Linear Current-voltage Characteristics Research Articles

Surface and Electrical Characterization of Non-Stoichiometric Semiconducting Thin-Film Coatings Based on Ti-Co Mixed Oxides Obtained by Gas Impulse Magnetron Sputtering

This article presents a detailed investigation of non-stoichiometric (Ti,Co)Ox thin films prepared using the Gas Impulse Magnetron Sputtering (GIMS) technique. The films were prepared with various Co contents (3 at.%, 19 at.%, 44 at.%, and 60 at.%) and characterized for their material composition, microstructure, and electrical properties. The films exhibited an ohmic behavior with linear current-voltage (I-V) characteristics, and their resistivity values ranged from approximately 10−3 to 104 Ω·cm. The highest resistivity was observed in the film with 3 at.% Co content. Thermoelectric measurements revealed that all of the prepared films displayed n-type semiconducting properties, with the Seebeck coefficient (S) tending close to zero. The resistivity of the films decreased as the temperature increased, affirming their semiconducting nature. The activation energy (Ea) values, determined using the Arrhenius formula, ranged from 0.0058 eV to 0.267 eV, with the highest Ea observed for films containing 3 at.% Co. Additionally, the films’ surface topography and microstructure were examined through Atomic Force Microscopy (AFM) and optical profiler techniques. The results showed that the films had smooth, crack-free surfaces with remarkable homogeneity. The surface diversification decreased with the increase in cobalt in the (Ti,Co)Ox films.

Superconducting nonlinear transport in optically driven high-temperature K3C60

Optically driven quantum materials exhibit a variety of non-equilibrium functional phenomena, which to date have been primarily studied with ultrafast optical, X-Ray and photo-emission spectroscopy. However, little has been done to characterize their transient electrical responses, which are directly associated with the functionality of these materials. Especially interesting are linear and nonlinear current-voltage characteristics at frequencies below 1 THz, which are not easily measured at picosecond temporal resolution. Here, we report on ultrafast transport measurements in photo-excited K3C60. Thin films of this compound were connected to photo-conductive switches with co-planar waveguides. We observe characteristic nonlinear current-voltage responses, which in these films point to photo-induced granular superconductivity. Although these dynamics are not necessarily identical to those reported for the powder samples studied so far, they provide valuable new information on the nature of the light-induced superconducting-like state above equilibrium Tc. Furthermore, integration of non-equilibrium superconductivity into optoelectronic platforms may lead to integration in high-speed devices based on this effect.

Improving electric-energy transmission performance of LixNi1-xO linear ceramic resistors by Li+/Ni3+ ion occupation regulation for pulsed power technology

Linear ceramic resistor receives extensive attention for its great advantages of fast energy transmission in pulsed power technology. However, the research about improving the linear current-voltage characteristics and pulsed charge-discharge speed of LixNi1-xO solid solution is rarely reported. Herein, we fabricated the LixNi1-xO linear ceramic resistors through a solid-phase reaction method, and the effects of the Li+ ion on the microstructure, current-voltage characteristics and pulsed charge-discharge properties were studied. Specifically, the electron holes exchange between divalent and trivalent nickel ions is enhanced by Li substitution, which contributed to the significantly decreasing in nonlinear coefficient (α) and resistivity. On a separate note, the rising edge of pulse charge-discharge is influenced by the linearity of LixNi1-xO ceramic resistors. The closer the nonlinear coefficient is to 1, the shorter the pulsed charge-discharge time is. Meanwhile, the defect dipoles induced by Li substitution may also contribute to the reduction of discharge time of LixNi1-xO linear ceramic resistors. As a result, the Li0.03Ni0.97O linear ceramic resistor with an excellent nonlinear coefficient of 1.03 exhibits extraordinary fast discharge speed (t0.9 = 94.8 ns) at 1.0 kV. The investigation on LixNi1-xO linear ceramic resistors suggests the potential of this material to be applied in the pulsed power system.

