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Study on Luminescence Property of Ag-doped ZnO Nanowire Arrays Prepared by Ion Implantation

Author: ChenXian
Tutor: ZhaoYong
School: Nanchang University
Course: Optics
Keywords: ion implantation Ag ion ZnO nanowires photoluminescence
CLC: TB383
Type: Master's thesis
Year: 2012
Downloads: 54
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Abstract


ZnO is a wide and direct band gap about3.37eV at room temperature II-VI semiconductor materials, which has large exciton binding energy of60meV. ZnO has been recognized as a promising material for use in many fields (such as ultraviolet detectors, laser diodes, lighting diodes, and solar cells, etc.) due to its various attractive properties, such as photoelectric, photosensitive, piezoelectric, voltage-sensitive and high light transmittance in the visible region. In order that ZnO can be applied in various optoelectronics devices, it is necessary that we should improve physical and chemical properties of ZnO by doping. At present, the ion implantation is an attractive doping technique for use in semiconductor materials.Silver ions with energy of63keV and a dose of5×10l5ions/cm2were implanted into ZnO nanowires deposited on silicon substrates by thermal evaporation of a mixture of ZnO and carbon powder. After ion implantation, Ag-implanted ZnO nanowires were annealed in air at different temperatures from600to1000℃. The structural and photoluminescent properties of the as-prepared and as-implanted ZnO nanowires were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area energy dispersive X-ray spectroscopy (SAEDX), and fluorescence spectrophotometer.After improving the reaction chamber, it is found that when reactant temperature is920℃and carrier gas flux is60sccm, we can obtain the best vertically aligned and crystalline ZnO nanowire arrays. Nanowire arrays got good alignment at920℃, because lower temperature is not favorable for dense growth of ZnO nanowires and higer temperature will result in secondary evaporation of Zn Atoms which is also not advantageous for good alignment and preferential growth of ZnO nanowire arrays. Nanowire arrays got the optimal density at the carrier gas flux of60sccm, this is related to the number of ZnO crystal separated out from alloy droplet in unit time. After improving the reaction chamber, the alignment, density and crystal quality of ZnO nanowire arrays have been greatly improved, this was mainly due to Zn vapor was compressed in a small quartz tube and the substrate per unit area got more Zn vapor particles. In photoluminescence property, as ZnO nanowires were synthesized in oxygen-deficient environment, oxygen vacancies which are responsible for green emission are the predominant defects. Rising temperature will result in the accelerated growth rate of ZnO nanwire and more oxygen vacancies in the thicker ZnO nanowire arrays, hence, green emission was increased by rising temperature and would get the maximum intensity at920℃, but was suppressed by further higher temperature which will result in number decreasing of oxygen vacancies by evaporation of Zn atoms from the nanowire surface. After improving the reaction chamber, nanowire arrays showed the strongest ultraviolet (UV) emission at the carrier gas flux of60sccm, it is revealed that ZnO nanowires obtained the most perfect crystal structrure.The analysis of TEM, HR-TEM and SAEDX demonstrate that doping of silver was fulfilled by ion implantation and the following annealing process. XRD pattern revealed that ZnO nanowires still remained hexagonal wurtzite structure after ion implantation. Ion implantation caused structure damage to ZnO nanowire and the structure damage could get recovered by choosing appropriate annealing temperature. PL emissions of the ZnO nanwire were suppressed at both UV and visible bands by ion implantation due to the non-radiative center defects induced by structure damage. Suppressing effects of ion implantation on the PL intensities can be removed by annealing. When annealing temperature exceeds about650℃, Ag-implanted ZnO nanowires show a stable UV emission stronger than as-prepared samples. This is attributed to substitution of Zn atom sites by Ag atoms which can result in quick diffusion of photocarriers. In the range from650to900℃, rising annealing temperature can also enhance visible emission of Ag-implanted ZnO nanowires due to the increased concentration of activated Zn and O vacancies.

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