Epitaxial Graphene Layers Research Articles

Bilayer Graphene Grown on 4H-SiC (0001) Step-Free Mesas

We demonstrate the first successful growth of large-area (200 × 200 μm(2)) bilayer, Bernal stacked, epitaxial graphene (EG) on atomically flat, 4H-SiC (0001) step-free mesas (SFMs) . The use of SFMs for the growth of graphene resulted in the complete elimination of surface step-bunching typically found after EG growth on conventional nominally on-axis SiC (0001) substrates. As a result heights of EG surface features are reduced by at least a factor of 50 from the heights found on conventional substrates. Evaluation of the EG across the SFM using the Raman 2D mode indicates Bernal stacking with low and uniform compressive lattice strain of only 0.05%. The uniformity of this strain is significantly improved, which is about 13-fold decrease of strain found for EG grown on conventional nominally on-axis substrates. The magnitude of the strain approaches values for stress-free exfoliated graphene flakes. Hall transport measurements on large area bilayer samples taken as a function of temperature from 4.3 to 300 K revealed an n-type carrier mobility that increased from 1170 to 1730 cm(2) V(-1) s(-1), and a corresponding sheet carrier density that decreased from 5.0 × 10(12) cm(-2) to 3.26 × 10(12) cm(-2). The transport is believed to occur predominantly through the top EG layer with the bottom layer screening the top layer from the substrate. These results demonstrate that EG synthesized on large area, perfectly flat on-axis mesa surfaces can be used to produce Bernal-stacked bilayer EG having excellent uniformity and reduced strain and provides the perfect opportunity for significant advancement of epitaxial graphene electronics technology.

Comparison of the formation process and properties of epitaxial graphenes on Si- and C-face 6H—SiC substrates

In this paper, the epitaxial graphene layers grown on Si- and C-face 6H—SiC substrates are investigated under a low pressure of 400 Pa at 1600 °C. By using atomic force microscopy and Raman spectroscopy, we find that there are distinct differences in the formation and the properties between the epitaxial graphene layers grown on the Si-face and the C-face substrates, including the hydrogen etching process, the stacking type, and the number of layers. Hopefully, our results will be useful for improving the quality of the epitaxial graphene on SiC substrate.

Photoresponse in epitaxial graphene with asymmetric metal contacts

We report photoresponse observations in epitaxial graphene (EG) devices with asymmetric metals (Au, Al) contacted in planar Au/EG/Al device format. The transient photocurrent measurements on the zero-bias device show photocurrent maxima at the Au/EG contact and minima at the EG/Al contact. This observed significant difference between the two types of junctions is responsible for the overall efficient device photoresponse. We have also found that the number of EG layers influences the photocurrent magnitude and response time regardless of incident photon energy or intensity. An external photoresponsivity (or efficiency) of ∼31.3 mA W−1 is achieved with a biased Au/EG/Al photodetector at excitation wavelength of 632.8 nm.

Molecular orientation of terephthalic acid assembly on epitaxial graphene: NEXAFS and XPS study

The adsorption of terephthalic acid molecules [C(6)H(4)(COOH)(2)), TPA] on a single layer of graphene grown epitaxially on Ni(111) has been investigated by means of near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS) at room temperature. The assignment of the NEXAFS resonances was aided by ab initio calculations for the free TPA molecule. For coverages up to a monolayer the molecular plane of TPA adopts a parallel orientation with regard to the epitaxial graphene (EG) layer. Deprotonation of TPA molecules at one monolayer coverage can be excluded. For TPA multilayers, the molecular plane is tilted on average by approximately 45° with respect to the sample surface.

Femtosecond Carrier Dynamics and Saturable Absorption in Functionalized Epitaxial Graphene

We have measured carrier relaxation times and nonlinear optical properties in three functionalized epitaxial graphene layers grown on SiC wafers by using ultrafast optical pump-probe spectroscopy and open aperture z-scan at 780nm. The results demonstrate that after different functionalization, the ratios of the slower relaxation in the range of 3.1-4.6 ps (related to carrier-phonon intraband and interband scattering) increase about 2-6 times compared to original graphene. These changes of relaxation obviously improved the nonlinear optical properties, making functionalized epitaxial graphene can be used as a tuneable saturable absorber.

