6. Low-Temperature Synthesis of Graphene and Fabrication of Top-Gated Field-Effect Transistors Using Transfer-Free Processes for Future LSIs

High-quality graphene synthesis by chemical vapor deposition (CVD) on a substrate has been achieved recently. Although synthesized graphene is often transferred to another substrate for electrical measurements, this transfer process may not be appropriate for applications using a large substrate, including large-scale integrated circuits (LSIs). Therefore, it is desirable that graphene channels are formed directly on a substrate without such transfer processes. Furthermore, graphene should be formed at low temperature to avoid possible adverse effects on the substrate.

In this study, thickness-controlled growth of few-layer and multilayer graphene was demonstrated at 650 °C by the thermal CVD method, and top-gated field-effect transistors (FETs) were fabricated directly on a large SiO₂/Si substrate without using graphene-transfer processes. Graphene was synthesized on patterned Fe films. The iron was subsequently etched after both ends of the graphene were fixed by source and drain electrodes, leaving the graphene channels bridging the electrodes all over the substrate. Top-gated FETs were then made after covering the channels with HfO₂. The fabricated devices exhibit ambipolar behavior and can sustain a high-density current. The growth mechanism of graphene was also investigated. In addition, a novel technique for synthesizing graphene directly on insulating substrate is also described.