Impact of Silicon Dioxide Periodic Grating Structure on Electrical Properties of Graphene
Authors: Long Wei-Yu , Shia Po-Han , LU Yuxuan , Lin Chih-Ting 
Published: August 2024

Our study breaks away from the conventional focus on 3D transistor structures, aiming to explore the potential of two-dimensional materials. Graphene, a key player of two-dimensional materials, draws significant attention due to its exceptional electrical properties and chemical stability. Rather than delving into the intrinsic properties of graphene, our research focuses on examining the impact that other materials in contact with graphene have on graphene’s behavior. With a thickness of just one atomic layer, graphene's band structure is easily influenced by materials in contact with it, giving rise to the Surface Charge Transfer Doping (SCTD) effect. Our study involves the utilization of e-beam lithography to define 140 nm gratings and RIE to etch into SiO2 substrates. Subsequently, graphene is transferred onto these SiO2 gratings. Within these grating regions, in other words, graphene is supported by SiO2, while other part of the graphene is suspended. By strategically forming alternatively supported/suspended type of graphene in period of 140 nm through these gratings, our aim is to unravel the impact on the electrical characteristics of graphene along both the vertical and horizontal axes. In vertical direction, we measured contact resistance between graphene and metal; in horizontal, we measured the difference of Id-Vg curve caused by the grating structure under graphene channel. This exploration emerges from the quest to harness novel properties in the face of evolving fabrication methodologies.

To have 140 nm grating, we chose e-beam lithography in defining the nano grating pattern. Initially, a clean wafer was coated with chromium, and e-beam resist Zep-520A was spin-coated and soft-baked. Subsequently, using e-beam lithography, we defined the periodic pattern. Following this, ICP-RIE was employed for dry etching of chromium, and the resist was removed. Then, ICP-RIE, with chromium as the hard mask, was utilized for dry etching of SiO2. Finally, the chromium was immersed in a chromium etchant solution, resulting in a periodic grating structure with a depth of 100nm on SiO2. This marks the completion of the first part of the fabrication process. In the second part of fabrication process, we transferred graphene, prepared through CVD, onto the SiO2 substrate with periodic gratings defined through etching. We employed optical lithography to define patterns for transmission line method (TLM) measurements. These patterns were aligned with positions defined by e-beam, ensuring that the graphene-metal contact positions precisely coincided with the underlying grating structure. Utilizing TLM, we measured the contact resistance, enabling the study of the impact of the grating on the electrical properties of graphene in the vertical direction. The fabrication flow can be schematically shown in Fig. 1.

By using ELS-7000 (E-beam writer) and Oxford ICP-RIE, we successfully achieved ~140 nm grating structure on silicon dioxide with depth of 100nm, which is observed on SEM as shown in Fig. 2. Then we measured the I-V curve by 4200A-SCS and calculated contact resistance by TLM. The preliminary results of the contact resistance measurement are shown in the Fig. 3. The I-V curve shows that as the channel length increases, the total resistance increases, which is aligned with the assumptions of TLM. We can further calculate the contact resistance through the Y-axis intercept of channel length-resistance curve. With the same process, we can explore the electrical variations of graphene with graphene-metal contact resistance and current-voltage relationship.

Figure 1