Transport Measurements Of Graphene Research Articles

Sub-micrometer four-point probe transport measurements on graphene

Usually, electronic transport measurements on two-dimensional materials, such as graphene and transition metal dichalcogenides, require deposition of electrodes on top of the material, in, for instance, the form of a Hall bar device. In this work, we show that by making use of a collinear micro-four-point probe, electrical transport measurements on small flakes of graphene can be performed without having to fabricate electrodes on top of the flakes. Using probes with probe pitches down to sub-micrometer scale, we show back-gate tuned transport measurements in graphene on silicon oxide and on hexagonal boron nitride. The charge carrier mobilities and the minimum conductivity of graphene are in good agreement with conventional transport measurements. In addition, we assess the possible damage caused by landing these probes on graphene.

High-voltage carrier transport measurements in graphene and MoS2

Pulsed current–voltage characteristics have been measured for CVD monolayer graphene and exfoliated MoS2 on HfO2 and Al2O3 (sapphire) under controlled lattice temperature conditions. The unintentional donor density in MoS2 is 3.6 × 1011cm−3 and hole density in graphene is 1.4 × 1012 cm−3. Graphene samples withstand 47 V bias when 10 ns electric pulses are applied and 131 V when 3 ns pulses are used. MoS2 samples suffer from thermal breakdown at 22 V on sapphire and 145 V on HfO2 upon application of 10 ns and 3 ns duration electrical pulses, respectively. The results are treated in terms of electron drift velocity estimated from the data on current and constant carrier density under the assumption of uniform electric field. The highest hole velocity of ∼1.8(3.1)×107 cm/s is estimated in graphene on sapphire(HfO2).

Spin transport in high-mobility graphene on WS2 substrate with electric-field tunable proximity spin-orbit interaction

Graphene supported on a transition metal dichalcogenide substrate offers a novel platform to study the spin transport in graphene in the presence of a substrate-induced spin-orbit coupling while preserving its intrinsic charge transport properties. We report the first nonlocal spin transport measurements in graphene completely supported on a 3.5-nm-thick tungsten disulfide (${\mathrm{WS}}_{\text{2}}$) substrate, and encapsulated from the top with an 8-nm-thick hexagonal boron nitride layer. For graphene, having mobility up to 16 000 ${\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, we measure almost constant spin signals both in electron and hole-doped regimes, independent of the conducting state of the underlying ${\mathrm{WS}}_{2}$ substrate, which rules out the role of spin-absorption by ${\mathrm{WS}}_{2}$. The spin-relaxation time ${\ensuremath{\tau}}_{\text{s}}$ for the electrons in graphene-on-${\mathrm{WS}}_{2}$ is drastically reduced down to $\ensuremath{\sim}10$ ps from ${\ensuremath{\tau}}_{\text{s}}\ensuremath{\sim}800$ ps in graphene-on-${\mathrm{SiO}}_{2}$ on the same chip. The strong suppression of ${\ensuremath{\tau}}_{\text{s}}$ along with a detectable weak antilocalization signature in the quantum magnetoresistance measurements is a clear effect of the ${\mathrm{WS}}_{2}$-induced spin-orbit coupling (SOC) in graphene. Via the top-gate voltage application in the encapsulated region, we modulate the electric field by 1 V/nm, changing ${\ensuremath{\tau}}_{\text{s}}$ almost by a factor of four, which suggests electric-field control of the in-plane Rashba SOC. Further, via the carrier-density dependence of ${\ensuremath{\tau}}_{\text{s}}$, we also identify the fingerprints of the D'yakonov-Perel' type mechanism in the hole-doped regime at the graphene-${\mathrm{WS}}_{2}$ interface.

Evaluation of disorder introduced by electrolyte gating through transport measurements in graphene

We evaluate the degree of disorder in electrolyte-gating devices through transport measurements in graphene. By comparing the mobility in ion- and standard metal-gated devices, we show that the deposition of the ionic liquid introduces charged impurities that limit the mobility in graphene to 3 × 103 cm2 V−1 s−1. At higher temperatures (>50 K), phonons in the ionic liquid further reduce the mobility, making its upper limit 2 × 103 cm2 V−1 s−1 at room temperature. Since the degree of disorder is independent of the base material, these results are valuable for understanding disorder effects in general devices using electrolyte gating.

Local transport measurements in graphene on SiO2 using Kelvin probe force microscopy

By using Kelvin Probe Force Microscopy with an additional applied electric field we investigate the local voltage drop in graphene on SiO2 under ambient conditions. We are able to quantify the variation of the local sheet resistance and to resolve localized voltage drops at line defects. Our data demonstrates that the resistance of line defects has been overestimated so far. Moreover, we show that wrinkles have the largest resistance, ρWrinkle < 80 Ωμm. Temperature-dependent measurements show that the local monolayer sheet resistance reflects the macroscopic increase in resistance with temperature while the defect resistance for folded wrinkles is best described by a temperature-independent model which we attribute to interlayer tunneling.

Evidence of the role of contacts on the observed electron-hole asymmetry in graphene

We perform electrical transport measurements in graphene with several sample geometries. In particular, we design ``invasive'' probes crossing the whole graphene sheet as well as ``external'' probes connected through graphene side arms. The four-probe conductance measured between external probes varies linearly with charge density and is symmetric between electron and hole types of carriers. In contrast measurements with invasive probes give a strong electron-hole asymmetry and a sublinear conductance as a function of density. By comparing various geometries and types of contact metal, we show that these two observations are due to transport properties of the metal/graphene interface. The asymmetry originates from the pinning of the charge density below the metal, which thereby forms a $p\text{\ensuremath{-}}n$ or $p\text{\ensuremath{-}}p$ junction, depending on the polarity of the carriers in the bulk graphene sheet. Our results also explain part of the sublinearity observed in conductance as a function of density in a large number of experiments on graphene, which has generally been attributed to short-range scattering only.

Numbers of donors and acceptors from transport measurements in graphene

A method is suggested to separately determine the surface density of positively and negatively charged impurities that limit the mobility in a graphene monolayer. The method is based on the exact result for the transport cross section, according to which the massless carriers are scattered more strongly when they are attracted to a charged impurity than when they are repelled from it.