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We present a study of the noise properties of single-layer exfoliated graphene as a function of gate bias. A tunnel/trap model is presented based on the interaction of graphene electrons with the underlying substrate. The model incorpo- rates trap position, energy, and barrier height for tunneling into a given trap—along with the band-structure of the graphene—and is in good accord with the general char- acteristics of the data.

One-dimensional nanostructures of graphene such as graphene nanoribbons (GNRs) can prove attractive for digital electronics in the form of interband tunneling transistors, as they are capable of high drive currents. Here, we report on the transport properties of p-n junctions formed in GNR field effect transistors (FETs). It is found that the current density in the devices is indeed high; in the 1–1.5 A/mm range have been measured, comparable to Si-MOSFETs and III-V Nitride HEMTs. The observed unique current–voltage characteristics of the double-gated GNR FETs having a lateral p-n junction as their channel is explained by a field-effect model. Due to the lack of sufficiently large bandgap in the 30 nm wide GNR, the device still cannot be turned off completely, but rectification is achieved. The results suggest that the fabrication of tunneling FETs made out of graphene is possible and their characteristics may meet the expectations. Copyright © 2012 John Wiley & Sons, Ltd.

Transient absorption microscopy was employed to image charge carrier dynamics in epitaxial multilayer graphene. The carrier cooling exhibited a biexponential decay that showed a significant dependence on carrier density. The fast and slow relaxation times were assigned to coupling between electrons and optical phonon modes and the hot phonon effect, respectively. The limiting value of the slow relaxation time at high pump intensity reflects the lifetime of the optical phonons. Significant spatial heterogeneity in the dynamics was observed due to differences in coupling between graphene layers and the substrate.

Spurred by the knowledge of isolation of graphene, other 2D transition-metal dichalcogenide materials in the form of MX 2 (where M=transition metal such as Mo, W, Ti, Nb, etc. and X=S, Se, or Te) have drawn considerable attention. The MX 2 family material consists of one or more sets of triple layers with one M and two X in a sandwich structure (X-M-X). Atoms within each layer are strongly held together by covalent-ion mixed bonds, while interlayer van der Waals forces are weak. Prior investigations of 2D materials have concentrated on optical properties [1]. Field-effect transistors (FETs) in MoS 2 and WSe 2 have been demonstrated with substantial gate modulation and stable current saturation [2, 3]. A recent calculation shows that the single layer WS 2 has the potential to outperform Si and other 2D crystals in FET-type applications due to its favorable bandstructure [4]. No prior device results have been reported for WS 2 for logic or optical devices. Here we report the first fabrication and demonstration of 2D WS 2 FETs and explore the effects of photosimulation on the transistor characteristics.

Terahertz technology promises myriad applications including imaging, spectroscopy and communications. However, one major bottleneck at present for advancing this field is the lack of efficient devices to manipulate the terahertz electromagnetic waves. Here we demonstrate that exceptionally efficient broadband modulation of terahertz waves at room temperature can be realized using graphene with extremely low intrinsic signal attenuation. We experimentally achieved more than 2.5 times superior modulation than prior broadband intensity modulators, which is also the first demonstrated graphene-based device enabled solely by intraband transitions. The unique advantages of graphene in comparison to conventional semiconductors are the ease of integration and the extraordinary transport properties of holes, which are as good as those of electrons owing to the symmetric conical band structure of graphene. Given recent progress in graphene-based terahertz emitters and detectors, graphene may offer some interesting solutions for terahertz technologies.

We investigate high-field transport in graphene nanoribbons (GNRs) on SiO2, up to breakdown. The maximum current density is limited by self-heating, but can reach >3 mA/um for GNRs ~15 nm wide. Comparison with larger, micron-sized graphene devices reveals that narrow GNRs benefit from 3D heat spreading into the SiO2, which enables their higher current density. GNRs also benefit from lateral heat flow to the contacts in short devices (< ~0.3 um), which allows extraction of a median GNR thermal conductivity (TC), ~80 W/m/K at 20 C across our samples, dominated by phonons. The TC of GNRs is an order of magnitude lower than that of micron-sized graphene on SiO2, suggesting strong roles of edge and defect scattering, and the importance of thermal dissipation in small GNR devices., this version (v3) is very similar to that ultimately published in PRL

