U.S. Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375, USA.
Nano Lett. 2012 Apr 11;12(4):1749-56. doi: 10.1021/nl203353f. Epub 2012 Mar 7.
We demonstrate the first successful growth of large-area (200 × 200 μm(2)) bilayer, Bernal stacked, epitaxial graphene (EG) on atomically flat, 4H-SiC (0001) step-free mesas (SFMs) . The use of SFMs for the growth of graphene resulted in the complete elimination of surface step-bunching typically found after EG growth on conventional nominally on-axis SiC (0001) substrates. As a result heights of EG surface features are reduced by at least a factor of 50 from the heights found on conventional substrates. Evaluation of the EG across the SFM using the Raman 2D mode indicates Bernal stacking with low and uniform compressive lattice strain of only 0.05%. The uniformity of this strain is significantly improved, which is about 13-fold decrease of strain found for EG grown on conventional nominally on-axis substrates. The magnitude of the strain approaches values for stress-free exfoliated graphene flakes. Hall transport measurements on large area bilayer samples taken as a function of temperature from 4.3 to 300 K revealed an n-type carrier mobility that increased from 1170 to 1730 cm(2) V(-1) s(-1), and a corresponding sheet carrier density that decreased from 5.0 × 10(12) cm(-2) to 3.26 × 10(12) cm(-2). The transport is believed to occur predominantly through the top EG layer with the bottom layer screening the top layer from the substrate. These results demonstrate that EG synthesized on large area, perfectly flat on-axis mesa surfaces can be used to produce Bernal-stacked bilayer EG having excellent uniformity and reduced strain and provides the perfect opportunity for significant advancement of epitaxial graphene electronics technology.
我们首次成功地在原子级平坦的 4H-SiC(0001)无阶跃台面（SFMs）上生长出大面积（200×200μm²）双层、伯纳尔堆叠、外延石墨烯（EG）。在 SFMs 上生长石墨烯可完全消除在传统准轴 SiC（0001）衬底上生长 EG 后通常发现的表面阶跃堆积。结果，EG 表面特征的高度至少降低了 50 倍，与传统衬底上的高度相比。使用拉曼 2D 模式对 SFM 上的 EG 进行评估表明，具有低且均匀的压缩晶格应变（仅为 0.05%）的伯纳尔堆叠。这种应变的均匀性得到了显著改善，与在传统准轴衬底上生长的 EG 相比，应变降低了约 13 倍。应变的幅度接近无应力剥离石墨烯薄片的值。在 4.3 至 300 K 的温度范围内对大面积双层样品进行的 Hall 传输测量表明，n 型载流子迁移率从 1170 增加到 1730cm² V⁻¹ s⁻¹，相应的薄层载流子密度从 5.0×10¹²cm⁻²降低到 3.26×10¹²cm⁻²。据信，传输主要通过顶层 EG 层发生，底层从衬底屏蔽顶层。这些结果表明，在大面积、完美平坦的轴台面表面上合成的 EG 可用于生产具有优异均匀性和降低应变的伯纳尔堆叠双层 EG，并为外延石墨烯电子技术的重大进展提供了绝佳机会。
