Papers by Author: Charles R. Eddy

Abstract: The electrical characteristics of oxygen functionalized epitaxial graphene and Ti/Au metal contact interfaces were systematically investigated as a function of temperature. As the temperature was increased from 300 K to 673 K, the contact resistance and the sheet resistance decreased by 75% and 33%, respectively. The resistance of oxygen functionalized graphene vs temperature exhibited Arrhenius type behavior with activation energy of 38 meV. The results showed no hysteresis effects in resistance measurements over the temperatures studied here, suggesting the contact interfaces remain stable at high temperatures.

Abstract: We investigated the chemical sensing mechanism of epitaxial graphene grown on 6H-SiC (0001) to different polar solvents and their behavior at higher temperatures. We show that at 300 K the sensitivity of the graphene sensor increases exponentially with the dipole moment of a solvent and decreases significantly as the temperature increased to 425 K. Using electrical measurements, we also show that graphene can effectively discriminate between polar protic and polar aprotic solvents with the shift in device electrical resistance at 300 K.

Abstract: In this work, the treatment of epitaxial graphene on SiC using electron beam generated plasmas produced in mixtures of argon and oxygen is demonstrated. The treatment imparts oxygen functional groups on the surface with concentrations ranging up to about 12 at.%, depending on treatment parameters. Surface characterization of the functionalized graphene shows incorporation of oxygen to the lattice by disruption of ∏-bonds, and an altering of bulk electrical properties.

Abstract: Graphene, a 2D material, has motivated significant research in the study of its in-plane charge carrier transport in order to understand and exploit its unique physical and electrical properties. The vertical graphene-semiconductor system, however, also presents opportunities for unique devices, yet there have been few attempts to understand the properties of carrier transport through the graphene sheet into an underlying substrate. In this work, we investigate the epitaxial graphene/4H-SiC system, studying both p and n-type SiC substrates with varying doping levels in order to better understand this vertical heterojunction.

Abstract: Integration of patterned ballast resistance into the anode of SiC PiNs is a solution to the dilemma of negative dV_f /dT for such diodes. In fabricated 4H-SiC PiN diodes, we demonstrate a cross-over from negative to positive temperature coefficient for current densities as low as 80 A/cm². Adjusting the percentage of the patterned anode area, the positive or neutral dV_f /dT can be achieved over a wide current-density range without substantial penalty in the forward voltage drop. This characteristic is crucial for high-power SiC packages with ganged-parallel rectifier arrays.

Abstract: The growth of epitaxial graphene on C-face 6H-SiC substrates is investigated using pro-cess conditions that can form small, local areas of graphene. The thickness of SiC lost to Si sublimation is not completely countered by the thickness of the resulting graphene and so graphene-covered basins (GCBs) are formed. The GCBs are most likely nucleated at threading dislocations from the substrate. The GCB morphology exhibits ridges, similar to those found on continuous films. The GCBs expand through erosion of the surrounding SiC substrate walls, eventually coalescing into continuous films. The ratio of the Raman D and G peaks was used to estimate the crystallite length scale and it was found to be about 200 nm for small GCBs and > 1 m for continuous films.

Abstract: Doubly-implanted SiC vertical MOSFETs were fabricated displaying a blocking voltage of 4.2kV and a specific on-resistance of 23 mΩ-cm2, on a 4.5mm x 2.25mm device. Design variations on smaller (1.1mm x 1.1mm) devices showed on-resistance as low as 17 mΩ-cm2 with a blocking voltage of 3.3kV. Analysis is presented of the on-resistance and temperature dependence (up to 175°C), as well as switching performance. Switching tests taken at 1000V and 6A showed turn-on and turn-off transients of approximately 20-40ns.

Abstract: The effects of proton irradiation on uv 4H-SiC single photon avalanche photodiodes (SPADs) are reported. The SPADs, grown by chemical vapor deposition, were designed for uv operation with dark count rates (DCR) of about 30 kHz and single photon detection efficiency (SPDE) of 4.89%. The SPADs were irradiated with 2 MeV protons to a fluence of 1012 cm-2. After irradiation, the I-V characteristics show forward voltage (<1.9 V) generation-recombination currents 2 to 3 times higher than before irradiation. Single photon counting measurements imply generation-recombination centers created in the band gap after irradiation. For threshold voltage ranging from 23 to 26 mV, the 4H-SiC SPAD showed low DCR (<54 kHz) and high SPDE (>1%) after irradiation. The SPADs demonstrated proton radiation tolerance for geosynchronous space applications.

Abstract: Homo- and heteroepitaxial 3C-SiC layers were grown on 4H-SiC step-free mesas. The yields of smooth, defect-free mesas were ~ 17% for both intentionally and unintentionally doped films, while those with screw dislocations and multiple stepped surfaces were ~ 22%. The electronic and structural properties of the mesas were found on a micrometer-sized length scale using µ-PL and µ-Raman, respectively. 3C-SiC mesas were found to have complete 3C-SiC coverage with some of the mesas having electronic defects, while other mesas were found to be defect-free.

Abstract: The effectiveness of an in-situ growth interrupt in nitrogen doped 8° off-cut epilayers was investigated using ultraviolet photoluminescence imaging. Low-doped n-type epilayers (<1016 cm-3) exhibited an abrupt increase in BPD to TED conversion at the growth interrupt and achieved 96-99% conversion overall (< 10 BPDs/cm-2), while high-doped epilayers had minimal conversion at the interrupt (< 1%) and overall (< 30%). This large discrepancy suggests nitrogen prohibits or alters the conversion mechanism at the growth interrupt. Therefore, a novel SEM technique was developed to "freeze-in" the interface morphology and help elucidate the conversion mechanism. Preliminary results suggest that preferential etching at the point of BPD intersection with the surface is greatly reduced in highly doped layers, which inhibits the conversion mechanism.