Suspended graphene variable capacitor

Abstract

The tuning of electrical circuit resonance with a variable capacitor, or varactor, finds wide application with the most important being wireless telecommunication. We demonstrate an electromechanical graphene varactor, a variable capacitor wherein the capacitance is tuned by voltage controlled deflection of a dense array of suspended graphene membranes. The low flexural rigidity of graphene monolayers is exploited to achieve low actuation voltage in an ultra-thin structure. Large arrays comprising thousands of suspensions were fabricated to give a tunable capacitance of over 10 pF/mm 2 , higher than that achieved by traditional micro-electromechanical system (MEMS) technologies. A capacitance tuning of 55% was achieved with a 10 V actuating voltage, exceeding that of conventional MEMS parallel plate capacitors. Capacitor behavior was investigated experimentally, and described by a simple theoretical model. Mechanical properties of the graphene membranes were measured independently using Atomic Force Microscopy (AFM). Increased graphene conductivity will enable the application of the compact graphene varactor to radio frequency systems.Mechanically tuned variable capacitance has been an effective means to tune resonant circuits since the advent of radio [1]. More compact varactors have since been developed in the form of an electrically tuned semiconductor junction capacitance [2]. Micro-electromechanical system (MEMS) implementations of varactors [3] combine the advantages of mechanical and semiconductor varactors in a single device architecture, including high electrical quality factor, high linearity, and the capacity for monolithic integration with silicon electronics [4]. The canonical MEMS varactor is the parallel plate structure consisting of a conducting membrane suspended over a fixed plate, actuated by electrostatic attraction under an applied bias potential. While simple in structure, typical parallel plate varactors suffer high operating voltage [5,6] and a limited capacitive tuning range. These limitations are typically overcome by complex mechanisms [7], which increase both the size and actuation voltage.Fundamentally, increasing the flexibility of a suspended element by reducing it's thickness will reduce the actuation voltage of a parallel plate varator [8]. Monolayer graphene membranes achieve the ultimate limit with an inferred elastic stiffness of E e 390 N/m [9]. In comparison a 15 nm thick Si 3 N 4 membrane has an elastic stiffness of E e 6.3 kN/m. In addition to lower actuation voltage, graphene nano-electromecanical systems (NEMS) occupy less area than traditional MEMS counterparts [10], and can be easily integrated with integrated silicon electronics using standard transfer techniques [11,12]. In the last decade, graphene NEMS have been widely investigated, including suspended graphene resonators [13][14][15], switches [16][17][18], and sensors [9]. While the theoretical limits of suspended graphene varactors has been investigated [8], large arrays of low spring constant suspensions has be...

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