Graphene structure deduced from LEED observations as a function of pressure and temperature. Graphene takes the IRD structure for most of the growth conditions (green hexagons), but at low pressure and high temperature, graphene takes the expected 6√3 structure (orange stars). Orange and green stars correspond to 6√3/IRD mixed structures, blue circles to low quality graphene, blue triangle to SiC surface reconstruction, and red circles to samples without graphene.
Examples of the different kinds of LEED patterns observed (primary energy: 120 eV) and AFM views of specific samples (1 × 1 μm2, z-scale = 2.5 nm). (a) Sample without graphene obtained at high temperature; (b) poor quality graphene observed at median temperature and low pressure; (c) SiC surface reconstruction observed at low temperature; (d) graphene on a (6√3 × 6√3)-R30° graphene/SiC interface reconstruction observed at high temperature and low pressure; (e) example of IRD graphene structure observed at median temperature and median or high pressure; (f) another example of IRD graphene structure observed at high temperature and high pressure; (g) example of mix between IRD and 6√3 patterns observed at median temperature and low pressure. For all LEED patterns, red arrows indicate 1,0 graphitic spots, blue triangles indicate 1,0 SiC spots, and purple hexagons indicate reconstruction spots.
FLG film thickness estimated from intensity ratio between SiC and graphene peaks as a function of the propane ratio. In the IRD region of the P-T diagram, the clear increase of the thickness with the propane ratio indicates that graphene growth is mainly supplied by propane flow (b). In contrast, in the 6√3 region, the small effect of the propane ratio suggests that SiC etching contributes to the growth of graphene (a).
Two different 6√3 structures can be observed by LEED [(a) and (b), primary energy: 120 eV] and XPS (c). The comparison with previous works suggests that conditions near the 6√3/IRD transition [LEED pattern in (b) and blue curve in (c)] lead to the hydrogenation of the SiC beneath the 6√3 graphene. In LEED patterns, red arrows indicate 1,0 graphitic spots, blue triangles indicate 1,0 SiC spots, and purple hexagons indicate 6√3 reconstruction spots.
Morphology observed using AFM on graphene films with the 6√3 structure grown at 100 mbar and 1350 °C (a) or 1550 °C (b). The improvement of the morphology when increasing growth temperature is attributed to kinetic effects.
Morphology observed using AFM on graphene films with the IRD structure grown at 800 mbar and 1250 °C (a), 1350 °C (b), 1450 °C (c), or 1550 °C (d). We can distinguish small defect lines (denoted as “D”) from wrinkles (denoted as “W”) on samples grown at lower temperatures.
STM observations of a sample grown at 1450 °C, 800 mbar. We can distinguish wrinkles (denoted as “W”), with heights superior to 2 nm (a), small defect lines (denoted as “D”), with heights inferior to 1 nm [(a), (b), and (c)], and moiré patterns [(b) and (c)].
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