P-N Junction Formation in Electron-beam Irradiated Graphene Step
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P-N Junction Formation in Electron-beam Irradiated Graphene Step Xiaomu Wang1, Chengliang Wang1 and Jian-Bin Xu1 1 Department of Electronic Engineering and Materials Science and Technology Research Center, The Chinese University of Hong Kong, Hong Kong SAR, China
ABSTRACT We show that graphene mono-/bilayer step operates as an abrupt p-n (p-p+) junction. Due to the thickness-dependent oxidation effect, the uniform channel can be adjusted to spatially asymmetrical junction by means of electron beam irradiation. The lithography-free process on OTMS modified substrate possesses the merit of clean surface and high performance. This conveniently fabricated graphene step device opens an opportunity to study the intrinsic interface across a well defined junction. INTRODUCTION Graphene, a new allotrope of carbon, which has been very recently discovered, attracts more and more attention due to its anomalous electrical properties and two-dimensional structure [14]. Graphene p-n junction is a crucial building block in the fabrication of sophisticated graphenebased circuits. Generally speaking, spatially varied doping is used to define abrupt graphene p-n (p-p+) junctions. As a result, graphene p-n junctions were previously fabricated by means of asymmetrical electrical doping [5-7] or chemical doping [8-10]. However, the fabrication process was complex, generally two-step masking or multiple bias were needed. In addition, the channel was partially covered for most of the time and thus made it difficult to study the surface of the 2D p-n junction. On the other hand, electron irradiation is proved to be an efficient way to introducing oxygen functional group or vacuum gap on graphene and thus alter the chemical, electrical and mechanical properties [11-13], which provides another possibility to form graphene abrupt p-n junction. Here we present an easy method to fabricate graphene p-n (p-p+) junction acheived by inhomogeneous electron beam modification of mono-/bilayer graphene step. Different from high-energy electron beam and homogenous grapheme sheet used in the previous works [11-13], we demonstrate that mild electron-beam (EB) irradiation modifies electronic properties of monoand bilayer graphene differently, i.e., it introduces significant defects to monolayer graphene but only dopes bilayer ones by charge transfer. Optically, Raman shift of monolayer graphene presents graphene oxide features but bilayer graphene remains graphene-like spectrum. We attribute the distinct irradiation results by diverse oxidation effect between mono- and bilayer graphene.
EXPERIMENT The graphene sample is prepared by mechanical exfoliation of HOPG. The single layer and bilayer part in a few layers graphene (FLG) are identified by optical microscopy and Raman spectroscopy. Graphene field effect transistor is formed by locating electrodes through 'lithography-free' method [14]. The average width to length ratio is about 1, as shown in figure 1 (a). After fabrication and test, the GFET is imaged by a scanning electron microscopy (SEM). The se
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