Lateral spin valves are used to generate and characterize pure spin currents. Non-local voltage measured in such structures provides information about spin polarization and spin decay rates. For wide high-transparency F/N contacts it was shown that the Johnson-Silsbee non-local effect is substantially enriched by closed-loop electric currents driven by local spin injection in the electrically dangling part of the valve. For valves with low-transparency F/I/N tunnel contacts such circular currents are strongly suppressed, yet we show that the voltage modifications persist, may be significant, and must be accounted for in the data analysis.
We report GaN n++/p++ interband tunnel junctions with repeatable negative differential resistance and low resistance. Reverse and forward tunneling current densities were observed to increase as Si and Mg doping concentrations were increased. Hysteresis-free, bidirectional negative differential resistance was observed at room temperature from these junctions at a forward voltage of ~1.6-2 V. Thermionic PN junctions with tunnel contact to the p-layer exhibited forward current density of 150 kA/cm^2 at 7.6 V, with a low series device resistance of 1 x 10^-5 ohm.cm^2.
The paper proposes a strict diffusion theory of the electrical contact resistance of a thermoelectric superlattice (TES) - metal couple.The limits of the contact resistance for traditional thermoeletric materials with parabolic band spectrum and for superlattices described by Fivaz model are estimated.
Edge contact geometries are thought to yield ultralow contact resistances in most non-ferromagnetic metal-graphene interfaces owing to their large metal-graphene coupling strengths. Here, we examine the contact resistance of edge- versus surface-contacted ferromagnetic metal-graphene interfaces (i.e. nickel- and cobalt-graphene interfaces) using both single-layer and few-layer graphene. Good qualitative agreement is obtained between theory and experiment. In particular, in both theory and experiment, we observe that the contact resistance of edge-contacted ferromagnetic metal-graphene interfac…
The performance of graphene-based transistors is often limited by the large electrical resistance across the metal-graphene contact. We report an approach to achieve ultra-low resistance metal contacts to graphene transistors. Through a process of metal-catalyzed etching in hydrogen, multiple nano-sized pits with zigzag edges are created in the graphene that form strong chemical bonds with deposited nickel metallization for source-drain contacts without the need for further annealing. This facile contact treatment prior to electrode metallization results in contact resistance as low as 100 ohm…
The possible origins of metal-bilayer graphene (BLG) contact resistance are investigated by taking into consideration the bandgap formed by interfacial charge transfer at the metal contacts. Our results show that a charge injection barrier (Schottky barrier) does not contribute to the contact resistance because the BLG under the contacts is always degenerately doped. We also showed that the contact-doping-induced increase in the density of states (DOS) of BLG under the metal contacts decreases the contact resistance owing to enhanced charge carrier tunnelling at the contacts. The contact dopin…
The gate-voltage (VG) dependence of the contact resistance (RC) in graphene field-effect transistors is characterized by the transmission line model. The RC-VG characteristics of Ag, Cu, and Au contacts display a dip around the charge neutrality point, and become even negative with Ag contacts. The dip structure is well reproduced by a model calculation that considers a metal-contact-induced potential variation near the metal contact edges. The apparently negative RC originates with the carrier doping from the metal contacts to the graphene channel and appears when the doping effect is more su…
We study the contact resistance and the transfer characteristics of back-gated field effect transistors of mono- and bi-layer graphene. We measure specific contact resistivity of ~7kohm*um2 and ~30kohm*um2 for Ni and Ti, respectively. We show that the contact resistance is a significant contributor to the total source-to-drain resistance and it is modulated by the back-gate voltage. We measure transfer characteristics showing double dip feature that we explain as the effect of doping due to charge transfer from the contacts causing minimum density of states for graphene under the contacts and …
Magnetic tunnel junctions (MTJs), composed of two ferromagnetic electrodes separated by a thin insulating barrier layer, are currently used in spintronic devices, such as magnetic sensors and magnetic random access memories. Recently, driven by demonstrations of ferroelectricity at the nanoscale, thin-film ferroelectric barriers were proposed to extend the functionality of MTJs. Due to the sensitivity of conductance to the magnetization alignment of the electrodes (tunnelling magnetoresistance) and the polarization orientation in the ferroelectric barrier (tunnelling electroresistance), these …
We report a systematic study of the contact resistance present at the interface between a metal (Ti) and graphene layers of different, known thickness. By comparing devices fabricated on 11 graphene flakes we demonstrate that the contact resistance is quantitatively the same for single-, bi-, and tri-layer graphene ($\sim800 \pm 200 Ωμm$), and is in all cases independent of gate voltage and temperature. We argue that the observed behavior is due to charge transfer from the metal, causing the Fermi level in the graphene region under the contacts to shift far away from the charge neutrality poin…