Local electrical conductivity of carbon black/PDMS nanocomposites subjected to large deformations

The full-field microscale electrical behavior of polydimethylsiloxane (PDMS) matrix composites with 12 wt% (below the electrical percolation threshold) and 20 wt% (above the electrical percolation threshold) carbon black (CB) nanoparticles was obtained with the aid of Conductive Atomic Force Microscopy (C-AFM) as a function of uniaxial tensile strain. A comparison between the macroscale and the microscale 2D strain fields showed that 25 × 25 μm2 specimen domains can serve as a representative surface element (RSE) to describe the macroscale mechanical response under finite deformations. The same surface domain size was shown to serve as RSE for the local electrical behavior of CB/PDMS nanocomposites at small and large strains. The transverse specimen contraction due to the Poisson’s effect increased the through-thickness conductivity of 12 wt% CB specimens, with a rapid increase in the total electric current measured at the scale of the RSE taking place above 23% uniaxial true strain. The locally dominant tunneling conductance in the undeformed specimen state transitioned to a combination of Ohmic (linear local current-voltage (I–V) characteristics) and tunneling (parabolic local I–V characteristics) at 35% uniaxial true strain. In comparison, the local I–V characteristics of undeformed PDMS composites with 20 wt% CB were a combination of Ohmic (contributing the majority of the total current) and tunneling. Upon application of 35% uniaxial true strain the vast majority of local conduction sites became Ohmic, while the total electric current at the scale of the RSE increased relatively linearly with applied strain.

Anomalous metallic state regulated by magnetic field at LaAlO<sub>3</sub>/SrTiO<sub>3</sub> heterointerface

<sec>Since the discovery of two-dimensional electron gas with high mobility at the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> heterointerface, many physical properties such as two-dimensional superconductivity, magnetism and spin-orbit coupling have been widely studied. The origin of the transition from quantum superconductor to metal at zero temperature in two-dimensional superconductor is still an open problem, which has been discussed intensely. According to the conventional theory, when the temperature is close to zero, the superconductor-insulator transition can be observed by applying a magnetic field or magnetic field effect of disorder, and the ground state should be superconducting or insulating.</sec><sec>However, when Jaeger et al. (Jaeger H M, Haviland D B, Orr B G, Goldman A M <ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.1103/PhysRevB.40.182">1989 <i>Phys. Rev. B</i> <b>40</b> 182</ext-link>) studied the relationship between superconductivity evolution and thickness and temperature in a superconducting granular film, they found that there exists an intermediate metal state that can destroy the direct transition between superconducting and insulating. The intermediate metal state is characterized by the existence of saturation resistance at superconducting transition temperature, and the corresponding ground state is called anomalous metallic state. In addition to the saturation of resistance at low temperature, the characteristics of an anomalous metallic state also include the linear current-voltage (<i>I-V</i>) characteristics in the low current range, the giant positive magnetoresistance (MR), the vanishing of Hall resistance (<i>R</i><sub><i>xy</i></sub>), and the tuning capability adjusted by changing a variety of parameters including degree of disorder, gate voltage and magnetic field.</sec><sec>In this work, we systematically investigate the electrical transport properties of LaAlO<sub>3</sub>/SrTiO<sub>3</sub> (001) heterointerface in a perpendicular magnetic field at low temperature. The <i>R-T</i> curves and the <i>I-V</i> characteristics in zero magnetic field show that LaAlO<sub>3</sub>/SrTiO<sub>3</sub> (001) heterointerface is in a superconducting state. However, after a small magnetic field is applied, the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> (001) heterointerface has the characteristics of resistance saturation at low temperature, linear <i>I-V</i> characteristics, giant positive MR, abnormal Hall response, indicating the clear characteristics of an anomalous metallic state. The sample undergoes a transition from quantum superconductor to metal at temperatures approaching to zero.</sec><sec>In addition, we observe that the anomalous metallic state in an unusually large region under the action of magnetic field, and our main observations are summarized in the <i>H-T</i> phase diagram. By analyzing the relationship between the resistance of the anomalous metallic regime and the magnetic field, and the vanish of Hall resistance, we infer that the anomalous metallic state observed in LaAlO<sub>3</sub>/SrTiO<sub>3</sub> (001) heterointerface can be explained by Bose metal model. According to our findings, the magnetic field regulated LaAlO<sub>3</sub>/SrTiO<sub>3</sub> (001) heterointerface appears as a special platform to study the details of anomalous metallic state in a controllable way.</sec>

Bisdithiocarbamate and Diamine Interlinked Gold Nanoparticle Networks: Characterization of Chemical Composition and Chemiresistive Properties

