Randall Cavalero

Nucleation of epitaxial graphene on SiC(0001).

A promising route for the synthesis of large-area graphene, suitable for standard device fabrication techniques, is the sublimation of silicon from silicon carbide at elevated temperatures (>1200 degrees C). Previous reports suggest that graphene nucleates along the (110n) plane, known as terrace step edges, on the silicon carbide surface. However, to date, a fundamental understanding of the nucleation of graphene on silicon carbide is lacking. We provide the first direct evidence that nucleation of epitaxial graphene on silicon carbide occurs along the (110n) plane and show that the nucleated graphene quality improves as the synthesis temperature is increased. Additionally, we find that graphene on the (110n) plane can be significantly thicker than its (0001) counterpart and appears not to have a thickness limit. Finally, we find that graphene along the (110n) plane can contain a high density of structural defects, often the result of the underlying substrate, which will undoubtedly degrade the electronic properties of the material. Addressing the presence of non-uniform graphene that may contain structural defects at terrace step edges will be key to the development of a large-scale graphene technology derived from silicon carbide.

Epitaxial graphene transistors: enhancing performance via hydrogen intercalation.

We directly demonstrate the importance of buffer elimination at the graphene/SiC(0001) interface for high frequency applications. Upon successful buffer elimination, carrier mobility increases from an average of 800 cm(2)/(V s) to >2000 cm(2)/(V s). Additionally, graphene transistor current saturation increases from 750 to >1300 mA/mm, and transconductance improves from 175 mS/mm to >400 mS. Finally, we report a 10× improvement in the extrinsic current gain response of graphene transistors with optimal extrinsic current-gain cutoff frequencies of 24 GHz.

Structure of few-layer epitaxial graphene on 6H-SiC(0001) at atomic resolution

Using directly interpretable atomic-resolution cross-sectional scanning transmission electron microscopy, we have investigated the structure of few-layer epitaxial graphene (EG) on 6H-SiC(0001). We show that the buried interface layer possesses a lower average areal density of carbon atoms than graphene, indicating that it is not a graphenelike sheet with the 63×63R30° structure. The EG interlayer spacings are found to be considerably larger than that of the bulk graphite and the surface of the SiC(0001) substrate, often treated as relaxed, is found to be strained. Discontinuity of the graphene layers above the SiC surface steps is observed, in contradiction with the commonly believed continuous coverage.

Effects of substrate orientation on the structural and electronic properties of epitaxial graphene on SiC(0001)

We investigate graphene transport and structural properties as a function of silicon carbide (SiC) wafer orientation. Terrace step edge density is found to increase with wafer misorientation from SiC(0001). This results in a monotonic increase in average graphene thickness, as well as a 30% increase in carrier density and 40% decrease in mobility up to 0.45° miscut toward (11¯00). Beyond 0.45°, average thickness and carrier density continues to increase; however, carrier mobility is similar to low-miscut angles, suggesting that the interaction between graphene and SiC(0001) may be fundamentally different that of graphene/SiC(11¯0n).

Properties of 6H–SiC crystals grown by hydrogen-assisted physical vapor transport

Effects of hydrogen addition to the growth ambient during physical vapor transport (PVT) growth of 6H–SiC were investigated using secondary ion mass spectrometry, deep level transient spectroscopy, and Hall effect measurement. The background nitrogen concentration and the free electron density decrease with increasing hydrogen content. The formation of electron traps (activation energies of 0.4eV, 0.6eV, 0.7eV, 0.9eV, and 1eV) was also strongly suppressed. The above results are interpreted as a consequence of hydrocarbon formation produced by the reaction of hydrogen with the SiC source and the graphite parts of the furnace. This leads to more congruent evaporation of SiC and the shift of the gas phase and the SiC deposit stoichiometry due to less Si-rich conditions than in standard PVT growth.

Record high conversion gain ambipolar graphene mixer at 10GHz using scaled gate oxide

This work presents a detailed study of the graphene RF mixer, comparing ambipolar and drain mixing for the first time. Output characteristics of the graphene transistor are analyzed and the effects of device scaling and interface state density on mixer performance are explained. We design a graphene RF transistor with gate length 750 nm, width 20 μm, and equivalent oxide thickness (EOT) ~2.5 nm to achieve record high conversion gain of -14 and -16 dB at LO power 0 dBm at 4.2 and 10 GHz, respectively, 100x higher than previously reported ambipolar mixing.

Epitaxial graphene on SiC(0001¯): Stacking order and interfacial structure

The fundamental structural properties of multilayer epitaxial graphene (MEG) on C-face SiC(0001¯) were revealed in a straightforward manner using cross-sectional transmission electron microscopy (TEM) and scanning TEM (STEM). The AB-stacking and the azimuthal rotational disorder of the graphene layers were directly identified by selected area electron diffraction and high-resolution TEM. The directly interpretable STEM revealed that the interlayer spacing between the first graphene layer and the top SiC bilayer is substantially larger than that of the bulk graphite. Such a large interlayer spacing combined with the regional partially decomposed top bilayers of the SiC substrate provides a plausible explanation to the weak bonding between the MEG film and the SiC(0001¯) substrate.

High Performance RF FETs Using High-k Dielectrics on Wafer-Scale Quasi-Free-Standing Epitaxial Graphene

We explore the effect of processing on graphene/metal ohmic contact resistance, the integration of high-κ dielectric seeds and overlayers on carrier transport in epitaxial graphene, and directly demonstrate the importance of buffer elimination at the graphene/SiC(0001) interface for high frequency applications. We present a robust method for forming high quality ohmic contacts to graphene, which improves the contact resistance by nearly 6000x compared to untreated metal/graphene interfaces. Optimal specific contact resistance for treated Ti/Au contacts is found to average -7 Ohm-cm2. Additionally, we introduce a novel seeding technique for depositing dielectrics by ALD that utilizes direct deposition of high-κ seed layers and can lead to an increase in Hall mobility up to 70% from as-grown. Finally, we demonstrate that buffer elimination at the graphene/SiC(0001) results in excellent high frequency performance of graphene transistors with fT > 130 GHz at 75 nm gate lengths.