Point contacts between normal and ferromagnetic metals are investigated using magneto-resistance and transport spectroscopy measurements combined with micromagnetic simulations. Pronounced hysteresis in the point-contact resistance versus both bias current and external magnetic field are observed. It is found that such hysteretic resistance can exhibit, in addition to bi-stable resistance states found in ordinary spin valves, tri-stable resistance states with a middle resistance level. We interpret these observation in terms of surface spin-valve and spin-vortex states, originating from a subs…
The extremely high carrier mobility and the unique band structure, make graphene very useful for field-effect transistor applications. According to several works, the primary limitation to graphene based transistor performance is not related to the material quality, but to extrinsic factors that affect the electronic transport properties. One of the most important parasitic element is the contact resistance appearing between graphene and the metal electrodes functioning as the source and the drain. Ohmic contacts to graphene, with low contact resistances, are necessary for injection and extrac…
We demonstrate gate-tunable resonant tunneling and negative differential resistance between two rotationally aligned bilayer graphene sheets separated by bilayer WSe2. We observe large interlayer current densities of 2 uA/um2 and 2.5 uA/um2, and peak-to-valley ratios approaching 4 and 6 at room temperature and 1.5 K, respectively, values that are comparable to epitaxially grown resonant tunneling heterostructures. An excellent agreement between theoretical calculations using a Lorentzian spectral function for the two-dimensional (2D) quasiparticle states, and the experimental data indicates th…
The metal-graphene contact resistance is a technological bottleneck for the realization of viable graphene based electronics. We report a useful model to find the gate tunable components of this resistance determined by the sequential tunneling of carriers between the 3D-metal and 2D-graphene underneath followed by Klein tunneling to the graphene in the channel. This model quantifies the intrinsic factors that control that resistance, including the effect of unintended chemical doping. Our results agree with experimental results for several metals.
The conductance-voltage spectrum of molecular nanostructures measured by scanning tunneling spectroscopy (STS) is generally assumed to reflect the local density of states of the molecule. This excludes the possibility of observing negative differential resistance (NDR). We report here the observation of NDR in the scanning tunneling microscope (STM) current-voltage (I-V) characteristics of self-assembled monolayer (SAM) of 4-p-Terphenylthiol molecules on gold substrate measured using a platinum probe. We argue that the NDR arises from narrow structures in the local density of states at the tip…
The relation between tunnel magneto-resistance (TMR) and spin polarization is explored for GaMnAs/GaAlAs/GaMnAs structures where the carriers experience strong spin-orbit interactions. TMR is calculated using Landauer approach. The materials are described in the 6 band $\bf {k}\cdot \bf{p}$ model which includes spin orbit interaction. Ferromagnetism is described in the virtual crystal mean field approximations. Our results indicate that TMR is a function of of spin polarization and barrier thickness. As a result of the stong spin orbit interactions, TMR also depends on the the angle between cu…
We report a new strategy for fabricating 2D/2D low-resistance ohmic contacts for a variety of transition metal dichalcogenides (TMDs) using van der Waals assembly of substitutionally doped TMDs as drain/source contacts and TMDs with no intentional doping as channel materials. We demonstrate that few-layer WSe2 field-effect transistors (FETs) with 2D/2D contacts exhibit low contact resistances of ~ 0.3 k ohm.um, high on/off ratios up to > 109, and high drive currents exceeding 320 uA um-1. These favorable characteristics are combined with a two-terminal field-effect hole mobility ~ 2x102 cm2 V-…
We report reduced contact resistance of single-layer graphene devices by using ultraviolet ozone (UVO) treatment to modify the metal/graphene contact interface. The devices were fabricated from mechanically transferred, chemical vapor deposition (CVD) grown, single layer graphene. UVO treatment of graphene in the contact regions as defined by photolithography and prior to metal deposition was found to reduce interface contamination originating from incomplete removal of poly(methyl methacrylate) (PMMA) and photoresist. Our control experiment shows that exposure times up to 10 minutes did not i…
Quantum transport properties of intermolecular nanotube contacts are investigated. We find that atomic structure in the contact region plays important roles and resistance of contacts varies strongly with geometry and nanotube chirality. Nanotube end-end contacts have low resistance and show negative differential resistance (NDR) behavior. Contact resistance can be dramatically decreased by exerting small pressure/force between the tubes if the contact is commensurate. Significant variation and nonlinearity of contact resistance may lead to new device applications.
