D. K. Gaskill

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 ∼150u2009000u2002cm2u2009V−1u2009s−1 on the (0001¯) face and ∼5800u2002cm2u2009V−1u2009s−1 on the (0001) face.

Morphology characterization of argon-mediated epitaxial graphene on C-face SiC

Epitaxial graphene layers were grown on the C-face of 4H–SiC and 6H–SiC using an argon-mediated growth process. Variations in growth temperature and pressure were found to dramatically affect the morphological properties of the layers. The presence of argon during growth slowed the rate of graphene formation on the C-face and led to the observation of islanding. The similarity in the morphology of the islands and continuous films indicated that island nucleation and coalescence is the growth mechanism for C-face graphene.

Epitaxial Graphene Growth on SiC Wafers

An in vacuo thermal desorption process has been accomplished to form epitaxial graphene (EG) on 4H- and 6H-SiC substrates using a commercial chemical vapor deposition reactor. Correlation of growth conditions and the morphology and electrical properties of EG are described. Raman spectra of EG on Si-face samples were dominated by monolayer thickness. This approach was used to grow EG on 50 mm SiC wafers that were subsequently fabricated into field effect transistors with fmax of 14 GHz.

Improvement of Morphology and Free Carrier Mobility through Argon-Assisted Growth of Epitaxial Graphene on Silicon Carbide

Graphene was epitaxially grown on both the C- and Si-faces of 4H- and 6H-SiC(0001) under an argon atmosphere and under high vacuum conditions. Following growth, samples were imaged with Nomarski interference contrast and atomic force microscopies and it was found that growth under argon led to improved morphologies on the C-face films but the Si-face films were not significantly affected. Free carrier transport studies were conducted through Hall effect measurements, and carrier mobilities were found to increase and sheet carrier densities were found to decrease for those films grown under argon as compared to high vacuum conditions. The improved mobilities and concurrent decreases in sheet carrier densities suggest a decrease in scattering in the films grown under argon.

4H-SiC Visible-Blind Single-Photon Avalanche Diode for Ultraviolet Detection at 280 and 350 nm

This paper reports on a 4H-SiC single-photon avalanche diode (SPAD) operating at UV wavelengths of 280 and 350 nm. The SPAD shows low dark currents of 20 and 57 fA at 80 V and 90% breakdown voltage, respectively. The quantum efficiency (QE) reaches its peak of 43% at 270 nm and is < 0.007% at 400 nm, indicating a high UV-to-visible rejection ratio of > 6100. The 4H-SiC SPAD shows a fast self-quenching and a high photon count rate of 1.44 MHz in the passive-quenching mode. At the wavelength of 280 nm, a single-photon detection efficiency (SPDE) of 2.83 % with a low dark count rate of 22 kHz is achieved at the reverse bias of 116.8 V. The SPDE at 350 nm is lower, which is 0.195%, owing to the correspondingly smaller QE. Optimization measurements were conducted on SPDE as a function of voltage bias and signal output threshold.

Development toward Wafer-Scale Graphene RF Electronics

We will present recent development of graphene FET technology on a wafer scale, including epitaxial graphene growth, device fabrication and characterization. The epitaxial growth of graphene on 2-inch wafers were fabricated via graphitization of Siface SiC(0001) substrates. The sheet electron carrier density of these layers were typically 10-13 cm2 at room temperature and had mobility of ~ 1500 cm2 V-1 s-1 or higher. Graphene FETs were fabricated with source and drain non-alloyed ohmic metal schemes. Metal gates were used on top of atomic-layer-deposited high-k (Al2O3) gate dielectric layer. DC and RF performance of the worlds first epitaxial graphene RF FETs is presented.

