S. Bala Kumar

A Simulation Study of Graphene-Nanoribbon Tunneling FET With Heterojunction Channel

The device physics and performance of heterojunction (HJ) graphene-nanoribbon (GNR) tunneling field-effect transistors (TFETs) with different designs are investigated in this letter. Due to the width-dependent energy bandgap (EG), a single GNR with spatially dependent width naturally yields an HJ structure to improve the device performance of a GNR TFET. By adding a small-EG region in the channel near the source and a large-EG region in the middle of the channel, the ON- and OFF-state currents (ION and IOFF, respectively) can be tuned. Last, we have studied the effect of channel length scaling on an HJ GNR TFET, and it has been observed that an ION/IOFF ratio of four orders of magnitude can be achieved with a channel length of 10 nm and a drain bias of 0.6 V.

Observation of Degenerate One-Dimensional Sub-Bands in Cylindrical InAs Nanowires

One-dimensional (1D) sub-bands in cylindrical InAs nanowires (NWs) are electrically mapped as a function of NW diameter in the range of 15-35 nm. At low temperatures, stepwise current increases with the gate voltage are clearly observed and attributed to the electron transport through individual 1D sub-bands. The 2-fold degeneracy in certain sub-band energies predicted by simulation due to structural symmetry is experimentally observed for the first time. The experimentally obtained sub-band energies match the simulated results, shedding light on both the energies of the sub-bands as well as the number of sub-bands populated per given gate voltage and diameter. This work serves to provide better insight into the electrical transport behavior of 1D semiconductors.

Modeling of a vertical tunneling graphene heterojunction field-effect transistor

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 difference 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 behaviors the vertical-FET as a function of the gate insulator thickness and the thickness of the tunneling channel material are examined.

Strain-Induced Conductance Modulation in Graphene Grain Boundary

Grain boundaries (GBs) are ubiquitous in polycrystalline graphene materials obtained by various growth methods. It has been shown previously that considerable electrical transport gap can be opened by grain boundaries. On the other hand, polycrystalline graphene with GBs is an atomically thin membrane that can sustain extraordinary amount of strain. Here, by using atomistic quantum transport numerical simulations, we examine modulation of electrical transport properties of graphene GBs. The results indicate the modulation of transport gap and electrical conductance strongly depends on the topological structure of the GB. The transport gap of certain GBs can be significantly widened by strain, which is useful for improving the on-off ratio in potential transistor applications and for applications as monolayer strain sensors.

Multilayer graphene under vertical electric field

We study the effect of vertical electric field (E-field) on the electronic properties of multilayer graphene. We show that the effective mass, electron velocity, and density-of-state of a bilayer graphene are modified under the E-field. We also study the transformation of the band structure of multilayer graphenes. E-field induces finite (zero) band gap in the even (odd)-layer ABA-stacking graphene. On the other hand, finite band gap is induced in all ABC-stacking graphene. We also identify the optimum E-field to obtain the maximum band gap in the multilayer graphenes. Finally, we compare our results with the experimental results of a field-effect-transistor.

Magnetoresistive effect in graphene nanoribbon due to magnetic field induced band gap modulation

The electronic properties of armchair graphene nanoribbons (AGNRs) can be significantly modified from semiconducting to metallic states by applying a uniform perpendicular magnetic field (B-field). Here, we theoretically study the band gap modulation induced by a perpendicular B-field. The applied B-field causes the lowest conduction subband and the topmost valence subband to move closer to one another to form the n=0 Landau level. We exploit this effect to realize a device relevant magnetoresistive (MR) modulation. Unlike in conventional spin-valves, this intrinsic MR effect is realized without the use of any ferromagnetic leads. The AGNRs with number of dimers, Na=3p+1[p=1,2,3,…] show the most promising behavior for MR applications with large conductance modulation, and hence, high MR ratio at the optimal source-drain bias. However, the MR is suppressed at higher temperature due to the spread of the Fermi function distribution. We also investigate the importance of the source-drain bias in optimizing th...

Layer thickness effect on the magnetoresistance of a current-perpendicular-to-plane spin valve

We performed a theoretical study and analysis of the effect of modifying the layer thicknesses of a current-perpendicular-to-plane (CPP) spin valve multilayer on its magnetoresistance (MR) ratio. An increase in the ferromagnetic (FM) layer thickness results in (i) an increase in the spin-dependent component of its total resistance, thereby resulting in higher MR, but also leads to (ii) greater spin relaxation in that layer and (iii) an anomalous MR effect in the high resistance regime, both of which suppress the MR ratio. The interplay of these effects results in a complex MR dependence on FM thickness, instead of the simple monotonic MR increase predicted by the two-current model. It also explains the existence of an optimum FM thickness for maximum MR ratio, as evidenced by experimental data. Finally, we consider the MR dependence on the strength and spin selectivity of interfacial resistances, which can either arise naturally or be engineered in the spin valve structure. The study of the combined effec...

Chiral tunneling in trilayer graphene

We study the effect of chiral-tunneling in Bernal and rhombohedral stacked trilayer-graphene (3LG). Based on the chirality of the electronic bands, at the K-point, (rhombohedral) Bernal-3LG exhibits 100% (50%) transparency across a heterojunction. Utilizing this property, we further investigate the effect of electron collimation in 3LG. Due to the difference in the Berry’s phase, we show that, rhombohedral-3LG is a better electron collimator, compared to monolayer and Bernal-bilayer graphene. Since, Bernal-3LG can be decomposed into two separate channels consisting of a monolayer and a modified Bernal-bilayer graphene; the Bernal-3LG is weaker electron collimator, compared to rhombohedral-3LG.

Transversal electric field effect in multilayer graphene nanoribbon

We study the effect of transversal electric-field (E-field) on the electronic properties of multilayer armchair-graphene-nanoribbon (AGNR). The bandgap in multilayer-AGNRs can be reversibly modulated with the application of E-field. At optimized widths, we obtain a semiconductor (SC) to metallic (M) and a M-SC transitions. The AGNR electronic bands undergo vivid transformations due to the E-field, leading to phenomena such as an increase in electron velocity, a change in the sign of the electron effective mass, and the formation of linear dispersion with massless Dirac fermions similar to 2D-graphene. These effects are very useful and can be utilized for device applications.

The effect of magnetic field and disorders on the electronic transport in graphene nanoribbons

We developed a unified mesoscopic transport model for graphene nanoribbons, which combines the nonequilibrium Greens function (NEGF) formalism with the real-space π-orbital model. Based on this model, we probe the spatial distribution of electrons under a magnetic field, in order to obtain insights into the various signature Hall effects in disordered armchair graphene nanoribbons (AGNR). In the presence of a uniform perpendicular magnetic field (B[Symbol: see text]-field), a perfect AGNR shows three distinct spatial current profiles at equilibrium, depending on its width. Under nonequilibrium conditions (i.e. in the presence of an applied bias), the net electron flow is restricted to the edges and occurs in opposite directions depending on whether the Fermi level lies within the valence or conduction band. For electrons at an energy level below the conduction window, the B[Symbol: see text]-field gives rise to local electron flux circulation, although the global flux is zero. Our study also reveals the suppression of electron backscattering as a result of the edge transport which is induced by the B[Symbol: see text]-field. This phenomenon can potentially mitigate the undesired effects of disorder, such as bulk and edge vacancies, on the transport properties of AGNR. Lastly, we show that the effect of [Formula: see text]-field on electronic transport is less significant in the multimode compared to the single-mode electron transport.