Chemiresistive composites of gold (Au) nanoparticles interlinked with different types of organic molecules were prepared automatically by layer-by-layer self-assembly using a microfluidic cell. For the assembly process, dodecylamine-stabilized Au nanoparticles with an average size of 3.7 nm as well as alkyl dithiols, alkyl diamines, and alkyl bisdithiocarbamates with different alkyl chain length (C6 and C8) were used. X-ray photoelectron spectroscopy was applied on prepared nanoparticle composites to study the film composition and the degree of interlinkage. For the measurement of electrical and vapor-sensing properties, silicon dies equipped with gold interdigitated electrodes were used. All films show linear current-voltage characteristics and conductivities in the range of 10–2 and 10–4 Ω−1 cm−1 at room temperature. The sensitivity of the film is investigated by dosing them with vapors of toluene, 1-propanol, 4-methyl-2-pentanone, and water in the concentration range from 100 to 5,000 ppm at 0% relative humidity. All composite films respond with an increase in their electrical resistance to the analytes. The sensors show a high signal-to-noise ratio which indicates a detection limit below 100 ppm for all test vapors. The response dynamics demonstrate a high reversibility and a fast sensing mechanism especially for dithiols and diamines with response and recovery times from 2 to 10 s. The dithiol sensors exhibit a high selectivity to toluene and 4-methyl-2-pentanone whereas the bisdithiocarbamate composites are suitable for the detection of water and 1-propanol. All materials are stable for (at least) several months.

Electron transport in boron functionalised armchair graphene nanoribbons: Potential interconnects

Density functional theory (DFT) coupled with non-equilibrium Green's function (NEGF) formalism is employed to explore the electronic and transport properties of armchair graphene nanoribbons (AGNR) doped with Boron (B) at 6.25%, 12.5%, 18.75% and 25% doping concentrations. The modeled (3,0) AGNR exhibit a semiconducting behaviour with a band gap of 1.06eV and as a effect of Boron doping (6.25%, 12.5% and 18.75%) shows fluctuating semiconducting or metallic behaviour. However, at around the boron concentration of 25% AGNR remains metallic it exhibit relatively better conductance than silver, gold, copper and its other B doped counterparts. Further, the interesting linear Current-Voltage characteristics and conductance of boron doped AGNR may lead to its application as interconnects.

Conduction and Electric Field Effect in Ultra-Thin Tungsten Films

Ultra-thin tungsten films were prepared using hotwire assisted atomic layer deposition. The film thickness ranged from 2.5 to 10 nm, as determined by spectroscopic ellipsometry and verified by scanning electron microscopy. The films were implemented in conventional Van der Pauw and circular transmission line method (CTLM) test structures, to explore the effect of film thickness on the sheet and contact resistance, temperature coefficient of resistance (TCR), and external electric field applied. All films exhibited linear current-voltage characteristics. The sheet resistance was shown to considerably vary across the wafer, due to the film thickness non-uniformity. The TCR values changed from positive to negative with decreasing the film thickness. A field-induced modulation of the sheet resistance up to $\sim 4.6\cdot 10^{-4}\,\,\text{V}^{-1}$ was obtained for a 2.5 nm thick film, larger than that generally observed for metals.

Improvement of UV photodetector properties of reactively sputtered TiO2−x films through vacuum annealing

We present the influence of vacuum annealing on RF magnetron-sputtered TiO2−x thin films grown at various oxygen partial pressures ($$p_{{{\text{O}}_{2} }}$$) of 2.0%, 4.0% and 6.0% to enhance the ultra-violet (UV) photodetector performance. Apart from the film deposited at 2.0% of $$p_{{{\text{O}}_{2} }}$$, all the as-grown TiO2−x films show an amorphous nature, whereas thermally annealed TiO2−x films at 400 °C show the rutile phase. As revealed by the linear current–voltage characteristics, the ohmic-contact behavior was observed between the TiO2−x layer and the electrode material. Benefiting from the above features, the photocurrent was significantly increased at 4.0% of $$p_{{{\text{O}}_{2} }}$$ due to the increase of oxygen vacancies and the suppression of electron–hole recombination. As a result, thermally annealed TiO2−x films are very useful for next-generation UV-photodetectors even at a lower power density of 1.72 mW/cm2. Therefore, the merits of the above findings present a promising strategy to enhance the UV photoresponse of thermally annealed TiO2−x films by optimizing the $$p_{{{\text{O}}_{2} }}$$ at 4.0%.