Effects of disorder on the optical properties of CVD grown polycrystalline graphene

We explore the effects of crystallite size (L(a)) on the linear and non-linear optical properties of chemical vapor deposition grown polycrystalline graphene. The π-plasmon resonance present at ∼4.75 eV (∼260 nm) in the optical absorption spectrum of graphene follows the empirical relationship λ(π) = 250.5 nm + 89.5 nm(2)/L(a), where λ(π) represents the π-plasmon wavelength. Furthermore, our Z-scan studies reveal that the crystallite size significantly changes the saturation intensity in CVD grown graphene. Notably, in comparison to epitaxial graphene layers grown on SiC wafers which exhibit a photogenerated carrier lifetime of few picoseconds, we find that the photogenerated carriers in our CVD grown graphene can exhibit lifetimes as long as nanoseconds.

Graphene Grown on Sapphire Surface by Using SiC Buffer Layer with SSMBE

Graphene layer is successfully grown on insulated sapphire substrate through depositing a SiC buffer layer and annealing at high temperature with solid source molecular beam epitaxy (SSMBE). The structural properties of the epitaxial graphene layer are characterized by reflection high energy diffraction (RHEED), Raman spectroscopy and near edge X-ray absorption fine structure (NEXAFS). Graphene diffraction streaks hve been seen in RHEED. The Raman and XEXAFS results reveal that well-order graphene layer adhere to the substrate surface.

Wafer-Scale Gigahertz Graphene Field Effect Transistors on SiC Substrates

Wafer-scale graphene field-effect transistors are fabricated using benzocyclobutene and atomic layer deposition Al2 O3 as the top-gate dielectric. The epitaxial-graphene layer is formed by graphitization of a 2-inch-diameter Si-face semi-insulating 6H-SiC substrate. The graphene on the silicon carbide substrate is heavily n-doped and current saturation is not found. For the intrinsic characteristic of this particular channel material, the devices cannot be switched off. The cut-off frequencies of these graphene field-effect transistors, which have a gate length of 1 μm, are larger than 800 MHz. The largest one can reach 1.24 GHz. There are greater than 95% active devices that can be successfully applied. We thus succeed in fabricating wafer-scale gigahertz graphene field-effect transistors, which paves the way for high-performance graphene devices and circuits.

High Mobility Epitaxial Graphene for Carbon-based Nanoelectronics

Epitaxial graphene has demonstrated a great potential for carbon-based electronic. Multiple nanoscale devices can be patterned on a scalable platform with high electronic mobility, large conductivity, and conductance modulation by an electrostatic gate. High quality seamless epitaxial graphene layers are grown on the entire surface of SiC substrates by thermal decomposition of the SiC crystal. For multilayer epitaxial graphene grown on the 4H-SiC (000-1) surface, transport and spectroscopy measurements demonstrate an effective decoupling of the adjacent graphene layers due to a non-graphitic ordered rotational stacking. Large switching ratios require a band gap but graphene is a gapless semimetal. Transport gaps have been demonstrated in chemical functionalized graphene and in narrow ribbons. But patterning techniques severely degrade graphene. Transport data on narrow graphene ribbons directly grown on silicon carbide substrate step edges at high temperature indicate reduced edge scattering and open the way to non diffusive electronics.

Thermoelectric effect in high mobility single layer epitaxial graphene

The thermoelectric response of high mobility (∼20 000 cm2/V s at 4 K) single layer epitaxial graphene on silicon carbide substrates has been experimentally investigated. The temperature dependence of the thermopower displays a strong deviation from the Mott relation at a carrier density of 1 × 1012 cm−2, reflecting the importance of the screening effect. In the quantum Hall regime, the amplitude of the thermopower peaks is lower than a quantum value, despite the high mobility of the sample. In addition, a temperature dependence for the amplitude is observed, unexpected by theories. The Nernst signal changes it sign as the magnetic field increases.