Plasma-assisted atomic layer deposition of nanolaminates for gate dielectric applications

Thin [(x)Al2O3 + (y)TiO2] nanolaminates (NLs) films of various TiO2 and Al2O3 volume fractions were deposited on n-Si substrates at 250u2009°C using remote plasma-assisted atomic layer deposition. While the overall thickness of the dielectric was held relatively constant at ∼16u2009nm, the relative ratio of Al2O3 to TiO2 in the NL was varied by changing the number of deposition cycles of each component. This permitted the evaluation of changes in the dielectric constant κ, index of refraction Nf, optical band gap, Eg, and the electrical performance of the resulting oxides. Capacitance–voltage and current–voltage results on 100u2009μm diameter circular capacitors were obtained. The data reveals that the high-content TiO2 films show limited evidence of oxide charge trapping and relatively large dielectric constants (k ∼ 15) with reduced reverse-biased leakage current, whereas the high-content Al2O3 films offer a larger optical band-gap and excellent insulating character with reduced leakage currents. In addition, the a...

Large-Area Epitaxial Graphene: Effect of Strain and Thickness on Electronic Properties

The recent success of graphene transistor operation in the giga-hertz range has solidified the potential of this material for high speed electronic applications. Realization of a graphene technology on the production scale; however, requires the ability to synthesize large area graphene, and rapidly characterize the materials structural and electronic quality. We report a direct link between carrier mobility and Raman topography of epitaxial graphene grown on silicon carbide. We have examined epitaxial graphene with mobility values of 25 - 1100 cm2/V-s, and show that the Hall mobility of epitaxial graphene on the Si-face of SiC (SiC(0001)) is not only highly dependent on thickness uniformity, but also on mono-layer strain uniformity. It is not until the thickness and strain uniformity is approaches 50% of the device width that one is able to achieve mobility values higher than 1000 cm2/V-s.

Plasma-Modified, Epitaxial Fabricated Graphene on SiC for the Electrochemical Detection of TNT

Using square wave voltammetry, we show an increase in the electrochemical detection of trinitrotoluene (TNT) with a working electrode constructed from plasma modified graphene on a SiC surface vs. unmodified graphene. The graphene surface was chemically modified using electron beam generated plasmas produced in oxygen or nitrogen containing backgrounds to introduce oxygen or nitrogen moieties. The use of this chemical modification route enabled enhancement of the electrochemical signal for TNT, with the oxygen treatment showing a more pronounced detection than the nitrogen treatment. For graphene modified with oxygen, the electrochemical response to TNT can be fit to a two-site Langmuir isotherm suggesting different sites on the graphene surface with different affinities for TNT. We estimate a limit of detection for TNT equal to 20 ppb based on the analytical standard S/N ratio of 3. In addition, this approach to sensor fabrication is inherently a high-throughput, high-volume process amenable to industrial applications. High quality epitaxial graphene is easily grown over large area SiC substrates, while plasma processing is a rapid approach to large area substrate processing. This combination facilitates low cost, mass production of sensors.

Universal conformal ultrathin dielectrics on epitaxial graphene enabled by a graphene oxide seed layer

The amphiphilic nature of graphene oxide (GO) is exploited as a seed layer to facilitate the ultrathin and conformal high-κ metal oxide (MOX) deposition on defect-free epitaxial graphene (EG) by atomic layer deposition (ALD). Three different high-κ metal oxides (Al2O3, HfO2 and TiO2) with various thicknesses (4–20u2009nm) were grown on ultrathin (1.5u2009nm) GO seed layers on EG. The quality of such dielectrics was examined by fabricating various metal-insulator-graphene (MIG) type devices. For MIG tunnel devices, on-off ratios of 104 and 103 were obtained for 4u2009nm Al2O3 and HfO2 dielectric layers, respectively. Additionally, no defect/trap assisted conduction behavior was observed. Graphene field effect transistors (GFETs) with bi-layer metal oxide stack (6u2009nm TiO2/14u2009nm HfO2) demonstrated a peak on-state current of 0.16u2009A/mm, an on-resistance of 6.8u2009Ωu2009mm, an Ion/Ioff ratio of ∼4, and a gate leakage current below 10u2009pA/mm at Vdsu2009=u20091u2009V and Vgsu2009=u20094u2009V. Capacitance-voltage measurement of the same GFETs exhibited a l...