Low resistivity ohmic contacts on lightly doped n-type β-Ga2O3 using Mg/Au

Mg/Au stacks were introduced to prepare ohmic contact at different annealing temperatures on Sn lightly doped β-Ga2O3. Linear current–voltage characteristics were realized when the annealing temperatures were 300, 400 and 500 °C. The transmission line measurements were used to extract the specific contact resistance (ρc) of the annealed samples. The ρc decreased as annealing temperature increased and a minimum ρc of 2.1 × 10−5 Ω cm2 was obtained for the sample annealed at 500 °C. However, the contact electrodes with the ρc of 1.3 × 10−4 Ω cm2 after annealing at 400 °C had better surface profile and stability. The ohmic contact mechanism and stability of electrodes were discussed in details.

Characteristics of electronic and spin-independent linear conductance in conjugated aromatic polymer based molecular device

Using the non-equilibrium Green's function formalism combined with density-functional theory, we performed a theoretical research to study the electronic and spin-dependent electron transport of a molecular device constructed from a 2-TBQP, 1D-conjugated aromatic polymer(CAP) and 1D-CAP with dihedral angle being 180° sandwiched between two semiinfinite zigzag-edged graphene nanoribbon (zGNR) electrodes. Results show that the proposed 1D-CAP model exhibits an interesting linear conductive behavior, which is much larger than other two models. The intriguing linear current-voltage characteristic can be explained by the well delocalized spatial distribution of frontier orbitals and the separation of electron transmission pathways. Moreover, it's also shown that the spin-charge transport can be modulated by performing the magnetic configuration in zGNR electrodes. The proposed models show many spin related behaviors, such as single spin conductance, dual spin filter and spin-independent linear conductance. The corresponding mechanisms of the above mentioned spin-resolved results can be analyzed and discussed in terms of the spin transmission spectra and the separation of isosurfaces of spin density of these systems.

Ionic Diode for Ultra-High Temperature Logic Circuit

Data collecting from extremely high temperature environment such as space, earth core, and manufacturing places will provide fruitful new information. This will contribute to further development of the wide variety of research fields ranging from basic sciences to cutting-edge technologies. For that, new type of logic circuit which can work at extremely high temperature is necessary. As a candidate of such high temperature logic circuits, wide band gap materials such as SiC, GaN, AlN, and diamond have been proposed and widely studied. Although these devices work properly at higher temperature than the conventional Si-based devices, much higher temperature, for instance 1273 K, is desired for the data collecting from an extreme environmental condition. However, such high temperature is undesirable for the electronic devices, because thermally excited electron and hole severely disturb proper operation. For the realization of the ultra-high temperature logic circuits, it is necessary to develop a new type of logic circuit device based on a new working mechanism. In this study, we propose an ionic diode as a promising candidate for the ultra-high temperature logic circuits. Basic idea of the ionic diode is the analogy to the p-n junction of the typical electronic devices. Similar to the electronic depletion layer at the interface of p-type and n-type semiconductors, the ionic depletion layer can be formed at the interface of a vacancy conductor and an interstitial ion conductor. Under the forward bias condition, interstitial ions and vacancies are supplied to the ion depletion layer which makes the ion depletion layer narrower. In this case, the interface is not resistive. Under the reverse bias condition, on the other hand, interstitial ions and vacancies are extracted from the ion depletion layer which makes the ion depletion layer wider. In this case, the interface become resistive. Then, one can expect an asymmetric current-voltage characteristics at this interface, so-called a rectifying effect. Since the thermal excitation of ionic defects are smaller than that of the electronic species, this ionic diode is considered to work properly even at high temperatures. In this study, ionic diodes were fabricated from oxygen vacancy conductors, yttria stabilized zirconia (YSZ) and gadolinia-doped ceria (GDC), and interstitial oxide ion conductors, La-doped PbWO4 (PLW) and La-Si apatite oxides (LSO). A dense pellet of these oxides were prepared and one side of the pellet was mirror polished. Pt electrodes were set on the opposite surface as a current electrode and on the side face of the pellet as a potential electrode. Bulk type cell was constructed by contacting polished surface of an oxygen vacancy conductor pellet and an interstitial oxygen conductor pellet. To make good contact, these pellets were mechanically pressed by a spring. The current-voltage characteristics of the bulk cells were evaluated at 873-1373 K in an artificial air condition. Since mobile ionic defects are interstitial oxygen and oxygen vacancy, the forward bias is polarizing the oxygen vacancy conductor positively and the interstitial oxygen conductor negatively. For the YSZ/PLW cell and the YSZ/LSO cell, a gradual current increase was observed under the forward bias condition while a gradual current decrease was observed under the reverse bias condition. The bulk cells of the YSZ/PLW and the YSZ/LSO showed weak but clear rectifying behavior, while the YSZ/YSZ and the PLW/PLW cells showed a linear current-voltage characteristics. The rectifying ratio (the forward current over the reverse current) of the YSZ/PLW and the YSZ/LSO reaches about 1.4 at 973 K and 1.3 at 1273 K, respectively. The possible cause of the low rectification ratio of the bulk cells are imperfect contacts of the pellets, large ohmic resistance of the thick pellets, and the formation of an interphase at the interface. Although the rectifying ratio is small at the present stage, we have succeeded to demonstrate an ultra-high temperature ionic diode. In the presentation, we are going to talk about our latest progress and some theoretical discussions.