Graphene is being investigated as a promising candidate for electronic devices. For digital electronic devices, a substantial bandgap is necessary. It is possible to open a bandgap in graphene by quantum confinement of the carriers in patterned graphene nanoribbons (GNRs); GNRs with width W nm have a bandgap E g ∼1.3/W eV [1]. This implies that sub-10 nm wide ribbons can enable room-temperature operation of GNRs as traditional semiconductors, but with ultimate vertical scaling, and still take advantage of high current drives. To date, GNRs have been fabricated from exfoliated graphene [2] and operated by back gates, or nanometer scale ribbons produced by ‘explosive’ methods [3] that are neither controlled nor reproducible. These methods are not suitable for large-area device fabrication. In this work, we report lithographically patterned GNRs on epitaxial graphene on SiC substrates. Specifically, we show the first top-gated GNR field-effect transistors (FETs) on epi-graphene substrates that exhibit the opening of a substantial energy bandgap (exceeding ∼0.15 eV at a ribbon width of 10 nm), respectable carrier mobility (700 – 800 cm2/Vs), high current modulation (10∶1 at 300 K), and high current carrying capacity (0.3 mA/µm at V DS = 1 V) at the same time. Both single GNR and GNR array devices are reported.

The exceptional electrical properties of graphene materials have led to an explosion of research investigating the potential of graphene as the foundation for a future generation of devices as well as developing methods of producing high quality graphene materials. Material quality and our ability to manipulate the properties of graphene will ultimately determine the success of graphene as a device platform. Recently, the formation of single layer graphene via catalyzed-chemical vapor deposition (CVD) on copper foils has generated much interest [1]. A few groups have reported the CVD growth of graphene on copper, and transport properties including quantum Hall effect [2,3] in layers subsequently transferred to insulating substrates. However, there have been no careful studies of FETs fabricated from them. In this work, we report the characteristics of single-layer graphene FETs whose channels were grown by CVD on copper.

Energy exchange between the electrons and phonons is particularly important to electron transport, and understanding this process will be vital for the realization of future graphene-based electronics. Epitaxial growth is a very promising approach for practical applications, as it has the ability to prepare graphene on a large scale and supported on a substrate. However, epitaxially grown graphene is highly inhomogeneous, with variations in the sample thickness occurring over length scale of a few micrometers. To pave the road for electronic devices based on epitaxial graphene, characterization methods with high spatial resolution are required. In this paper, we describe transient absorption microscopy as a novel tool to characterize graphene, and to interrogate the charge carrier dynamics. The carrier cooling exhibited a biexponential decay that showed a significant dependence on carrier density. The fast and slow relaxation times were assigned to coupling between electrons and optical phonon modes and the hot phonon effect, respectively. The limiting value of the slow relaxation time at high pump intensity reflects the lifetime of the optical phonons. Significant spatial heterogeneity in the dynamics was observed due to differences in coupling between graphene layers and the substrate. This is attributed to differences in coupling between the graphene and the substrate. These results point to transient absorption microscopy as a potentially important tool for characterizing graphene.

The unique electronic properties of graphene have ignited active research in the past few years. Though in theory its electronic structure and transport properties are well known, the applicability as channel replacement material in conventional CMOS technology is still an open question. High mobility [1] and high current carrying capacity [2,3] make graphene very attractive, but on the other hand low I ON /I OFF ratio and the lack of sufficient saturation in I ON are yet unsolved drawbacks. In the past we demonstrated these properties in double-gated graphene nanoribbon field effect transistors (GNR FETs). We successfully achieved I ON /I OFF ratios of 106 at cryogenic temperatures using either top- or back-gates and sublinear output characteristic was observed [3]. Now we present the detailed analysis of the distinct operational modes of such devices at high applied source-drain field.

Graphene, a single sheet of graphite, has aroused intense interest since its isolation in 2004. Two-dimensional (2D) graphene sheets are nearly metallic, while ultrathin graphene nanoribbons (GNRs) show semiconducting properties with the energy bandgap scaling inversely with the ribbon width. Although sub-10-nm GNRs have been achieved by chemical approaches [1, 2], the ability to form GNRs lithographically will facilitate the compatibility with the conventional planar integrated circuit (IC) manufacturing. Hydrogen silsesquioxane (HSQ) is widely utilized as etching masks to fabricate GNRs due to its high resolution in e-beam lithography (EBL). HSQ can be removed by buffered hydrofluoric (BHF) acid after the GNR formation. However, the normally used SiO 2 substrate supporting the exfoliated graphene can be etched by BHF as well. In this work we present the fabrication of GNRs using Al metal mask in the O 2 plasma etching. GNRs as thin as ~ 20 nm have been achieved. The conductance modulation by the back gate is in the order of 106 at 4.2 K. Besides, quantized conductance was observed in the thin GNRs which form quasi-1D transport systems due to the carrier constriction by the ribbon edge boundaries. Landauer's formula was applied to fit the experimental data and excellent agreement was achieved.

