Nishant Sule

Clustered impurities and carrier transport in supported graphene

We investigate the effects of charged impurity distributions and carrier-carrier interactions on electronic transport in graphene on SiO

Effect of sputtered lanthanum hexaboride film thickness on field emission from metallic knife edge cathodes

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EMC/FDTD/MD simulation of carrier transport and electrodynamics in two-dimensional electron systems

by employing a self-consistent coupled simulation of carrier transport and electrodynamics. We show that impurity clusters of characteristic width 40--50 nm generate electron--hole puddles of experimentally observed sizes. In the conductivity versus carrier density dependence, the residual conductivity and the linear-region slope are determined by the impurity distribution, and the measured slope can be used to estimate the impurity density in experiment. Furthermore, we show that the high-density sublinearity in the conductivity stems from carrier-carrier interactions.

Examination of field emission from Lanthanum Hexaboride coated knife edge cathodes

We report experiments and analysis of field emission from metallic knife-edge cathodes, which are sputter-coated with thin films of lanthanum hexaboride (LaB6), a low-work function material. The emission current is found to depend sensitively on the thickness of the LaB6 layer. We find that films thinner than 10 nm greatly enhance the emitted current. However, cathodes coated with a thicker layer of LaB6 are observed to emit less current than the uncoated metallic cathode. This result is unexpected due to the higher work function of the bare metal cathode. We show, based on numerical calculation of the electrostatic potential throughout the structure, that the external (LaB6/vacuum) barrier is reduced with respect to uncoated samples for both thin and thick coatings. However, this behavior is not exhibited at the internal (metal/LaB6) barrier. In thinly coated samples, electrons tunnel efficiently through both the internal and external barrier, resulting in current enhancement with respect to the uncoated...

11.4: Examination of field emission from copper knife edge cathodes with low-work function coatings

We present the implementation and application of a multiphysics simulation technique to carrier dynamics under electromagnetic excitation in supported two-dimensional electronic systems. The technique combines ensemble Monte Carlo (EMC) for carrier transport with finite-difference time-domain (FDTD) for electrodynamics and molecular dynamics (MD) for short-range Coulomb interactions among particles. We demonstrate the use of this EMC/FDTD/MD technique by calculating the room-temperature dc and ac conductivity of graphene supported on SiO2.

Examination of cathode emission area variation with applied electric field

We report experiments and analysis of field emission from metallic knife-edge cathodes coated with thin film Lanthanum Hexaboride (LaB6), a low work function material (∼2.5 eV). The emission current density is found to depend sensitively on film properties, particularly the thickness of the LaB6 layer. Films thinner than approximately 10 nm greatly enhance the emitted current. However, cathodes coated with a thicker layer of LaB6 are observed to emit less current than the uncoated metallic cathode even though the bare metal cathode possesses a higher work function (> 4 eV). A hypothesis is proposed to explain this surprising experimental finding, and a computational model is developed. This simulation model incorporates field emission as well as solid state electron transport from the metal substrate through the LaB6 thin film, and is found to support the proposed hypothesis and agree with experimental observations.

Simulation of carrier dynamics in graphene on a substrate at terahertz and mid-infrared frequencies

We report experiments and analysis of field emission from copper knife-edge (CKE) cathodes, both bare and coated with low work function (∼2.5 eV) Lanthanum hexaboride (LaB6) thin films. The bare CKE cathode exhibits evidence of space charge limited emission currents at high field strengths. The LaB6 coated cathodes exhibit a nonlinear Fowler-Nordhiem (FN) type emission. An intermediate saturation region is observed from field emission data, which is more prominent at elevated (185 C) temperature. Surprisingly, the LaB6 coated cathodes are observed to emit less current than the higher work function (> 4 eV) bare CKE cathode. A hypothesis and corresponding model including both field emission and solid state electron transport from the Cu substrate, through the LaB6 thin film, is proposed to explain the experimental observations.

Measurements and Analysis of Advanced Field Emission Cold Cathodes

A recently published field emission cathode parameter extraction method [X. He et al., J. Appl. Phys. 102, 056107 (2007)] provides unambiguous and reliable cathode parameters. The method utilizes a transfer matrix method (TMM) to solve a one-dimensional model that includes both thermionic and field electron emission. However, there are modest differences between our TMM simulation results and experimental data near the transition point between thermal-dominated and field-dominated electron emission. We hypothesize that this discrepancy is due a dependence of effective emitting area with applied electric field. Incorporating surface field enhancement factors and emission area variations that are physically intuitive as the applied field is varied within our TMM simulation, we obtain much better agreement with the experimental data. This result supports the hypothesis that the effective emission area varies as a function of the applied electric field between the A-K (anode-cathode) gap.

Coupled simulation of carrier transport and electrodynamics: the EMC/FDTD/MD technique

We calculate the complex conductivity of graphene in the terahertz (THz) to mid-infrared (mid-IR) frequency range using a numerical simulation that couples the two-dimensional (2D) ensemble Monte Carlo technique (EMC) for carrier transport, the three-dimensional (3D) finite-difference time-domain (FDTD) technique for electrodynamics, and molecular dynamics (MD) for short range Coulomb interactions. We demonstrate the effect of the typically used silicon-dioxide substrate on the high-frequency carrier dynamics in graphene and show good agreement between recent experimental results and our numerical simulations.

Multiphysics simulations of carrier transport and electrodynamics in two-dimensional electron systems

We report measurements and analyses of field emission from both copper and aluminum cathodes. To analyze the data, we have developed a numerical model of electron emission. We note that localized heating of high current density field emission cathodes can provide regime where thermionic as well as field emission effects must be considered. Our analysis of experimental data fitting in both the thermionic and field emission dominant regime provides a robust method for determination of effective work function and field enhancement factor for field emission cathode.