Glenn G. Jernigan

Chemical Vapor Sensing with Monolayer MoS2

Two-dimensional materials such as graphene show great potential for future nanoscale electronic devices. The high surface-to-volume ratio is a natural asset for applications such as chemical sensing, where perturbations to the surface resulting in charge redistribution are readily manifested in the transport characteristics. Here we show that single monolayer MoS(2) functions effectively as a chemical sensor, exhibiting highly selective reactivity to a range of analytes and providing sensitive transduction of transient surface physisorption events to the conductance of the monolayer channel. We find strong response upon exposure to triethylamine, a decomposition product of the V-series nerve gas agents. We discuss these results in the context of analyte/sensor interaction in which the analyte serves as either an electron donor or acceptor, producing a temporary charge perturbation of the sensor material. We find highly selective response to electron donors and little response to electron acceptors, consistent with the weak n-type character of our MoS(2). The MoS(2) sensor exhibits a much higher selectivity than carbon nanotube-based sensors.

Epitaxial-Graphene RF Field-Effect Transistors on Si-Face 6H-SiC Substrates

We report dc and the first-ever measured small-signal radio-frequency (RF) performance of epitaxial-graphene RF field-effect transistors (FETs), where the epitaxial-graphene layer is formed by graphitization of 2-in-diameter Si-face semi-insulating 6H-SiC (0001) substrates. The gate is processed with a metal gate on top of a high-k Al2 O3 gate dielectric deposited via an atomic-layer-deposition method. With a gate length (Lg) of 2 mum and an extrinsic transconductance of 148 mS/mm, the extrinsic current-gain cutoff frequency (fT) is measured as 4.4 GHz, yielding an extrinsic fT ldr Lg of 8.8 GHz middot mum. This is comparable to that of Si NMOS. With graphene FETs fabricated in a layout similar to those of Si n-MOSFETs, on-state current density increases dramatically to as high as 1.18 A/mm at Vds = 1 V and 3 A/mm at Vds = 5 V. The current drive level is the highest ever observed in any semiconductor FETs.

Hall effect mobility of epitaxial graphene grown on silicon carbide

Epitaxial graphene (EG) films were grown in vacuo by silicon sublimation from the (0001) and (0001¯) faces of 4H-SiC and 6H-SiC. Hall effect mobilities and sheet carrier densities of the films were measured at 300 and 77 K and the data depended on the growth face. About 40% of the samples exhibited holes as the dominant carrier, independent of face. Generally, mobilities increased with decreasing carrier density, independent of carrier type and substrate polytype. The contributions of scattering mechanisms to the conductivities of the films are discussed. The results suggest that for near-intrinsic carrier densities at 300 K epitaxial graphene mobilities will be ∼150 000 cm2 V−1 s−1 on the (0001¯) face and ∼5800 cm2 V−1 s−1 on the (0001) face.

Technique for the dry transfer of epitaxial graphene onto arbitrary substrates.

To make graphene technologically viable, the transfer of graphene films to substrates appropriate for specific applications is required. We demonstrate the dry transfer of epitaxial graphene (EG) from the C-face of 4H-SiC onto SiO(2), GaN and Al(2)O(3) substrates using a thermal release tape. Subsequent Hall effect measurements illustrated that minimal degradation in the carrier mobility was induced following the transfer process in lithographically patterned devices. Correspondingly, a large drop in the carrier concentration was observed following the transfer process, supporting the notion that a gradient in the carrier density is present in C-face EG, with lower values being observed in layers further removed from the SiC interface. X-ray photoemission spectra collected from EG films attached to the transfer tape revealed the presence of atomic Si within the EG layers, which may indicate the identity of the unknown intrinsic dopant in EG. Finally, this transfer process is shown to enable EG films amenable for use in device fabrication on arbitrary substrates and films that are deemed most beneficial to carrier transport, as flexible electronic devices or optically transparent contacts.

