Monolayer Epitaxial Graphene Research Articles

Chemical-doping-driven crossover from graphene to "ordinary metal" in epitaxial graphene grown on SiC.

Atmospheric chemical doping can be used to modify the electronic properties of graphene. Here we report that the chemical atmospheric doping (derived from air, oxygen and water vapor) of low-carrier-density monolayer epitaxial graphene on SiC can be readily tuned by a simple low-temperature (T ≤ 450 K), in situ vacuum gentle heating method. Interestingly, such an approach allows, for the first time, the observation of a crossover from graphene (μt/μq ≈ 2) to an "ordinary metal" (μt/μq ≈ 1) with decreasing carrier density, where μt and μq are transport mobility and quantum mobility, respectively. In the low carrier density limit, our results are consistent with the theoretical prediction that μt is inversely proportional to charged impurity density. Our data also suggest that atmospheric chemical doping can be used to vary intervalley scattering in graphene which plays a crucial role in backward scattering events.

Unusual renormalization group (RG) flow and temperature-dependent phase transition in strongly-insulating monolayer epitaxial graphene

By changing the measurement temperature (T), one can vary the effective sample size so as to study the renormalization group (RG) (or T-driven) flow of a semiconductor, a topological insulator, or a graphene device in the complex conductivity plane.

Monolayer graphene/SiC Schottky barrier diodes with improved barrier height uniformity as a sensing platform for the detection of heavy metals.

A vertical diode structure comprising homogeneous monolayer epitaxial graphene on silicon carbide is fabricated by thermal decomposition of a Si-face 4H-SiC wafer in argon atmosphere. Current–voltage characteristics of the graphene/SiC Schottky junction were analyzed by applying the thermionic-emission theory. Extracted values of the Schottky barrier height and the ideality factor are found to be 0.4879 ± 0.013 eV and 1.01803 ± 0.0049, respectively. Deviations of these parameters from average values are smaller than those of previously observed literature data, thereby implying uniformity of the Schottky barrier height over the whole diode area, a stable rectifying behaviour and a good quality of ohmic palladium–graphene contacts. Keeping in mind the strong sensitivity of graphene to analytes we propose the possibility to use the graphene/SiC Schottky diode as a sensing platform for the recognition of toxic heavy metals. Using density functional theory (DFT) calculations we gain insight into the nature of the interaction of cadmium, mercury and lead with graphene as well as estimate the work function and the Schottky barrier height of the graphene/SiC structure before and after applying heavy metals to the sensing material. A shift of the I–V characteristics of the graphene/SiC-based sensor has been proposed as an indicator of presence of the heavy metals. Since the calculations suggested the strongest charge transfer between Pb and graphene, the proposed sensing platform was characterized by good selectivity towards lead atoms and slight interferences from cadmium and mercury. The dependence of the sensitivity parameters on the concentration of Cd, Hg and Pb is studied and discussed.

Alkali doping of graphene: The crucial role of high-temperature annealing

The doping efficiency of lithium deposited at cryogenic temperatures on epitaxial and CVD monolayer graphene has been investigated under ultra-high vacuum conditions. Change of charge carrier density was monitored by gate voltage shift of the Dirac point and by Hall measurements, in low and high doping regimes. It was found that pre-annealing the graphene greatly enhanced the maximum levels of doping that could be achieved: doping saturated at $\Delta n = 2\times 10^{13}$ e$^-$/cm$^2$ without annealing, independent of sample type or previous processing; after a 900 K anneal, the saturated doping rose one order of magnitude to $\Delta n = 2\times 10^{14}$ e$^-$/cm$^2$.

Variable range hopping and nonlinear transport in monolayer epitaxial graphene grown on SiC

We report experimental results on variable range hopping (VRH) and nonlinear transport in monolayer epitaxial graphene. In the linear regime in which the conductance is independent of voltage, the resistance curve derivative analysis method can be used to unequivocally determine whether Mott VRH or Efros–Shklovskii VRH is the dominant transport mechanism in our devices. In the nonlinear regime in which the conductance shows a strong dependence on voltage, we find that our experimental results can be successfully described by existing theoretical models. We suggest that the observed vastly different exponents in the threshold voltage–temperature dependence require further experimental and theoretical studies.

Comparative Study of Monolayer and Bilayer Epitaxial Graphene Field-Effect Transistors on SiC Substrates**Supported by the National Natural Science Foundation of China under Grant No 61306006.

