| As a wide direct band gap semiconductor, the band gap of Zinc oxide (ZnO) is3.37eV and the binding energy of exciton is as high as60meV at room temperature (RT). Accordingly, ZnO is considered to be a strong candidate for ultraviolet (UV) light emitting diode (LED) and laser diode (LD). As an optoelectronic functional material, however, there are still some unsolved problems hindering its further applications in optoelectronic field. The details are as follows:(1) When nitrogen (N) is used as dopant for p doping of ZnO, it is always faced with a troublesome that the N-N pair at oxygen site (N2(O)) was proved to be a shallow donor, which is considered as an important compensation source for acceptors in ZnO. Many post treatments were reported to decrease the Ni(O) donors in N-doped ZnO and significant effects were achieved. However, these routes will result in escaping of acceptors in a certain degree, or introduce some unexpected impurities. So how to suppress the N2content in precursors come to be a key for fabricating ZnO based photoelectric devices.(2) Low mobility and hole concentration remain as obstructions for applications in the field of optoelectronics for ZnO by far. So, to research and to remove the impurities introduced into the ZnO lattice through the doping process are problems crying out for solutions.(3) Over the past decade, although p type ZnO has been reported by for several literatures, acknowledged method for fabricating stable p-type ZnO is still not obtained. Nevertheless, almost entire applications in the field of optoelectronics base on the realizing stable and reproducible p-type ZnO. Therefore, to investigate the factors influencing on the stability of the p type conduction and the physical mechanism become chief of all the problems to realize short wavelength optoelectronic devices based on ZnO.According to the present research difficulties on ZnO, conclusions are drawn as follows:(1) By controlling the N*/N2*species ratio in precursors, p-type ZnO:N was fabricated by plasma-assisted molecular beam epitaxy (P-MBE) using RF activated nitric oxide (NO) as dopant. By decreasing the flux of NO, the N*/N2*ratio in the precursor is boosted effectually, leading to the increase of NO/N2(O) ratio in ZnO:N films. Correspondingly, the n-type conduction of the thin films degrades gradually and turned to p-type. The method is confirmed to depress the N2(O) doping efficiently in precursors and expediently for fabrication of p-type ZnO:N.(2) Plasma-assisted molecular beam epitaxy (P-MBE), the cleanest epitaxial growth technique up to now. However, we confirmed that the donorlike defect, CN complex, which can be derived from N2and N plasma reacting with graphite, could be one of the most important compensation source for fabrication p-type ZnO thin film by using P-MBE method, which hindering the realization of ZnO based optoelectronic devices.(3) By comparing and studying the different electric characteristic evolution of nitrogen doped p-type ZnO thin films grown on a-/c-plane sapphire substrate by plasma assisted molecular beam epitaxy (P-MBE), the instability of p-type ZnO film were investigated through the misfit between the film and the substrate. It concluded that biaxial compressive stress in the deposited film caused by large lattice mismatch between ZnO and c-Al2O3results in the evolution of N chemical state, and it was ascribed to the origin of the instability of the electrical properties.
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