Quasi-free-standing Monolayer Graphene Research Articles

Crater-free monolayer graphene above the 2D Si film on SiC(0001) formed via SiSn cointercalation and Sn deintercalation

When Si atoms are intercalated under the graphene-like zero layer (ZL) generated on the SiC(0001) substrate, the resulting quasi-free-standing monolayer graphene (QFMLG) has lots of crater-like defects due to a strong Si-C reaction. Here, a method for creating crater-free QFMLG above a two-dimensional Si film is investigated using scanning tunneling microscopy and photoemission spectroscopy. The method involves forming a SiSn interfacial film as a precursor via cointercalation performed by sequential Sn and Si deposition on the ZL at room temperature and annealing at 650–700 °C. The Sn atoms prevent reactive Si atoms from directly contacting the ZL, which would otherwise cause SiC defects. The sample is then annealed at 750 °C to selectively remove the Sn atoms from the SiSn interfacial film, leaving a well-ordered Si film. The SiSn film composed of a bottom Si layer and top Sn atoms has a short-range-ordered “2 × 2” or a long-range-ordered 2×7 superstructure depending on the annealing temperature and the Sn coverage. Both the superstructures are semiconducting and induce less n-doped QFMLG than the pure Si film.

Transfer doping of epitaxial graphene on SiC(0001) using Cs

Control of the charge carrier concentration is essential for applications of graphene. Here, we demonstrate the doping of epitaxial graphene on SiC(0001) via charge transfer from an adsorbed layer of Cs atoms with sub-monolayer coverage. The electronic structure of the graphene is analyzed using x-ray and angle-resolved photoelectron spectroscopy. In H-intercalated, quasi-freestanding monolayer graphene (QFMLG), the Dirac point can be tuned continuously from p-type to strong n-type doping. For strong n-type doping, analysis of the core level binding energies implies a deviation from a rigid band shift. This might be explained by an increased screening of the atomic core potential due to the higher number of charge carriers per C atom in the graphene layer. Furthermore, charge transfer into the SiC substrate leads to a change in band bending at the SiC/QFMLG interface, which saturates into a flat band scenario at higher Cs coverage. An analysis of the Fermi surfaces suggests an increasing electron-phonon-coupling in strongly doped QFMLG. In monolayer graphene (MLG), which is intrinsically n-type doped due to the presence of the buffer layer at the SiC interface, n-type doping can be enhanced by Cs evaporation in a similar fashion. In contrast to QFMLG, core level spectra and Dirac cone position in MLG apparently show a rigid band shift even for very high doping, emphasizing the importance of the substrate.

Dielectric function of epitaxial quasi-freestanding monolayer graphene on Si-face 6H-SiC in a broad spectral range

We present a study of the optical properties of quasi-freestanding epitaxial monolayer graphene grown on the Si face of 6H-SiC (0001) in a broad spectral range from midinfrared to ultraviolet. After growth, the sample was intercalated by hydrogen in order to ensure that no dangling bonds between the substrate and graphene layer remain. Spectral dependence of the dielectric function of graphene was parametrized based on spectroscopic ellipsometry fits. We show that, from the viewpoint of optical properties, the investigated sample is very close to exfoliated graphene. We provide information about the spectral dependence of the complex dielectric function of quasi-freestanding graphene in a broad spectral range.

Structural identification of graphene films and nanoislands on 6H-SiC(0001) by direct height measurement

By combining non-contact atomic force microscopy (nc-AFM) and Kelvin probe microscopy (KPFM) in ultra high vacuum environment (UHV), we directly measure the height and work function of graphene monolayer on the Si-face of 6H-SiC(0001) with a precision that allows us to differentiate three different types of graphene structures : zero layer graphene (ZLG), Quasi free-standing monolayer graphene (QFMLG) and bilayer graphene (BLG). The height and work function of ZLG are 2.62 ± 0.22 Å and 4.42 ± 0.05 eV respectively, when they are 4.09 ± 0.11 Å and 4.63 ± 0.05 eV for QFMLG. The work function is 4.83 ± 0.05 eV for the BLG. Unlike any other available technique, the local nc-AFM/KPFM dual probe makes it possible to directly identify the nature of nanometer-sized graphene islands that constitute the early nuclei of graphene monolayer grown on 6H-SiC(0001) by chemical vapor deposition.

