Landauer Approach Research Articles

Spin splitter and spin filter in a three terminals device under Zeeman effect in a tight binding approach

In this work we propose a theoretical study of impurities effects in charge transport through a linear and an aromatic three terminals device under an external magnetic field. We use the Landauer approach, with the electronic structure treated at tight binding (TB) level and model the self-energy with the wide band limit approximation. For the linear chain, we use decimation techniques to renormalize the system, reducing it into a 3 × 3 matrix, so as to make an analytical treatment. For aromatic systems we opt for numerical calculations. Our main results suggest that no matter the system, by introducing impurities into the chain together with an applied magnetic field, there is a possibility to have filter spin (through the system) or splitter spins (on each branch), due to the Zeeman effect. The results agree with other theoretical works for systems with also intrinsic asymmetry of the device but caused by the different polarization into distinct branches. Also, our theoretical approach reveals, in a clear way, how the impurity on each branch makes the system behaves like a spin filter or a spin splitter, depending on the onsite energies of the impurities.

A novel ballistic model for the subthreshold behavior of nanosheet transistors

A novel ballistic model for the subthreshold current of nanosheet transistors is successfully developed based on the Landauer approach with the three-dimensional number of channels. The ballistic threshold voltage can also be achieved through the calculated free charge density induced by the three-dimensional density-of-states equal to the substrate doping concentration. It indicates that under the low drain voltage corresponding to the Fermi distribution function, the subthreshold current is mainly governed by the low contact potential. However, under the high drain voltage corresponding to the Fermi distribution function, the thermal voltage, instead of the contact potential between the source and drain, initiates the subthreshold current. Besides subthreshold current and threshold voltage, the ballistic conductance and subthreshold swing are also revealed in the subthreshold conduction. It indicates that the thin silicon, thin gate oxide, heavy substrate doping density, and high work function will alleviate the ballistic effects, decrease the subthreshold current/swing, and increase the threshold voltage/ballistic resistance.

A Quasi-Ballistic Model for Short Channel Monolayer Graphene Field Effect Transistor Including Scattering Effects

In this work, the Landauer approach-based short channel model of a graphene field effect transistor has been presented. The quasi-ballistic and ballistic transport mechanism is explained by this approach for nanoscale transistors. The concept of the virtual source is considered to obtain the carrier density in active regions. Here, we apply the Landauer approach for short-channel graphene field effect transistors relating to the parameter in the virtual source model. The mobility of the model has been improved including various scattering effects like impurity scattering, acoustic phonon scattering, and optical phonon scattering. The dynamic resistance model is considered due to asymmetry transport in the electron branch and hole branch. The proposed model has been verified with several experimental works for model validation. The comparisons of the model simulation with several experimental results that have been reported in the literature match well for different short channel lengths of graphene.

A ballistic model for subthreshold current and threshold voltage of the dual-material-gate nanosheet MOSFET

— A ballistic model for subthreshold current of the dual-material-gate (DMG) nanosheet (NS) MOSFET is developed based on the Landauer approach, three-dimensional scaling equation, and the continuity of subthreshold current at the interface between the control gate and screen gate in the channel, The work function difference between the screen gate and control gate can be converted into the new interface-quasi-fermi-potential (IQFP) at the junction between different gate regions. Through the IQFP, the ballistic subthreshold current for DMG NS MOSFET is successfully developed. With the determined subthreshold current, the ballistic threshold voltage can also be achieved by the constant current method. Under the low and high drain voltage corresponding to their IQFPs, the subthreshold behavior is mainly governed by the contact potential and thermal voltage, respectively. Furthermore, Ohm's law regarding ballistic resistance for the DMG NS MOSFET can still be valid in the subthreshold conduction.

