Mono- and few-layer nanocrystalline graphene grown on sapphire by molecular beam epitaxy

We have studied the growth of graphene by MBE. We have synthesized nanocrystalline graphene films directly on c-plane Al₂O₃ substrates by carbon evaporation from a highly-oriented-pyrolytic-graphite filament. The prepared graphene films are continuous over the entire substrate surface up to two-inch wafer sizes.

Raman spectroscopy was employed to access the structural characteristics of the MBE-grown graphene. Figure 1 displays typical Raman spectra obtained for films grown at a substrate temperature of 1000 ºC and growth times ranging from 240 to 480 min. The appearance of well-defined and intense G and 2D peaks is a clear evidence for the formation of a graphene structure. Its nanocrystalline nature is revealed by the existence of the D and D` peaks, which are mainly related to the existence of domain boundaries. A systematic analysis of the Raman peaks parameters shows that the films have a high in-plane order with crystalline domain sizes exceeding 30 nm. In addition, it reveals the homogeneity of the layers over large surface areas, which is exemplified here by a Raman mapping of the intensity ratio between the 2D and G peaks acquired over a surface area of 1.6X1.6 mm² [see Fig. 1(b)]. X-ray photoelectron spectroscopy analysis (not shown) of the same samples reveals that the average thickness of the graphene films varies from about 1 to 3 monolayers for the growth time range from 240 to 480 minutes. This proves the principle that MBE can be used to synthesize not only mono- but also few layers of graphene of similar quality. It also suggests that the growth proceeds in a layer-by-layer manner.

The planarity as well as the layered nature of the graphene films is confirmed by transmission electron microscopy (TEM). Fig. 2 depicts an image where the few-layer graphene structure can be seen. Also, there are some local corrugations within the atomic layers. In the corrugation-free areas the graphene layers are separated by about 3.3 ± 0.2 Å, as expected for multilayer graphene structures.

Finally, transport measurements revealed that the nanocrystalline graphene layers exhibit mobility values up to 140 cm²/Vs at room temperature. Even though a mobility of 140 cm²/Vs is still low for what is expected for graphene as an electronic material, it shows the potential of MBE as a method to synthesize conducting graphene layers directly on an insulator.

The results demonstrate the feasibility of using MBE for achieving a controlled and large-area synthesis of graphene (mono- and few-layer) directly on an insulating substrate. Based on them, it is reasonable to state that the optimization of the growth conditions will certainly allow the growth of high-quality graphene layers (i.e. with crystalline domains exceeding hundreds of nanometers) not only on Al₂O₃ but on a variety of technologically relevant insulating templates.