Materials Science Forum Vols. 717-720

Abstract: Patterned Few Layers Graphene (FLG) films were grown by local solid phase epitaxy from nickel silicide supersaturated with carbon. The process was realised by annealing of thin Ni films deposited on the carbon-terminated surface of 6H-SiC semi-insulating wafer followed by wet processing to remove the resulting nickel silicide. Raman spectroscopy was used to investigate both the formation and subsequent removal of nickel silicide during processing. Characterisation of the resulting FLG films was carried out by Raman spectroscopy and Atomic Force Microscopy (AFM). The thickness of the final FLG film estimated from the Raman spectra varied from 1 to 3 monolayers for initial Ni layers varying from 3 to 20 nm thick. AFM observations revealed process-induced surface roughening in FLG films, however, electrical conductivity measurements by Transmission Line Model (TLM) structures confirmed that roughness does not compromise the film sheet resistance.

Abstract: We present epitaxial graphene (EG) growth on non-polar a-plane and m-plane 6H-SiC faces where material characterization is compared with that known for EG grown on polar faces. AFM surface morphology exhibits nanocrystalline graphite like features for non-polar faces, while polar silicon face shows step like features. This differing behavior is attributed to the lack of a hexagonal template on the non-polar faces. Non-polar faces also exhibit greater disorder and red shift of all Raman peaks (D, G and 2D) with increasing temperature. This is attributed to decreasing stress with increasing temperature. These variations provide evidence of different EG growth mechanisms on non-polar and polar faces, likely due to differences in surface free energy. We also present differences between a-plane ( ) EG and m-plane ( ) EG in terms of morphology, thickness and Raman characteristics.

Abstract: The electrical properties of epitaxial graphene (EG) grown on 8° off-axis 4H-SiC (0001) by annealing at 1600 °C in inert gas ambient (Ar) were studied. The sheet resistance of the EG layers (R_sh=740±50Ω/sq) and the specific contact resistance of Ni contacts to EG (ρ_c≈6x10^-5 Ωcm²) were evaluated on micrometer scale by measurements on transmission line model (TLM) structures. Si₃N₄ was evaluated as a gate dielectric, showing excellent coverage to EG and a limited effect on its conductivity. The high n-type doping (~10¹³ cm^-2) of EG, as well as the field effect mobility (μ) dependence on n were determined using top gated field effect transistors (FETs) with Si₃N₄ gate dielectric. Electron mean free path (l_loc) and mobility (μ_loc) were also locally determined, on submicrometer scale, by scanning probe microscopy, showing a broad distribution of μ_loc values, with the most probable value very similar to macroscopic carrier mobility μ.

Abstract: Graphene, a 2D material, has motivated significant research in the study of its in-plane charge carrier transport in order to understand and exploit its unique physical and electrical properties. The vertical graphene-semiconductor system, however, also presents opportunities for unique devices, yet there have been few attempts to understand the properties of carrier transport through the graphene sheet into an underlying substrate. In this work, we investigate the epitaxial graphene/4H-SiC system, studying both p and n-type SiC substrates with varying doping levels in order to better understand this vertical heterojunction.

Abstract: The analytical expression for the density-of-states (DOS) of single-layer graphene interacting with the SiC surface (epitaxial graphene) is obtained. The silicon carbide DOS is described within the scope of the Haldane-Anderson model. It is shown that due to the interaction with the substrate the gap of about 0.01-0.06 eV arises in the epitaxial graphene DOS. The estimation indicates that the electron charge of about (−10-3) e/atom transfers from the substrate to graphene.

Abstract: Epitaxial graphene (EG) grown on SiC(0001) resides on the so-called buffer layer. This carbon rich (6√3×6√3)R30° reconstruction is covalently bound to the topmost silicon atoms of the SiC. Decoupling the graphene buffer layer from the SiC interface is a well studied topic since successful intercalation has been shown for hydrogen [1-3]. Recently, intercalation was also shown for oxygen [4, 5]. We present ARPES, XPS and Raman spectroscopy studies to determine the quality of oxygen intercalated buffer layer samples in terms of decoupling and integrity of the transformed graphene layer. The decoupling effect is demonstrated by ARPES measurements showing a graphene-like π band. XPS shows whether the oxidation takes place in the buffer layer or at the interface. Raman spectroscopy is well suited to investigate oxygen induced defects in graphene-like material.

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

Abstract: In this work, the treatment of epitaxial graphene on SiC using electron beam generated plasmas produced in mixtures of argon and oxygen is demonstrated. The treatment imparts oxygen functional groups on the surface with concentrations ranging up to about 12 at.%, depending on treatment parameters. Surface characterization of the functionalized graphene shows incorporation of oxygen to the lattice by disruption of ∏-bonds, and an altering of bulk electrical properties.

Abstract: Electrochemical functionalization of treated epitaxial graphene samples on Si-face 6H-SiC are presented in this work. Three semi-insulating 6H-SiC substrates cut from different boules with varying off cut angle (on axis, 0.5° and 1.0° degrees off axis in the [112‾0] direction) were diced into 10mm x 10mm samples and quality EG grown on top. A home-build electrochemical cell was used with current applied though a 10% H₂SO₄ solution, with a Pt wire and exposed graphene as the anode and cathode respectively. Functionalization was determined using Raman spectroscopy and measured by an increase in D/G ratio, increase in fluorescence background and introduction of C-H bond peak at ~2930 cm^-1. Components of the Raman spectra before and after functionalization of all samples used were analyzed to show a substrate dependent effect on functionalization with values such as D/G ratio and normalized fluorescence slope varying between the substrates.

Abstract: We investigate molecular adsorption doping by electron withdrawing NO2 and electron donating NH3 on epitaxial graphene. Amperometric measurements show conductance changes upon introduction of molecular adsorbents on epitaxial graphene. Conductance changes are a trade-off between carrier concentration and scattering, and manifest at direct current and optical frequencies. We therefore investigate changes in the infrared (IR) reflection spectra to correlate these two frequency domains, as reflectance changes are due to a change of EG surface conductance. We match theory with experimental IR data and extract changes in carrier concentration and scattering due to gas adsorption. Finally, we separate the intraband and interband scattering contributions to the electronic transport under gas adsorption. The results indicate that, under gas adsorption, the influence of interband scattering cannot be neglected, even at DC.