Graphene grown on a copper (Cu) substrate by chemical vapor deposition (CVD) is typically required to be transferred to another substrate for the fabrication of various electrical devices. PMMA-mediated wet process is the most widely used method for CVD-graphene-transfer. However, PMMA residue and wrinkles that inevitably remain on the graphene surface during the transfer process are critical issues degrading the electrical properties of graphene. In this paper, we report on a PMMA-mediated graphene-transfer method that can effectively reduce the density and size of the PMMA residue and the height of wrinkles on the transferred graphene layer. We found out that acetic acid is the most effective PMMA stripper among the typically used solutions to remove the PMMA residue. In addition, we observed that an optimized annealing process can reduce the height of the wrinkles on the transferred graphene layer without degrading the graphene quality. The effects of the suggested wet transfer process were also investigated by evaluating the electrical properties of field-effect transistors fabricated on the transferred graphene layer. The results of this work will contribute to the development of fabrication processes for high-quality graphene devices, given that the transfer of graphene from the Cu substrate is essential process to the application of CVD-graphene.
Carrier mobility is one of the most important parameters to evaluate the quality and uniformity of graphene. The mobility of graphene is typically extracted from the transconductance of a field-effect transistor fabricated with the graphene layer. However, the mobility value evaluated by this method is imprecise when the contact resistance is non-negligible, or the contact resistance is modulated by the gate bias, which is the case for typical graphene field-effect transistors. Here, we suggest a method for extracting the precise intrinsic field-effect mobility by considering the effective bias across the channel and its gate-induced modulation. We show that the contact resistances of typical graphene field-effect transistors are significantly modulated by gate bias and conventional methods can, therefore, cause a considerable error in the evaluation of the mobility. The proposed method in which the contact-induced error is removed gives a channel-length-independent intrinsic field-effect mobility. This method can be generally used to correctly evaluate the field-effect mobility of nano-scale or low-dimensional materials.
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