Monolayers of transition metal dichalcogenides (TMDCs) exhibit excellent electronic and optical properties. However, the performance of these two-dimensional (2D) devices are often limited by the large resistance offered by the metal contact interface. Till date, the carrier injection mechanism from metal to 2D TMDC layers remains unclear, with widely varying reports of Schottky barrier height (SBH) and contact resistance (Rc), particularly in the monolayer limit. In this work, we use a combination of theory and experiments in Au and Ni contacted monolayer MoS2 device to conclude the following…
For the first time, polyethyleneimine (PEI) doping on multilayer MoS2 field-effect transistors are investigated. A 2.6 times reduction in sheet resistance, and 1.2 times reduction in contact resistance have been achieved. The enhanced electrical characteristics are also reflected in a 70% improvement in ON current, and 50% improvement in extrinsic field-effect mobility. The threshold voltage also confirms a negative shift upon the molecular doping. All studies demonstrate the feasibility of PEI molecular doping in MoS2 transistors, and its potential applications in layer-structured semiconduct…
A model for the extraction of the charge density dependent mobility and variable contact resistance in thin film transistors is proposed by performing a full derivation of the current-voltage characteristics both in the linear and saturation regime of operation. The calculated values are validated against the ones obtained from direct experimental methods. This approach allows unambiguous determination of both contact and channel resistance from the analysis of the current voltage characteristics of a single device, with no a-priori assumption on the two parameters. It solves the inconsistenci…
Theory of electrical spin injection from a ferromagnetic (FM) metal into a normal (N) conductor is presented. We show that tunnel contacts (T) can dramatically increase spin injection and solve the problem of the mismatch in the conductivities of a FM metal and a semiconductor microstructure. We also present explicit expressions for the spin-valve resistance of FM-T-N- and FM-T-N-T-FM-junctions with tunnel contacts at the interfaces and show that the resistance includes both positive and negative contributions (Kapitza resistance and injection conductivity, respectively).
We have studied the finite bias transport properties of a 2H-1T' MoS$_2$ lateral metal-semiconductor (M-S) junction by non-equilibrium Green's functions calculations, aimed at contacting the 2D channel in a field effect transistor. Our results indicate that (a) despite the fundamentally different electrostatics of line and planar dipoles, the Schottky barrier heights respond similarly to changes in doping and applied bias in 2D and 3D M-S junctions, (b) 2H-1T' MoS$_2$ lateral junctions are free from Fermi level pinning, (c) armchair interfaces have superior contacting properties vs.\ zigzag in…
The impact of semiconductor surface roughness on the electrical and thermal contact resistances and thermoEMF of thermoelectric material (TEM)-metal transient contact layer is studied theoretically. The distribution of hollows and humps on the rough surface is simulated by the truncated Gaussian distribution. The impact of distribution parameters on the electrical contact resistance and thermoEMF of thermoelectric material-metal contact is studied.
Aim of this paper is to find out a simple model, which provides more insight into the relation between the tunneling current and the chemical force arising during formation of atomic contact in between a SPM probe and surface atom.
