Investigation of Room Temperature Plasma-Etching CVD-grown Graphene on Copper Substrates
Author:Jong Yeon Lee
Mentor:Nai-Chang Yeh, Professor of Physics, California Institute of Technology
Graphene has remarkable electronic and physical properties, such as the massless relativistic dispersion relation near its Dirac points at the edges of the Brillouin zone, and the presence of inequivalent valleys that allows charge carriers to travel without back scattering, which result in high electronic mobility. However, current fabrication processes such as Chemical Vapor Deposition cause strain and distortion, so that strain-induced effects such as local pseudo magnetic field and charging arise, and lower the mobility and graphene. Because copper, which is common substrate for CVD method, has higher thermal expansion coefficient than that of graphene, strain and distortion are thought to arise when we cool down sample after fabrication process from 1000°C to room temperature. Therefore, to decrease these mobility-lowering effects, plasma-etching technique was introduced to treat the surface of transition metal substrates, followed by CVD-growth of graphene at room temperature. These room temperature grown graphene samples were mainly examined through Raman Spectroscopy because Raman spectral shifts of the G and 2D bands can provide information about the average biaxial strain on the graphene. Conductance measurement and Scanning Tunneling Microscopy/Spectroscopy were also performed for detail investigation for grain boundary effect and strain-induced electronic band structure change. Moreover, to investigate substrate dependence in CVD method, single crystalline copper substrates with (111) and (100) crystalline orientations were used for comparison with graphene grown on polycrystalline copper foils. Experimental results allow comparison between high-temperature (~1000°C) CVD grown graphene and room temperature grown graphene, showing significantly reduced strain and surface roughness in graphene sample fabricated by our new method.