Stable Electron Emission from ZnO Nanoemitters Grown with Pseudo-Catalyst
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Stable Electron Emission from ZnO Nanoemitters Grown with Pseudo-Catalyst Su-Hua Yang,1 Yi-Ming Hsu1, Ming-Wei Tsai1, and Ting-Jen Hsueh2 1
Department of Electronic Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan, R.O.C.
2
National Nano Devices Laboratories, Tainan 741, Taiwan, R.O.C.
ABSTRACT Catalyst-free vapor phase transport was applied for the growth of ZnO nanoemitters. A single-crystalline ZnO:Al seed layer was deposited and used as a pseudo-catalyst. The desired morphology of nanostructures can be achieved by means of modifying the growth rates of crystal planes via adjustment in the growth conditions. The field emission characteristics of ZnO nanoemitters satisfied the Fowler-Nordheim relationship. The high aspect ratio of nanoemitters had a low turn-on electric field of 0.18 MV/m at emission current density of 0.1 μA/cm2. A stable electron emission with a variation of less than 14% was measured. INTRODUCTION The fabrications and applications of nanostructures have attracted a great deal of attention because they possess unique and fascinating physical and chemical properties. Nanostructures have a variety of possible applications in optoelectronic devices, in which field-emission-related flat-panel lightings (FELs) and displays are regarded as the most promising applications [1,2]. FEL is a well-known illumination technology, which employs a field-emitter cathode and a phosphor screen anode. The emission of electrons is performed when an appropriate electric field is applied to the FEL, which follows the Fowler-Nordheim (F-N) equation [3]: 3
2 1. 54 10 6 E 2 7 v( y) J exp[ 6 . 87 10 ] E t 2 ( y)
A / cm 2 ,
(1)
describing electrons passing across an energy barrier under the effect of an electric field E at the emitter tip. t ( y ) and v( y ) are the correction factors related to the electric field. The electric field at the emitter tip can be represented as E = βV, β denotes the field enhancement factor and V represents the voltage applied to the anode and emitter electrodes. In order to achieve high electron emission efficiency, field-emitters must have a low work function, high aspect ratio, and large emission area as well as stable physical and chemical properties. Accordingly, nanowires, nanorods, and nanoneedles are grown for this purpose [4−6]. For the materials of field-emitters, ZnO is of particular interest owing to its high exciton binding energy of 60 meV, large bandgap of 3.37 eV, and cost-effectiveness, eco-friendliness, and nontoxic nature [7,8]. However, the hexagonal wurtzite ZnO nanoemitters grow into many possible configurations depending on the synthesis parameters. With regard to the morphology
and field-emission behaviors of ZnO nanoemitters, vertically aligned growth along with nanoscale radius of the tips are preferred. In this study, catalyst-free technique was applied for the growth of ZnO nanoemitters to avoid impurity doping from the catalyst itself. The growth mechanism of ZnO nanoemitters was investigated and the el
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