Graphene Formation on the Carbon face of Silicon Carbide
In this thesis, we study the structure of epitaxial graphene formed on polar faces of SiC - the (0001) face, also known as the Si-face, and the (0001) face, known as the C-face. On both polar surfaces, graphene films are prepared in ultra-high vacuum (UHV), in environments either of argon or cryogenically purified neon, or in a low-pressure background of disilane. Characterization of graphene is done by using atomic force microscopy (AFM), low-energy electron diffraction (LEED), and low-energy electron microscopy (LEEM). In addition, a dynamical LEED structure calculation is performed to analyze the interface structures formed between graphene films and the C-face of SiC. When SiC is heated at high temperature, Si atoms preferentially sublimate from the surface, leaving behind excess C atoms that self-rearrange to form graphene on the surface. We find, in agreement with other reports, that the structure and the electrical properties of graphene films formed on the two polar faces are very different, in many aspects. For example, in all environments we have employed, the graphene formation rate is slower on the Si-face than that on the C-face under the same preparation condition. Also, graphene films formed on the Si-face are generally more homogeneous and the domain sizes are larger, compared to those formed on the C-face (by “domain”, we mean a surface area with constant thickness of multilayer graphene). On the Si-face, graphene lattice vectors are rotated 30º with respect to the SiC lattice vectors as observed by LEED, while on the C-face, graphene films are usually rotationally disordered which gives rise to streaking in the diffraction pattern. In addition, the electrical properties of graphene films are also quite different between these two polar surfaces. These differences between the two polar surfaces, we believe, can be attributed to the differences of interface structures between graphene films and the underlying substrate. In this thesis, we report our interpretation of these differences in terms of the detailed interface structures that form for graphene on the two polar surfaces. For the Si-face, graphene films prepared in vacuum are of moderate quality; thickness uniformity e.g. for a film with average thickness of two graphene monolayer (ML) ranges between about 1 and 3 ML, and the surface morphology of the underlying SiC is found to contain numerous small pits. In this thesis, we report improvements in the morphology of graphene films for the Si-face utilizing environments of disilane, 1 atm of argon or 1 atm cryogenically purified neon. The presence of disilane, argon, or neon gas decreases Si sublimation rate, thus increasing the temperature required for graphene formation. The higher graphitization temperature enhances the mobility of diffusing species, which in turn results in an improved morphology of the graphene films. For the case of the C-face, graphene prepared in vacuum is of considerably worse quality than for the Si-face; a film with average thickness of 2 ML will contains areas covered by 0 – 5 ML of graphene. In order to improve the quality of graphene, the same preparation techniques (graphitization in 1 atm of argon, 1 atm purified neon, or in disilane) as we have used for the Siface are employed for the C-face. When graphene is prepared in argon on the C-face, the morphology is found to become much worse (unlike the improvement found for the Si-face). We find that the surface becomes unintentionally oxidized before the graphene formation (due to residual oxygen in the argon), and hence become resistant to graphitization. This unintentional oxidization results in inhomogeneous islands of thick graphene forming over the surface. In contrast, utilizing purified neon can eliminate the unintentional oxidation while permitting increased preparation temperatures, and thus is found to improve the morphology of graphene on the C-face. Use of a low-pressure background of disilane yields a similar improvement. The morphology of graphene on both polar surfaces of SiC in these various environments is reported in detail in this thesis. In terms of interface structure, the situation is presently well understood for the Si-face: the interface consists of a C-rich layer having 6√3 × 6√3 − 30° symmetry, which is covalently bonded to the underlying SiC substrate. This interface on the Si-face acts as an electronic “buffer” layer between graphene films and substrate and provides a template for subsequent graphene formation. It is noteworthy that this interface on the Si-face occurs for all growth conditions. In contrast, formation of interface structures on the C-face is sensitive to both the starting surface of SiC and graphene preparation conditions. In this thesis, the graphene/SiC interface on the C-face is studied by varying the preparation conditions (sample temperature T, and silicon partial pressure PSi). In vacuum, a 3 × 3 reconstruction is found before and after graphitization. At relatively low PSi of 5 ´ 10-6 Torr, a 2 × 2 reconstruction is found, both before and after graphene formation. When graphene is formed on the C-face using 5 ´ 10-5 Torr of disilane (or using 1 atm of neon), a new interface structure forms between the graphene films and the underlying substrate, which displays √43 × √43 − ± 7.6° ( √43 for short) symmetry as revealed by in situ LEED immediately after graphitization. When subsequent oxidation of the surface is performed, the interface structure transforms to one with √3 × √3 − 30° symmetry. Electron reflectivity measurements coupled with the recent published first-principles computations indicate that the new interface structures consists a graphene-like layer that forms between the graphene and the underlying substrate, similar to that found on the Si-face. This graphene-like layer has the √43 symmetry due to bonding to the underlying SiC, but upon oxidation, these bonds are broken and the layer becomes “decoupled” from the SiC. The decoupled graphene-like layer then becomes a graphene layer. From a dynamical LEED structure calculation for the oxidized C-face surface, it is found that the interface structure transforms to that of a graphene layer sitting on top of a silicate (Si2O3) layer, with the silicate layer having the well-known structure as previously studied on bare SiC(0001) surfaces. A separate project discussed in this thesis is determination of size, shape, and composition of InAs/GaAs quantum dots (QD) by scanning tunneling microscopy (STM) and finite-element calculation. Cross-sectional STM images and finite-element calculations reveal individual InAs QDs having a lens shape with maximum base diameter of 10.5 nm and height of 2.9 nm. Comparison between the STM data and the computational results of the displacement of the dot profile out from the cleavage surface, together with measurements of its local lattice parameter, leads to an accurate determination of the cation composition as varying from 65% indium at the base of the QD to 95% at its center and back to 65% at its apex.