. The crystalline and optical properties of thin films were investigated by the X-ray diffraction (XRD), scanning electron microscope (SEM) and fluorescence spectro-photometer. In this way some experimental data and the theoretical basis are provided for the application of ZnO films. The results are summarized as follows:1. ZnO thin film was deposited on Si substrate with the insertion of ZnO buffer layer, which was annealed at300℃in vacuum. ZnO thin film was grown by RF magnetron sputtering deposition technique and the ZnO/ZnO-buffer/Si films have been further annealed with different annealing temperatures. The microstructure and luminescence properties of ZnO thin films were investigated by X-ray diffraction (XRD) and photoluminescence (PL) spectrum, respectively. The results reveal that the sample, which was grown on buffer layer of the deposition time is6min, has the best crystalline quality. The crystalline quality is markedly improved with annealing temperature increasing. All films have an average optical transparency over90%in the visible range. The optical band gaps gradually decrease from3.221eV to3.184eV within the increase of annealing temperature. Five main peaks located at about384nm (UV),420nm (violet),455nm (blue),473nm (blue) and530nm (green) photoluminescence are observed from the PL spectra. The UV emission is attributed to free exciton-related activity. The violet luminescence is associated with radiative defects. The blue emission is ascribed to oxygen vacancies and interstitial Zn. The green emission is due to electron transition from deep donor levels of oxygen vacancies to shallow acceptor levels of Zn vacancies. The455nm blue peak shifts from455nm to447nm as the annealing temperature increases from400℃to600℃. The intensity of530nm green first increases then decreases as annealing temperature increases, which attributes to the reduce of Zn vacancies.2. Al-doped ZnO thin film was deposited on ZnO buffered and non-buffered Si substrate, and the ZnO buffer layer annealed at300℃in vacuum. AZO thin films have been further annealed at500℃in vacuum. The microstructure and luminescence properties of AZO thin films were investigated by X-ray diffraction (XRD) and photoluminescence (PL) spectrum, respectively. The results show that AZO film grown on buffer layer has the best crystalline quality. The crystalline quality is markedly improved with buffer layer. All films have an average optical transparency over90%in the visible range. One strong UV photoluminescence is observed from the PL spectra. The UV emission peak is narrowed by introducting ZnO buffer layer, which is attributed to the decrease of the grain boundary interface defects. So the good crystal quality improved UV light.3. The effects of annealing atmosphere and temperature on the structural and photoluminescence properties of Al-doped ZnO (AZO) films which deposited on the buffered substrates by radio frequency magnetron sputtering were investigated. X-ray diffraction pattern of the annealed films on buffered substrate exhibited wurtzite crystal structure with a strong (002) preferred orientation, and the results showed a better crystalline quality obtained through annealing process. Based on the photoluminescence (PL) properties, AZO film annealed in oxygen displayed the stronger ultraviolet (UV) emission than that annealed in vacuum. The ratio of UV-to-visible emission was greatly influenced by annealing condition. Mechanism analysis of PL shows that visible emission band should be derived from the defects, which is greatly associated with oxygen concentration during annealing. The relationship between blue/green luminescence and oxygen concentration in ZnO thin films was discussed. |
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