graphene-SiC Interface Research Articles

Quasi-freestanding AA-stacked bilayer graphene induced by calcium intercalation of the graphene-silicon carbide interface

We study quasi-freestanding bilayer graphene on silicon carbide intercalated by calcium. The intercalation, and subsequent changes to the system, were investigated by low-energy electron diffraction, angle-resolved photoemission spectroscopy (ARPES) and density-functional theory (DFT). Calcium is found to intercalate only at the graphene-SiC interface, completely displacing the hydrogen terminating SiC. As a consequence, the system becomes highly n-doped. Comparison to DFT calculations shows that the band dispersion, as determined by ARPES, deviates from the band structure expected for Bernal-stacked bilayer graphene. Instead, the electronic structure closely matches AA-stacked bilayer graphene on calcium-terminated SiC, indicating a spontaneous transition from AB- to AA-stacked bilayer graphene following calcium intercalation of the underlying graphene-SiC interface.

Charge transport in semi insulating bulk 4H-Silicon carbide: Effect of metallization and wafer homogeneity

We investigated the effect of metallization and spatial homogeneity on the charge collection of sensors prepared from high purity semi-insulating 4H-SiC bulk wafer. We used Au, Ni, Cr or graphene electrical contacts subsequently prepared on the same chip. We also tested sensors prepared from different positions on the wafer. Laser-induced transient current technique (L-TCT) and alpha spectroscopy were used to characterize the charge transport in the sensors. It was found that different sensors polarize at DC bias in the time interval of 0.2–270 s. We observed significant deviation of current transients measured on the sensor with a graphene contact from the sensors with metal contacts, which is attributed to the surface plasmon formed on the graphene-SiC interface. We demonstrated that the sensor polarization is independent of the metallization, whereas it strongly depends on the location on the wafer from which the sensor was cut. This testifies to a significant spatial inhomogeneity of the wafer. We also tested the electrical current relaxation after step-DC biasing as a simple method for the characterization of the sensor polarization. We found that the relaxation profile has extremes in the similar positions as the L-TCT profiles of collected charge. We also showed that using pulse biased L-TCT we are able to choose suitable sensors with slow polarization and to design optimal pulsing conditions including necessary depolarization times for the alpha spectroscopy measurement and L-TCT techniques to achieve long-term stable charge collection. In case of 241Am alpha spectroscopy charge collection efficiency 5–24 % was observed. We framed up model depicting all observed phenomena assuming the space charge formation induced by a blocking cathode in an n-type material and related lifetime reduction. We evaluated the space charge formed in the sensors after 104 s biasing, which ranged from 1.7 × 1014 cm−3 to 7 × 1014 cm−3 in respective sensors.

Influence of intercalated Gd atoms on graphene-4H-SiC(0001) properties

In this work, the influence of intercalated gadolinium (Gd) atoms on graphene-4H-SiC(0001) properties are presented, using X-ray photoelectron spectroscopy, low energy electron diffraction and density functional theory calculations. Detailed analysis, ranging from adsorption to temperature induced intercalation under the graphene, of the chemical composition, surface structures, charge transfer and changes in the graphene band structure is performed. The investigation is carried out for various initial Gd film thicknesses and temperature of annealing, which allows to obtain and observe interactions between elements in the subsequent stages of intercalation. Calculations show that the deeper Gd is placed into the substrate the more energetically favourable configuration is obtained, which indicates the possibility of intercalation process to occur. Moreover, intercalated Gd strongly affects the electronic properties of both graphene and buffer layer. Simultaneously, experiment results show that Gd induces n-type doping in graphene. Additionally, temperature induced intercalation causes the buffer layer decoupling and transformation into new graphene one. The presence of gadolinium at the graphene-SiC interface results also in chemical shifts to the lower binding energy range and follows the trend in which the more gadolinium intercalates the higher shift is observed.

Impact of oxygen adsorption on the electronic properties and contact type of a defective epitaxial graphene-SiC interface

Adsorption and dissociation of molecular oxygen on a defective graphene buffer layer (BL) grown on Si-terminated SiC(0001) are herein studied by means of periodic DFT calculations coupled with the nudged elastic band method. We considered a single vacancy (SV) and a divacancy (DV) defective BL on SiC, with or without the uppermost epitaxial monolayer graphene (EG). The adsorption energy of O2 on SV and DV surfaces, producing SV-O and DV-O oxidized systems, is between 2 and 3 times as strong as that on a defect-free BL surface and lacks an energy barrier. In addition, we demonstrate that the n-doping measured in the EG layer of SiC-BL-EG systems, which stems from the charge transfer in the SiC-BL interface, can be fully compensated by oxidizing the DV defective BL. The DV-O-EG interface forms a p-type Schottky contact with a small Schottky barrier height. Instead, a p-type Ohmic contact was measured at the SV-O-EG interface.