SiGe-intercalated graphene on SiC(0001): Interfacial structures and graphene doping depending on coverage and composition ratio of the alloy

To find out the relation between the charge transfer doping of quasi-free-standing monolayer graphene (QFMLG) formed on 6H-SiC(0001) and the structures of the intercalated SiGe alloy film, Ge atoms were intercalated additionally/substitutionally under QFMLG above the already-built bilayer Si film with a coverage of 1.2 ML. When 0.4-ML Ge is additionally intercalated, the SiGe film with a 31×23 structure is generated and induces weakly n-doped QFMLG whose Dirac point (ED) is at 0.04 eV below the Fermi level (EF). When the additionally intercalated Ge amount is over 0.6 ML, the SiGe film with the 2×1.5n3 and/or 4×33 structure composed of adatoms and a bilayer is formed below QFMLG. It turns out that Ge atoms preferentially occupy the top layer, while Si atoms occupy the bottom layer. Once a well-ordered trilayer film is formed at a total Si and Ge coverage of 2.3 MLs, though the concentration of Ge is gradually increased by the substitution of Ge atoms for Si atoms, the ED of QFMLG is fixed at 0.20 eV above the EF. Such a result confirms that the amount of charge transferred from the well-ordered trilayer SiGe film to QFMLG is irrelevant to the composition ratio.

Vertical structure of Sb-intercalated quasifreestanding graphene on SiC(0001)

Using the normal incidence x-ray standing wave technique as well as low energy electron microscopy we have investigated the structure of quasi-freestanding monolayer graphene (QFMLG) obtained by intercalation of antimony under the $\left(6\sqrt{3}\times6\sqrt{3}\right)R30^\circ$ reconstructed graphitized 6H-SiC(0001) surface, also known as zeroth-layer graphene. We found that Sb intercalation decouples the QFMLG well from the substrate. The distance from the QFMLG to the Sb layer almost equals the expected van der Waals bonding distance of C and Sb. The Sb intercalation layer itself is mono-atomic, flat, and located much closer to the substrate, at almost the distance of a covalent Sb-Si bond length. All data is consistent with Sb located on top of the uppermost Si atoms of the SiC bulk.

Momentum microscopy of Pb-intercalated graphene on SiC: Charge neutrality and electronic structure of interfacial Pb

Intercalation is an established technique for tailoring the electronic structure of epitaxial graphene. Moreover, it enables the synthesis of otherwise unstable two-dimensional (2D) layers of elements with unique physical properties compared to their bulk versions due to interfacial quantum confinement. In this work, we present uniformly Pb-intercalated quasi-freestanding monolayer graphene on SiC, which turns out to be essentially charge neutral with an unprecedented $p$-type carrier density of only $(5.5\pm2.5)\times10^9$ cm$^{-2}$. Probing the low-energy electronic structure throughout the entire first surface Brillouin zone by means of momentum microscopy, we clearly discern additional bands related to metallic 2D-Pb at the interface. Low-energy electron diffraction further reveals a $10\times10$ Moir\'e superperiodicity relative to graphene, counterparts of which cannot be directly identified in the available band structure data. Our experiments demonstrate 2D interlayer confinement and associated band structure formation of a heavy-element superconductor, paving the way towards strong spin-orbit coupling effects or even 2D superconductivity at the graphene/SiC interface.

Critical View on Buffer Layer Formation and Monolayer Graphene Properties in High-Temperature Sublimation

In this work we have critically reviewed the processes in high-temperature sublimation growth of graphene in Ar atmosphere using closed graphite crucible. Special focus is put on buffer layer formation and free charge carrier properties of monolayer graphene and quasi-freestanding monolayer graphene on 4H–SiC. We show that by introducing Ar at higher temperatures, TAr, one can shift the formation of the buffer layer to higher temperatures for both n-type and semi-insulating substrates. A scenario explaining the observed suppressed formation of buffer layer at higher TAr is proposed and discussed. Increased TAr is also shown to reduce the sp3 hybridization content and defect densities in the buffer layer on n-type conductive substrates. Growth on semi-insulating substrates results in ordered buffer layer with significantly improved structural properties, for which TAr plays only a minor role. The free charge density and mobility parameters of monolayer graphene and quasi-freestanding monolayer graphene with different TAr and different environmental treatment conditions are determined by contactless terahertz optical Hall effect. An efficient annealing of donors on and near the SiC surface is suggested to take place for intrinsic monolayer graphene grown at 2000 ∘C, and which is found to be independent of TAr. Higher TAr leads to higher free charge carrier mobility parameters in both intrinsically n-type and ambient p-type doped monolayer graphene. TAr is also found to have a profound effect on the free hole parameters of quasi-freestanding monolayer graphene. These findings are discussed in view of interface and buffer layer properties in order to construct a comprehensive picture of high-temperature sublimation growth and provide guidance for growth parameters optimization depending on the targeted graphene application.

