Randall Cavalero

Pennsylvania State University, University Park, Maryland, United States

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Publications (10)36.38 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 · 3.30 Impact Factor
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    ABSTRACT: 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.
    Electron Devices Meeting (IEDM), 2012 IEEE International; 01/2012
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    ABSTRACT: In recent years, hexagonal boron nitride (h-BN) has gained interest as a material for use in graphene based electronics, where its ultra-smooth two-dimensional structure, lack of dangling bonds, and high energy surface optical phonon modes are desirable when considering the effect of dielectric materials in introducing additional sources of scattering for carriers within graphene. Initial work has indicated that use of h-BN in place of SiO2 supporting substrates can lead to 2-3x improvements in device performance [1,2], suggesting that h-BN may be an excellent choice as top-gate dielectric for graphene devices. In this work, we integrate h-BN with quasi-freestanding graphene (QFEG) for the first time and demonstrate a 2x improvement in radio frequency (RF) performance and the highest fT·Lg product yet reported for h-BN integrated graphene devices (25 GHz·μm).
    Device Research Conference (DRC), 2012 70th Annual; 01/2012
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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 · 13.59 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 · 3.30 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 · 3.30 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 · 12.88 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

Publication Stats

126 Citations
36.38 Total Impact Points


  • 2010–2012
    • Pennsylvania State University
      • • Center for Electro-Optics (EOC)
      • • Department of Materials Science and Engineering
      University Park, Maryland, United States
  • 2009
    • William Penn University
      Worcester, Massachusetts, United States