Graphene-based Nanoelectronics Research Articles

Effect of strain on tunneling time in graphene magnetic barrier

We solve the Dirac equation in three regions of graphene to get the solutions of the energy spectrum in connection to the strain, energy gap, and magnetic field. The Goos-H\"anchen shifts and group delay time will be obtained by applying the stationary phase approximation after the wave functions at the interfaces have been matched. Our results suggest that the group delay time is influenced by the presence of strain along the armchair and zigzag directions. We show that the gate voltage and strain have the ability to change the group delay from subluminality to superluminality. This may have significant uses in high-speed graphene-based nanoelectronics.

A survey on pristine and intercalation doped graphene nanoribbon interconnect for future VLSI circuits

The paper presents a review of recent works on pristine and intercalation doped graphene nanoribbon interconnects. Since the last decade, there have been tremendous research interests on graphene-based nanoelectronics. Graphene nanoribbon (GNR) has been projected as an interconnect material to replace the traditional copper interconnects. Since graphene is a planar material, CMOS compatible processes for patterning and making contacts to it, can be developed. Though, fabricating side-contacts in multilayered GNR (MLGNR) is a challenging task, fabrication of top-contact MLGNR is technologically viable. In addition, intercalation doping in top-contact MLGNR improves effective conductivity further. A number of models have been developed to optimize emerging on-chip interconnect technologies and benchmark them against conventional technologies. In this review, physical and electrical models of pristine and intercalation doped MLGNRs have been discussed in brief. A chronological survey towards analytical and spice compatible modeling of pristine and intercalation doped top-contact (TC) and side-contact (SC) MLGNR interconnect, has been presented in this article. A brief review of experimental work on pristine and intercalated GNRs as on-chip interconnect has also been incorporated in this survey. The stability, signal integrity (SI), and power integrity (PI) analysis of pristine and intercalation doped MLGNR interconnect with the presented models have been depicted further. The performance assessments have been compared with conventional copper, and CNT-based interconnects as well. It has been concluded that the side-contact and intercalation doped top-contact MLGNRs can outperform conventional copper-based interconnects.

Graphene-based monoatomic chain spintronics: contact-derived half-metallicity, sp2 vs sp

Because of the remarkable semi-metallicity and edge states, graphene-based nanoelectronics has become an intense arena in low-dimensional atomic device science and technology. Nowadays, various single atomic chains have been successfully fabricated, which can serve as multi-functional interconnects to bridge atomic devices. Using first principles approach, the spintronic applications of graphene-based carbon atomic chains and boron nitride atomic chains MTJs (magnetic tunnelling junctions) are systematically investigated in this work. The results show that the half-metallicity is only contact-dominated ( sp and sp 2 contacts), which cannot be switched by gate voltages, strain, the length of atomic chains, or the width of zigzag graphene nanoribbon leads. The intrinsic physics is that the electronic coupling in sp junctions provides partial transmission states for both spin currents, while one spin state is completely blocked by the electronic coupling in sp 2 junctions. Therefore, the sp 2 junctions present half-metallicity and remarkable tunnelling magneto-resistance (10 5 %–10 6 %), demonstrating the potential applications as spin-valve. This work discloses the origin of the contact-dominated effect in graphene-based spintronics which is unknown before. • The half-metallicity of graphene-ACs/BNACs junctions is dominated by contacts. • The sp 2 junctions are fully polarzied and not the case in sp junctions. • The tunnelling magneto-resistance of sp 2 junctions can remarkably reach 10 5 %-10 6 %.

Highly Ordered Boron Nitride/Epigraphene Epitaxial Films on Silicon Carbide by Lateral Epitaxial Deposition.

The realization of high-performance nanoelectronics requires control of materials at the nanoscale. Methods to produce high quality epitaxial graphene (EG) nanostructures on silicon carbide are known. The next step is to grow van der Waals semiconductors on top of EG nanostructures. Hexagonal boron nitride (h-BN) is a wide bandgap semiconductor with a honeycomb lattice structure that matches that of graphene, making it ideally suited for graphene-based nanoelectronics. Here, we describe the preparation and characterization of multilayer h-BN grown epitaxially on EG using a migration-enhanced metalorganic vapor phase epitaxy process. As a result of the lateral epitaxial deposition (LED) mechanism, the grown h-BN/EG heterostructures have highly ordered epitaxial interfaces, as desired in order to preserve the transport properties of pristine graphene. Atomic scale structural and energetic details of the observed row-by-row growth mechanism of the two-dimensional (2D) epitaxial h-BN film are analyzed through first-principles simulations, demonstrating one-dimensional nucleation-free-energy-barrierless growth. This industrially relevant LED process can be applied to a wide variety of van der Waals materials.

