II. Graphene Nanophotonic Modulators for Near-Infrared Applications
2.4. Experimental Investigation of the Solid-Electrolyte-Gated Graphene-Covered Metal-
2.4.1. Structure and Experiment Methods of the EAM
The MISIM waveguide for this research consists of a Si strip and copper (Cu) blocks horizontally sandwiching the Si strip with thin SiO2 insulator layers in between. The structure of the MISIM waveguide is described in Figure 2.4.1(a), and the MISIM waveguide is covered with the single graphene and solid electrolyte. The width and height of the Si strip are 250 nm and 160 nm respectively.
The thickness of Cu blocks is 260 nm. The SiO2 insulator layers are 30 nm thick. The mode profile of the MISIM waveguide, which is calculated by using a simulation program (Mode Solutions, Lumerical) is also shown in Figure 2.4.1(b). The electric field of the MISIM waveguide mode is strongly confined in the SiO2 layers due to the Si strip and Cu blocks. The field intensity at the position, where graphene is located (z = 230 nm), is almost as strong as the field intensity in the middle of the SiO2 layer of the MISIM waveguide (z = 115 nm). This strongly confined electric field at the SiO2 layers makes light- graphene interaction strong which is proved below. The MISIM waveguide is fabricated by using standard CMOS fabrication processes [61]. Lastly, the MISIM waveguide is connected to a 450-nm- wide conventional Si strip waveguide through the linear taper with the length of 600 nm which Si strip width is varied from 160 nm to 450 nm. The coupling loss of the as-fabricated coupler without graphene is about 0.5 dB [61]. Figure 2.4.1(c) is the scanning electron microscope (SEM) image of the graphene- covered MISIM waveguide. Figure 2.4.1(d) is SEM image of the cross-section of the MISIM waveguide after milling with a focused ion beam (FIB).
Figure 2.4.1. Device structure of the solid-electrolyte-gated graphene-covered MISIM waveguide. (a) Operation schematic of the device with cross-sectional structure of the MMISIM waveguide. (b) Mode profile of the MISIM waveguide in this device. (c) SEM image of the graphene-covered MISIM waveguide with false colors. (d) SEM image of cross-section of the MISIM waveguide after FIB milling.
The MISIM waveguides are fabricated on an 8-inch silicon-on-insulator (SOI) wafer by using a fabrication process service provided by National Nanofab Center. The thickness of the Si layer and buried oxide of the SOI wafer were 250 nm and 2 μm respectively. Identical MISIM waveguide chips with dimensions of 15 mm × 20 mm were fabricated on the SOI wafer. Standard CMOS fabrication processes were used: Optical lithography with 248 nm UV, reactive-ion etching (RIE), CVD of dielectrics, sputtering of Cu, and CMP process. After that the wafer was divided into chips by a dicing saw. The MISIM waveguide is modified in order to fabricate the MISIM waveguide covered with the solid-electrolyte-gated graphene. Actually, there is surrounding SiO2 layer around Si strip of the MISIM waveguide above the level of Cu top surface. If graphene is transferred on such SiO2 layer, graphene cannot interact with the MISIM waveguide mode effectively. So, the SiO2 layer above Cu surface was etched for 5 seconds with a buffered oxide etchant solution (BOE, Buffered Oxide Etch 6:1, J. T. Baker).
After modifying MISIM waveguide, the wet-transfer process of graphene and formation of the solid electrolyte were conducted. First, graphene grown by the chemical vapor deposition (CVD) on Cu foil is prepared (Graphene Square). Then poly(methyl methacrylate) (PMMA, PMMA 495k A2, MicroChem) was covered with a spin-coater on one side of Cu foil. Backside graphene at the Cu foil is removed by RIE with the gas mixture of O2 and Ar. Then, PMMA-coated graphene-Cu foil was floated on an aqueous solution of ammonium persulfate with a weight percent of 10% to deionized (DI) water for 2 hours. After complete etching of Cu, PMMA-graphene film on the solution was picked up with slide glasses and moved to large amount of DI water. By floating the film on DI water for 5 hours, remained Cu etchant on the film is rinsed out. PMMA-graphene stacked film was transferred on to the MISIM waveguide chip, and it was heated on a hot plate at a temperature of 150 °C for 10 minutes.
Finally, PMMA was removed by placing the graphene-transferred chip in acetone for 12~24 hours.
After complete the wet-transfer of graphene, photolithography was conducted to make a graphene pattern on the MISIM waveguide with a photoresist (AZ5214E). Then, RIE with O2 plasma was carried out to etch unnecessary region which is outside of photoresist pattern. Residual photoresist is removed by dipping the sample in acetone. The width of graphene strip on the MISIM waveguide is 30 μm after photolithography and RIE. Finally, contact electrodes were form by e-beam evaporation, and their material types and thicknesses are 5-nm-thick titanium (Ti) and 200-nm-thick gold (Au). In case of the solid electrolyte, the solid electrolyte is a mixture of LiClO4, poly(ethylene oxide) (PEO), and methanol with a weight ratio of 0.1:1:40. The mixture was spin-coated on the chip and heated on a hot plat at the temperature of 100 °C for 20 minutes in order to remove residual methanol in the solid electrolyte.
For measuring the MISIM waveguide, tunable laser source is used as a light source. The polarization of the input light is adjusted to transverse-electric (TE) mode by using the polarization controller. After polarization controller, the input light is coupled to the MISIM waveguide with a lensed
output lensed fiber is connected to the photodetector and the optical powermeter. For the DC measurement of the EAM, the DC power supply is used for applying voltage on the electrodes, and for the modulation characteristics, the function generator is used for as AC source and the oscilloscope is used for collecting optical power response from the optical powermeter.