V. Nam Do

Hanoi University of Science and Technology, Hà Nội, Ha Nội, Vietnam

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Publications (24)37.31 Total impact

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    ABSTRACT: We study several graphene devices able to generate non-linear effects in the current─voltage characteristics and in particular negative differential resistance (NDR) effects. This theoretical investigation is based on numerical charge transport simulation in the Green's function approach applied to a tight-binding Hamiltonian for particles in graphene. Depending on the device, the physical mechanism involved in the NDR effect may be different: (i) the mismatch of modes between left and right sides of a P+/P zigzag ribbon junction, (ii) the modulation of interband tunnelling in P/N junctions (tunnel diodes and tunnel field-effect transistors) or (iii) the modulation of chiral tunnelling in ‘conventional’ graphene transistors. We emphasize the advantages of exploiting different approaches of bandgap engineering in the form of graphene nanoribbons (GNRs) or nanomesh lattices (GNM), the latter resulting from a periodic array of nanoholes in graphene sheets. In particular, such nanostructuring allows us to design position-dependent bandgaps in devices, which is shown to make possible the optimization of device operation and, here, to get very high peak-to-valley ratio of the NDR. In the case of GNR nanostructuring, it is shown that appropriate bandgap engineering can even make the current─voltage characteristics of tunnel diodes weakly sensitive to the atomic edge disorder. Finally, GNM lattices are shown to be a very promising way to open large bandgaps in wide sheets of graphene and to introduce bandgaps locally with a view to optimizing the device operation and performance.
    02/2014; 47(9).
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    ABSTRACT: A numerical scheme based on the tight-binding description for pz-electrons in graphene was developed to study the formation and behaviors of plasmons in this two-dimensional electron system. The random phase approximation has been used to calculate the dielectric function for arbitrary temperature and doping level. We show that at zero-doping, only one kind of plasmons of long wavelength is observed at sufficiently high temperature. At finite doping, such plasmons exist even at zero temperature, but strongly damped, due to the interplay between the intra- and inter-band transition processes. Particularly, we show a significant dependence of the plasmon spectrum on the wave-vector direction in the regime of high doping, which is the reflection of the anisotropy of the energy surfaces far from the Dirac point.
    Physica E Low-dimensional Systems and Nanostructures 01/2014; 58:101–105. · 1.52 Impact Factor
  • V. Nam Do, H. Anh Le
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    ABSTRACT: A physical model is proposed to clarify the electron transport through graphene-metal interfaces. It is based on an effective description of the coupling between the graphene π-bands and the metal sd- and d-bands. Applying this model to vertically symmetrical metal-graphene-metal structures, we show that the current-voltage characteristics can be either linear or non-linear with a negative or positive differential resistance depending on the dominance of the d-like or s-like electrons in the metal as well as the graphene-metal coupling.
    Applied Physics Letters 10/2012; 101(16). · 3.79 Impact Factor
  • V. N. Do, P. Dollfus
    Journal of Applied Physics 12/2010; 108(11):9901-. · 2.21 Impact Factor
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    ABSTRACT: An effective approach of quantum transport of Dirac carriers in mono- and bi-layer graphene structures and devices is presented. Initially based on the Green's function formalism to treat the Dirac Hamiltonian of massless particles in two-dimensional mono-layer graphene, the model has been extended to to small bandgap materials and to bi-layer graphene with massive carriers. It is applied to investigate some transport problems as the minimum conductivity, the tunneling properties the spin-polarized transport through single-barrier structures, and the operation of graphene field-effect transistors.
    Computational Electronics (IWCE), 2010 14th International Workshop on; 11/2010
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    ABSTRACT: An atomistic Green's function approach to simulating electron and phonon transport in graphene nanoribbons (GNRs) is presented. Phonons are described by an empirical Force-Constant model including interactions up to the fifth nearest neighbour while the nearest neighbour tight-binding Hamiltonian is used for electrons. The model was applied to investigate the edge dependence of GNR thermoelectric properties. The factor of merit ZT appears to be strongly enhanced in mixed-edge ribbons.
    Computational Electronics (IWCE), 2010 14th International Workshop on; 11/2010
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    V Nam Do, P Dollfus
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    ABSTRACT: In this work, the effect of coupling between metallic electrodes and graphene is discussed. We demonstrate that the transport properties of graphene at the charge neutrality point are very sensitive to this coupling. By introducing a model based on two real parameters, namely the real and the imaginary parts of the self-energy which describes the metal-graphene coupling, the obtained results of charge conductivity versus the Fermi energy reproduce well the essential features of experimental data such as the asymmetry between electrons and holes. Additionally, the possible role of scattering processes in the enhancement of the density of states, and thus of the minimum of conductivity, at the charge neutrality point is also discussed. This work is believed to be helpful for further studies of graphene-based devices wherein metallic electrodes may have a major impact on electrical characteristics.
