We fabricate a graphene p-n-p heterojunction and exploit the coherence of weakly-confined Dirac quasiparticles to resolve the underlying scattering potential using low temperature scanning gate microscopy. The tip-induced perturbation to the heterojunction modifies the condition for resonant scattering, enabling us to detect localized Fabry-Perot cavities from the focal point of halos in scanning gate images. In addition to halos over the bulk we also observe ones spatially registered to the physical edge of the graphene. Guided by quantum transport simulations we attribute these to modified resonant scattering at the edges within elongated cavities that form due to focusing of the electrostatic field.
We observe conductivity oscillations with aperiodic spacing to only one side of the tunneling current in a dual-gated graphene field effect transistor with an n-p-n type potential barrier. The spacing and width of these oscillatoins were found to be inconsistent with pure Farbry-Perot-type interferences, but are in quantitative agreement with theoretical predictions that attribute them to resonant tunneling through quasi-bound impurity states. This observation may be understood as another signature of Klein tunneling in graphene heterojunctions and is of importance for future development and m…
A typical graphene heterojunction device can be divided into two classical zones, where the transport is basically diffusive, separated by a "quantum active region" (e.g., a locally gated region), where the charge carriers are scattered according to the laws of quantum mechanics.In this paper we derive a mathematical model of such a device, where the classical regions are described by drift-diffusion equations and the quantum zone is seen as an interface where suitable transmission conditions are imposed that take into account the quantum scattering process. Numerical simulations show good agr…
We have studied the optical properties of two-dimensional (2D) Schottky photodiode heterojunctions made of chemical vapor deposited (CVD) graphene on n- and p-type Silicon (Si) substrates. Much better rectification behavior is observed from the diodes fabricated on n- Si substrates in comparison with the devices on p-Si substrates in dark condition. Also, graphene/n-Si photodiodes show a considerable responsivity of 270 mA/W within the silicon spectral range in DC reverse bias condition. The present results are furthermore compared with that of a molybdenum disulfide (MoS2)/p-type silicon phot…
We show a large magnetoresistance(MR) effect in a graphene heterostructure consisting of an metallic(M) and semiconductor(SC)-type armchair-graphene-nanoribbon(aGNR). In the heterostructure, the transmission across the first subband of the SC-aGNR and M-aGNR is forbidden under zero magnetic-field, due to the orthogonality of the wavefunctions. A finite magnetic-field introduces the quantum hall-like effect, which distorts the wavefunctions. Thus, a finite transmission occurs across the heterojunction, giving rise to a large MR effect. We study the dependence of this MR on temperature and elect…
Vertical tunneling field-effect-transistor (FET) based on graphene heterojunctions with layers of hBN is simulated by self-consistent quantum transport simulations. It is found that the asymmetric p-type and n-type conduction is due to work function deference between the graphene contact and the tunneling channel material. Modulation of the bottom-graphene-contact plays an important role in determining the switching characteristic of the device. Due to the electrostatic short-channel-effects stemming from the vertical-FET structure, the output I-V characteristics do not saturate. The scaling b…
The quantum transport formalism based on tight-binding models is known to be powerful in dealing with a wide range of open physical systems subject to external driving forces but is, at the same time, limited by the memory requirement's increasing with the number of atomic sites in the scattering region. Here we demonstrate how to achieve an accurate simulation of quantum transport feasible for experimentally sized bulk graphene heterojunctions at a strongly reduced computational cost. Without free tuning parameters, we show excellent agreement with a recent experiment on Klein backscattering …
We present the first principle calculations of the electrical properties of graphene sheet/h-BN heterojunction(GS/h-BN) and 11-armchair graphene nanoribbon heterojunction(11-AGNR/h-BN), which were carried out using the density functional theory(DFT) method and the non-equilibrium Green's function(NEGF) technique. Since 11-AGNR belongs to the conductive (3n-1)-family of AGNR, both are metallic nanomaterials with two transverse arrays of h-BN, which is a wide-gap semi-conductor. The two h-BN arrays act as double barriers. The transmission functions(TF) and I-V characteristics of GS/h-BN and 11-A…
A new type of states in graphene-based planar heterojunctions has been studied in the envelope wave function approximation. The condition for the formation of these states is the intersection between the dispersion curves of graphene and its gap modification. This type of states can also occur in smooth graphene-based heterojunctions.
