Optimizing the Electrical Properties of CVD Graphene by Substrate and Doping Control

Abstract

After the first artificially-isolated single-atom-thick material, which is graphene, the new era of two-dimensional materials has begun. With its extraordinary physical and chemical properties, graphene has drawn considerable attention as a subject of the fundamental research by condensed matter physicists. Thereafter, the realization of large scale graphene synthesis on catalytic substrates, such as Ni, Fe, and especially Cu, by chemical vapor deposition (CVD) makes researchers in numerous fields rush into graphene study. Though large-scale graphene films synthesized by the CVD method shows tremendous potential of applications for transparent electrodes with high electrical, optical properties and mechanical stability, the CVD-grown graphene films still have higher sheet resistance, compared to the exfoliated graphene. One of the limiting factors to degrade graphene quality is polycrystallinity and grain boundary of graphene, which originated from the randomly oriented nucleation and growth of graphene islands. Another factor is the ruga morphology of graphene, which is induced by the different thermal expansion coefficient between graphene and catalyst. The cooling down process makes catalyst surface corrugated and it leads to quasi periodic nanoripple arrays of graphene after graphene transfer onto target substrates. The rippled graphene gives rise to flexural phonon scattering, which play a limiting role in charge mobility and sheet resistance. The present thesis addresses the synthesis of graphene by CVD on copper, which has become the most popular catalyst for graphene growth. A significant study on the graphene growth and property control is reported in the present thesis. This dissertation provides the details of my work on all projects related to synthesize and characterization of the graphene synthesis by CVD on copper and its application for field effect transistor. Especially, we will discuss the improvement of the sheet resistance of graphene by controlling synthesis condition. We also controlled the work function of graphene films via vapor phase doping process. First, tension-controlled graphene growth is presented herein. The methods for optimizing the crystalline orientations of Cu foils by tension control are explained in detail. In addition, the optimized Cu foils allow the growth of larger single-crystalline graphene with higher charge carrier mobility. Second, we demonstrated tuning the cooling rate can control the electrical properties of CVD-grown graphene. A higher cooling rate gives rise to large suspended graphene formation, which in turn results in reduced ripple density and its heights after transfer onto SiO2/Si substrates. Finally, an improved graphene doping method is also described. we have investigated the effect of the dopant structure and number of amines group in dopant. The doping concentration was stronger as increasing amino group in linear ethylene amine structure. However, the branched structure showed weaker doping, although it has the most amino functional groups in the series of ethylene amine dopants. In conclusion, we believe that our findings will provide crucial ideas to design a system for the CVD growth of high-quality graphene films on roll-to-roll Cu foils which would be of great importance for the continuous mass-production of graphene films for practical applications in the future.