Structural studies of epitaxial graphene formed on SiC{0001} surfaces
Creators: name: Luxmi
Publisher: name: Carnegie Mellon University
Year: 2010
Subjects: subject: 20399 Classical Physics not elsewhere classified
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: In this thesis, we report on the structural studies of epitaxial graphene formed on polar faces of SiC; the (0001) and the ( 0001 ) surfaces (the so-called Si-face and C-face, respectively). Graphene films are prepared by heating the SiC substrates either in ultra-high vacuum (UHV) or in 1-atm-argon environment. Prior to graphitization, substrates are hydrogenetched for removing residual polishing damages. The resulting graphene films are characterized using atomic force microscopy (AFM), Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), low-energy electron microscopy (LEEM), Raman spectroscopy, and electrical measurements. Field-effect mobilities of the transistors made from vacuum-annealed graphene films exceed 4000 cm<sup>2</sup>/Vs at room temperature. It is found, in agreement with other reports, that the graphene growth and properties are very different on the two polar faces of SiC. Graphene formation rate is faster on the C-face compared to the Si-face. For a given annealing temperature and time, thick film forms on the C-face compared to the Si-face. On the Si-face, graphene lattice vectors are rotated 30° with respect to the SiC lattice vectors as seen in the LEED pattern which shows a hexagonal arrangement of six distinct spots. However on the C-face, graphene films are rotationally disordered which gives rise to streaking in the diffraction pattern. We still observe six discrete spots in the pattern, but additional spots (streaks) are also seen located at angles of 30° ± φ relative to the six discrete spots. Angles of φ ranging from 6 to 13° have been observed, although most typically we find φ asymptotically equal to 7°. We have examined the evolution of surface morphology of graphene films prepared in UHV as a function of annealing temperature on both the faces. The results for the Si-face graphene films are found to be in good agreement with what is reported by other groups. However, we present novel results for the morphology and structural properties of the C-face graphene films. On the Si-face graphene films, pits form during the initial stages of graphitization (due to the development of buffer layer) and steps-terraces, seen after hydrogen-etching, are not ordered. On the C-face graphene films, it is observed that a uniform step-terrace arrangement is preserved during the initial stage which develops into a terraced morphology at a later stage. Terraces of varying heights are seen and with further annealing, thicker films with ridges (possibly arising from a thermal expansion mismatch between the SiC and the graphene) are formed. An additional aspect of the C-face graphene films morphology is found to be associated with the surface properties of the starting wafer. It is observed that for wafers which show large number of pits (after etching or graphitization), the surface is covered with large amounts of disordered graphene, also called nanocrystalline graphite (NCG). However for wafers which display fewer pits, the surface is found to be covered with little amounts of NCG. As investigated in LEEM, small areas of constant graphene thickness, which we call domains, are found to extend laterally over 1-2 μm on the C-face with variation of up to 5 monolayers between domains. This large variation in thickness is suggestive of three-dimensional growth of graphene. In the case of the Si-face graphene films, larger domains are formed with variation in thickness of only 1 monolayer between domains (away from step bunches) suggestive of layer-by-layer graphene growth. We have interpreted the difference in the growth modes for the two faces in terms of limited surface kinetics. It is likely that for the C-face, lower temperatures employed in graphitization inhibit coarsening of adjacent domains. Correlated AFM and LEEM data on the C-face graphene films suggests that domains are bounded by step bunches which could possibly lead to discontinuities in the graphene films. Due to low temperatures, the driving force for the planarization of the morphology or for the uniform distribution of graphene thicknesses is missing on the C-face. Due to higher temperatures needed for obtaining graphene of comparable thickness on the Si-face compared to the C-face, steps are more mobile leading to a flatter morphology and a layer-by-layer growth of graphene films is promoted on this face. At higher annealing temperatures, the films thickness on the C-face is much greater than for the Si-face, but both films display the characteristic ridges associated with strain relaxation and both surfaces display comparable amounts of step bunching. The reason for the thicker film on the C-face is, we believe, simply because the ( 0001 ) surface and ( 0001 )/graphene interface have higher energies (i.e. are more unstable), respectively, than the (0001) surface and (0001)/graphene interface. Additionally, more defects in the C-face films such as the discontinuities and/or rotational domain boundaries could lead to easier Si diffusion through the graphene, which would also favor thicker growth. Thus, the different morphologies between the Si- and C-faces found for films of the same thickness simply arises from the lower graphene formation temperatures used in the latter case, which inhibits coarsening between adjacent domains. In order to increase the growth temperature for the C-face, while maintaining a fixed growth rate, we switched to an ambient atmosphere of argon from UHV, following other workers’ research, for annealing the SiC substrates. In the presence of argon, Si sublimation rate is significantly reduced which leads to an increase in the annealing temperature for producing graphene of given thickness. Increase in temperature enhances the mobility of diffusing species which in turn improves the homogeneity of the film. We have been successful in forming monolayer graphene with increased domain size on the Si-face of SiC in the presence of argon. However, for the C-face the morphology becomes much worse, with the surface displaying markedly inhomogeneous nucleation of the graphene. It is demonstrated that these surfaces are unintentionally oxidized, which accounts for the inhomogeneous growth.