Session Q12: Focus Session: Graphene: Growth, Mechanical Exfoliation, and Properties - SiC and Growth Kinetics

From above or from below? Determining how graphene layers form on SiC(0001)

SiC decomposes when heated above 1200 C in vacuum. Silicon desorbs, while the carbon left behind can coalesce to form graphene. Growth of graphene on the SiC(0001) surface (``Si face'') and the SiC(000-1) (``C face'') is very different. On the Si face, graphene growth is epitaxial, while on the C face the growth is generally much less ordered. On the Si face, the observed epitaxy suggests that new graphene layers form under existing one, that is, at the SiC/graphene interface. The lack of epitaxy on the C face suggests that the growth mode on this surface might be different. To test this, we grew ultra-thin epitaxial SiC films (1 nm) on both SiC(0001) and SiC(000-1) via CVD using isotopically pure carbon-13. We then formed graphene via high-temperature thermal decomposition. We used medium energy ion scattering to determine where the carbon-13 was located within the graphene film. For both the Si face and C-face, we find that the carbon-13 is located predominantly in the outmost graphene layer, confirming that graphene grows ``from the inside out'' on both surfaces [1]. This work was performed in collaboration with Matt Copel and Ruud Tromp. \\[4pt] [1] Phys. Rev. Lett. 107, 166101 (2011)   

STM/STS study of ridges on epitaxial graphene/SiC

The graphitization of hexagonal SiC surfaces provides a viable alternative for the synthesis of wafer-sized graphene for mass device production. During later stages of growth, ridges are often observed on the graphene layers as a result of bending and buckling to relieve the strain between the graphene and SiC substrate. In this work, we show, by atomic resolution STM/STS, that these ridges are in fact bulged regions of the graphene layer, forming one-dimentional (nanowire) and zero-dimentional (quantum dot) nanostructures. We further show that their structures can be manipulated by the pressure exerted by the STM tip during imaging. These results and their impact on the electronic properties of epitaxial graphene on SiC(0001) will be presented at the meeting.   

Scalable templated growth of graphene patterns on SiC and their electronic properties

Conventional graphene growth research focuses on SiC on-axis surfaces: SiC (0001) and (000-1). In our previous work, we showed that graphene can be selectively grown on off-axis SiC crystal facets, demonstrated the possibility for templated graphene growth. Here we show scalable production of various devices made with this technique, such as graphene nanoribbons, Hall bars and Aharonov--Bohm rings. Graphene and SiC crystal facets are characterized with SEM and SPM tools. Shubnikov-de Haas oscillations and other phase coherent transport phenomena are observed at low temperature. These observations indicate that the structured epitaxial graphene growth can be a viable method for graphene electronics.   

A study of epitaxial graphene/SiC(0001) functionalized by nitrogen doping

In this work, we have carried out nitridation of epitaxial graphene/SiC(0001) using N$_{2}$ plasma. The effects of processing conditions on the structure of graphene have been investigated by x-ray photoemission spectroscopy and Raman spectroscopy, and changes in the electronic structures of the nitrogen-doped graphene have been studied using scanning tunneling microscopy. We find that the exposure of epitaxial graphene to nitrogen plasma not only leads to N incorporation, but also creates carbon vacancies, resulting in the formation of N-vacancy complexes.   

Creation and sculpting of graphene with ion and electron beams

This talk will cover our recent work on the creation of graphene by ion implantation of carbon into copper substrates followed by a prescribed annealing procedure. We also discuss nanopore nucleation with ion beams and the direct observation of nanopore growth in an aberration corrected TEM. We discuss the cross-sections and knock-on energy transfers required for edge atom removal and demonstrate the controlled growth of monodisperse nanopores in graphene with atomic precision.   

Spatially Selective Graphene Formation on Si Substrate

A uniform large area graphene is useful for applications such as electrode in a touch screen or substrate for growing another material. For applications where graphene is used either as active material (FET for instance) or electrode in a 2-D device array, to form a 2D (electronic) superlattice or photonic crystal, it is critically important to be able to form graphene at selective locations, in desirable size and shape on a substrate, without relying on mechanical cutting. It would be even more significant if the substrate is a Si wafer for coupling with the mature microelectronic technology. We have developed a technique that can achieve these goals. A thin SiC film is first deposited on a Si substrate using MBE. Then, at ambient condition, a focused laser beam is used to convert SiC to graphene at the selected location with the shape and size that can be defined by either a lithographic method or simply by a focused laser beam. The graphene conversion has been verified by structural characterization (TEM, SEM/EDS, etc.) and Raman spectroscopy.   