Several data sets of electrical breakdown in air of single-wall carbon nanotubes (SWNTs) on insulating substrates are collected and analyzed. A universal scaling of the Joule breakdown power with nanotube length is found, which appears independent of the insulating substrates used or their thickness. This suggests the thermal resistances at the interface between SWNT and insulator, and between SWNT and electrodes, govern heat sinking from the nanotube. Analytical models for the breakdown power scaling are presented, providing an intuitive, physical understanding of the breakdown process. The e…
Contact resistances between organic semiconductors and metals can dominate the transport properties of electronic devices incorporating such materials. We report measurements of the parasitic contact resistance and the true channel resistance in bottom contact poly(3-hexylthiophene) (P3HT) field-effect transistors with channel lengths from 400 nm up to 40 $μ$m, from room temperature down to 77 K. For fixed gate voltage, the ratio of contact to channel resistance decreases with decreasing temperature. We compare this result with a recent model for metal-organic semiconductor contacts. Mobilitie…
The large tunneling anisotropic magneto-resistance of a single $p^{++}$-(Ga,Mn)As/$n^{+}$-GaAs Zener-Esaki diode is evidenced in a perpendicular magnetic field over a large temperature and voltage range. Under an applied bias, the tunnel junction transparency is modified, allowing to continuously tune anisotropic transport properties between the tunneling and the ohmic regimes. Furthermore, an asymmetric bias-dependence of the anisotropic tunneling magneto-resistance is also observed: a reverse bias highlights the full (Ga,Mn)As valence band states contribution, whereas a forward bias only pro…
Capillary and van der Waals forces cause nanotubes to deform or even collapse under metal contacts. Using ab-initio bandstructure calculations, we find that these deformations reduce the bandgap by as much as 30\%, while fully collapsed nanotubes become metallic. Moreover degeneracy lifting, due to the broken axial symmetry and wavefunctions mismatch between the fully collapsed and the round portions of a CNT, leads to a three times higher contact resistance. The latter we demonstrate by contact resistance calculations within the tight-binding approach.
The negative differential resistance (NDR) tunnel diodes are promising alternative devices for beyond-CMOS computing as they offer several potential applications when integrated with transistors. We propose a novel semiconductor-free NDR tunnel diode concept that exhibits an ultra-high peak-to-valley current ratio (PVCR) value. Our proposed NDR diode consists of two cold metal electrodes separated by a thin insulating tunnel barrier. The NDR effect stems from the unique electronic band structure of the cold metal electrodes, i.e., the width of the isolated metallic bands around the Fermi level…
We study the ballistic transmission and the contact resistance ($R_c$) of a graphite-graphene contact in a top contact geometry from first principles. We find that the calculated $R_c$'s depend on the amount of graphene-graphite overlap, but quickly saturate for transfer lengths of the order of 20 Å. For contacts overlapping more than this transfer length, the $R_c$ can be lower than the 100 Ω$\cdot$μm mark. On the other hand, edge graphite-graphene contacts are expected to have very low contact resistance.
The tunneling current between independently contacted graphene sheets separated by boron nitride insulator is calculated. Both dissipative tunneling transitions, with momentum transfer due to disorder scattering, and non-dissipative regime of tunneling, which appears due to intersection of electron and hole branches of energy spectrum, are described. Dependencies of tunneling current on concentrations in top and bottom graphene layers, which are governed by the voltages applied through independent contacts and gates, are considered for the back- and double-gated structures. The current-voltage…
Although monolayer black phosphorus (BP) or phosphorene has been successfully exfoliated and its optical properties have been explored, most of electrical performance of the devices is demonstrated on few-layer phosphorene and ultra-thin BP films. In this paper, we study the channel length scaling of ultra-thin BP field-effect transistors (FETs), and discuss a scheme for using various contact metals to change transistor characteristics. Through studying transistor behaviors with various channel lengths, the contact resistance can be extracted from the transfer length method (TLM). With differe…
The contact resistance between two dissimilar semiconductors is determined by the carrier transmission through their interface. Despite the ubiquitous presence of interfaces, quantitative simulation of charge transport across such interfaces is difficult, limiting the understanding of interfacial charge transport. This work employs Green's functions to study the charge transport across representative Si/Ge interfaces. For perfect interfaces, it is found that the transmittance is small and the contact resistance is high, not only because the mismatch of carrier pockets makes it hard to meet the…
We report the fabrication of both n-type and p-type WSe2 field effect transistors with hexagonal boron nitride passivated channels and ionic-liquid (IL)-gated graphene contacts. Our transport measurements reveal intrinsic channel properties including a metal-insulator transition at a characteristic conductivity close to the quantum conductance e2/h, a high ON/OFF ratio of >107 at 170 K, and large electron and hole mobility of ~200 cm2V-1s-1 at 160 K. Decreasing the temperature to 77 K increases mobility of electrons to ~330 cm2V-1s-1 and that of holes to ~270 cm2V-1s-1. We attribute our abilit…