Optical contrast in the near-field limit for structural characterization of graphene nanoribbons

Graphene nanoribbons (GNRs) are unique structures with interesting optical and electronic properties which have great potential of application in fields such as optoelectronics and nanoelectronics. Synthesis and characterization techniques of GNRs are quite important for the development of such technologies. In the case of the structural characterization, it is essential to develop quick and non-destructive approaches in order to determine properties such as thickness and uniformity of GNRs. In the present work we report the structural characterization of GNRs grown on stepped SiC (0001) surfaces by using a differential reflectance contrast (DRC) technique based on a near-field scanning optical microscope (NSOM). With this approach, it is possible to assess GNRs widths with dimension as small as 60 nm with a thickness of one or two graphene monolayers. Our results show that the DRC technique is powerful to analyze the morphology of GNRs grown on SiC (0001) substrates which is a promising wafer-scale plattform for the development of graphene-based nanoelectronics.

Spectroscopic and Electrical Characterizations of Low-Damage Phosphorous-Doped Graphene via Ion Implantation.

Development of n-/p-type semiconducting graphenes is a critical route to implement in graphene-based nanoelectronics and optronics. Compared to the p-type graphene, the n-type graphene is more difficult to be prepared. Recently, phosphorous doping was reported to achieve air-stable and high mobility of n-typed graphene. The phosphorous-doped graphene (P-Gra) by ion implantation is considered as an ideal method for tailoring graphene due to its IC compatible process; however, for a conventional ion implanter, the acceleration energy is in the order of kiloelectron volts (keV), thus severely destroys the sp2 bonding of graphene owing to its high energy of accelerated ions. The introduced defects, therefore, degrade the electrical performance of graphene. Here, for the first time, we report a low-damage n-typed chemical vapor deposition (CVD) graphene by an industrial-compatible ion implanter with an energy of 20 keV where the designed protection layer (thin Au film) covered on as-grown CVD graphene is employed to efficiently reduce defect formation. The additional post-annealing is found to heal the crystal defects of graphene. Moreover, this method allows transferring ultraclean and residue-free P-Gra onto versatile target substrates directly. The doping configuration, crystallinity, and electrical properties on P-Gra were comprehensively studied. The results indicate that the low-damaged P-Gra with a controllable doping concentration of up to 4.22 at % was achieved, which is the highest concentration ever recorded. The doped graphenes with tunable work functions (4.85-4.15 eV) and stable n-type doping while keeping high-carrier mobility are realized. This work contributes to the proof-of-concept for tailoring graphene or 2D materials through doping with an exceptional low defect density by the low energy ion implantation, suggesting a great potential for unconventional doping technologies for next-generation 2D-based nanoelectronics.

Electronic transport on graphene armchair-edge nanoribbons with Fermi velocity and potential barriers

The transport properties of Dirac fermions through armchair-edge graphene nanoribbons (AGNRs) with a single and double rectangular Fermi velocity vF and electrostatic potential U barriers is investigated. We employ a transfer matrix method (TMM) to compute the transmission coefficient of the full set of propagating mode which is used to obtain the conductance and Fano factor spectra for both metallic and semiconducting nanoribbons. We show that a reduced Fermi velocity within the barrier region can partially suppress the backscattering resulting from the electrostatic potential. In a double barrier structure, the emergence of high-order transmitting modes is shown to substantially reduce the Fano factor in the spectral region around U. These results indicate that the simultaneous tuning of vF and U in barrier regions can be explored to control the electronic transport in graphene-based nanoelectronics structures.

