Randall Cavalero

Pennsylvania State University, University Park, Maryland, United States

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Publications (10)38.99 Total impact

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    ABSTRACT: 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.
    Materials Science Forum 05/2012; 717-720:669-674. DOI:10.4028/www.scientific.net/MSF.717-720.669
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    ABSTRACT: The fundamental structural properties of multilayer epitaxial graphene (MEG) on C-face SiC(000) 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(000) substrate.
    Applied Physics Letters 01/2012; 100(3). DOI:10.1063/1.3678021 · 3.52 Impact Factor
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    ABSTRACT: 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.
    Nano Letters 08/2011; 11(9):3875-80. DOI:10.1021/nl2019855 · 13.03 Impact Factor
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    ABSTRACT: 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 (100). 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(10n).
    Applied Physics Letters 06/2011; 98(22):222109-222109-3. DOI:10.1063/1.3597356 · 3.52 Impact Factor
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    ABSTRACT: The practicality and success of a graphene technology depends on the ability to regularly and controllably synthesize graphene; integrate it with metals and dielectrics; and to develop device designs that take advantage of graphene's unique properties. We demonstrate graphene synthesis on SiC(0001) and Sapphire with 1.5% variation in sheet resistance across 100mm wafers. Hall mobility measurements indicate that direct growth of graphene on sapphire leads to a 2x increase in mobility (2200 cm^2/Vs) compared to silicon sublimation from SiC(0001). Additionally, we have developed high quality ohmic contacts to graphene, which improves the contact resistance by nearly 6000x (5x10-8 Ohm-cm^2) compared to untreated metal/graphene interfaces. Finally, we discuss integration of ultra-thin high-k dielectrics and their impact on graphene transport. Atomic layer deposited oxide heterostructures (seed not equal to overlayer) have deleterious effects on Hall mobility while homostructures lead to an increase in Hall mobility. Importantly, 5nm thick EBPVD HfO2 gate dielectrics are successfully demonstrated and show improved Hall mobility, on-off ratio, and transconductance relative to Al2O3 gates and heterostructure gates.
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    ABSTRACT: 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 6×6R30° 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.
    Applied Physics Letters 11/2010; 97(20):201905-201905-3. DOI:10.1063/1.3517505 · 3.52 Impact Factor
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    ABSTRACT: 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.
    ACS Nano 12/2009; 4(1):153-8. DOI:10.1021/nn901248j · 12.03 Impact Factor
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    ABSTRACT: This paper has been withdrawn due to the adherance to the double submission policies of a refereed journal. Our apologies. Comment: Includes supplimental material
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    ABSTRACT: A novel halide chemical vapor deposition process has been developed for growth of single crystal 6H and 4H SiC boules. This process takes advantage of the thermal stability of halogenated precursors and a unique reactor design to produce SiC crystals up to 75 mm in diameter at growth rates up to 250 mm/h. Growth rate was significantly improved by using CH4 instead of C3H8 as the carbon source gas. Growth of the 6H and 4H polytypes was evaluated as a function of seed polytype, off-cut and surface polarity at a growth temperature of 2020 °C, which is well above typical epitaxy conditions and well below typical sublimation growth conditions. The effect of the substrate and nitrogen flow rate on nitrogen incorporation were characterized by secondary ion mass spectrometry. Radial uniformity of doping was assessed by mercury probe capacitance–voltage measurements. Bulk crystals with an average boron concentration of 1.5e15 atoms/cm3 and nitrogen doping level above 1.0e19 atoms/cm3 were readily achieved. High-resolution X-ray diffraction showed that growth of the 4H polytype requires growth on the carbon face of 4H seed crystals.
    Journal of Crystal Growth 01/2006; DOI:10.1016/j.jcrysgro.2005.11.044 · 1.69 Impact Factor
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    ABSTRACT: Thermodynamic analysis of the effects of hydrogen addition to the growth ambient during physical vapor transport growth of SiC is presented. In the presence of hydrogen, the efficiency of carbon transport should greatly improve due to the interaction between hydrogen and graphite resulting in formation of hydrocarbons and due to more congruent evaporation of the SiC source material. The changes induced by hydrogen are more pronounced at lower growth temperatures and higher background pressures. A temperature range of 2000–2350°C and total pressures of 10–100Torr were considered. Among the consequences of the increased C/Si ratio in the vapor phase, the marked decrease of the nitrogen incorporation efficiency and the suppression of formation of deep traps related to Si-rich growth conditions are expected to be the most prominent. Growth experiments with hydrogen concentrations in the 0–50% range show that indeed the nitrogen concentration decreases by several times and the density of all electron traps in SiC becomes lower by about an order of magnitude with the addition of hydrogen.
    Journal of Crystal Growth 01/2006; 287(2):339-343. DOI:10.1016/j.jcrysgro.2005.11.022 · 1.69 Impact Factor