Monolayer and bilayer graphenes have generated tremendous excitement as the potentially useful electronic materials due to their unique features. We report on monolayer and bilayer epitaxial graphene field-effect transistors (GFETs) fabricated on SiC substrates. Compared with monolayer GFETs, the bilayer GFETs exhibit a significant improvement in dc characteristics, including increasing current density IDS, improved transconductance gm, reduced sheet resistance Ron, and current saturation. The improved electrical properties and tunable bandgap in the bilayer graphene lead to the excellent dc performance of the bilayer GFETs. Furthermore, the improved dc characteristics enhance a better rf performance for bilayer graphene devices, demonstrating that the quasi-free-standing bilayer graphene on SiC substrates has a great application potential for the future graphene-based electronics.

Characterization and physical modeling of MOS capacitors in epitaxial graphene monolayers and bilayers on 6H-SiC

Capacitance voltage (CV) measurements are performed on planar MOS capacitors with an Al2O3 dielectric fabricated in hydrogen intercalated monolayer and bilayer graphene grown on 6H-SiC as a function of frequency and temperature. Quantitative models of the CV data are presented in conjunction with the measurements in order to facilitate a physical understanding of graphene MOS systems. An interface state density of order 2 ⋅ 1012 eV−1 cm−2 is found in both material systems. Surface potential fluctuations of order 80-90meV are also assessed in the context of measured data. In bilayer material, a narrow bandgap of 260meV is observed consequent to the spontaneous polarization in the substrate. Supporting measurements of material anisotropy and temperature dependent hysteresis are also presented in the context of the CV data and provide valuable insight into measured and modeled data. The methods outlined in this work should be applicable to most graphene MOS systems.

Buffer-eliminated, charge-neutral epitaxial graphene on oxidized 4H-SiC (0001) surface

Buffer-eliminated, charge-neutral epitaxial graphene (EG) is important to enhance its potential in device applications. Using the first principles Density Functional Theory calculations, we investigated the effect of oxidation on the electronic and structural properties of EG on 4H-SiC (0001) surface. Our investigation reveals that the buffer layer decouples from the substrate in the presence of both silicate and silicon oxy-nitride at the interface, and the resultant monolayer EG is charge-neutral in both cases. The interface at 4H-SiC/silicate/EG is characterized by surface dangling electrons, which opens up another route for further engineering EG on 4H-SiC. Dangling electron-free 4H-SiC/silicon oxy-nitride/EG is ideal for achieving charge-neutral EG.

Structural and Electronic Properties of Nitrogen-Doped Graphene.

We investigate the structural and electronic properties of nitrogen-doped epitaxial monolayer graphene and quasifreestanding monolayer graphene on 6H-SiC(0001) by the normal incidence x-ray standing wave technique and by angle-resolved photoelectron spectroscopy supported by density functional theory simulations. With the location of various nitrogen species uniquely identified, we observe that for the same doping procedure, the graphene support, consisting of substrate and interface, strongly influences the structural as well as the electronic properties of the resulting doped graphene layer. Compared to epitaxial graphene, quasifreestanding graphene is found to contain fewer nitrogen dopants. However, this lack of dopants is compensated by the proximity of nitrogen atoms at the interface that yield a similar number of charge carriers in graphene.

Surface conductance of graphene from non-contact resonant cavity

A method is established to reliably determine surface conductance of single-layer or multi-layer atomically thin nano-carbon graphene structures. The measurements are made in an air filled standard R100 rectangular waveguide configuration at one of the resonant frequency modes, typically at TE103 mode of 7.4543GHz. Surface conductance measurement involves monitoring a change in the quality factor of the cavity as the specimen is progressively inserted into the cavity in quantitative correlation with the specimen surface area. The specimen consists of a nano-carbon-layer supported on a low loss dielectric substrate. The thickness of the conducting nano-carbon layer does not need to be explicitly known, but it is assumed that the lateral dimension is uniform over the specimen area. The non-contact surface conductance measurements are illustrated for a typical graphene grown by chemical vapor deposition process, and for a high quality monolayer epitaxial graphene grown on silicon carbide wafers for which we performed non-gated quantum Hall resistance measurements. The sequence of quantized transverse Hall resistance at the Landau filling factors ν=±6 and ±2, and the absence of the Hall plateau at ν=4 indicate that the epitaxially grown graphene is a high quality mono-layer. The resonant microwave cavity measurement is sensitive to the surface and bulk conductivity, and since no additional processing is required, it preserves the integrity of the conductive graphene layer. It allows characterization with high speed, precision and efficiency, compared to transport measurements where sample contacts must be defined and applied in multiple processing steps.