Surface reconstruction and graphene formation on face-to-face 6H-SiC at 2000 $^{\circ}$C

Improved epitaxial graphene films have been widely reported when the sublimation rate of Si is reduced by ambient Ar gas, vapor phase silane, or confined Si vapor. We describe graphene growth on (0001) 6H-SiC samples annealed ``face-to-face'' [1]; in our modified method the separation is limited only by the flatness of the surfaces. After annealing in 100 kPa Ar gas at 2000 $^{\circ}$C for 300 s, atomic force microscopy (AFM) and electrostatic force microscopy (EFM) show graphene coverage is typically between one and a few layers. Samples without prior hydrogen etching undergo surface reconstruction in the graphitization process, resulting in atomically flat terraces with step bunching. Estimates of the sequestered carbon in the form of graphene are compared to calculated levels due to sublimation and diffusion rates where the sublimated gas is dominated by Si atoms below 2100 $^{\circ}$C. The 2000 $^{\circ}$C samples are contrasted against samples processed between 1700 $^{\circ}$C and 1900 $^{\circ}$C and transport results on large-scale graphene devices are presented.\\[4pt] [1] X.Z Yu, C.G. Hwang, C.M. Jozwiak, A. Kohl, A.K. Schmid and A. Lanzara, New synthesis method for the growth of epitaxial graphene, Journal of Electron Spectroscopy and Related Phenomena 184 (2011) 100-106.   

Imaging grain boundary scattering of graphene in real space

Graphene grain boundaries are unavoidable defects in most growth methods, in particular chemical vapor deposition and thermal decomposition on the SiC(000\underline {1}) surface. How electrons are scattered by those grain boundaries has not been experimentally demonstrated at the nanoscale. Here we report atomic-scale images of grain boundary scattering measured by scanning tunneling potentiometry (STP). Monolayer graphene sheets were synthesized on the SiC(000\underline {1}) surface by thermal decomposition in a background of disilane, using low energy electron microscopy to monitor the graphene thickness during its formation. High resolution scanning tunneling microscopy (STM) reveals graphene grain boundaries and various grain orientations, and STP shows variations in voltage across grains and terraces as current flows across the graphene layer. We have identified two types of grain boundary. One shows a trench structure in STM images; potential mapping shows prominent potential drops. These boundaries between grains appear to be weak links and the dominant scattering locations. The other type of boundary shows a continuous lattice between the grains, with periodic dislocations accommodating the grain misorientation. Potential mapping indicates much weaker scattering despite the grain misorientation. We will discuss transport in polycrystalline graphene based on these measurements.   

Imaging of Electron Beam Induced Current in Epitaxial graphene

It has been observed that there forms a Schottky junction between graphene and SiC in epitaxial graphene due to the work function difference. As a result, it is viable to apply the electron beam induced current (EBIC) technique on epitaxial graphene due to the fact that it needs a built-in field and ample electron generation volume to generate EBIC. EBIC is an important characterization technique, which identifies electrically active impurities/defects, detects local built-in field, and measures minority carrier diffusion length. In this paper, we use a FEI SEM equipped with a current amplifier to investigate the spatial mapping of EBIC. The incident electron beam generates excited electron-hole pairs in SiC and the minority carriers are collected through the Schottky junction before flowing into graphene. EBIC imaging reveals mesoscopic domains of bright and dark contrast areas due to local EBIC polarity and magnitude, which is believed to be the result of spatial fluctuation in the carrier density in graphene. We also investigate the electron energy dependence, which modulates the EBIC magnitude. With an analytical drift-diffusion current model, we are able to extract the minority carrier diffusion length in the SiC, which is on the order of micro meter.   

Writing Graphene Nanoribbons on SiC by Selective Graphitization

We describe a new technique for selective graphene growth onto 4H- and 6H-SiC by ion implan- tation. The presented technique is as easy as patterning (ion implanting) regions where graphene layers are desired followed by annealing to 100 C below the graphitization temperature (T$_{G})$ of SiC. We find that ion implantation of SiC lowers the T$_{G}$ of SiC, allowing selective graphene growth at temperatures below the T$_{G}$ of pristine SiC and above T$_{G}$ of implanted SiC. Presented results provide a new technique to pattern device structures ranging from nanometers to microns in size without using conventional lithography and chemical processing.   

Nucleation of uniform mono- and bilayer epitaxial graphene on SiC(000$\overline{1}$)

Early stage of epitaxial graphene growth on SiC(000$\overline{1}$) has been investigated. Using the confinement controlled sublimation (CCS) method, we has achieved well controlled growth and been able to see the formation of mono- and bilayer graphene islands. The growth features reveal the intriguing growth mechanism. In particular, a new ``stepdown'' growth mode has been identified. Graphene can propagate tens of micrometers across many SiC steps, while, most importantly, step bunching is avoided and the initial regular stepped SiC surface morphology is preserved. The stepdown growth demonstrates a route towards uniform epitaxial graphene in wafer size without sacrificing the initial substrate surface morphology.