Transformation of the buffer layer grown on 4H-SiC to single-layer graphene by ex situ hydrogen intercalation

The buffer layer samples grown on the Si-face of the 4H- and 6H-SiC substrates were heated in a hydrogen flow at different temperatures (600–900 °C) and heating times (40–60 min) in order to obtain quasi-freestanding monolayer graphene. Their structural properties were characterized before and after heating by using the methods of Raman spectroscopy, atomic force microscopy, Kelvin probe force microscopy, and low-energy electron diffraction. The dependence of the degree of coverage of the sample with quasi-freestanding graphene and the number of defects in the resulting films on the heating temperature was studied. As a result of optimization of the technological parameters, it is shown that the highest quality of the resulting quasi-freestanding graphene can be achieved by using the following parameters: heating time of 40 minutes and temperature of 800 °C.

Effects of two kinds of intercalated In films on quasi-free-standing monolayer graphene formed above SiC(0001)

The effects of In atoms intercalated between the n-type 6H–SiC(0001) substrate and the (63×63)R30∘ zero layer (ZL) on the interface morphology, chemical composition and electron band structure were investigated by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), core-level/valence-band photoemission spectroscopy (PES) and angle-resolved photoemission spectroscopy (ARPES). As a result of In intercalation, two kinds of ordered In films depending on the thickness of In as well as the annealing temperature were formed under quasi-free-standing monolayer graphene (QFMLG) transformed from ZL. One is a bilayer film, which is stable under 800 °C. The other is a monolayer film composed of In adatoms of a (3×3)R30∘ structure, which survives over 800 °C. The latter induces electron doping of the QFMLG stronger than the former. In addition, the QFMLG on the (3×3)R30∘ film becomes more n-doped under higher annealing temperature, which is due to a vacancy increment of the In film.

Probing the uniformity of hydrogen intercalation in quasi-free-standing epitaxial graphene on SiC by micro-Raman mapping and conductive atomic force microscopy

In this paper, micro-Raman mapping and conductive atomic force microscopy (C-AFM) were jointly applied to investigate the structural and electrical homogeneity of quasi-free-standing monolayer graphene (QFMLG), obtained by high temperature decomposition of 4H-SiC(0001) followed by hydrogen intercalation at 900 °C. Strain and doping maps, obtained by Raman data, showed the presence of sub-micron patches with reduced hole density correlated to regions with higher compressive strain, probably associated with a locally reduced hydrogen intercalation. Nanoscale resolution electrical maps by C-AFM also revealed the presence of patches with enhanced current injection through the QFMLG/SiC interface, indicating a locally reduced Schottky barrier height (ΦB). The ΦB values evaluated from local I–V curves by the thermionic emission model were in good agreement with the values calculated for the QFMLG/SiC interface using the Schottky–Mott rule and the graphene holes density from Raman maps. The demonstrated approach revealed a useful and non-invasive method to probe the structural and electrical homogeneity of QFMLG for future nano-electronics applications.

Probing the structural transition from buffer layer to quasifreestanding monolayer graphene by Raman spectroscopy

The structural transition of a graphene buffer layer epitaxially grown on $6H$ silicon carbide (SiC) to quasifreestanding monolayer graphene by intercalation of oxygen and water molecules at low concentrations is studied by temperature-dependent Raman spectroscopy. We present a detailed investigation of the defect density and strain and doping evolution in the graphene crystal lattice. The structural transition from the buffer layer to monolayer graphene with high defect densities occurs at temperatures from 400 to 500 \ifmmode^\circ\else\textdegree\fi{}C, revealing the nanocrystalline regime of stage 2 of the amorphization trajectory, followed by the transition into stage 1 as evidenced by a gradual reduction of defects in graphene during subsequent annealing up to 900 \ifmmode^\circ\else\textdegree\fi{}C.