The transport properties of graphene with saturation currents as high as 1.5 A/mm and electron mobilities of 15 000 cm2/Vs [1] place this material in the limelight as an attractive candidate of next-generation nanoelectronic devices. While DC performance has been extensively studied just a few small signal characterizations have been done so far. The widely observed high carrier mobility naturally focuses our attention towards high frequency performance of graphene based devices. Gigahertz operation of exfoliated [2] and epitaxial [3] graphene FETs has been reported recently. Here we present the high frequency performances of graphene devices based on epitaxially grown graphene on SiC substrates. The measured f T .L G product of 8 GHz.μm and f max of 16 GHz is in very good agreement with previously reported values [3]. Surprisingly, in spite of the much lower electron mobility and transconductance, these FETs demonstrate remarkable small signal performance comparable to the small signal performance of higher mobility exfoliated graphene devices.

The unique electronic properties of graphene have ignited active research in the past few years. Though in theory its electronic structure and transport properties are well known the applicability as channel replacement material in conventional CMOS technology is still an open question. High mobility [1] and high current carrying capacity [2] makes graphene very attractive but on the other hand low I ON /I OFF ratio and the lack of sufficient saturation are yet unsolved drawbacks. In the past we were able to drive back‐gated 2D graphene transistors to saturation regime [2]. Now we present the realization of these properties in double‐ gated graphene nanoribbon field effect transistors (GNR FETs). We were able to achieve I ON /I OFF ratio of 103 using either top‐ or back‐gates and we analyzed the high field characteristics of such devices.

We report the realization of top-gated graphene nanoribbon field effect transistors (GNRFETs) of ∼10 nm width on large-area epitaxial graphene exhibiting the opening of a band gap of ∼0.14 eV. Contrary to prior observations of disordered transport and severe edge-roughness effects of graphene nanoribbons (GNRs), the experimental results presented here clearly show that the transport mechanism in carefully fabricated GNRFETs is conventional band-transport at room temperature and inter-band tunneling at low temperature. The entire space of temperature, size, and geometry dependent transport properties and electrostatics of the GNRFETs are explained by a conventional thermionic emission and tunneling current model. Our combined experimental and modeling work proves that carefully fabricated narrow GNRs behave as conventional semiconductors and remain potential candidates for electronic switching devices.

Top-gated epitaxial-graphene nanoribbon (GNR) field-effect transistors on SiC wafers were fabricated and characterized at room temperature. The devices exhibited extremely high current densities (∼10 000 mA/mm) due to the combined advantages of the one-dimensionality of GNRs and the SiC substrate. These advantages included good heat dissipation as well as the high optical phonon energy of the GNRs and SiC substrate. An analytical model explains the measured family of ID–VDS curves with a pronounced ‘kink’ at a high electric field. The effective carrier mobility as a function of the channel length was extracted from both the ID–VDS modeling and the maximum transconductance from the ID–VGS curve. The effective mobility decreased for small channel lengths (

Graphene nanoribbon (GNR) field-effect transistors (FETs) with widths down to 12 nm have been fabricated by electron beam lithography using a wafer-scale chemical vapor deposition (CVD) process to form the graphene. The GNR FETs show drain-current modulation of approximately 10 at 300 K, increasing to nearly 106 at 4 K. The strong temperature dependence of the minimum current indicates the opening of a bandgap for CVD-grown GNR-FETs. The extracted bandgap is estimated to be around 0.1 eV by differential conductance methods. This work highlights the development of CVD-grown large-area graphene and demonstrates the opening of a bandgap in nanoribbon transistors.

Top-gated epitaxial graphene nanoribbon (EGNR) field effect transistors (FETs) were fabricated on epitaxial graphene substrates which demonstrated the opening of a substantial bandgap. Hydrogen silsesquioxane (HSQ) was used for the patterning of 10 nm size linewidth as well as a seed layer for atomic layer deposition (ALD) of a high-k dielectric aluminum oxide (Al2O3). It is found that the resolution of the patterning is affected by the development temperature, electron beam dose, and substrate materials. The chosen gate stack of HSQ followed by Al2O3 ALD permits stable device performance and enables the demonstration of the EGNR-FET.

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Graphene nanoribbons (GNRs) were fabricated by metal mask lithography and plasma etching. GNRs with width ∼20 nm show field-effect conductance modulation of ∼12 at room temperature and >106 at 4.2 K. Conductance quantization due to quantum confinement in low field transport was observed. Landauer formula was utilized to fit the experimental data and excellent agreement was obtained. The extracted subband energy separation was found to deviate from the predicted values of perfect armchair GNRs. Transmission probability is much smaller than unity due to scattering by GNR edge/bulk disorder and impurities, indicating a mean free path ∼40 nm. High field family I-Vs exhibited current saturation tendency and current density as high as 2 A/mm has been measured at low temperature.