Using ab-initio methods we investigate the possibility of three-terminal graphene "T-junction" devices and show that these all-graphene edge contacts are energetically feasible when the 1D interface itself is free from foreign atoms. We examine the energetics of various junction structures as a function of the atomic scale geometry. Three-terminal equilibrium Green's functions are used to determine the transmission spectrum and contact resistance of the system. We find that the most symmetric structures have a significant binding energy, and we determine the contact resistances in the junction…
We investigate spin relaxation in graphene by systematically comparing the roles of spin absorption, other contact-induced effects (e.g. fringe fields, etc.), and bulk spin relaxation for graphene spin valves with MgO barriers, Al2O3 barriers, and transparent contacts. We obtain effective spin lifetimes by fitting the Hanle spin precession data with two models that include or exclude the effect of spin absorption. Results indicate that additional contact-induced spin relaxation other than spin absorption dominates the contact effect. For tunneling contacts, we find reasonable agreement between…
Potential steps naturally develop in graphene near metallic contacts. We investigate the influence of these steps on the transport in graphene Field Effect Transistors. We give simple expressions to estimate the voltage-dependent contribution of the contacts to the total resistance and noise in the diffusive and ballistic regimes.
We present transfer-length-method measurements of the contact resistance between Cu and graphene, and a method to significantly reduce the contact resistance by vacuum annealing. Even in samples with heavily contaminated contacts, the contacts display very low contact resistance post annealing. Due to the common use of Cu, and it's low chemical reactivity with graphene, thermal annealing will be important for future graphene devices requiring non-perturbing contacts with low contact resistance.
We investigate a wafer scale tunnel junction fabrication method, where a plasma etched via through a dielectric layer covering bottom Al electrode defines the tunnel junction area. The ex-situ tunnel barrier is formed by oxidation of the bottom electrode in the junction area. Room temperature resistance mapping over a 150 mm wafer give local deviation values of the tunnel junction resistance that fall below 7.5 % with an average of 1.3 %. The deviation is further investigated by sub-1 K measurements of a device, which has one tunnel junction connected to four arrays consisting of N junctions (…
We have observed that submicron sized Al--AlO{$_x$}--Al tunnel junctions can be stabilized completely by annealing them in vacuum at temperatures between $350^{\circ}$C and $450^{\circ}$C. In addition, low temperature characterization of the samples after the annealing treatment showed a marked improvement of the tunneling characteristics due to disappearance of unwanted resonances in the current. Charging energy, tunneling resistance, barrier thickness and height all increase after the treatment. The superconducting gap is not affected, but supercurrent is reduced in accordance with the incre…
We perform first-principles calculations of electron transport across a nickel-graphene interface. Four different geometries are considered, where the contact area, graphene and nickel surface orientations and the passivation of the terminating graphene edge are varied. We find covalent bond formation between the graphene layer and the nickel surface, in agreement with other theoretical studies. We calculate the energy-dependent electron transmission for the four systems and find that the systems have very similar edge contact resistance, independent of the contact area between nickel and grap…
In this work, an evaluation of the contact quality of black phosphorus (BP) field-effect transistors (FETs) from different technologies previously reported is performed by means of an efficient and reliable contact resistance extraction methodology based on individual device practical characteristics. A good agreement is achieved between the extracted values with the Y-function method used here and reference values obtained with other methods considering internal values as well as with more expensive methods involving fabricated test structures. The method enables a direct evaluation of differ…
We calculate the tunnelling resistance at the quantum critical point of a mesoscopic SQUIDs array in the presence of magnetic flux. We find the analytical relation between the magnetic flux induced dissipation strength and the Luttinger liquid parameter of the system. While the experimental finding for the system is around 40-50 mK, we find the behavior of the system even at lower temperatures through the analysis of renormalization group. Apart from the length scale dependent superconductor-insulator transition, we also predict the evidence of length scale independent metallic state. This stu…
We analyze the effect of contact resistance on the Lorenz number measurement based on direct electronic thermal conductivity experiments. The contact resistance can significantly limit the experimental measured value when the Lorenz number is enhanced, but not as much so when it is suppressed, should the Wiedemann-Franz law be violated. The result provides the conditions of the potential false negative error and highlights the importance of improving the contact resistance in studying non-Fermi liquid behavior in thermal transport experiments.