This paper demonstrates the high-quality tunnel barrier characteristics and layer number controlled tunnel resistance of a transition metal dichalcogenide (TMD) measuring just a few monolayers in thickness. Investigation of vertical transport in WS2 and MoS2 TMDs in graphene/TMD/metal heterostructures revealed that WS2 exhibits tunnel barrier characteristics when its thickness is between 2 to 5 monolayers, whereas MoS2 experiences a transition from tunneling to thermionic emission transport with increasing thickness within the same range. Tunnel resistance in a graphene/WS2/metal heterostructu…
A simple model based on the divide and conquer rule and tight-binding (TB) approximation is employed for studying the role of finite size effect on the electronic properties of elongated graphene nanoribbon (GNR) heterojunctions. In our model, the GNR heterojunction is divided into three parts: a left (L) part, middle (M) part, and right (R) part. The left part is a GNR of width $W_{L}$, the middle part is a GNR of width $W_{M}$, and the right part is a GNR of width $W_{R}$. We assume that the left and right parts of the GNR heterojunction interact with the middle part only. Under this approxi…
We study the effect of the strain on the band structure and the valley-dependent transport property of graphene heterojunctions. It is found that valley-dependent separation of electrons can be achieved by utilizing the strain and on-site energies. In the presence of the strain, the values of the transmission can be effectively adjusted by changing the strengths of the strain, while the transport angle basically keeps unchanged. When an extra on-site energy is simultaneously applied to the central scattering region, not only are the electrons of valleys K and K' separated into two distinct tra…
We present a theory of quantum-coherent transport through a lateral p-n-p structure in graphene, which fully accounts for the interference of forward and backward scattering on the p-n interfaces. The backreflection amplitude changes sign at zero incidence angle because of the Klein phenomenon, adding a phase $π$ to the interference fringes. The contributions of the two p-n interfaces to the phase of the interference cancel with each other at zero magnetic field, but become imbalanced at a finite field. The resulting half a period shift in the Fabry-Perot fringe pattern, induced by a relativel…
Considering the difference of energy bands in graphene and silicene, we put forward a new model of the graphene-silicene-graphene (GSG) heterojunction. In the GSG, we study the valley polarization properties in a zigzag nanoribbon in the presence of an external electric field. We find the energy range associated with the bulk gap of silicene has a valley polarization more than 95%. Under the protection of the topological edge states of the silicene, the valley polarization remains even the small non-magnetic disorder is introduced. These results have certain practical significance in applicati…
Metal-semiconductor interfaces, known as Schottky junctions, have long been hindered by defects and impurities. Such imperfections dominate the electrical characteristics of the junction by pinning the metal Fermi energy. We report measurements on a boron nitride encapsulated graphene-tungsten diselenide (WSe$_2$) Schottky junction which exhibits ideal diode characteristics and a complete lack of Fermi-level pinning. The Schottky barrier height of the device is rigidly tuned by electrostatic gating of the WSe$_2$, enabling experimental verification of the Schottky-Mott limit in a single device…
Quantum Hall (QH) and quantum spin Hall (QSH) phases have very different edge states and, when going from one phase to the other, the direction of one edge state must be reversed. We study this phenomena in graphene in presence of a strong perpendicular magnetic field on top of a spin-orbit (SO) induced QSH phase. We show that, below the SO gap, the QSH phase is virtually unaffected by the presence of the magnetic field. Above the SO gap, the QH phase is restored. An electrostatic gate placed on top of the system allows to create a QSH-QH junction which is characterized by the existence of a s…
We model the quantum Hall effect in heterostructures made of two gapped graphene stripes with different gaps, $Δ_1$ and $Δ_2$. We consider two main situations, $Δ_1=0,Δ_2\neq0$ and $Δ_1=-Δ_2$. They are different in a fundamental aspect: only the latter feature kink states that, when intervalley coupling is absent, are protected against backscattering. We compute the two terminal conductance of heterostructures with channel length up to 430 nm, in two transport configurations, parallel and perpendicular to the interface. By studying the effect of disorder on the transport along the boundary, we…
