Dependence of Field-Effect Mobility of Graphene Grown by Thermal Chemical Vapor Deposition on Its Grain Size

Graphene was synthesized on a Cu film by chemical vapor deposition (CVD) and its grain size was analyzed by using dark-field transmission electron microscopy. The grain size was mainly controlled by changing the partial pressure of hydrocarbons in H₂/Ar. The grain size increased as the partial pressure of C₂H₄ decreased, but eventually leveled off. The size saturation may be related to the long growth time at a low partial pressure. It was also revealed that growth using CH₄ provided graphene with larger grain sizes than that using C₂H₄ at the same partial pressure. Back-gate transistors were then fabricated using graphene with various grain sizes, and the dependence of field-effect mobility on the grain size was investigated. The mobility roughly scales with the grain size, but the intergrain angles and/or small holes at grain boundaries also seem to affect the carrier mobility. It was also found that low mobility was often caused by fractures and/or wrinkles in graphene channels.