Linear magnetoresistance in monolayer epitaxial graphene grown on SiC

We have observed classical linear magnetoresistance (LMR), which persists to room temperature, in clean monolayer epitaxial graphene grown on SiC. Such results are consistent with the resistor network model based on density inhomogeneity in a disordered two-dimensional system, though the observed LMR is non-saturating possibly due to formation of a quantum Hall-like state beyond the highest measurement magnetic field. Given the prospect of epitaxial graphene in high-frequency transistors, our experimental data pave the way for integration of magnetic sensing devices with high-frequency devices based on wafer-scale graphene.

Self-Assembly of Graphene Nanoblisters Sealed to a Bare Metal Surface.

The possibility to intercalate noble gas atoms below epitaxial graphene monolayers coupled with the instability at high temperature of graphene on the surface of certain metals has been exploited to produce Ar-filled graphene nanosized blisters evenly distributed on the bare Ni(111) surface. We have followed in real time the self-assembling of the nanoblisters during the thermal annealing of the Gr/Ni(111) interface loaded with Ar and characterized their morphology and structure at the atomic scale. The nanoblisters contain Ar aggregates compressed at high pressure arranged below the graphene monolayer skin that is decoupled from the Ni substrate and sealed only at the periphery through stable C-Ni bonds. Their in-plane truncated triangular shapes are driven by the crystallographic directions of the Ni surface. The nonuniform strain revealed along the blister profile is explained by the inhomogeneous expansion of the flexible graphene lattice that adjusts to envelop the Ar atom stacks.

Insulator-quantum Hall transitionin monolayer epitaxial graphene.

We report on magneto-transport measurements on low-density, large-area monolayer epitaxial graphene devices grown on SiC. We observe temperature (T)-independent crossing points in the longitudinal resistivity ρxx, which are signatures of the insulator-quantum Hall (I-QH) transition, in all three devices. Upon converting the raw data into longitudinal and Hall conductivities σxx and σxy, in the most disordered device, we observed T-driven flow diagram approximated by the semi-circle law as well as the T-independent point in σxy near e2/h. We discuss our experimental results in the context of the evolution of the zero-energy Landau level at low magnetic fields B. We also compare the observed strongly insulating behaviour with metallic behaviour and the absence of the I-QH transition in graphene on SiO2 prepared by mechanical exfoliation.

Charge Trapping in Monolayer and Multilayer Epitaxial Graphene

We have studied the carrier densitiesnof multilayer and monolayer epitaxial graphene devices over a wide range of temperaturesT. It is found that, in the high temperature regime (typicallyT≥ 200 K),ln⁡(n)shows a linear dependence of 1/T, showing activated behavior. Such results yield activation energiesΔEfor charge trapping in epitaxial graphene ranging from 196 meV to 34 meV. We find thatΔEdecreases with increasing mobility. Vacuum annealing experiments suggest that both adsorbates on EG and the SiC/graphene interface play a role in charge trapping in EG devices.

Anomalous thermomagnetic effects in an epitaxial and irradiated graphene monolayer

We study valley-dependent anomalous thermomagnetic effects, including the Nernst and Ettingshausen effects, in a graphene monolayer that is subjected to a staggered sublattice potential and off-resonant circularly polarized light. It is found that a topological phase transition in this system can significantly affect the signs of the Nernst conductivity as well as the Ettingshausen thermal conductivity, which provides an alternative method to characterize the phase transition between band and topological insulators. At the topological phase-transition point, pure valley-polarized electric and heat currents are generated. In contrast to traditional thermomagnetism, an anomalous thermomagnetic figure of merit is formulated and used to characterize conversion efficiency. The theoretical approach, including numerical calculations and analytical treatment, can also be used to study the same properties of other graphenelike materials.

Coincident-site lattice matching during van der Waals epitaxy.

Van der Waals (vdW) epitaxy is an attractive method for the fabrication of vdW heterostructures. Here Sb2Te3 films grown on three different kind of graphene substrates (monolayer epitaxial graphene, quasi freestanding bilayer graphene and the SiC (6√3 × 6√3)R30° buffer layer) are used to study the vdW epitaxy between two 2-dimensionally (2D) bonded materials. It is shown that the Sb2Te3 /graphene interface is stable and that coincidence lattices are formed between the epilayers and substrate that depend on the size of the surface unit cell. This demonstrates that there is a significant, although relatively weak, interfacial interaction between the two materials. Lattice matching is thus relevant for vdW epitaxy with two 2D bonded materials and a fundamental design parameter for vdW heterostructures.