The study of electron transport in low-dimensional systems is of importance, not only from a fundamental point of view, but also for future electronic and spintronic devices. In this context heterostructures containing a two-dimensional electron gas (2DEG) are a key technology. In particular GaAs/AlGaAs heterostructures, with a 2DEG at typically 100 nm below the surface, are widely studied. In order to explore electron transport in such systems, low-resistance ohmic contacts are required that connect the 2DEG to macroscopic measurement leads at the surface. Here we report on designing and meas…
Metal contacts have been identified to be a key technological bottleneck for the realization of viable graphene electronics. Recently, it was observed that for structures that possess both a top and a bottom gate, the electron-hole conductance asymmetry can be modulated by the bottom gate. In this letter, we explain this observation by postulating the presence of an effective thin interfacial dielectric layer between the metal contact and the underlying graphene. Electrical results from quantum transport calculations accounting for this modified electrostatics corroborate well with the experim…
The ability to tune the thermal resistance of carbon nanotube mechanical supports from insulating to conducting could permit the next generation of thermal management devices. Here, we demonstrate fabrication techniques for carbon nanotube supports that realize either weak or strong thermal coupling, selectively. Direct imaging by in-situ electron thermal microscopy shows that the thermal contact resistance of a nanotube weakly-coupled to its support is greater than 250 K*m/W and that this value can be reduced to 4.2(+5.6/-2.1) K*m/W by imbedding the nanotube in metal contacts.
We consider a two-dimensional magnetic tunnel junction of the FM/I/QW(FM+SO)/I/N structure, where FM, I and QW(FM+SO) stand for a ferromagnet, an insulator and a quantum wire (QW) with both magnetic ordering and Rashba spin-orbit (SOC), respectively. The tunneling magneto-resistance (TMR) exhibits strong anisotropy and switches sign as the polarization direction varies relative to the QW axis, due to interplay among the one-dimensionality, the magnetic ordering, and the strong SOC of the QW. The results may provide a possible explanation for the sign-switching anisotropic TMR recently observed…
We report an approach to achieve low-resistance contacts to MoS2 transistors with the intrinsic performance of the MoS2 channel preserved. Through a dry transfer technique and a metal-catalyzed graphene treatment process, nickel-etched-graphene electrodes were fabricated on MoS2 that yield contact resistance as low as 200 ohm-um. The substantial contact enhancement (~2 orders of magnitude) as compared to pure nickel electrodes, is attributed to the much smaller work function of nickel-graphene electrodes, together with the fact that presence of zigzag edges in the treated graphene surface enha…
Contact resistance is a severe performance bottleneck for electronic devices based on two-dimensional layered (2D) semiconductors, whose contacts are Schottky rather than Ohmic. Although there is general consensus that the injection mechanism changes from thermionic to tunneling with gate biasing, existing models tend to oversimplify the transport problem, by neglecting the 2D transport nature and the modulation of the Schottky barrier height, the latter being of particular importance in back-gated devices. In this work, we develop a semi-analytical model based on Bardeen's transfer Hamiltonia…
For metallic point contacts with Be and Al the magnetoquantum oscillations in the contact resistance have been investigated as a function of the applied voltage over the contact. For one set of point contacts the oscillation amplitude is found to vary nonmonotonously with the applied voltage with similarities to the point-contact spectrum of the electron-phonon interaction. The other part of the investigated point contacts shows a decrease of the oscillation amplitude with increasing bias voltage. For the understanding of the voltage dependence of the amplitude of the point-contact magnetoresi…
Magnetoquantum oscillations of resistance as functions of the potential difference applied across the contact are studied for metallic point contacts made of $Al$ and $Be$. The amplitude of resistance oscillations in a magnetic field increases with voltage and is identical to the EPI spectrum of the point contact for one group of contacts, and with the bias voltage across the contact for another. The increase in the oscillation amplitude as well as its decrease has a nonmonotonic dependence on energy. The scattering of electrons by nonequilibrium phonons and the Fermi-liquid effects in the non…
