An investigation of how electron/photon beam exposures affect the intercalation rate of Na deposited on graphene prepared on Si-face SiC is presented. Focused radiation from a storage ring is used for soft X-ray exposures while the electron beam in a low energy electron microscope is utilized for electron exposures. The microscopy and core level spectroscopy data presented clearly show that the effect of soft X-ray exposure is significantly greater than of electron exposure, i.e., it produces a greater increase in the intercalation rate of Na. Heat transfer from the photoelectrons generated during soft X-ray exposure and by the electrons penetrating the sample during electron beam exposure is suggested to increase the local surface temperature and thus the intercalation rate. The estimated electron flux density is 50 times greater for soft X-ray exposure compared to electron exposure, which explains the larger increase in the intercalation rate from soft X-ray exposure. Effects occurring with time only at room temperature are found to be fairly slow, but detectable. The graphene quality, i.e., domain/grain size and homogeneity, was also observed to be an important factor since exposure-induced effects occurred more rapidly on a graphene sample prepared in situ compared to on a furnace grown sample.

The effects induced by Na deposited on furnace grown graphene on SiC(0001) and after subsequent annealing are investigated using LEEM, mu-LEED, mu-PES and XPEEM. Intercalation in between carbon layers and at the interface is observed to occur both on the 1 ML and 2 ML areas directly after Na deposition. Annealing at a temperature around 100 degrees C is found to strongly promote Na intercalation. Exposure to the electron beam or the focused synchrotron radiation in the LEEM/XPEEM is also found to promote the intercalation, which is confirmed to begin at domain boundaries between the 1 ML and 2 ML areas, and also as stripe/streak-like features on the 1 ML areas. The XPEEM data show Na adsorption on the surface and intercalation at the interface to be quite non-uniform. When annealing at higher temperatures Na starts to de-intercalate and leave the sample, but Na is still detectable on the sample after annealing at 240 degrees C.

Graphene grown on C-face SiC substrates using two procedures, high and low growth temperature and different ambients, was investigated using Low Energy Electron Microscopy (LEEM), X-ray Photo Electron Electron Microscopy (XPEEM), selected area Low Energy Electron Diffraction (μ-LEED) and selected area Photo Electron Spectroscopy (μ-PES). Both types of samples showed formation of μm-sized grains of graphene. The sharp (1 × 1) μ-LEED pattern and six Dirac cones observed in constant energy photoelectron angular distribution patterns from a grain showed that adjacent layers are not rotated relative to each other, but that adjacent grains in general have different azimuthal orientations. Diffraction spots from the SiC substrate appeared in μ-LEED patterns collected at higher energies, showing that the rotation angle between graphene and SiC varied. C 1s spectra collected did not show any hint of a carbon interface layer. A hydrogen treatment applied was found to have a detrimental effect on the graphene quality for both types of samples, since the graphene domain/grain size was drastically reduced. From hydrogen treated samples, μ-LEED showed at first a clear (1 × 1) pattern, but within minutes, a pattern containing strong superstructure spots, indicating the presence of twisted graphene layers. The LEED electron beam was found to induce local desorption of hydrogen. Heating a hydrogenated C-face graphene sample did not restore the quality of the original as-grown sample.

Synchrotron-based core level and angle resolved photoemission spectroscopy was used to study the formation of ytterbium (Yb) oxide at the graphene-SiC substrate interface. Oxide formation at the interface was accomplished in two steps, first intercalation of Yb into the interface region and then oxygen exposure while heating the sample at 260 degrees C to oxidize the Yb. After these processes, core level results revealed the formation of Yb oxide at the interface. The Yb 4f spectrum showed upon oxidation a clear valence change from Yb2+ to Yb3+. After oxidation the spectrum was dominated by emission from oxide related Yb3+ states and only a small contribution from silicide Yb2+ states remained. In addition, the very similar changes observed in the oxide related components identified in the Si 2p and Yb 4f spectra after oxidation and after subsequent heating suggested formation of a Si-Yb-O silicate at the interface. The electronic band structure of graphene around the (K) over bar -point was upon Yb intercalation found to transform from a single pi band to two pi bands. After Yb oxide formation, an additional third pi band was found to appear. These pi bands showed different locations of the Dirac point (E-D), i.e., two upper bands with E-D around 0.4 eV and a lower band with E-D at about 1.5 eV below the Fermi level. The appearance of three pi-bands is attributed to a mixture of areas with Yb oxide and Yb silicide at the interface.