Europium, one of the rare earth elements, exhibits +2 and +3 valence states and has been widely used for magnetic modification of materials. Based on density functional theory calculations, we predict the 2D EuBr/graphene heterojunctions to exhibit metallicity, huge intrinsic-ferromagnetism nearly 7.0 μB per Eu and the special monovalent Eu ions. Electron localization function (ELF), difference charge densities and Bader charge analyses demonstrate that there are cation-π interactions between the EuBr films and graphene, which explains the stability of these unusual heterojunctions. Graphene w…
The realization of p-n junctions in graphene, combined with the gapless and chiral nature of its massless Dirac fermions has led to the observation of many intriguing phenomena such as quantum Hall effect in bipolar regime, Klein tunneling, and Fabry-Pérot interferences all of which involve electronic transport across p-n junctions. Ballistic snake states propagating along the p-n junctions have been predicted to induce conductance oscillations, manifesting their twisting nature. However, transport studies along p-n junctions have so far only been performed in low mobility devices. Here, we re…
The structural similarity between hexagonal boron nitride (h-BN) and graphene nanoribbons allows for the formation of heterojunctions with small chain stress. The combination of the insulation nature of the former and the quasi-metallic property of the latter makes this kind of heterostructure particularly interesting for flat optoelectronics. Recently, it was experimentally demonstrated that the shapes of the graphene and h-BN domains can be controlled precisely, and sharp graphene/h-BN interfaces can be created. Here, we investigated the electronic and structural properties of graphene (h-BN…
Heterojunction, a photoelectric conversion center, plays a critical role in photo detection. A compare study is performed on field effect phototransistors (FEpT) with two typical heterojunctions based on graphene and PbSe quantum dots (QDs) hybrid, including layer-heterojunction (LH) and bulk-heterojunction (BH). LH-FEpT exhibits a higher mobility (μn) 677 cm2V-1s-1, whereas μn of BH-FEpT is as low as 314 cm2V-1s-1, however the higher responsivity (106 A/W) is achieved by the former. Because R is proportional to the product of mobility and transfer rate, and high transfer rate of LH-FEpT overc…
In the past decade graphene has been one of the most studied material for several unique and excellent properties. Due to its two dimensional nature, physical and chemical properties and ease of manipulation, graphene offers the possibility of integration with the exiting semiconductor technology for next-generation electronic and sensing devices. In this context, the understanding of the graphene/semiconductor interface is of great importance since it can constitute a versatile standalone device as well as the building-block of more advanced electronic systems. Since graphene was brought to t…
We calculate quantum transport for metal-graphene nanoribbon heterojunctions within the atomistic self-consistent Schrödinger/Poisson scheme. Attention is paid on both the chemical aspects of the interface bonding as well the one-dimensional electrostatics along the ribbon length. Band-bending and doping effects strongly influence the transport properties, giving rise to conductance asymmetries and a selective suppression of the subband formation. Junction electrostatics and p-type characteristics drive the conduction mechanism in the case of high work function Au, Pd and Pt electrodes, while …
We report novel graphene nanoribbon (GNR)/semiconductor nanowire (SNW) heterojunction light-emitting diodes (LEDs) for the first time. The GNR and SNW have a face-to-face contact structure, which has the merit of bigger active region. ZnO, CdS, and CdSe NWs were employed in our case. At forward biases, the GNR/SNW heterjunction LEDs could emit light with wavelengths varying from ultraviolet (380 nm) to green (513 nm) to red (705 nm), which were determined by the band-gaps of the involved SNWs. The mechanism of light emitting for the GNR/SNW heterojunction LED was discussed. Our approach can ea…
Heterostructures of two-dimensional (2D) and three-dimensional (3D) materials form efficient devices for utilizing the properties of both classes of materials. Graphene/silicon (G/Si) Schottky diodes have been studied extensively with respect to their optoelectronic properties. Here, we introduce a method to analyze measured capacitance-voltage data of G/Si Schottky diodes connected in parallel with G/silicon dioxide/Si (GIS) capacitors. We also demonstrate the accurate extraction of the built-in potential ($Φ$$_{bi}$) and the Schottky barrier height from the measurement data independent of th…
