| AVS 58th Annual International Symposium and Exhibition | |
| Graphene and Related Materials Focus Topic | Wednesday Sessions |
| Session GR-WeA |
| Session: | Graphene Characterization including Microscopy and Spectroscopy |
| Presenter: | N. Srivastava, Carnegie Mellon University |
| Authors: | N. Srivastava, Carnegie Mellon University G. He, Carnegie Mellon University R.M. Feenstra, Carnegie Mellon University |
| Correspondent: | Click to Email |
The graphene/SiC interface structure is quite well understood on the SiC(0001) surface (the Si-face) but the situation is less clear on the SiC(000-1) surface (the C-face). For the C-face some groups report a 3×3 and/or 2×2 interface structure with weak interaction with the underlying substrate.1 A single study however found an interface layer that was strongly bonded to the SiC.2 We demonstrate that the interface layer on the C-face depends on the means of graphene formation. For graphitization in vacuum we observe a 3×3 interface in agreement with other groups. However for graphitization in a Si-rich environment we observe a new interface indicative of a buffer layer similar to that seen on the Si-face.
In this work, graphene films are formed by heating the C-face in vacuum or in a disilane environment. It is found that different interface structures occur for the two preparation conditions. In particular, in 5×10-5 Torr of disilane we find a graphene-like buffer layer forming at the interface, analogous to the well known behavior of the Si-face. We therefore find that graphene formation on the C-face and Si-face are not so much different (although they appear to be when using vacuum preparation): A buffer layer that acts as a template for graphene formation exists in both cases, so long as equilibrium conditions are employed (i.e. with the disilane environment).
Studies are performed using atomic force microscopy (AFM), low-energy electron diffraction (LEED), and low-energy electron microscopy (LEEM). For graphene prepared in vacuum, LEED patterns show a characteristic 3×3 pattern together with graphene streaks. In contrast, for the graphene produced in 5×10-5 Torr of disilane, LEED patterns reveals a complex √43×√43-R±7.6° arrangement. This structure is somewhat similar to the well known 6√3×6√3-R30° “buffer layer” of the Si-face, with satellite spots surrounding the primary Si spots, and is interpreted as arising from a C-rich buffer layer with 8x8 graphene unit cells on the SiC (with rotation angle of ±7.6° rather than 30° for the Si-face). After air exposure the √43×√43-R±7.6° pattern changes, with the intensity of the graphene streaks increasing and the √43×√43 spots themselves disappearing and being replaced by √3×√3-R30° spots. This latter behavior is interpreted as oxidation of the SiC surface beneath the buffer layer,3 again similar to what occurs on the Si-face. LEEM reflectivity curves on the surface reveal features similar to those for the 6√3×6√3-R30° layer on the Si-face.4 Importantly, selected area diffraction on those surface areas, after oxidation, reveals a wavevector magnitude precisely equal to that of graphene, thus proving that a decoupled buffer layer does indeed exist on the surface. It is argued that the C-face buffer layer represents the equilibrium structure of the interface, whereas the 3×3 interface forms due to kinetic limitations.
[1] Emtsev et al., Phys. Rev. B 77, 155303 (2008).
[2] Varchon et al., Phys. Rev. Lett 99, 126805 (2007).
[3] Oida et al., Phys. Rev. B 82, 041411 (2010).
[4] Hibino et al., Phys. Rev. B 77, 075413 (2008).