Introduction
Optically transparent antennas have attracted much attention because they can be located in various places without altering the landscape. Such antennas are fabricated using transparent conductive materials, such as indium tin oxide (ITO) [1], gallium-doped zinc oxide (GZO) [2], indium zinc tin oxide (IZTO) [3], and metal mesh structures [4]. We focused on graphene as an electrode material for optically transparent antennas. The high carrier mobility [5], optical transparency [6],and flexibility [7] make graphene attractive as an electrode material for optically transparent antennas. We previously reported that an invisible dipole antenna fabricated using monolayer chemical vapor deposition (CVD) graphene radiates microwaves in the microwave band [8] [9]. The optical transparency of the fabricated graphene antenna was about 97.7%. Optically transparent antennas fabricated using 6-layer-stacked CVD graphene with a sheet resistance of 18 \(\Omega/sq\) and optical transparency of 85% has recently been reported [10]. Optically transparent microwave absorbers based on graphene [11] [12] and printed graphene-based antennas [13] [14], which are opaque to visible light, have also been reported.
The electrical contact properties at the interface between the metal-based feeding lines and graphene-based optically transparent antennas are important to obtain high radiation efficiency. The metal/graphene contact structure is the most commonly used to feed the microwave power to graphene-based antennas. In the DC band, the contact resistance at the interface between metal and graphene may significantly limit the performance of graphene-based deveices because the current injection from metal to graphene is suppressed due to the small density of states in graphene [15]. The graphene-based invisible antenna investigated in our previous studies [8] [9] consists of dipole elements fabricated using graphene as a half-wavelength dipole antenna and a coplanar waveguide (CPW) fabricated using Au and graphene as a feeding line. CPWs are favoarable structures for feeding graphene-based antennas because all electrodes of CPWs can be integrated on the surface side of the substrates, which are compatible with the transfer techniques of graphene and photolithography techniques. The transmission properties of CPWs fabricated with metal and graphene have been reported [16]. In the microwave band, the contact properties between metal and graphene are represented by the parallel contact resistanceRc and parallel contact capacitanceCc [17]. However, the properties of the metal/graphene contact in the microwave band have not been discussed and not been optimized in detail, though they are quite important to optimize the performance of the graphene-based antennas.
We focused on the properties of the metal/graphene contact in the microwave band and fabricated CPWs composed of Au electrodes and monolayer CVD graphene. To characterize the properties of the metal/graphene contact, the equivalent circuit model of the fabricated CPWs was used for the analyzing the contact properties from 1 to 15 GHz, in which the metal/graphene contact and the graphene area are represented by lumped components, which were experimentary determined. The calculated reflection coefficients for the equivalent circuit model were compared with the measured ones. Finally, we compared the impedance of Cc and Rc .
Experiments
Fabrication and measurement set up