Tunable Schottky barrier in graphene/graphene-like germanium carbide van der Waals heterostructure

The structural and electronic properties of van der Waals (vdW) heterostructrue constructed by graphene and graphene-like germanium carbide were investigated by computations based on density functional theory with vdW correction. The results showed that the Dirac cone in graphene can be quite well-preserved in the vdW heterostructure. The graphene/graphene-like germanium carbide interface forms a p-type Schottky contact. The p-type Schottky barrier height decreases as the interlayer distance decreases and finally the contact transforms into a p-type Ohmic contact, suggesting that the Schottky barrier can be effectively tuned by changing the interlayer distance in the vdW heterostructure. In addition, it is also possible to modulate the Schottky barrier in the graphene/graphene-like germanium carbide vdW heterostructure by applying a perpendicular electric field. In particular, the positive electric field induces a p-type Ohmic contact, while the negative electric field results in the transition from a p-type to an n-type Schottky contact. Our results demonstrate that controlling the interlayer distance and applying a perpendicular electric field are two promising methods for tuning the electronic properties of the graphene/graphene-like germanium carbide vdW heterostructure, and they can yield dynamic switching among p-type Ohmic contact, p-type Schottky contact, and n-type Schottky contact in a single graphene-based nanoelectronics device.

Ballistic tracks in graphene nanoribbons

High quality graphene nanoribbons epitaxially grown on the sidewalls of silicon carbide (SiC) mesa structures stand as key building blocks for graphene-based nanoelectronics. Such ribbons display 1D single-channel ballistic transport at room temperature with exceptionally long mean free paths. Here, using spatially-resolved two-point probe (2PP) measurements, we selectively access and directly image a range of individual transport modes in sidewall ribbons. The signature of the independently contacted channels is a sequence of quantised conductance plateaus for different probe positions. These result from an interplay between edge magnetism and asymmetric terminations at opposite ribbon edges due to the underlying SiC structure morphology. Our findings demonstrate a precise control of transport through multiple, independent, ballistic tracks in graphene-based devices, opening intriguing pathways for quantum information device concepts.

Energy bandgap engineering of graphene nanoribbon by doping phosphorous impurities to create nano-heterostructures: A DFT study

Energy bandgap engineering is used to produce semiconductor heterostructure systems that perform processes such as the resonant tunneling in nanoelectronics and the solar energy conversion in solar cell applications. However, the performance of such systems degrades as their size is reduced. Graphene-based nanoelectronics has appeared as a candidate to enable high performance down to the single-molecule scale. Here, graphene nanoribbons (GNRs) can have bandgaps that are tunable by using chemical doping method. We have been predicted that bandgap engineering within a GNR may be achieved by varying the number and geometrical pattern of phosphorus (P) impurities in the GNS, so that we have carried out first-principles calculations of the energetic and electronic properties of P-doped GNR based on density functional theory (DFT) with Gaussian 09W packages. The geometric and electronic properties of the GNR with and without various dopants of phosphorus impurities were performed and discussed. Our results show that the electronic properties of GNR do not only depend on the phosphorus impurity concentrations, but also depend on the geometrical pattern of phosphorus impurities in the GNR. As a result, we can bandgap engineering of GNR by doping phosphorous impurities to create semiconductor heterostructure, which can be used in many important applications.

Raman spectroscopy of gallium ion irradiated graphene

The successful integration of graphene in future technologies, such as filtration and nanoelectronics, depends on the ability to introduce controlled nanostructured defects in graphene. In this work, Raman spectroscopy is used to investigate the induction of disorder in graphene via gallium ion beam bombardment. Two configurations of CVD-grown graphene samples are used: (i) graphene supported on a Si/SiO2 substrate, and (ii) graphene suspended on porous TEM grids. It is observed that the supported graphene experiences more damage in response to lower beam doses than suspended graphene. This phenomenon is attributed to the behaviour of the energetic ions impinging the sample. In suspended graphene, the ions pass through the graphene membrane once and disperse to the atmosphere, while in supported graphene, the ions embed themselves in the substrate causing swelling and backscattering events, hence increasing the induced disorder. In supported graphene, the ratio between the Gaussian D and G peaks attributed to amorphous carbon, and the Lorentzian D and G peaks attributed to graphene, (IDG/IDL) and (IGG/IGL), are suggested to be used to quantify the degree of amorphization. The results are relevant to the development of nanostructured graphene-based filtration or desalination membranes, as well as for graphene-based nanoelectronics.