Joining Landauer approach and classical multi electronic master-equation description in nanoscale transport for non-interactive systems

In this work we show that in the non-interacting limit, i.e., when the electronic structure of the N-particle states is frozen, in the wide-band limit (WBL) approximation, the classical master equation (CME) with the electron transfer rates obtained via Fermi golden rule and the elastic scattering approach (ES) are equivalent. This fact is encouraging, since it allows the insertion of elastic mechanisms into quantum transport via the CME approach thus, describing weak and strong quantum transport regimes on an equal footing within this theory.

Electronic-, Optical-, and Temperature-Dependent Carrier Mobility Simulations of Perovskite-Type Liganded PbS Quantum Dot Array

Recent experimental results suggest that higher mobility of perovskite-type ligand passivated PbS quantum dots (QDs) could be useful for efficient solar cell applications. However, theoretical understanding of the mechanism through first principal modeling is still lacking. In this study, electronic-, optical-, and temperature-dependent carrier mobility for perovskite ligand passivated PbS QD array is calculated by using the first-principles density functional theory (DFT) combined with the nonequilibrium Green’s function (NEGF) technique and a molecular dynamics (MD)-Landauer approach. It is found that formamidinium (FA)-liganded QDs have higher mobility and enhanced optical absorption comparing to that of Cl-liganded QDs. The difference could be understood through the intermediate band featured electronic structure.

Determining the Electronic Structure and Thermoelectric Properties of MoS2/MoSe2 Type‐I Heterojunction by DFT and the Landauer Approach

AbstractThe electronic structure and thermoelectric properties of MoX2 (X = S, Se) Van der Waals heterojunctions are reported, with the intention of motivating the design of electronic devices using such materials. Calculations indicate the proposed heterojunctions are thermodynamically stable and present a band gap reduction from 1.8 eV to 0.8 eV. The latter effect is highly related to interactions between metallic d‐character orbitals and chalcogen p‐character orbitals. The theoretical approach allows to predict a transition from semiconducting to semi‐metallic behavior. The band alignment indicates a type‐I heterojunction and band offsets of 0.2 eV. Transport properties show clear n‐type nature and a high Seebeck coefficient at 300 K, along with conductivity values (σ/τ) in the order of 1020. Lastly, using the Landauer approach and ballistic transport, the proposed heterojunctions can be modeled as a channel material for a typical one‐gate transistor configuration predicting subthreshold values of ≈60 mV dec−1 and field–effect mobilities of ≈160 cm−2 V−1 s−1.

Chiral-Induced Spin Selectivity and Non-equilibrium Spin Accumulation in Molecules and Interfaces: A First-Principles Study.

Electrons moving through chiral molecules are selected according to their spin orientation and the helicity of the molecule, an effect known as chiral-induced spin selectivity (CISS). The underlying physical mechanism is not yet completely understood. To help elucidate this mechanism, a non-equilibrium Green's function method, combined with a Landauer approach and density functional theory, is applied to carbon helices contacted by gold electrodes, resulting in spin polarization of transmitted electrons. Spin polarization is also observed in the non-equilibrium electronic structure of the junctions. While this spin polarization is small, its sign changes with the direction of the current and with the handedness of the molecule. While these calculations were performed with a pure exchange-correlation functional, previous studies suggest that computationally more expensive hybrid functionals may lead to considerably larger spin polarization in the electronic structure. Thus, non-equilibrium spin polarization could be a key component in understanding the CISS mechanism.

Even-odd effects in the electrical conductance of a three terminals device composed by linear chains: An analytical approach

In this work we propose a theoretical study of odd-even effects in charge transport through a linear three terminal chain device. We use the Landauer approach, with the electronic structure treated at tight binding (TB) level and model the self-energy as an energy independent parameter. Moreover, we use decimation techniques to renormalize the system, reducing it into a 3 × 3 matrix, so as to make an analytical treatment. By varying the size of the chain, we alter not only the parity of the full system but also the parity of each possible pathway and show how odd-even effect appears in the transmission function, local density of states and the behavior of the electrical current as function of the parity. Our theoretical approach reveals, in a clear way, how the parity of each branch competes with the parity of the full system to the conductance values near the Fermi level.