The effects of Na deposited on monolayer graphene on SiC(001) were investigated by synchrotron-based photoelectron spectroscopy and angle resolved photoelectron spectroscopy. The experimental results show that Na prefers to adsorb on the graphene layer after deposition at room temperature. Nonetheless, part of the Na atoms are able to intercalate in between the graphene and the buffer layer and some go even further into the substrate interface as indicated by the shift of the bulk SiC component in the C 1s and Si 2p core level spectra. The ARPES spectrum exhibits a lowering of the Dirac point indicating increased n-type doping of the monolayer graphene induced by the deposited Na atoms. Upon subsequently heating the sample, we found that a slightly elevated temperature is essential in order to promote Na intercalation. A fully Na intercalation at the graphene-SiC interface is obtained after heating at a temperature of about 75 degrees C. The intercalated Na decouples the buffer layer and transforms it into a second graphene layer so two pi-bands are observed in the ARPES spectra. Interestingly, the two bands show different locations of the Dirac point but both exhibit linear dispersion in the vicinity of the (K) over bar point and not the hyperbolic dispersion observed for AB stacked bi-layer graphene. When heating the sample to about 125 degrees C or higher, Na is found to leave the interface and the second graphene layer is transformed back to the carbon buffer layer.

We report graphene thickness, uniformity and surface morphology dependence on thegrowth temperature and local variations in the off-cut of Si-face 4H-SiC on-axis substrates. Thetransformation of the buffer layer through hydrogen intercalation and the subsequent influence onthe charge carrier mobility are also studied. A hot-wall CVD reactor was used for in-situ etching,graphene growth in vacuum and the hydrogen intercalation process. The number of graphene layersis found to be dependent on the growth temperature while the surface morphology also depends onthe local off-cut of the substrate and results in a non-homogeneous surface. Additionally, the influence of dislocations on surface morphology and graphene thickness uniformity is also presented.

Graphene samples were grown on the C-face of SiC, at high temperature in a furnace and an Ar ambient, and were investigated using LEEM, XPEEM, LEED, XPS and ARPES. Formation of fairly large grains (crystallographic domains) of graphene exhibiting sharp (1x1) patterns in μ-LEED was revealed and that different grains showed different azimuthal orientations. Selective area constant initial energy photoelectron angular distribution patterns recorded showed the same results, ordered grains and no rotational disorder between adjacent layers. A grain size of up to a few μm was obtained on some samples.

The intercalation and deintercalation mechanisms of Si deposited on monolayer graphene grown on SiC(0001) substrates and after subsequent annealing steps are investigated using low-energy electron microscopy (LEEM), photoelectron spectroscopy (PES), and micro-low-energy electron diffraction (mu-LEED). After Si deposition on samples kept at room temperature, small Si droplets are observed on the surface, but no intercalation can be detected. Intercalation is revealed to occur at an elevated temperature of about 800. C. The Si is found to migrate to the interface region via defects and domain boundaries. This observation may provide an answer to the problem of controlling homogeneous bi-/multilayer graphene growth on nearly perfect monolayer graphene samples prepared on SiC(0001). Likewise, Si penetrates more easily small monolayer graphene domains because of the higher density of domain boundaries. Upon annealing at 1000-1100 degrees C, formation of SiC on the surface is revealed by the appearance of a characteristic surface state located at about 1.5 eV below the Fermi level. A streaked mu-LEED pattern is also observed at this stage. The SiC formed on the surface is found to decompose again after annealing at temperatures higher than 1200 degrees C.

Detailed studies of Li deposition on monolayer graphene grown on the Si-face SiCsurface were performed using LEEM, μ-LEED, PES and ARPES. Li was found to intercalatedirectly after the deposition at room temperature. However, excess Li was also observed on thesurface and found to form a compound with carbon atoms. This compound is suggested to give riseto a new (√3x√3) R30° surface reconstruction. After annealing the (√3x√3) R30° reconstructionvanished and only a (1x1) graphene diffraction pattern was visible. At the same time a severechange was observed in the graphene morphology, especially from the ex-situ grown graphene, i.e.,extended areas of cracks/wrinkles were observed. These wrinkles/cracks did not disappear evenafter heating at temperature of 500-1000°C when no Li signal was detected.

The effects of Li deposition on hydrogenated bilayer graphene on SiC(0001) samples, i.e. on quasi-freestanding bilayer graphene samples, are studied using low energy electron microscopy, micro-low-energy electron diffraction and photoelectron spectroscopy. After deposition, some Li atoms form islands on the surface creating defects that are observed to disappear after annealing. Some other Li atoms are found to penetrate through the bilayer graphene sample and into the interface where H already resides. This is revealed by the existence of shifted components, related to H–SiC and Li–SiC bonding, in recorded core level spectra. The Dirac point is found to exhibit a rigid shift to about 1.25 eV below the Fermi level, indicating strong electron doping of the graphene by the deposited Li. After annealing the sample at 300–400 °C formation of LiH at the interface is suggested from the observed change of the dipole layer at the interface. Annealing at 600 °C or higher removes both Li and H from the sample and a monolayer graphene sample is re-established. The Li thus promotes the removal of H from the interface at a considerably lower temperature than after pure H intercalation.