Recovery of edge states of graphene nanoislands on an iridium substrate by silicon intercalation

Finite-sized graphene sheets, such as graphene nanoislands (GNIs), are promising candidates for practical applications in graphene-based nanoelectronics. GNIs with well-defined zigzag edges are predicted to have spin-polarized edge-states similar to those of zigzag-edged graphene nanoribbons, which can achieve graphene spintronics. However, it has been reported that GNIs on metal substrates have no edge states because of interactions with the substrate.We used a combination of scanning tunneling microscopy, spectroscopy, and density functional theory calculations to demonstrate that the edge states of GNIs on an Ir substrate can be recovered by intercalating a layer of Si atoms between the GNIs and the substrate. We also found that the edge states gradually shift to the Fermi level with increasing island size. This work provides a method to investigate spin-polarized edge states in high-quality graphene nanostructures on a metal substrate.

Doping and band gap control at poly(vinylidene fluoride)/graphene interface

Using the density-functional first-principles calculations, we investigate the electronic structures of poly(vinylidene fluoride) PVDF/graphene composite systems. The n- and p-doping of graphene can be flexibly switched by reversing the ferroelectric polarization of PVDF, without scarifying the intrinsic π-electron band dispersions of graphene that are usually undermined by chemical doping. The doping degree is also dependent on the thickness of PVDF layers, which will get saturated when PVDF is thick enough. In PVDF/bilayer graphene (BLG) heterostructure, the doping degree directly determines the local energy gap of the charged BLG. The sandwich structure of PVDF/BLG/PVDF can further enhance the local energy gap as well as keep the electric neutrality of BLG, which will be of great application potentials in graphene-based nanoelectronics.

Facile Formation of Interconnected Multi-Walled Carbon Nanotube-Graphene Nanocomposite for Nanoelectronics Applications

Novel nanocomposite made of one-dimensional (1-D) multi-walled carbon nanotube (MWCNT) and two-dimensional (2-D) graphene was prepared. MWCNT was spin coated onto copper foil and followed by chemical vapor deposition (CVD) growth of graphene. The MWCNT-Graphene nanocomposite was transferred onto target substrate by using a standard polymer-based transfer technique. HRTEM and Raman spectroscopy showed high crystallinity of fused MWCNT and graphene layer. Low defect-related D-peak was also observed even after the nanocomposite underwent high temperature processing. As compared to pristine graphene, electrical characterization of MWCNT-Graphene nanocomposite also revealed the reduction of sheet resistance by ~71% and almost 2-fold improvement in room-temperature carrier mobility. These improvements are surmised due to additional conducting channels formed by MWCNT in the graphene layer. Hence, higher electrical conductivity can be expected. With the introduction of MWCNT across the graphene layer, highly desirable electrical properties can be achieved and as such leveraging the viability of graphene-based nanoelectronics devices.

Modification of electronic properties of graphene by using low-energy K+ ions

Despite its superb electronic properties, the semi-metallic nature of graphene with no band gap (Eg) at the Dirac point has been a stumbling block for its industrial application. We report an improved means of producing a tunable band gap over other schemes by doping low energy (10 eV) potassium ions (K+) on single layer graphene formed on 6H-SiC(0001) surface, where the noble Dirac nature of the π-band remains almost unaltered. The changes in the π-band induced by K+ ions reveal that the band gap increases gradually with increasing dose (θ) of the ions up to Eg = 0.65 eV at θ = 1.10 monolayers, demonstrating the tunable character of the band gap. Our core level data for C 1s, Si 2p, and K 2p suggest that the K+-induced asymmetry in charge distribution among carbon atoms drives the opening of band gap, which is in sharp contrast with no band gap when neutral K atoms are adsorbed on graphene. This tunable K+-induced band gap in graphene illustrates its potential application in graphene-based nano-electronics.

Imaging coherent transport in chemical vapor deposition graphene wide constriction by scanning gate microscopy

We use a scanning gate microscopy to perturb coherent transport in chemical vapor deposition (CVD) graphene wide constriction. Particularly, we observe conductance oscillations in the wide constriction region (W ∼ 800 nm) characterized by spatial conductance variations, which imply formation of the nanometer-scale ring structure due to the merged domains and intrinsic grain boundaries. Moreover, additional hot charges from high current can suppress the coherent transport, suggesting that the hot carriers with a wide spreading kinetic energy could easily tunnel merged domains and intrinsic grain boundaries in CVD-grown graphene due to the heating effect, a great advantage for applications in graphene-based interference-type nano-electronics.

