Roles of Laser-Ablation of Mn in Initial Stage of Growth of ZnO Nanorods by Chemical Vapor Deposition
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Roles of Laser-Ablation of Mn in Initial Stage of Growth of ZnO Nanorods by Chemical Vapor Deposition Takashi Hirate, Hiroshi Miyashita, Takashi Kimpara, Kazumoto Takizawa and Tomomasa Satoh Department of Electrical, Electronics and Information Engineering Faculty of Engineering, Kanagawa University 3-27 Rokkakubashi, Kanagawa-ku, Yokohama, 221-8686, Japan ABSTRACT We studied effects of laser ablation on morphology of ZnO nanorods fabricated by low-pressure thermal chemical vapor deposition combined with laser ablation of Mn. In this paper, we studied the effects of laser-ablation time period and energy density of laser beam at an initial stage of growth of ZnO nanorods. Both of laser ablation and chemical vapor deposition are simultaneously started, and only laser ablation is terminated after a certain period of time. After the laser ablation is terminated, chemical vapor deposition is continued. It is concluded from the experimental results that even if laser-ablation time period is only 30 sec and energy density of the laser is only 0.023 Joule/cm2/pulse, the laser ablation shows remarkable effects on growth of ZnO nanorods. INTRODUCTION ZnO is an attractive II-VI compound semiconductor material for various optoelectronic devices due to its wide band gap of 3.37 eV at room temperature and its large excitonic binding energy of about 60 meV. Recently, growth of various ZnO nano-structures such as nanorod, nanobelt, nanowall, etc. has been reported [1-8]. Reports on initial stage of growth of these ZnO nanostructures, however, are few. We have studied on fabrication of well oriented ZnO nanorods grown by low-pressure thermal chemical vapor deposition (CVD) method cooperated with a laser ablation that is developed by us [1,8]. When a Mn target was ablated, it was found that the laser ablation had drastic effects on morphology of ZnO nanorods [8]. In this paper, we studied effects of laser ablation of Mn on morphology of ZnO at an initial stage of growth of ZnO nanorods. It was recognized from the experimental results that a laser ablation at an initial stage of ZnO nanorods is very effective on construction of nanorods. EXPERIMENTAL ZnO nanorods were grown on a p-/p++-Si (111) substrate by chemical vapor deposition method combined with laser ablation that was developed by us [1, 8]. The growth equipment is schematically shown in figure 1. This is basically a low-pressure thermal CVD system. The precursors to synthesize ZnO are metal Zn (purity 99.999%) vapor and O2 (purity 99.99%) that are
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Laser Beam (Pulsed Nd:YAG Laser[λ=1.064μm]) Heater 2 Heater 1 (for deposition) Lens (for Zn evaporation) Qurtz Reactor
Evacuation
N2
Thermocouple
O2+N2 Inlet Mn Target p-Si Substrate
Zn
Figure 1. Growth equipment for ZnO nanorods. introduced into a deposition chamber through each orifice whose diameter is 1 mm. N2 (purity 99.99%) is used as carrier gas to transport Zn vapor from a Zn vaporization chamber to a deposition chamber. A polycrystalline Mn (99.9%) target (10x10x10 mm3) is set at the
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