Top-Gated Epitaxial Graphene FETs on Si-Face SiC Wafers With a Peak Transconductance of 600 mS/mm

In this letter, we present state-of-the-art performance of top-gated graphene n-FETs and p-FETs fabricated with epitaxial graphene layers grown on Si-face 6H-SiC substrates on 50-mm wafers. The current-voltage characteristics of these devices show excellent saturation with on-state current densities (I_on) of 0.59 A/mm at V_ds = 1 V and 1.65 A/mm at V_ds = 3 V. I_on/I_off ratios of 12 and 19 were measured at V_ds = 1 and 0.5 V, respectively. A peak extrinsic g_m as high as 600 mS/mm was measured at V_ds = 3.05 V, with a gate length of 2.94 ¿m. The field-effect mobility versus effective electric field (E_eff) was measured for the first time in epitaxial graphene FETs, where record field-effect mobilities of 6000 cm²/V·s for electrons and 3200 cm²/V·s for holes were obtained at E_eff ~ 0.27 MV/cm .

Correlating Raman Spectral Signatures with Carrier Mobility in Epitaxial Graphene: A Guide to Achieving High Mobility on the Wafer Scale

We report a direct correlation between carrier mobility and Raman topography of epitaxial graphene (EG) grown on silicon carbide (SiC). We show the Hall mobility of material on SiC(0001) is highly dependent on thickness and monolayer strain uniformity. Additionally, we achieve high mobility epitaxial graphene (18100 cm(2)/(V s) at room temperature) on SiC(0001) and show that carrier mobility depends strongly on the graphene layer stacking.

Quantum linear magnetoresistance in multilayer epitaxial graphene.

We report the first observation of linear magnetoresistance (LMR) in multilayer epitaxial graphene grown on SiC. We show that multilayer epitaxial graphene exhibits large LMR from 2.2 K up to room temperature and that it can be best explained by a purely quantum mechanical model. We attribute the observation of LMR to inhomogeneities in the epitaxially grown graphene film. The large magnitude of the LMR suggests potential for novel applications in areas such as high-density data storage and magnetic sensors and actuators.

The kinetics and mechanism of scanned probe oxidation of Si

We report measurements of the kinetics of scanned probe oxidation under conditions of high humidity and pulsed bias. For a hydrophobic Si surface the oxidation rate for short pulse times (∼10 ms) is controlled by the density of H2O molecules in the ambient humidity surrounding the tip-sample interface. At longer pulse times (∼0.1 s) liquid H2O bridges this interface and the maximum oxidation rate increases by a factor of ∼104 because of the increased density of H2O molecules. We propose that the rate-limiting step of the oxidation process is the production of O anions from the ambient humidity.

Comparison of Epitaxial Graphene on Si-face and C-face 4H SiC Formed by Ultrahigh Vacuum and RF Furnace Production

We present X-ray photoelectron spectroscopy, van der Pauw Hall mobilities, low-temperature far-infrared magneto transmission (FIR-MT), and atomic force microscopy (AFM) results from graphene films produced by radiative heating in an ultrahigh vacuum (UHV) chamber or produced by radio frequency (RF) furnace annealing in a high vacuum chemical vapor deposition system on Si- and C-face 4H SiC substrates at 1200-1600 degrees C. Although the vacuum level and heating methods are different, graphene films produced by the two methods are chemically similar with the RF furnace annealing typically producing thicker graphene films than UHV. We observe, however, that the formation of graphene on the two faces is different with the thicker graphene films on the C-face RF samples having higher mobility. The FIR-MT showed a 0(-1) --> 1(0) Landau level transition with a square root B dependence and a line width consistent with a Dirac fermion with a mobility >250,000 cm(2) x V(-1) x s(-1) at 4.2 K in a C-face RF sample having a Hall-effect carrier mobility of 425 cm(2) x V(-1) x s(-1) at 300 K. AFM shows that graphene grows continuously over the varying morphology of both Si and C-face substrates.

Low-Phase-Noise Graphene FETs in Ambipolar RF Applications

In this letter, we present both the 1/f noise and phase noise performance of top-gated epitaxial graphene field-effect transistors (FETs) in nonlinear circuit applications for the first time. In the case of frequency doublers, the fundamental signal is suppressed by 25 dB below the second harmonic signal. With a phase noise of -110 dBc/Hz measured at a 10-kHz offset, a carrier-to-noise degradation (ΔCNR) of 6 dB was measured for the frequency doubler. This implies noiseless frequency multiplication without additional 1/f noise upconversion during the nonlinear process. The frequency multiplication was demonstrated above the gigahertz range. The 1/f noise of top-gated epitaxial graphene FETs is comparable or lower than that of exfoliated graphene FETs.