Tuning the Electrical Properties of Graphene via Nitrogen Plasma-Assisted Chemical Modification.

The control in electrical properties of graphene is essentially required in order to realize graphenebased nanoelectronics. In this study, N-doped graphene was successfully obtained via nitrogen plasma treatment. Graphene was synthesized on copper foil using thermal chemical vapor deposition. After N2 plasma treatment, the G-band of the graphene was blueshifted and the intensity ratio of 2D- to G-bands decreased with increasing the plasma power. Pyrrolic-N bonding configuration induced by N2 plasma treatment was studied by X-ray photoelectron spectroscopy. Remarkably, electrical characterization including Hall measurement and I-V characteristics of the N-doped graphene exhibit semiconducting behavior as well as the n-type doping effect.

Graphene engineering by neon ion beams

Achieving the ultimate limits of lithographic resolution and material performance necessitates engineering of matter with atomic, molecular, and mesoscale fidelity. With the advent of scanning helium ion microscopy, maskless He+ and Ne+ beam lithography of 2D materials, such as graphene-based nanoelectronics, is coming to the forefront as a tool for fabrication and surface manipulation. However, the effects of using a Ne focused-ion-beam on the fidelity of structures created out of 2D materials have yet to be explored. Here, we will discuss the use of energetic Ne ions in engineering graphene nanostructures and explore their mechanical, electromechanical and chemical properties using scanning probe microscopy (SPM). By using SPM-based techniques such as band excitation (BE) force modulation microscopy, Kelvin probe force microscopy (KPFM) and Raman spectroscopy, we are able to ascertain changes in the mechanical, electrical and optical properties of Ne+ beam milled graphene nanostructures and surrounding regions. Additionally, we are able to link localized defects around the milled graphene to ion milling parameters such as dwell time and number of beam passes in order to characterize the induced changes in mechanical and electromechanical properties of the graphene surface.

Phonon thermal conductivity of graphene

The study of graphene thermal conductivity is of great importance, as its anomalous thermal and electrical conductivities (the largest among the all known materials so far) provide very good perspectives for graphene-based nanoelectronics devices. Thermal conductivity of graphene is phonon-based, since its electronic-based thermal conductivity represents less than 1% of the total thermal conductivity at room temperature. For the consideration of the thermal conductivity of graphene the Boltzmann equation in the approximation of relaxation time is used. The relaxation time is determined, with three mechanisms of phonon scattering accounted simultaneously: at defects, at borders, and on phonons. Temperature dependence of thermal conductivity is determined numerically in the range from 15 K to 400 K. The results obtained are in accordance with some other available results found in literature, obtained either experimentally or by numerical calculations.

(Invited) Non-Covalent Functionalization of Epitaxial Graphene for Atomic Layer Deposition of Dielectric Oxides

The successful integration of graphene with dielectric materials is an important step in the development of high-performance graphene-based nanoelectronics. For optimal performance, dielectric layers on graphene need to be ultrathin, conformal, and the growth process needs to be well-controlled and reproducible. In practice, atomic layer deposition (ALD) – a thin film growth process that utilizes self-limiting surface reactions for digital thin film thickness control – is an industrially relevant route to grow dielectric films and nanostructures for electronic applications. However, the chemical inertness and hydrophobicity of graphene typically cause direct ALD processes to yield films of poor quality (defected, non-uniform, and poorly adhered), thereby limiting the ultimate device performance. The key to successful integration of graphene and ALD film growth resides, therefore, in the ability to tailor the graphene surface in such a way that it is amenable to ALD chemistries, but in a manner that does not perturb the properties of the graphene itself. Here, we utilize non-covalently functionalized monolayers to seed uniform film growth by ALD as well as to facilitate directed growth of dielectric nanostructures, such as nanowire arrays. We specifically highlight the use of synchrotron characterization, including X-ray surface scattering and grazing-incidence small-angle scattering, to assess the influence that the functional layers have on thin film, interfacial, and nanoscale structure.