Due to their wide band gaps, III-N materials can exhibit behaviors ranging from the semiconductor class to the dielectric class. Through an analogy between a Metal/AlGaN/AlN/GaN diode and a MOS contact, we make use of this dual nature and show a direct path to capture the energy band diagram of the nitride system. We then apply transparency calculations to describe the forward conduction regime of a III-N heterojunction diode and demonstrate it realizes a tunnel diode, in contrast to its regular Schottky Barrier Diode designation. Thermionic emission is ruled out and instead, a coherent electr…
The sliding energy landscape of the heterogeneous graphene/h-BN interface is studied by means of the registry index. For a graphene flake sliding on top of h-BN the anisotropy of the sliding energy corrugation with respect to the misfit angle between the two naturally mismatched lattices is found to reduce with the flake size. For sufficiently large flakes the sliding energy corrugation is expected to be at least an order of magnitude lower than that obtained for matching lattices regardless of the relative interlayer orientation thus resulting in a stable low-friction state. This is in contra…
The success of all-graphene electronics is severely hindered by the challenging realization and subsequent integration of semiconducting channels and metallic contacts. Here, we comprehensively investigate the electronic transport across width-modulated heterojunctions consisting of a graphene quantum dot of varying lengths and widths embedded in a pair of armchair-edged metallic nanoribbons, of the kind recently fabricated via on-surface synthesis. We show that the presence of the quantum dot enables the opening of a width-dependent transport gap, thereby yielding built-in one-dimensional met…
Graphene has attracted increasing interests due to its remarkable properties, however, the zero band gap of monolayer graphene might limit its further electronic and optoelectronic applications. Herein, we have successfully synthesized monolayer silicon-doped graphene (SiG) in large area by chemical vapor deposition method. Raman spectroscopy and X-ray photoelectron spectroscopy measurements evidence silicon atoms are doped into graphene lattice with the doping level of 3.4 at%. The electrical measurement based on field effect transistor indicates that the band gap of graphene has been opened …
Heat dissipation is a very critical problem for designing nano-functional devices, including MoS2/Graphene heterojunctions. In this paper we investigate thermal transport in MoS2/Graphene hybrid nanosheets under various heating conditions, by using molecular dynamics simulation. Diverse transport processes and characteristics, depending on the conducting layers, are found in these structures. The thermal conductivities can be tuned by interlayer coupling, environment temperature and interlayer overlap. The highest thermal conductivity at room temperature is achieved as more than 5 times of tha…
Recent progress in preparing well controlled 2D van der Waals heterojunctions has opened up a new frontier in materials physics. In this paper we address the intriguing energy gaps that are sometimes observed when a graphene sheet is placed on a hexagonal boron nitride substrate, demonstrating that they are produced by an interesting interplay between structural and electronic properties, including electronic many-body exchange interactions. Our theory is able to explain the observed gap behavior by accounting first for the structural relaxation of graphene's carbon atoms when placed on a boro…
The observation of quantum conductance oscillations in mesoscopic systems has traditionally required the confinement of the carriers to a phase space of reduced dimensionality. While electron optics such as lensing and focusing have been demonstrated experimentally, building a collimated electron interferometer in two unconfined dimensions has remained a challenge due to the difficulty of creating electrostatic barriers that are sharp on the order of the electron wavelength. Here, we report the observation of conductance oscillations in extremely narrow graphene heterostructures where a resona…
Graphene-based vertical heterostructures, particularly stacks incorporated with other layered materials, are promising for nanoelectronics. The stacking of two model Dirac materials, graphene and topological insulator, can considerably enlarge the family of van der Waals heterostructures. Despite well understanding of the two individual materials, the electron transport properties of a combined vertical heterojunction are still unknown. Here we show the experimental realization of a vertical heterojunction between Bi2Se3 nanoplate and monolayer graphene. At low temperatures, the electron trans…