Coherent and incoherent contributions to molecular electron transport.

We numerically isolate the limits of validity of the Landauer approximation to describe charge transport along molecular junctions in condensed phase environments. To do so, we contrast Landauer with exact time-dependent non-equilibrium Green's function quantum transport computations in a two-site molecular junction subject to exponentially correlated noise. Under resonant transport conditions, we find Landauer accuracy to critically depend on intramolecular interactions. By contrast, under nonresonant conditions, the emergence of incoherent transport routes that go beyond Landauer depends on charging and discharging processes at the electrode-molecule interface. In both cases, decreasing the rate of charge exchange between the electrodes and molecule and increasing the interaction strength with the thermal environment cause Landauer to become less accurate. The results are interpreted from a time-dependent perspective where the noise prevents the junction from achieving steady-state and from a fully quantum perspective where the environment introduces dephasing in the dynamics. Using these results, we analyze why the Landauer approach is so useful to understand experiments, isolate regimes where it fails, and propose schemes to chemically manipulate the degree of transport coherence.

(Invited) Enhanced Electron Phonon Scattering in Si Nanowires Covered By Oxide

In order to maintain gate controllability of electric current in nanoscale transistors, nanosheet or nanowire channels are required for future nodes. In Si covered by oxides, deformation potential greater than that in bulk has been used to reproduce electron mobility. Increase of the deformation potential results in decrease of phonon-limited mobility. However, physical origin of the increase of phonon scattering in nanostructures was not completely understood. Although atomistic calculations with H-terminated Si nanosheets partially explained the increase of phonon scattering, the amount of the increase of phonon scattering was not sufficient to reproduce experimental mobility or empirical values of deformation potential [1]. In this work, we calculated mobility in oxide covered semiconductor nanowires by Molecular dynamics (MD)-Landauer approach [2]. Based on MD simulations followed by transport calculations of tight-binding nonequilibrium Green’s function model, effects of the oxide layers on phonon scattering in the semiconductor nanowires was considered.The derived phonon-limited mobility of the free-standing Si nanowire (205 cm2V-1s-1) was almost the same as previously reported value in H-terminated Si nanowire [2], confirming the validity of our calculations. In the SiO2-covered Si nanowires, the mobility was decreased by half due to the SiO2 (95 cm2V-1s-1). Corresponding effective deformation potential was extracted by comparing the mobility calculated from MD-Landauer approach with the mobility calculated from Boltzmann transport equation. The corresponding deformation potential in SiO2-covered Si nanowires (D ac=22 eV) roughly agrees with the empirical deformation potentials in SOI films of the same thickness [3]. From the comparison between the free-standing and the SiO2-covered nanowires, we found the atomic displacements in the SiO2-covered Si nanowires were abnormally increased at the surface, which would result in the enhance of phonon scattering. Therefore, in order to precisely model the transport properties in future node’s transistors, atomistic descriptions of the interface between semiconductor and oxide were indispensable.[1] J. Li, JAP, 120, 174301, 2016.[2] T. Markussen, PRB, 95, 245210, 2017.[3] T. Ohashi, IEEE JEDS, 4, 278, 2016. Figure 1