    Journal of Physics Condensed Matter 10/2010; 22(42):425301. · 2.36 Impact Factor
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    ABSTRACT: High crystalline quality ZnS nanowires were fabricated using the thermal evaporation method. They were then oxidized in air at different temperatures to form a one-dimensional protuberant ZnO/ZnS structure. It was argued that the oxidation at low enough temperature can significantly improve the quality of the ZnS nanowires by passivating dangling bonds on the nanowire surface as the absorption of oxygen atoms. This study provides a simple approach for synthesizing optically active ZnO/ZnS heterostructures.
    Materials Letters. 07/2010; 64(14):1650–1652.
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    V. Nam Do, P. Dollfus
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    ABSTRACT: We investigate the transport properties of p<sup>+</sup>/p junctions based on zigzag-edge graphene strips by means of numerical quantum simulation. The p<sup>+</sup> and p domains are created by field effect using appropriate gate electrodes. A negative differential resistance behavior is predicted regardless of the evenness/oddness of the zigzag line number of the ribbon with peak-to-valley current ratio reaching the value of 10 at room temperature. Besides the role of the parity selective rule, the phenomenon is explained as resulting from the suppression of the coherent transition due to the mismatch of modes in the left and right sides of the junction. The influence of various factors governing the peak-to-valley current ratio is analyzed. In particular, it is found that the negative differential resistance may be severely affected by the roughness of ribbon edges.
    Journal of Applied Physics 04/2010; · 2.21 Impact Factor
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    ABSTRACT: Using the nonequilibrium Green’s functions formalism in a tight binding model, the spin-dependent transport in armchair graphene nanoribbons controlled by a ferromagnetic gate is investigated. Beyond the oscillatory behavior of conductance and spin polarization with respect to the barrier height, which can be tuned by the gate voltage, we especially analyze the effects of width-dependent band gap and of the nature of contacts. The oscillation of spin polarization in graphene nanoribbons with a large band gap is strong in comparison with that in infinite graphene sheets. Very high spin polarization (close to 100%) is observed in normal-conductor/graphene/normal-conductor junctions. Moreover, we find that the difference in electronic structure between normal conductor and graphene generates confined states which have a strong influence on the transport properties of the device. This study suggests that the device should be carefully designed to obtain a high controllability of spin-polarized current.
    Journal of Applied Physics 10/2009; · 2.21 Impact Factor
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    V. Nam Do, P. Dollfus
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    ABSTRACT: Using the nonequilibrium Green’s function theory, transport properties of nanoscale graphene structures deposited on a SiO <sub>2</sub>/ Si substrate have been investigated taking into account the influence of both lattice defects and charged impurities. The calculation argues the metallic lead-graphene coupling responsible for the asymmetric transport of electrons and holes, and shows that the conductivity is generally suppressed by these scattering processes. However, at the charge neutrality point, the screening seems to weaken such a suppression, leading to the minimum conductivity value of 4e<sup>2</sup>/πh even for the impurity density higher than 10<sup>12</sup> cm <sup>-2</sup> , while it is strongly diminished to zero for the vacancy density of 10<sup>11</sup> cm <sup>-2</sup> . Obtained results for the conductivity and the charge mobility are also discussed to highlight available experimental data.
    Journal of Applied Physics 08/2009; · 2.21 Impact Factor
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    ABSTRACT: We analyze the spin-dependent transport in single ferromagnetic gate structures based on armchair graphene nanoribbon (GNR) using the non-equilibrium Green's function method in a tight binding model. It is shown that the spin polarized current oscillates as a function of the gate-induced barrier height. For perfect GNRs, the larger the energy band gap, the stronger the oscillation of the spin polarization. However, though the edge roughness of the ribbons tends to enlarge the band gap, it also strongly reduces the conductance which finally degrades the spin polarized current.
    Journal of Physics Conference Series 01/2009; 19332.
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    ABSTRACT: The Wigner Monte Carlo approach is shown to provide an efficient way to study quantum transport in the presence of scattering and to connect semi-classical to quantum transport. The study of resonant tunneling diodes highlights the physics of the impact of scattering on resonant tunneling, and on electron decoherence and localization. The simulation of nano-MOSFET evidences a mixed regime, where both quantum transport and scattering play a significant role.
