Defect Engineering for Graphene Tunable Doping
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Defect Engineering for Graphene Tunable Doping Henry Medina, Yung-Chang Lin and Po-Wen Chiu Department of Electrical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan ABSTRACT Here we describe a doping approach that enables selective and variable doping on graphene. The doping level reflected in the successive shift of the Raman G mode can be progressively changed by varying the coverage of molecular adsorption on graphene. We make use of lattice defects which serve as anchor groups for the non-covalent functionalization on graphene to enhance molecule adsorption on defective sites at the elevated processing temperatures and also orbital overlap between graphene and adsorbates (melamine). Low density of defects, which can be monitored by seeing the intensity ratio of the D to G mode in the Raman spectra, was generated by exposing graphene to short Ar plasma pulses, followed by dopant adsorption. The controllable creation of defects makes the precise doping on graphene feasible. Systematic characterizations by Raman scattering show that holes are transferred to graphene, with the doping level depending on the surface coverage of melamine. The charge transfer is also identified by the downshift of the charge neutrality point in the transfer characteristics. INTRODUCTION Graphene, a honeycomb arrangement of carbon atoms organized in a two-dimensional layer, has awakening the interest of research around the world[1]. It has shown its potential as one of the most promising materials for the next generation of electronics due to its high carrier mobility at room temperature[2] and the absence of the fatal short-channel effect that sets a fundamental constraint for silicon miniaturization. However, before being used in practical electronic devices, there are still some technical and fundamental hurdles to leap up. Well-controlled doping level in modern silicon devices is one of the most important factors that push silicon into the mainstream of the semiconductor industry. Likewise, it is also one of the main issues that should be properly addressed for graphene before being considered as a possible candidate for silicon replacement. There have been numbers of reports for graphene doping. The electro-thermal reactions in graphene under ammonia atmosphere is an example of n-type doping[3]. However, this approach is restricted by the reaction only along the edges of graphene, with fixed and low doping concentration. IBM group has also demonstrated electrical doping in a dual gate configuration, i.e., using the back gate for the universal tuning of carrier concentration in graphene channel and the top gate for device operation[4]. The main disadvantage of this process is the complex device architecture with back and top gates for electronic design plus the extra power consumption required to control the carrier density. In our previous work, we have shown the feasibility of inducing n-type doping in graphene with controllable doping level by using ammonia plasma a the nitrogen-containing functional groups[5]. H
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