Lateral heterostructures of two-dimensional materials may exhibit various intriguing emergent properties. Yet when specified to the orientationally aligned heterojunctions of zigzag graphene and hexagonal boron nitride (hBN) nanoribbons, realizations of the high expectations on their properties encounter two standing hurtles. First, the rapid accumulation of strain energy prevents large- scale fabrication. Secondly, the pronounced half-metallicity predicted for freestanding graphene nanoribbons is severely suppressed. By properly tailoring orientational misalignment between zigzag graphene and…
Graphene/silicon (G/Si) heterojunction based devices have been demonstrated as high responsivity photodetectors that are potentially compatible with semiconductor technology. Such G/Si Schottky junction diodes are typically in parallel with gated G/silicon dioxide (SiO$_2$)/Si areas, where the graphene is contacted. Here, we utilize scanning photocurrent measurements to investigate the spatial distribution and explain the physical origin of photocurrent generation in these devices. We observe distinctly higher photocurrents underneath the isolating region of graphene on SiO$_2$ adjacent to the…
A high performance triple-heterojunction (3HJ) design has been previously proposed for tunneling FETs (TFETs). Compared with single heterojunction (HJ) TFETs, the 3HJ TFETs have both shorter tunneling distance and two transmission resonances that significantly improve the ON-state current ($I_{\rm{ON}}$). Coherent quantum transport simulation predicts, that $I_{\rm{ON}}=460\rm{μA/μm}$ can be achieved at gate length $Lg=15\rm{nm}$, supply voltage $V_{\rm{DD}}=0.3\rm{V}$, and OFF-state current $I_{\rm{OFF}}=1\rm{nA/μm}$. However, strong electron-phonon and electron-electron scattering in the hea…
Based on first-principles calculations we predict that periodically repeated junctions of armchair graphene nanoribbons of different widths form superlattice structures. In these superlattice heterostructures the width and the energy gap are modulated in real space and specific states are confined in certain segments. Orientation of constituent nanoribbons, their width and length, the symmetry of the junction are the structural parameters to engineer electronic properties of these quantum structures. Not only the size modulation, but also composition modulation, such as periodically repeated, …
A heterojunction tunneling field effect transistor based on armchair graphene nanoribbons is proposed and studied using ballistic quantum transport simulation based on 3D real space nonequilibrium Green's function formalism. By using low band gap nanoribbons as the source/drain material, the hetero- structure shows better performance including higher on current, lower off current, and improved steep subthreshold swings compared with homostructure. It is also found the device performance greatly depends on the source/drain dopping density. High doping density leads to fewer density states in th…
The superior intrinsic properties of graphene have been a key research focus for the past few years. However, external components, such as metallic contacts, serve not only as essential probing elements, but also give rise to an effective electron cavity, which can form the basis for new quantum devices. In previous studies, quantum interference effects were demonstrated in graphene heterojunctions formed by a top gate. Here phase coherent transport behavior is demonstrated in a simple two terminal graphene structure with clearly-resolved Fabry-Perot oscillations in sub-100 nm devices. By aggr…
Graphene/silicon heterostructures have attracted tremendous interest as a new platform for diverse electronic and photonic devices such as barristors, solar cells, optical modulators, and chemical sensors. The studies to date largely focus on junctions between graphene and lightly-doped silicon, where a Schottky barrier is believed to dominate the carrier transport process. Here we report a systematic investigation of carrier transport across the heterojunctions formed between graphene and highly-doped silicon. By varying the silicon doping level and the measurement temperature, we show that t…
In this review, we discuss the impact of interfaces and heterojuctions on the electronic and thermoelectric transport properties of materials. We review recent progress in understanding electronic transport in two-dimensional (2D) materials ranging from graphene to transition metal dichalcogenides (TMDs), their homojunctions (grain boundaries), lateral heterojunctions (such as graphene/MoS$_2$ lateral interfaces), and vertical van der Waals (vdW) heterostructures. We also review work in thermoelectric properties of 2D heterojunctions, as well as their applications in creating devices such as r…
Topologically protected chiral states at a mass-inverted quantum dot in graphene are studied by analyzing both tight-binding and kernal polynomial method calculations. The mass-inverted quantum dot is introduced by considering a heterojunction between two different mass domains, which is similar to the domain wall in bilayer graphene. The numerical results show emergent metallic channels across the mass gap when the signs of the mass terms are opposite. The eigenstates of the metallic channels are revealed to be doubly degenerate---each state propagates along opposite directions, maintaining t…
