Fabrication And Characterization Of Graphene Field-Effect Transistor

As a new two-dimensional (2D) material, graphene has attracted extensive attention from the researchers in kinds of fields since it was found for the first time in 2004. Graphene is a 2D crystal consisting of one-layer carbon atoms. It has many excellent properties, such as ultra-high carrier mobility and saturation velocity, ballistic transport under sub-micrometer scale, good mechanical, thermal and optical properties. As the dimension of silicon based integrated circuit (IC) approaching the limit of Moore’s law, graphene is expected to replace silicon and to become the fundamental material of the next generation IC. Besides, because of its ultra-high’ carrier mobility, graphene is very suitable for radio frequency (RF) applications. Graphene field-effect transistor (GFET) is a basic device of graphene based RF transistor, sensor and IC. Thus, in this dissertation we thoroughly analyzed the research background and current situation of GFET, and investigated the microfabrication process and electrical properties of GFET. Particularly, we studied the dielectric of GFET. The main contents and conclusions are summarized as follows.(1) In chapter 1, we presented the properties and preparation methods of graphene and demonstrated the structure and principle of GFET, The application fields, research status and existing problems of GFET were analyzed. In addition, the research ideas and content of this thesis were introduced briefly.(2) In chapter 2, we characterized the graphene samples. The defect-related D peak was not found in the test range and the intensity ratio of G peak and 2D peak was 0.216, suggesting the graphene sample has high crystal quality. The ultraviolet lithography and photoresist were introduced. And we optimized the process parameters. The transmission line method(TLM) was used to investigate the contact resistance between different metals and graphene. Titanium (Ti) and gold (Au) were chosen as the contacting metal of graphene. Finally, we presented the fabrication process of our GFET and the principle of measurement. The back-gated GFET was fabricated and tested. The Dirac doping was not found in the test scope, probably because of the doping effect of the residual photoresist left during the fabrication and the absorbed oxygen and water on graphene.(3) In chapter 3, we demonstrated the significance of dielectric for GFET and contrasted the effect on graphene of dielectrics preparing by the different methods. The simple, nondegradative thermal evaporation method and easily sublimated silicon monoxide (SiO) was introduced. By analyzing the electrical parameters of the GFET before and after the evaporation of SiO, we found that the thermally evaporated SiO has little effect on graphene. The Ti-SiO-Ti sandwich structure was made and tested. The results showed the thermally evaporated SiO film of 30 nm thickness has a high dielectric constant (εr= 5.3) and high breakdown electrical field (EB= 4.1 MV/cm), suggesting it can function as the dielectric of GFET. The we presented the fabrication process of top-gated GFET. The top-gated GFET with thermally evaporated SiO as dielectric was fabricated and measured. The measure results showed the thermally evaporated SiO can play the role of dielectric in GFET and the nondegradative thermal evaporating method was also appropriate for other fragile 2D material based applications. And we tried to fabricate a GFET with SiO playing the roles of sacrifice layer and dielectric layer at the same time. This method threw a new light on the optimization and simplification of GFET fabrication process. The measure results showed the SiO can function as a sacrifice layer, which avoided the contact between graphene and photoresist, thus the contact resistance between the them was reduced. However, due to the different coefficients of thermal expansion, the SiO films on the graphene channel were inclined to deform, resulting in the failure of GFET. The problem can be solved by changing the fabrication process.(4) In chapter 4, we summarized the conclusions and innovative points of this dissertation, and previewed the further studies.