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