We explore the chiral transmission of electrons across graphene heterojunctions for electronic switching using gate geometry alone. A sequence of gates is used to collimate and orthogonalize the chiral transmission lobes across multiple junctions, resulting in negligible overall current. The resistance of the device is enhanced by several orders of magnitude by biasing the gates into the bipolar $npn$ doping regime, as the ON state in the near homogeneous $nn^-n$ regime remains highly conductive. The mobility is preserved because the switching involves a transmission gap instead of a structura…
Synthesis of Metal-Semiconductor Heterojunctions in Single Graphene Nanoribbons
University of California, Berkeley（加州大学伯克利分校）；Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室）；Kavli Energy NanoSciences Institute at the University of California Berkeley and the Lawrence Berkeley National Laboratory（加州大学伯克利分校与劳伦斯伯克利国家实验室卡弗里能源纳米科学研究所）；Department of Chemistry, Columbia University（哥伦比亚大学化学系）；Department of Physics, University of Illinois Urbana-Champaign（伊利诺伊大学厄巴纳-香槟分校物理系）；Department of Physics, Cornell University（康奈尔大学物理系）；Department of Materials Science and Nanoengineering, Rice University（莱斯大学材料科学与纳米工程系）
We address the tunneling current in a graphene-hBN-graphene heterostructure as function of the twisting between the crystals. The twisting induces a modulation of the hopping amplitude between the graphene layers, that provides the extra momentum necessary to satisfy momentum and energy conservation and to activate coherent tunneling between the graphene electrodes. Conservation rules limit the tunneling to states with wavevectors lying at the conic curves defined by the intersection of two Dirac cones shifted in momentum and energy. There is a critical voltage where the intersection is a stra…
Graphene nanoribbons provide an opportunity to integrate phase-coherent transport phenomena with nanoelectromechanical systems (NEMS). Due to the strain induced by a deflection in a graphene nanoribbon resonator, coherent electron transport and mechanical deformations couple. As the electrons in graphene have a Fermi wavelength λ~ a_0 = 1.4 Å, this coupling can be used for sensitive displacement detection in both armchair and zigzag graphene nanoribbon NEMS. Here it is shown that for ordered as well as disordered ribbon systems of length L, a strain ε~ (w/L)^2 due to a deflection w leads to a …
We present an ab initio study of the structural, electronic, and quantum transport properties of B-N-complex edge-doped graphene nanoribbons (GNRs). We find that the B-N edge codop-ing is energetically a very favorable process and furthermore can achieve novel doping effects that are absent for the single B or N doping. The compensation effect between B and N is predicted to generally recover the excellent electronic transport properties of pristine GNRs. For the zigzag GNRs, however, the spatially localized B-N defect states selectively destroy the doped-side spin-polarized GNR edge currents …
The coupling of charge carrier motion and pseudospin via chirality for massless Dirac fermions in monolayer graphene has generated dramatic consequences, such as the unusual quantum Hall effect and Klein tunneling. In bilayer graphene, charge carriers are massive Dirac fermions with a finite density of states at zero energy. Because of their non-relativistic nature, massive Dirac fermions can provide an even better test bed with which to clarify the importance of chirality in transport measurement than massless Dirac fermions in monolayer graphene. Here, we report an electronic cloaking effect…
Local magnetic moments can be created in graphene by incorporating different defects. The possibility of regulating dynamics of magnetization in graphene, by employing the Purcell effect, is analyzed. The role of the system parameters in magnetization reversal is studied. The characteristics of such a reversal can be varied in a wide range, which can be used for various applications in spintronics. It is shown that fast magnetization reversal generates coherent radiation.
We present a theory of coherent quantum transport in ferromagnetic/ non-magnetic/ ferromagnetic heterojunctions. We predict quantum coherence to give rise to a quantum spin valve effect that, unlike its familiar classical analog, occurs even in the absence of a net spin current through the heterostructure. Thus the relationship between spin and charge transport is qualitatively different in the presence of quantum interference than in the (semi)classical regime. This has important implications for the design of quantum coherent spintronic devices and the interpretation of experiments.
