Dielectric-tuned Diamondlike Carbon Materials for High-performance Self-aligned Graphene-channel Field Effect Transistor
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Dielectric-tuned Diamondlike Carbon Materials for High-performance Self-aligned Graphene-channel Field Effect Transistors Susumu Takabayashi1,2, Meng Yang3, Shuichi Ogawa3, Yuji Takakuwa3, Tetsuya Suemitsu1,2 and Taiichi Otsuji1,2 1 Research Institute of Electrical Communication (RIEC), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan 2 JST-CREST, K’s Olancho Bldg., 7 Olancho, Chiyoda-ku, Tokyo 102-0076, Japan 3 Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ABSTRACT The ‘DLC-GFET’, a graphene field effect transistor with a diamondlike carbon (DLC) topgate dielectric film, is presented. The DLC film was formed ‘directly’ onto the graphene channel without forming passivation interlayers using our original photoemission-assisted plasmaenhanced chemical vapor deposition (PA-CVD), where the plasma was precisely controlled by photoemission from the sample with quite low electric power to minimize plasma damage to the channel. The DLC-GFET exhibits clear ambipolar characteristics with a slightly positive shift of the neutral points (Dirac voltages). Relatively high transconductances were obtained as 14.6 (8.8) mS/mm in the n (p) channel modes, respectively, with a thick gate dielectric of 48 nm and a long gate length of 5 μm, promising vertical scaling-down to improve the high-frequency performance. The positive shift of the Dirac voltage is due to unintentional hole doping from oxygen species in the DLC film into the graphene channel, promising a minute modulation doped structure with oxygen to overcome high resistance in the access region. Hence, a DLC film deposited by PA-CVD is a candidate for the gate dielectric on graphene. INTRODUCTION Graphene is a simple carbonaceous material consisting of a two-dimensionally ordered honeycomb structure with sp2 carbons. Due to the nature of massless Dirac Fermions, electrons and holes exhibit equally outstanding carrier transport properties with mobilities over 200,000 cm2/V s [1]. Thus, graphene channel field effect transistors (GFET’s) can break the operating speed limit of conventional silicon- and III-V compound semiconductor-based transistors [2]. Recently, performances exceeding 100-GHz cutoff-frequency (fT) have been reported for topgate-type GFET’s with short gate lengths and ultrathin high-κ gate dielectrics such as HfO2 [3]. Despite the introduction of high-κ gate dielectrics, gate modulation remains insufficient because oxidized materials like HfO2 and Al2O3 easily damage graphene [4]. A low-κ noncovalent functionalization layer is forced to be introduced underneath the oxidized material to prevent the damage, which severely degrades the dielectric property. One of the main strategies to improve the GFET performance is to develop a direct top-gate dielectric deposition technique that does not degrade the electron transport characteristics of graphene. Like graphene, diamondlike carbon (DLC) is also a carbonaceous material, but having an amorphous structure com
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