Andreas Sandin

Optical and structural characterization of epitaxial graphene on vicinal 6H-SiC(0001)–Si by spectroscopic ellipsometry, Auger spectroscopy, and STM

The authors report results of spectroscopic ellipsometry (SE) measurements in the near-IR, visible, and near-UV spectral ranges using a Woollam dual rotating-compensator ellipsometer, analyzing data in terms of both epitaxial graphene and interface contributions. The SiC samples were cleaned by standard methods of CMP and HF etching prior to mounting in UHV and growing epitaxial graphene by thermal annealing at ∼1400 °C. Most samples were vicinally cut 3.5° off (0001) toward [11−20]. STM measurements show that the initial regular step edges were replaced by somewhat irregular edges after graphene growth. From growth-temperature and Auger data the authors estimate that the graphene is ∼3–4 ML thick. The authors find significant differences among the spectral features of the interface “buffer” layer and those of graphene. Specifically, the hyperbolic-exciton peak reported previously at ∼4.5 eV in graphene shifts to a similarly shaped peak at ∼4 eV in the interface buffer layer. The authors attribute this sh...

Incomplete screening by epitaxial graphene on the Si face of 6H–SiC(0001)

A biased scanning tunneling microscope (STM) tip is used to study the ability of carriers in graphene to screen external electrostatic fields by monitoring the effect of tunneling-junction width on the position of image potential-derived surface states. These states are unusually sensitive to local electric fields due to the STM tip in both single layer and bilayer epitaxial graphene. This is attributed to the incomplete screening of applied fields in epitaxial graphene on SiC(0001). Our observations imply that charged impurity scattering is likely to be a dominant factor in the transport properties of epitaxial graphene on SiC.

Smooth MgO films grown on graphite and graphene by pulsed laser deposition

Pulsed laser deposition was used to grow thin (1–100 nm) magnesium oxide films directly on graphite and epitaxial graphene on SiC(0001). The authors observe very smooth (typical rms roughness of ∼0.4 nm) film morphologies that are nearly independent of film thickness and conformal to the substrate for films grown on room temperature substrates. Surface roughness is less than 1 nm for thicknesses up to 100 nm and is independent of oxygen background pressure during growth. X-ray diffraction shows no evidence of crystallinity for films grown on room temperature substrates but shows ⟨100⟩ texture for films grown on heated substrates that also have very rough surface morphologies. X-ray photoelectron spectroscopy shows hydroxylation of films due to air exposure that can only be partially removed by annealing, indicating the presence of atomic defects in the films.

Aberration Corrected Microscopy of CVD Graphene and Spectroscopic Ellipsometry of Epitaxial Graphene and CVD Graphene for Comparison of the Dielectric Function

Graphenes importance in post-CMOS device research drives the need for a variety of metrology methods for film characterization. Chemical Vapor Deposition (CVD) on metallic foils and the thermal decomposition of SiC have become two of the dominant fabrication methods of large area graphene due to their industrial scalability. The former method has required transfer of the graphene film to secondary substrates (i.e. SiO2/Si) for electrical and optical characterization, but is conducive to TEM imaging due to the fact that the film can be etched from the growth foil and directly transferred to support grids. The latter method does not require film transfer for optical or electrical characterization and can control layer number by selection of process parameters as well as Si-face vs. Cface growth. The work will present STEM imaging of CVD graphene performed at 60 kV with atomic resolution achieved through aberration correction. Single-crystal areas are identified as well as defect structures.

Enhanced optical second-harmonic generation from current-biased graphene on the substrates of Si and SiC

We find that optical second-harmonic generation (SHG) in reflection from a chemical-vapor-deposition (CVD) graphene monolayer transferred onto a SiO2/Si(001) substrate is enhanced about 3 times by the flow of direct (dc) electric current in graphene. We also find that optical SHG in reflection from a 4-layer-graphene film epitaxially grown on a vicinal SiC(0001) substrate is enhanced 25% by the flow of dc electric current in graphene. Measurements of rotationalanisotropy SHG from both samples revealed that the current-induced SHG varies strongly with the measurement location on graphene along the current flow direction. The enhancement of SHG from the graphene/SiO2/Si(001) sample is due to current-associated charge trapping at the graphene/SiO2 interface, which introduces a vertical electric field across the SiO2/Si interface that produces electric field-induced SHG. The enhancement of SHG from the graphene/vicinal-SiC(0001) sample is due to the current-associated electric field at the graphene/SiC interface that produces electric field-induced SHG. The functions of the current-induced SHG varying with the measurement location are different for the CVD graphene/SiO2/Si(001) sample and the epitaxial graphene/vicinal-SiC(0001) sample.