Effect of Rare-Earth Metal Oxide Nanoparticles on the Conductivity of Nanocrystalline Titanium Dioxide: An Electrical and Electrochemical Approach

Doping of rare-earth metal oxides into nanocrystalline titanium dioxide (NTD) films is known to improve performance for photovoltaic and photocatalytic applications; however, the reasons for this improvement are not well understood. To explore the enhancement mechanism, an electrical and electrochemical study of rare-earth oxide-doped NTD films was performed. Doped films were found to be 40–50 times more conductive than undoped films, with linear current–voltage characteristics and decreased light sensitivity. Cyclic voltammograms of doped samples show an enhanced scan rate dependence in the deep trap regime due to a slower charge trapping rate. Finally, electrochemical impedance measurements reveal a decrease in space charge density and a shift in the flat-band potential. Taken together, these results suggest that charge transfer from the rare earth oxide neutralizes the deep trap states in the NTD film, decreasing charge scattering, and improving the NTD performance as an electron acceptor and electron ...

Estimation of intrinsic and extrinsic capacitances of graphene self-switching diode using conformal mapping technique

Due to a very high carrier concentration and low band gap, graphene based self-switching diodes do not demonstrate a very high rectification ratio. Despite that, it takes the advantage of graphene’s high carrier mobility and has been shown to work at very high microwave frequencies. However, the AC component of these devices is hidden in the very linear current–voltage characteristics. Here, we extract and quantitatively study the device capacitance that determines the device nonlinearity by implementing a conformal mapping technique. The estimated value of the nonlinear component or curvature coefficient from DC results based on Shichman–Hodges model predicts the rectified output voltage, which is in good agreement with the experimental RF results.

Influence of DC-biasing on the performance of graphene spin valve

Generating and controlling the spin valve signal are key factors in ‘spintronics’, which aims to utilize the spin degree of electrons. For this purpose, spintronic devices are constructed that can detect the spin signal. Here we investigate the effect of direct current (DC) on the magnetoresistance (MR) of graphene spin valve. The DC input not only decreases the magnitude of MR but also distorts the spin valve signal at higher DC inputs. Also, low temperature measurements revealed higher MR for the device, while the magnitude is noticed to decrease at higher temperatures. Furthermore, the spin polarization associated with NiFe electrodes is continuously increased at low DC bias and low temperatures. We also demonstrate the ohmic behavior of graphene spin valve by showing linear current-voltage (I-V) characteristics of the junction. Our findings may contribute significantly in modulating and controlling the spin transport properties of vertical spin valve structures.