Weak Localization and Universal Conductance Fluctuations on Epitaxial Graphene Grown on the C-Face of 8°off-Axis 4H-SiC Substrates

We report magnetotransport measurements in single epitaxial graphene layers grown on the C-face of an 8° off-axis 4H-SiC substrate using high temperature annealing conditions with a graphite cap covering the sample. The graphene sheets were found p-type doped, with mobilities varying between 1000 and 11000 cm²/V.s from device to device at 1.6 K. We examine the signature of weak localization and universal conductance fluctuations at weak magnetic field and we show that the phase coherence lengths extracted from the two phenomena are in satisfactory agreement.

Polymer Material as a Gate Dielectric for Graphene Field-Effect-Transistor Applications

Epitaxial-graphene field-effect transistor (EG-FET) with a polymer gate dielectric was fabricated and their electrical characteristics were investigated. The epitaxial graphene layer was formed on a semi-insulating 6H-SiC substrate by a high-temperature annealing in ultrahigh vacuum. The formation of graphene was confirmed by low-energy electron diffraction (LEED), Raman-scattering spectroscopy and X-ray photoelectron spectroscopy (XPS). The polymer gate dielectric (ZEP520a) layer was formed by spin coating, which exhibits good dielectric properties without noticeable structural degradation of the graphene layer. The EG-FETs with this polymer gate dielectric shows an n-type characteristic, with the field-effect mobility of 580 cm2 V-1 s-1.

Sharp interface in epitaxial graphene layers on 3C-SiC(100)/Si(100) wafers

Graphene ranks highly as a promising material for future nanoelectronic devices because of its exceptional electron-transport properties. It appears as a material of choice for high-frequency applications. We report the growth and structure of epitaxial graphene layers on 3$C$-SiC(100)/Si(100) wafers using low-energy electron microscopy. Selective-area low-energy electron diffraction highlights the presence of two graphene domains, rotated by \ifmmode\pm\else\textpm\fi{}15\ifmmode^\circ\else\textdegree\fi{} with respect to the SiC lattice. Micro-Raman spectroscopy demonstrates the characteristic signature of few layer graphene on the SiC. X-ray photoemission spectroscopy evidences a sharp interface between graphene and 3$C$-SiC(100). It appears that epitaxial graphene layers obtained on 3$C$-SiC(100)/Si(100) have properties similar to those obtained using classical 6$H$ or 4$H$-SiC substrates with the advantage of being compatible with the current Si processing technology.

Controlling doping in graphene through a SiC substrate: A first-principles study

Controlling the type and density of charge carriers by doping is the key step for developing graphene electronics. However, direct doping of graphene is rather a challenge. Based on first-principles calculations, a concept of overcoming doping difficulty in graphene via substrate is reported.We find that doping could be strongly enhanced in epitaxial graphene grown on silicon carbide substrate. Compared to free-standing graphene, the formation energies of the dopants can decrease by as much as 8 eV. The type and density of the charge carriers of epitaxial graphene layer can be effectively manipulated by suitable dopants and surface passivation. More importantly, contrasting to the direct doping of graphene, the charge carriers in epitaxial graphene layer are weakly scattered by dopants due to the spatial separation between dopants and the conducting channel. Finally, we show that a similar idea can also be used to control magnetic properties, for example, induce a half-metallic state in the epitaxial graphene without magnetic impurity doping.

Stability and charge transfer at the interface between SiC(0 0 0 1) and epitaxial graphene

Using density functional calculations, we address the energetics of the interface between the SiC(0 0 0 1) substrate and the first covalently bonded epitaxial graphene layer. We consider a 6 3 × 6 3 R 30 ° geometry showing the experimental periodicity, a simplified 3 × 3 R 30 ° geometry presenting a strained graphene layer, and an almost commensurate 4 × 4 geometry where SiC and graphene have the same orientation. The total energies of the structurally relaxed interface systems indicate that the 6 3 × 6 3 R 30 ° geometry is the most stable, in agreement with its experimental occurrence. The binding energy is found to correlate with the vertical spread of the C atoms in the graphene layer, with a larger extension corresponding to a higher binding energy. For the 6 3 × 6 3 R 30 ° geometry, the height variation of the graphene layer displays the experimentally observed modulation with an apparent 6 × 6 periodicity. The charge transfer also correlates with the height of the graphene atoms, being more significant in graphene regions which are strongly attached to the SiC substrate.

