NANO LETTERS
Comparison of Epitaxial Graphene on Si-face and C-face 4H SiC Formed by Ultrahigh Vacuum and RF Furnace Production
2009 Vol. 9, No. 7 2605-2609
Glenn G. Jernigan,* Brenda L. VanMil, Joseph L. Tedesco, Joseph G. Tischler, Evan R. Glaser, Anthony Davidson III, Paul M. Campbell, and D. Kurt Gaskill U.S. NaVal Research Laboratory, Electronics Science and Technology DiVision Code 6800, 4555 OVerlook AVenue SW, Washington, DC 20375 Received March 13, 2009; Revised Manuscript Received May 8, 2009
ABSTRACT We present X-ray photoelectron spectroscopy, van der Pauw Hall mobilities, low-temperature far-infrared magneto transmission (FIR-MT), and atomic force microscopy (AFM) results from graphene films produced by radiative heating in an ultrahigh vacuum (UHV) chamber or produced by radio frequency (RF) furnace annealing in a high vacuum chemical vapor deposition system on Si- and C-face 4H SiC substrates at 1200-1600 °C. Although the vacuum level and heating methods are different, graphene films produced by the two methods are chemically similar with the RF furnace annealing typically producing thicker graphene films than UHV. We observe, however, that the formation of graphene on the two faces is different with the thicker graphene films on the C-face RF samples having higher mobility. The FIR-MT showed a 0(-1) f 1(0) Landau level transition with a B dependence and a line width consistent with a Dirac fermion with a mobility >250 000 cm2·V-1·s-1 at 4.2 K in a C-face RF sample having a Hall-effect carrier mobility of 425 cm2·V-1·s-1 at 300 K. AFM shows that graphene grows continuously over the varying morphology of both Si and C-face substrates.
Graphene, a two-dimensional sheet of carbon atoms arranged in a hexagonal lattice with unique electrical properties, has become a potential candidate for post complementary metal oxide semiconductors electronics1,2 and has already demonstrated capability in high-frequency electronic applications.3 In order for graphene to become a viable technology, large area material is needed. The discovery of graphene formation on the surface of a SiC substrate by thermal desorption of Si4 offers the ability to make graphene reproducibly on substrates as large as 75 mm diameter. The graphene formed by this method is referred to as epitaxial graphene5 to differentiate the material from exfoliated graphene, which due to small sample sizes is not technologically relevant. This work investigates two practical methods utilized for epitaxial graphene formation, (1) ultrahigh vacuum (UHV) Si desorption, and (2) annealing in a radio frequency (RF) heated furnace under high vacuum. Graphene formation on SiC was originally accomplished by Si desorption in UHV (pressure 250 000 cm2·V-1·s-1.
the graphene was formed by annealing at 1600 °C. We observe, however, a large discrepancy between the UHV and RF furnace grown C-face samples. The RF furnace C-face sample consistently produces the highest mobility among the four types of samples, and these samples are the thickest. Conversely, the UHV C-face samples have the lowest mobility among the four sample types and at times do not have measurable conductivity, even though XPS indicates the presence of graphene. The mobility of the RF furnace C-face samples decreases with higher temperatures and longer annealing times, contrary to what we observed for the Si-face samples. One trend among the four sample types that we do observe is the mobility increases as the sheet carrier density decreases. This indicates that scattering over the sample area results in lower mobility values. Carrier mobility values as determined by electrical characterization can be drastically different from values determined by cyclotron resonance observed in far-infrared magneto transmission experiments. Figure 3 shows a representative FIR-MT result for graphene produced on the C-face by RF furnace growth. We observe an absorption feature that shifts in energy as the square root of B, which is consistent with carriers acting like “massless” Dirac fermions in agreement with the literature.23,24 From its field dependence, we identified this feature as the 0(-1) f 1(0) Landau level transition. Our measured linewidths at low magnetic field (e.g., 3.5 meV at 250 mT) are similar to 2608
transition linewidths recently reported by another group.24 We can conclude from this that some graphene layer(s) have carrier mobility greater than 250 000 cm2·V-1·s-1. This sample, however, had a Hall-effect mobility at room temperature of only 425 cm2·V-1·s-1. The discrepancy has been explained by statements that the electrical measurements probe highly conductive layers near the SiC interface whereas the spectroscopic measurements probe subsequent surface layers that are essentially neutral.24 In order to develop an electronics technology based on graphene, we pursue improvements in the electrically determined carrier mobility on large area samples, as we have demonstrated that micrometer-scale graphene Hall effect structures can show significantly higher room temperature mobility values16 (as high as 18 100 cm-2·V-1·s-1 for a 10 µm × 10 µm structure, which is comparable to other published results25). Photolithographically defined stripes in graphene films formed at 1600 °C on the four sample types (Si- and C-face, UHV and RF furnace) are shown in the AFM images of Figure 4. Thickness determination made by step-height measurements with the AFM corroborated the values obtained by XPS. Handling of graphene must be done with care as the thin films are susceptible to damage, as can be observed by the abrasions in the graphene stripes in the AFM images. We also observe in the images that steps and other morphological features occurring in the substrate are replicated in the graphene film. This is most obvious in the C-face RF furnace-produced graphene, where the substrate surface morphology has dramatically changed. The initial surface morphology for all substrates after hydrogen etching, but before graphitization, consisted of a uniformly stepped Nano Lett., Vol. 9, No. 7, 2009
surface with the step height for the Si-face surface being ∼0.5 nm (two Si-C bilayers or a half unit cell distance) and for the C-face surface being ∼1.0 nm (four Si-C bilayers or a full unit cell distance). The image of the C-face UHV sample provides a possible explanation for its low mobility results. In particular, the C-face UHV graphene films are granular in nature resulting from an islanding growth mode. The C-face RF furnace films appear smooth, which may be the result of being thicker. Thus, improvements in mobility may be obtained by improving surface morphology.26 We have shown by XPS that the chemical nature of UHV and RF furnace-produced graphene films formed on 4H SiC are similar but that the graphene films formed on the Si-and C-faces of SiC are different. The Si-face graphene layers form after an interfacial layer is created, while graphene forms directly on the C-face. The C-face samples show a strong surface plasmon, which may be due to being more conductive than the Si-face samples, and a shift in the C 1s binding energy, which may be associated with impurities or screening effects. Si-face graphene films can be grown to be a single layer or a few layers thick. The thick multilayer graphene of the C-face RF furnace samples have the highest carrier mobilities, and we have shown that FIR-MT of those films indicate the presence of layers with carrier mobility greater than 250,000 cm2·V-1·s-1. Graphene carrier mobility appears affected by scattering as lower carrier densities result in higher mobilities. AFM measurements show that graphene films conform to the morphology of the underlying substrate, indicating that smoother films should demonstrate even higher mobilities and thereby facilitate graphene’s device development. Acknowledgment. This work was supported by the Office of Naval Research. B.L.V. and J.L.T. acknowledge support from the ASEE for Postdoctoral Research Fellowships.
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