    Simulation of Semiconductor Processes and Devices, 2008. SISPAD 2008. International Conference on; 10/2008
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    ABSTRACT: Motivated by recent studies on the use of graphene for new concepts of electronic/spintronic devices, the authors develop an efficient calculation method based on the nonequilibrium Green’s function to solve the quantum relativisticlike Dirac’s equation that governs the low-energy excited states in graphene. The approach is then applied to investigate the electronic transport and the spin polarization in a single-graphene barrier structure. The obtained results are presented and analyzed in detail aiming to highlight typical properties of the considered graphene structure as well as the efficiency of the developed approach that both may be helpful for further development in electronic devices and in spintronics.
    Journal of Applied Physics 10/2008; · 2.21 Impact Factor
  • V. Nam Do
    Applied Physics Letters 01/2008; 92(21):216101-216101-2. · 3.79 Impact Factor
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    ABSTRACT: Wigner quasi-distribution function is an appropriate quantum mechanics formulation to study the transition from semi-classical to quantum transport in nano-devices since it can accurately describe quantum transport including the decoherence due to scatterings. We have recently developed an efficient approach to solving the Wigner transport equation using a Monte Carlo (MC) algorithm that has been applied to Resonant Tunnelling Diodes and nano-MOSFET simulation. The approach is here extended to incorporate degeneracy effects that are important in highly doped MOSFETs. The calculation is compared with Non Equilibrium Green's Function and the semi-classical Boltzmann equation. Relative importance of quantum transport and decoherent scattering is discussed at low and room temperatures. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 12/2007; 5(1):150 - 153.
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    ABSTRACT: Using the nonequilibrium Green’s function formalism, the authors investigate the effect of the electron-phonon interaction on the current and shot noise in one dimensional resonant tunneling structures. Besides the well-known current behavior, they particularly show that the shot noise may be enhanced over the Poissonian value due to the phonon-assisted tunneling effect. The observed super-Poissonian noise is then interpreted as a result of the competition between the coherent and sequential current components.
    Applied Physics Letters 08/2007; · 3.79 Impact Factor
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    V. Nam Do, P. Dollfus
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    ABSTRACT: Using the nonequilibrium Green’s function method, gate current characteristics are investigated for nanometer-scaled double-gate metal-oxide-semiconductor field-effect transistor. The mode-space approximation is, at the first stage of the calculation, used to obtain self-consistently the potential profile and the charge distribution in the structure. This solution is then used to solve the two-dimensional transport equation to extract the desired quantities. In addition to the dependence of the gate-leakage current on the gate bias and on the oxide thickness, our calculation shows the oscillation behavior of the leakage current versus the drain voltage. It is explained as the result of the strong quantization of electronic states inside the device, giving a resonant-like character to the tunneling of charges from source and drain contacts to the gates. This effect is strongly dependent on the gate length.
    Journal of Applied Physics 05/2007; · 2.21 Impact Factor
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    V. Nam Do, P. Dollfus, V. L. Nguyen
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    ABSTRACT: We propose an extension of the Landauer-Buttiker scattering theory to include effects of interaction in the active region of a mesoscopic conductor structure. The current expression obtained coincides with those derived by different methods. A new general expression for the noise is also established. These expressions are then discussed in the case of strongly sequential tunneling through a double-barrier resonant tunneling structure. Comment: 11 pages, 2 figures, rediscussion
    Physical review. B, Condensed matter 03/2007; · 3.77 Impact Factor
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    ABSTRACT: A new self-consistent quantum simulator based on the Monte Carlo solution of Wigner transport equation is used to analyze the operation of 6 nm-long DG-MOSFETs. By comparison with other simulation approaches, the work emphasizes the important role of scattering and quantum effects on the electrical characteristics of such nano-devices. The results are confronted to ITRS specifications and the various effects of aggressive oxide thickness thinning on device performance are discussed
    Electron Devices Meeting, 2006. IEDM '06. International; 01/2007

Publication Stats

156 Citations
37.31 Total Impact Points

Institutions

  • 2010–2014
    • Hanoi University of Science and Technology
      • Advanced Institute for Science and Technology
      Hà Nội, Ha Nội, Vietnam
  • 2006–2010
    • Université Paris-Sud 11
      • Institut d'Electronique Fondamentale
      Orsay, Île-de-France, France
  • 2009
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France