B21-P-07Elucidating structure and thermal reaction of low-dimeansional Bi growth on monolayer epitaxial graphene

The doping graphene with heavy metals can modify the number of carriers and spin-orbit coupling (SOC) can be exploited for novel electronic and spintronic applications. 1-2 This study elucidates the epitaxial growth structure and electronic properties of Bi on monolayer epitaxial graphene (MEG) surface by using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Bi adatoms undergo a structural transition from linear structures at lower Bi coverage to two-dimensional (2D) triangular islands at thicker Bi coverage, which is affected by the corrugated substrate. Upon Bi deposition, a little charge transfer occurs and a characteristic peak can be observed in the tunneling spectrum, reflecting the distinctive electronic structure of the Bi adatoms. After annealing to ∼500K, 2D triangular Bi islands aggregate into Bi nanoclusters (NCs) of uniform size. A well-controlled fabrication method is thus demonstrated. The approaches adopted herein provide perspectives for fabricating and characterizing periodic networks on MEG and related systems, which are useful in realizing graphene-based electronic, energy, sensor and spintronic devices.

Low contact resistance in epitaxial graphene devices for quantum metrology

We investigate Ti/Au contacts to monolayer epitaxial graphene on SiC (0001) for applications in quantum resistance metrology. Using three-terminal measurements in the quantum Hall regime we observed variations in contact resistances ranging from a minimal value of 0.6 Ω up to 11 kΩ. We identify a major source of high-resistance contacts to be due bilayer graphene interruptions to the quantum Hall current, whilst discarding the effects of interface cleanliness and contact geometry for our fabricated devices. Moreover, we experimentally demonstrate methods to improve the reproducibility of low resistance contacts (<10 Ω) suitable for high precision quantum resistance metrology.

Tailoring low-dimensional structures of bismuth on monolayer epitaxial graphene.

To improve graphene-based multifunctional devices at nanoscale, a stepwise and controllable fabrication procedure must be elucidated. Here, a series of structural transition of bismuth (Bi) adatoms, adsorbed on monolayer epitaxial graphene (MEG), is explored at room temperature. Bi adatoms undergo a structural transition from one-dimensional (1D) linear structures to two-dimensional (2D) triangular islands and such 2D growth mode is affected by the corrugated substrate. Upon Bi deposition, a little charge transfer occurs and a characteristic peak can be observed in the tunneling spectrum, reflecting the distinctive electronic structure of the Bi adatoms. When annealed to ~500 K, 2D triangular Bi islands aggregate into Bi nanoclusters (NCs) of uniform size. A well-controlled fabrication method is thus demonstrated. The approaches adopted herein provide perspectives for fabricating and characterizing periodic networks on MEG and related systems, which are useful in realizing graphene-based electronic, energy, sensor and spintronic devices.

Tip induced mechanical deformation of epitaxial graphene grown on reconstructed 6H–SiC(0001) surface during scanning tunneling and atomic force microscopy studies

The structural and mechanical properties of an epitaxial graphene (EG) monolayer thermally grown on top of a 6H–SiC(0001) surface were studied by combined dynamic scanning tunneling microscopy (STM) and frequency modulation atomic force microscopy (FM-AFM). Experimental STM, dynamic STM and AFM images of EG on 6H–SiC(0001) show a lattice with a 1.9 nm period corresponding to the (6 × 6) quasi-cell of the SiC surface. The corrugation amplitude of this (6 × 6) quasi-cell, measured from AFM topographies, increases with the setpoint value of the frequency shift Δf (15–20 Hz, repulsive interaction). Excitation variations map obtained simultaneously with the AFM topography shows that larger dissipation values are measured in between the topographical bumps of the (6 × 6) quasi-cell. These results demonstrate that the AFM tip deforms the graphene monolayer. During recording in dynamic STM mode, a frequency shift (Δf) map is obtained in which Δf values range from 41 to 47 Hz (repulsive interaction). As a result, we deduced that the STM tip, also, provokes local mechanical distortions of the graphene monolayer. The origin of these tip-induced distortions is discussed in terms of electronic and mechanical properties of EG on 6H–SiC(0001).