A Paper-Like Inorganic Thermal Interface Material Composed of Hierarchically Structured Graphene/Silicon Carbide Nanorods.

With the increasing integration of devices in electronics fabrication, there are growing demands for thermal interface materials (TIMs) with high through-plane thermal conductivity for efficiently solving thermal management issues. Graphene-based papers consisting of a layer-by-layer stacked architecture have been commercially used as lateral heat spreaders; however, they lack in-depth studies on their TIM applications due to the low through-plane thermal conductivity (<6 W m-1 K-1). In this study, a graphene hybrid paper (GHP) was fabricated by the intercalation of silicon source and the in situ growth of SiC nanorods between graphene sheets based on the carbothermal reduction reaction. Due to the formation of covalent C-Si bonding at the graphene-SiC interface, the GHP possesses a superior through-plane thermal conductivity of 10.9 W m-1 K-1 and can be up to 17.6 W m-1 K-1 under packaging conditions at 75 psi. Compared with the current graphene-based papers, our GHP has the highest through-plane thermal conductivity value. In the TIM performance test, the cooling efficiency of the GHP achieves significant improvement compared to that of state-of-the-art thermal pads. Our GHP with characteristic structure is of great promise as an inorganic TIM for the highly efficient removal of heat from electronic devices.

Effect of hydrogen passivation on the decoupling of graphene on SiC(0001) substrate: First-principles calculations

Intercalation of hydrogen is important for understanding the decoupling of graphene from SiC(0001) substrate. Employing first-principles calculations, we have systematically studied the decoupling of graphene from SiC surface by H atoms intercalation from graphene boundary. It is found the passivation of H atoms on both graphene edge and SiC substrate is the key factor of the decoupling process. Passivation of graphene edge can weaken the interaction between graphene boundary and the substrate, which reduced the energy barrier significantly for H diffusion into the graphene-SiC interface. As more and more H atoms diffuse into the interface and saturate the Si dangling bonds around the boundary, graphene will detach from substrate. Furthermore, the energy barriers in these processes are relatively low, indicating that these processes can occur under the experimental temperature.

Interfacial Charge States in Graphene on SiC Studied by Noncontact Scanning Nonlinear Dielectric Potentiometry.

We investigate pristine and hydrogen-intercalated graphene synthesized on a 4H-SiC(0001) substrate by using noncontact scanning nonlinear dielectric potentiometry (NC-SNDP). Permanent dipole moments are detected at the pristine graphene-SiC interface. These originate from the covalent bonds of carbon atoms of the so-called buffer layer to the substrate. Hydrogen intercalation at the interface eliminates these covalent bonds and the original quasi-(6×6) corrugation, which indicates the conversion of the buffer layer into a second graphene layer by the termination of Si bonds at the interface. NC-SNDP images suggest that a certain portion of the Si dangling bonds remains even after hydrogen intercalation. These bonds are thought to act as charged impurities reducing the carrier mobility in hydrogen-intercalated graphene on SiC.

Effects of Al on epitaxial graphene grown on 6H-SiC(0001)

Aluminum was deposited on epitaxial monolayer-grown graphene on SiC(0001). The effects of annealing up to 1200 °C on the surface and interface morphology, chemical composition, and electron band structure were analyzed in situ by synchrotron-based techniques at the MAX Laboratory. After heating at around 400 °C, Al islands or droplets are observed on the surface and the collected Si 2p, Al 2p, and C 1s core levels spectra indicate Al intercalation at the graphene SiC interface. Also, the original single π-band splits into two, indicating decoupling of the carbon buffer layer and the formation of a quasi-free-standing bilayer-like electronic structure. Further heating at higher temperatures from 700 to 900 °C yields additional chemical reactions. Broader core level spectra are then observed and clear changes in the π-bands near the Dirac point are detected. More electron doping was detected at this stage since one of the π-bands has shifted to about 1.1 eV below the Fermi level. Different ordered phases of (7 × 7), (4 × 4), (1 × 1)Al, and (1 × 1)G were also observed on the surface in this temperature range. The original single π-band was restored after heating at ~1200 °C, although an Al signal was still able to be detected.

Na Induced Changes in the Electronic Band Structure of Graphene Grown on C-Face SiC

Studies of the effects induced on the electron band structure after Na deposition, and subsequent heating, on a C-face 2 MLs graphene sample are reported. Na deposition shifts the Dirac point downwards from the Fermi level by about 0.5 eV due to electron doping. After heating at temperatures from around 120℃ to 300℃,thep-band appears considerably broadened. Collected Si 2p and Na 2p spectra then indicate Na intercalation in between the graphene layers and at the graphene SiC interface. The broadening is therefore interpreted to arise from the presence of two slightly shifted, but not clearly resolved,p-bands. Constant energy photoelectron distribution patterns, E(kx,ky);s, extracted from the clean 2MLs graphene C-face sample look very similar to earlier calculated distribution patterns for monolayer, but not Bernal stacked bilayer, graphene. After Na deposition the patterns extracted at energies below the Dirac point appear very similar so the doping had no pronounced effect on the shape or intensity distribution. At energies above the Dirac point the extracted angular distribution patterns show the flipped, “mirrored”, intensity distribution predicted for monolayer graphene at these energies. An additional weaker outer band is also discernable at energies above the Dirac point, which presumably is induced by the deposited Na.