Homogeneous Large-Area Quasi-Free-Standing Monolayer and Bilayer Graphene on SiC

In this study, we first show that the argon flow during epitaxial graphene growth is an important parameter to control the quality of the buffer and the graphene layer. Atomic force microscopy (AFM) and low-energy electron diffraction (LEED) measurements reveal that the decomposition of the SiC substrate strongly depends on the Ar mass flow rate while pressure and temperature are kept constant. Our data are interpreted by a model based on the competition of the SiC decomposition rate, controlled by the Ar flow, with a uniform graphene buffer layer formation under the equilibrium process at the SiC surface. The proper choice of a set of growth parameters allows the growth of defect-free, ultra-smooth and coherent graphene-free buffer layer and bilayer-free monolayer graphene sheets which can be transformed into large-area high-quality quasi-freestanding monolayer and bilayer graphene (QFMLG and QFBLG) by hydrogen intercalation. AFM, scanning tunneling microscopy (STM), Raman spectroscopy and electronic transport measurements underline the excellent homogeneity of the resulting quasi-freestanding layers. Electronic transport measurements in four-point probe configuration reveal a homogeneous low resistance anisotropy on both {\mu}m- and mm scales.

Doping modulation of quasi-free-standing monolayer graphene formed on SiC(0001) through Sn1-xGex intercalation

In order to modulate the transfer doping of quasi-free-standing monolayer graphene (QFMLG) formed on SiC(0001), Ge atoms were intercalated additionally into QFMLG already formed by Sn intercalation between ZL and 6H-SiC(0001). By postannealing the Ge-deposited surface at 600 °C, the Sn1-xGex film with the 4×33 structure, composed of a bilayer and adatoms with dangling bonds under QFMLG, has been formed. It turns out that, in this Sn1-xGex film, Ge atoms preferentially occupy the bottom layer bound to the top Si atoms of the substrate, while Sn atoms occupy the top adatom sites. Strong correlation among the electrons localized at these adatom sites induces a semiconducting alloy film. As the postannealing temperature is increased up to 800 °C, the concentration of Ge in the intercalated film of the same 4×33 structure is gradually increased and the Dirac point also shifts gradually from −0.16 eV to +0.20 eV relative to the Fermi level. Such a result confirms that the transfer doping of QFMLG on SiC(0001) has been modulated by varying the alloy composition of the Sn1-xGex interfacial film.

ZO phonon of a buffer layer and Raman mapping of hydrogenated buffer on SiC(0001)

AbstractWe have measured spatial Raman maps of hydrogen intercalated quasi‐free standing monolayer graphene (QFSMLG) on SiC(0001). We compare Raman spectra of QFSMLG with spectra of bare buffer layer, single‐layer graphene, and bare SiC substrate. We also present the evolution of QFSMLG Raman spectra with the temperature and duration of hydrogen intercalation. We present new Raman modes, and, on the basis of polarization resolved measurements, we attribute them to the totally symmetric out‐of‐plane optical phonon (ZO) modes of the buffer layer at the Γ and M points. We show that these modes are eliminated by hydrogen intercalation; thus, they indicate onset of buffer layer decoupling from the SiC substrate. The spatial mapping of Raman scattering reveals details of the optimal hydrogen intercalation at elevated temperatures. Further increase of the intercalation temperature leads to etching of the buffer layer and underlying SiC substrate. Therefore, we show that interplay between temperature and intercalation time is a promising route towards increased graphene grain size with reduced lattice strain.

Unraveling localized states in quasi free standing monolayer graphene by means of Density Functional Theory

Quasi free standing monolayer graphene (QFMLG), grown on SiC by Si evaporation from the Si-rich SiC(0001) face and H intercalation, displays irregularities in STM and AFM images appearing as localized features, associated with vacancies in the H layer coverage. Size, shape, concentration, and distribution of these features depend on hydrogenation conditions. In order to understand this phenomenology and possibly control it, we perform a Density Functional Theory study of QFMLG with defects in the H coverage of different size and shape and arranged in different configurations. We show that H vacancies generate localized states with highly specific electronic structure. Based on the comparison of simulated and measured STM images we are able to associate different vacancies of large size (7-13 missing H) to the observed features, unraveling the structural diversity of defects of H coverage in QFMLG. Our study indicates a tendency of single H vacancies to aggregate and to locate on a regular superlattice, providing insight into the kinetics of the hydrogenation process. The energy of the localized states associated with these vacancies depends on their size and shape, showing dependence of the electronic properties on the environmental conditions during the sample production.

