Bilayer Graphene Grown\non 4H-SiC (0001) Step-Free\nMesas

Abstract

We demonstrate the first successful growth of large-area\n(200 ×\n200 μm²) bilayer, Bernal stacked, epitaxial graphene\n(EG) on atomically flat, 4H-SiC (0001) step-free mesas (SFMs) . The\nuse of SFMs for the growth of graphene resulted in the complete elimination\nof surface step-bunching typically found after EG growth on conventional\nnominally on-axis SiC (0001) substrates. As a result heights of EG\nsurface features are reduced by at least a factor of 50 from the heights\nfound on conventional substrates. Evaluation of the EG across the\nSFM using the Raman 2D mode indicates Bernal stacking with low and\nuniform compressive lattice strain of only 0.05%. The uniformity of\nthis strain is significantly improved, which is about 13-fold decrease\nof strain found for EG grown on conventional nominally on-axis substrates.\nThe magnitude of the strain approaches values for stress-free exfoliated\ngraphene flakes. Hall transport measurements on large area bilayer\nsamples taken as a function of temperature from 4.3 to 300 K revealed\nan n-type carrier mobility that increased from 1170 to 1730 cm² V^–1 s^–1, and a corresponding\nsheet carrier density that decreased from 5.0 × 10¹² cm^–2 to 3.26 × 10¹² cm^–2. The transport is believed to occur predominantly through the top\nEG layer with the bottom layer screening the top layer from the substrate.\nThese results demonstrate that EG synthesized on large area, perfectly\nflat on-axis mesa surfaces can be used to produce Bernal-stacked bilayer\nEG having excellent uniformity and reduced strain and provides the\nperfect opportunity for significant advancement of epitaxial graphene\nelectronics technology.