Silicon intercalation into the graphene-SiC interface

In this work we use low-energy electron microscopy, x-ray photoemission electron microscopy, and x-ray photoelectron spectroscopy to study how the excess Si at the graphene-vacuum interface reorders itself at high temperatures. We show that silicon deposited at room temperature onto multilayer graphene films grown on the SiC$(000\overline{1})$ rapidly diffuses to the graphene-SiC interface when heated to temperatures above 1020${\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. In a sequence of depositions, we have been able to intercalate $\ensuremath{\sim}$6 ML of Si into the graphene-SiC interface.

Changes in structural and electronic properties of graphene grown on 6H-SiC(0001) induced by Na deposition

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 °C. The intercalated Na decouples the buffer layer and transforms it into a second graphene layer so two π-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¯ point and not the hyperbolic dispersion observed for AB stacked bi-layer graphene. When heating the sample to about 125 °C or higher, Na is found to leave the interface and the second graphene layer is transformed back to the carbon buffer layer.

The structure of graphene grown on the SiC surface

Graphene grown on the SiC surface is unique. Unlike graphene grown on the (0 0 0 1) surface, graphene grown on the surface has higher electron mobilities and an unusual non-Bernal stacking. Its different electronic properties are associated with its stacking and the graphene–SiC interface. In this paper we discuss what is known about the structure of this material. In particular we will discuss the ordering in this material and how it is related to the interface structure. We update new ideas about the interface and stacking and contrast it with works from other groups. New evidence for how Si is removed from the interface is also given that provides some insight into the growth process and shows that graphene nucleation is not confined to screw dislocations. This has important implications for the viability of patterned graphene growth.

Microscopic correlation between chemical and electronic states in epitaxial graphene on SiC(0001¯)

We present energy filtered electron emission spectromicroscopy with spatial and wave-vector resolution on few-layer epitaxial graphene on SiC$(000\overline{1})$ grown by furnace annealing. Low-energy electron microscopy shows that more than 80% of the sample is covered by 2--3 graphene layers. C1s spectromicroscopy provides an independent measurement of the graphene thickness distribution map. The work function, measured by photoelectron emission microscopy (PEEM), varies across the surface from 4.34 to 4.50 eV according to both the graphene thickness and the graphene-SiC interface chemical state. At least two SiC surface chemical states (i.e., two different SiC surface structures) are present at the graphene/SiC interface. Charge transfer occurs at each graphene/SiC interface. $k$-space PEEM gives 3D maps of the $|{\mathrm{k\ifmmode \bar{}\else \={}\fi{}}}_{\ensuremath{\parallel}}|$ $\ensuremath{\pi}\ensuremath{-}{\ensuremath{\pi}}^{*}$ band dispersion in micron-scale regions showing that the Dirac point shifts as a function of graphene thickness. Bragg diffraction of the Dirac cones via the superlattice formed by the commensurately rotated graphene sheets is observed. The experiments underline the importance of lateral and spectroscopic resolution on the scale of future electronic devices in order to precisely characterize the transport properties and band alignments.

Interface structure of graphene on SiC: an ab initio and STM approach

High temperature treatment of SiC surfaces is a well-established technique for producing graphene directly on top of an insulating substrate. In this domain an important question is the influence of the substrate on the atomic and electronic structure of the graphene layers. This requires a detailed investigation of the interactions at the graphene–SiC interface. Surface science techniques and ab initio calculations are well suited for that purpose. In this paper, we present a brief review of the recent investigations performed in this domain by scanning tunnelling microscopy (STM) and ab initio simulations. It is largely based on the work performed in our group, but it also provides a survey of the literature in these fields. Both the so-called Si and C face of the hexagonal 6H(4H)SiC{0 0 0 1} substrates will be considered, as they show markedly different types of behaviour.

Why Multilayer Graphene on4H−SiC(0001¯)Behaves Like a Single Sheet of Graphene

We show experimentally that multilayer graphene grown on the carbon terminated SiC(0001[over ]) surface contains rotational stacking faults related to the epitaxial condition at the graphene-SiC interface. Via first-principles calculation, we demonstrate that such faults produce an electronic structure indistinguishable from an isolated single graphene sheet in the vicinity of the Dirac point. This explains prior experimental results that showed single-layer electronic properties, even for epitaxial graphene films tens of layers thick.