Highly Biased Linear Condition Method for Separately Extracting Source and Drain Resistance in MOSFETs

A highly biased linear current method (HBLCM) for separately extracting source and drain resistance ( ${R}_{S}$ and ${R}_{D}$ ) in MOSFETs is proposed. The technique can be applied to a single device by using simple modeling. Compared to other methods, it provides accurate values of ${R}_{S}$ and ${R}_{D}$ because it considers carrier mobility degradation. The method basically uses linear current versus gate voltage ( ${I}_{\text {DS}} - {V}_{\text {GS}}$ and ${I}_{\text {SD}} - {V}_{\text {GD}}$ ) characteristics before and after the source/drain interchange ( ${I}_{\text {DS}}$ and ${I}_{\text {SD}}$ ). Afterward, by using the traditional Y-function and subsequent resistance modeling in a highly biased linear condition, ${R}_{S}$ and ${R}_{D}$ can be separately extracted. In order to evaluate and verify the accuracy of HBLCM, an external resistor was intentionally connected to a source electrode of a device, and the resulting change in source resistance was detected using the proposed method. Moreover, to demonstrate an application of the proposed method, internal resistance deliberately created by hot-carrier injection (HCI) was linked to a drain electrode, thereby changing drain resistance. The changed drain resistance was also sensed by the HBLCM. Afterward, the HCI-stressed device was cured by electrothermal annealing driven by Joule heating, and the recovery was again clearly observed using the proposed method.

Use of fumed silica nanoparticles to attain polymer free novel quasi-solid state electrolyte for high-efficiency dye-sensitized solar cells

Novel gel electrolyte is prepared by replacing conventional polymers solely by fumed silica nanoparticles. In the gel composition, ethylene carbonate and propylene carbonate are used as solvents with the tetrapropylammonium iodide/iodine redox couple. The molar ratio of total oxygen moles of ethylene carbonate and propylene carbonate to iodide moles of tetrapropylammonium iodide (O:I−) is varied from 80:1 to 40:1 and the best room temperature ionic conductivity of 6.90×10−3 S cm−1 is achieved for the system with the molar ratio at 50:1. The dye-sensitized solar cell (DSC) employing this gel electrolyte gives a light-to-electricity conversion efficiency of 7.46% under irradiation of 100 mW cm−2. The electrolyte and the fabricated DSCs are characterized using linear sweep voltammetry, FT-IR spectroscopy, current-voltage characteristics and photocurrent action spectra.

Improvement of Ohmic contacts on Ga2O3 through use of ITO-interlayers

The use of ITO interlayers between Ga2O3 and Ti/Au metallization is shown to produce Ohmic contacts after annealing in the range of 500–600 °C. Without the ITO, similar anneals do not lead to linear current–voltage characteristics. Transmission line measurements were used to extract the specific contact resistance of the Au/Ti/ITO/Ga2O3 stacks as a function of annealing temperature. Sheet, specific contact, and transfer resistances all decreased sharply from as-deposited values with annealing. The minimum transfer resistance and specific contact resistance of 0.60 Ω mm and 6.3 × 10−5 Ω cm2 were achieved after 600 °C annealing, respectively. The conduction band offset between ITO and Ga2O3 is 0.32 eV and is consistent with the improved electron transport across the heterointerface.

Nonequilibrium restoration of duality symmetry in the vicinity of the superconductor-to-insulator transition

The magnetic field driven superconductor to insulator transition in thin films was theoretically analyzed via a vortex-charge duality transformation applied to the Hamiltonian. Vortices condensation was conjectured as the underline physical mechanism of the insulating phase. Experimental evidence supported duality symmetry across the magnetic-field driven superconductor to insulator transition in amorphous Indium Oxide films. Counterintuitively, duality symmetry is broken at low temperatures where the insulating phase develops strongly non linear current-voltage characteristics. Here, we follow the breakdown of duality symmetry down to very low temperatures and demonstrate the restoration of duality symmetry out of equilibrium.

Interlayer reliant magnetotransport in graphene spin valve

Here the magnetotransport properties of vertical spin valve structures incorporating graphene (Gr), Gr/Au and Gr/Al2O3 intervening layers are elucidated. An in-plane magnetic field is obliquely applied to the device with the purpose to vary the relative magnetizations of ferromagnetic electrodes (Co and Ni). The relative magnetoresistance (MR) of Co/Gr/Ni is enhanced from ∼0.16% to 0.57% by simply passivating the bottom Ni electrode with thin Au film. On the other hand, depositing Al2O3 on the bottom ferromagnetic layer in such a spin valve junction not only increases the magnitude of MR (∼−0.52%) but also reverses its polarity. Furthermore, the linear current-voltage characteristics for graphene and graphene/Au spin valve devices specifies ohmic contact, while non-linear curves indicate tunneling behavior for graphene/Al2O3 device.