Nanoscale structural characterization of epitaxial graphene grown on off-axis 4H-SiC (0001)

In this work, we present a nanometer resolution structural characterization of epitaxial graphene (EG) layers grown on 4H-SiC (0001) 8° off-axis, by annealing in inert gas ambient (Ar) in a wide temperature range (Tgr from 1600 to 2000°C). For all the considered growth temperatures, few layers of graphene (FLG) conformally covering the 100 to 200-nm wide terraces of the SiC surface have been observed by high-resolution cross-sectional transmission electron microscopy (HR-XTEM). Tapping mode atomic force microscopy (t-AFM) showed the formation of wrinkles with approx. 1 to 2 nm height and 10 to 20 nm width in the FLG film, as a result of the release of the compressive strain, which builds up in FLG during the sample cooling due to the thermal expansion coefficients mismatch between graphene and SiC. While for EG grown on on-axis 4H-SiC an isotropic mesh-like network of wrinkles interconnected into nodes is commonly reported, in the present case of a vicinal SiC surface, wrinkles are preferentially oriented in the direction perpendicular to the step edges of the SiC terraces. For each Tgr, the number of graphene layers was determined on very small sample areas by HR-XTEM and, with high statistics and on several sample positions, by measuring the depth of selectively etched trenches in FLG by t-AFM. Both the density of wrinkles and the number of graphene layers are found to increase almost linearly as a function of the growth temperature in the considered temperature range.

Conductive Atomic Force Microscope Nanopatterning of Epitaxial Graphene on SiC(0001) in Ambient Conditions

Conductive atomic force microscope (cAFM) nanopatterning is demonstrated on epitaxial graphene on SiC (0001) under ambient conditions. Nanopatterning kinetics and chemistry suggest that ambient cAFM nanopatterning induces local oxidization with the surface, interface, and bulk layers of epitaxial graphene on SiC (0001) playing distinct roles in the depth profile of the final nanopatterned structure.

Multidimensional characterization, Landau levels and Density of States in epitaxial graphene grown on SiC substrates

Using high-temperature annealing conditions with a graphite cap covering the C-face of, both, on axis and 8° off-axis 4H-SiC samples, large and homogeneous single epitaxial graphene layers have been grown. Raman spectroscopy shows evidence of the almost free-standing character of these monolayer graphene sheets, which was confirmed by magneto-transport measurements. On the best samples, we find a moderate p-type doping, a high-carrier mobility and resolve the half-integer quantum Hall effect typical of high-quality graphene samples. A rough estimation of the density of states is given from temperature measurements.

Epitaxial Graphene Growth on <I>α</I>-SiC: Probing the Effect of Surface Orientation

We review the results of growing few layer graphene on alpha-SiC with different surface orientations. To this end we have used successively a pure {000-1} C face, a 8 degrees-off one, a nonpolar {11-20} surface and, finally, a 8 degrees-off Si face and a pure (exactly {0001}-oriented) Si one. Without any need for complex hydrogen annealing, we have shown that a pure C or a 8 degrees-off C or even a pure {11-20} surface allows the growth of single layer epitaxial graphene sheets that are strain-free, p-type, low doped and exhibit reasonably high carrier mobility. This is not the case on pure Si or 8 degrees-off Si faces on which strained material is constantly found with heavy n-type doping and low carrier mobility.

Probing residual strain in epitaxial graphene layers on 4H-SiC(0001¯) with Raman spectroscopy

Raman microspectroscopy was used to measure compressive strain within epitaxial graphene (EG) grown on the carbon-terminated SiC(0001¯) face as a function of annealing time for a growth temperature of 1400 °C. A maximum strain of −0.5% was seen at the longest time of 55 min. This differs from the −0.9% expected for strain caused by cooling from the growth temperature due to the differential thermal contraction between the SiC and EG layer, despite good agreement between this model and data on EG on SiC(0001). We suggest that this is due to the different EG bonding mechanisms on the two SiC faces.