Atomic and electronic structure of Si dangling bonds in quasi-free-standing monolayer graphene

Si dangling bonds at the interface of quasi-free-standing monolayer graphene (QFMLG) are known to act as scattering centers that can severely affect carrier mobility. Herein, we investigate the atomic and electronic structure of Si dangling bonds in QFMLG using low-temperature scanning tunneling microscopy/spectroscopy (STM/STS), atomic force microscopy (AFM), and density functional theory (DFT) calculations. Two types of defects with different contrast were observed on a flat graphene terrace by STM and AFM; in particular, their STM contrast varied with the bias voltage. Moreover, these defects showed characteristic STS peaks at different energies, 1.1 and 1.4 eV. The comparison of the experimental data with the DFT calculations indicates that the defects with STS peak energies of 1.1 and 1.4 eV consist of clusters of three and four Si dangling bonds, respectively. The relevance of the present results for the optimization of graphene synthesis is discussed.

Uniform coverage of quasi-free standing monolayer graphene on SiC by hydrogen intercalation

Buffer layer (BL) was grown on the Si-face of semi-insulating SiC substrates by thermal decomposition. The Raman spectra were correlated with the surface potential obtained by Kelvin probe force microscopy to assuredly confirm the complete coverage of the BL on SiC substrates. Subsequently, quasi-free standing monolayer graphene (QFSMG) was achieved by hydrogen intercalation. And moreover, different hydrogen annealing temperature was chosen in order to study the process of hydrogen intercalation. Raman and X-ray photoelectron spectroscopy measurements distinctly revealed the changes of QFSMG with varied hydrogen annealing temperature. In particular, a large number of Raman data were collected to indicate the differences of uniformity. Additionally, the peak of Si–H bonding vibration mode was observed by surface enhanced Raman scattering, which was the direct evidence to show the success of hydrogen intercalation. All results indicated that the optimized annealing temperature was about 900 °C for obtaining uniform coverage of high-quality QFSMG with low density of defects.

Synthesis of Freestanding Graphene on SiC by a Rapid-Cooling Technique.

Graphene has a negative thermal expansion coefficient; that is, when heated, the graphene lattice shrinks. On the other hand, the substrates typically used for graphene growth, such as silicon carbide, have a positive thermal expansion coefficient. Hence, on cooling graphene on SiC, graphene expands but SiC shrinks. This mismatch will physically break the atomic bonds between graphene and SiC. We have demonstrated that a graphenelike buffer layer on SiC can be converted to a quasifreestanding monolayer graphene by a rapid-cooling treatment. The decoupling of graphene from the SiC substrate was actually effective for reducing the electric carrier scattering due to interfacial phonons. In addition, the rapidly cooled graphene obtained in this way was of high-quality, strain-free, thermally stable, and strongly hole doped. This simple, classical, but quite novel technique for obtaining quasifreestanding graphene could open a new path towards a viable graphene-based semiconductor industry.

Origin of ambipolar graphene doping induced by the ordered Ge film intercalated on SiC(0001)

Transfer doping mechanism of the quasi-free-standing monolayer graphene (QFMLG), depending on the thickness as well as the atomic structures of the intercalated Ge film, has been revealed by combined investigations of scanning tunneling microscopy and synchrotron photoemission spectroscopy. The ordered Ge film has been formed between QFMLG and the n-type 6H-SiC(0001) substrate by depositing Ge on the (63×63)R30° zero layer and postannealing. At postannealing temperatures lower than 900 °C, the interfacial structures inducing p-type QFMLG, like 2×1.5n3 and 4×33, are formed. On the other hand, at those higher than 900 °C, the interfacial structures inducing n-type QFMLG, like 7×43 [expressed as a (5−376) matrix], 8×33 and 21×21 [expressed as a (5115) matrix], are formed as a result of partial deintercalation of Ge atoms. Such a transition from p-type to n-type doping turns out to be associated with an increase in the density of states of an upper Hubbard band originating from strong correlation of the electrons at the Ge atoms on the first and the second Ge interfacial layers.