Analytical consideration of particle transport in 1D nanostructures

The paper presents an analytical study of one-dimensional fluxes of ballistic quasiparticles in the presence of scattering centers. Such a situation can be realized at very low temperatures or systems of very small sizes – nanostructures. To describe such a situation, the approach of heat transfer by radiation, which goes back to Casimir, is used, in which the interaction of phonons with image boundaries is taken into account, or, for example, the Landauer approach, where the probability of phonon transition from the initial state to the final state is introduced. At the same time, the intermediate regime, the mean free path of phonons due to their interaction with each other, is comparable to the size of the samples; to this day, it remains a rather difficult problem for a theoretical or numerical solution. In this work, we propose the probabilistic approach in the Landauer model to describe heat transfer in the one-dimensional ballistic motion of quasiparticles. Within the framework of the theory of random walks, a model of successive scattering centers is considered. An explicit analytical expression is obtained for the dependence of the flux of quasiparticles on the probability of scattering and the number of scattering centers. In order to explain the physical sense of the obtained result the comparison with the result of iterative approach is made. As well the results are used for description of the problem of the heat flux in multilayered structures, in which one should take into account not only the thermal resistance inside the layers, but also the Kapitsa resistance between the layers. The practical application of the obtained results to one-dimensional nanostructures and to quasi-one-dimensional heat-conducting systems is discussed, various limiting cases are considered and a comparison with experimental data is made.

Thermal Transport across Metal/β-Ga2O3 Interfaces.

In this work, we study the thermal transport at β-Ga2O3/metal interfaces, which play important roles in heat dissipation and as electrical contacts in β-Ga2O3 devices. A theoretical Landauer approach was used to model and elucidate the factors that impact the thermal transport at these interfaces. Experimental measurements using time-domain thermoreflectance (TDTR) provided data for the thermal boundary conductance (TBC) between β-Ga2O3 and a range of metals used to create both Schottky and ohmic electrical contacts. From the modeling and experiments, the relation between the metal cutoff frequency and the corresponding TBC is observed. Moreover, the effect of the metal cutoff frequency on TBC is seen as the most significant factor followed by chemical reactions and defects between the metal and the β-Ga2O3. Among all β-Ga2O3/metal interfaces, for Schottky contacts, Ni/β-Ga2O3 interfaces show the highest TBC, while for ohmic contacts, Cr/β-Ga2O3 interfaces show the highest TBC. While there is a clear correlation between TBC and the phonon cutoff frequency of metal contacts, it is also important to control the chemical reactions and other defects at interfaces to maximize the TBC in this system.

On the theory of charge transport and entropic effects in solvated molecular junctions.

Experimental studies on single-molecule junctions are typically in need of a simple theoretical approach that can reproduce or be fitted to experimentally measured transport data. In this context, the single-level variant of the Landauer approach is most commonly used, but methods based on Marcus theory are also gaining popularity. Recently, a generalized theory unifying these two approaches has also been developed. In the present work, we extend this theory so that it includes entropic effects (which can be important when polar solvents are involved but are likely minor for solid-state systems). We investigate the temperature-dependence of the electric current and compare it to the behavior predicted by the Landauer and the conventional Marcus theory. We argue that this generalized theory provides a simple yet effective framework for understanding charge transport through molecular junctions. Furthermore, we explore the role of the entropic effects in different transport regimes and suggest experimental criteria for detecting them in solvated molecular junctions. Finally, in order to account for nuclear tunneling effects, we also demonstrate how lifetime broadening can be introduced into the Marcus-Levich-Dogonadze-Jortner-type description of electron transport.

Management of Phonon Transport in Lateral Direction for Gap-Controlled Si Nanopillar/SiGe Interlayer Composite Materials

The phonon transport in the lateral direction for gap-controlled Si nanopillar/SiGe interlayer composite materials was investigated to eliminate heat generation in the channel area for advanced MOS transistors. The gap-controlled Si NP/SiGe composite layer showed 1/100 times lower thermal conductivity than Si bulk. Then, the phonon transport behavior in lateral direction could be predicted by the combination between 3-omega measurement method for thermal conductivity and Landauer approach for phonon transport in Si NP/Si0.7Ge0.3 interlayer composite structure. We found that the NP structure could regulate the phonon transport in the lateral direction by changing the NP gaps. As such, this structure achieves the first step toward phonon transport management in the same electron transportation direction of planar-type MOSFETs and represents a promising solution to heat generation for advanced CMOS devices.

Quantum transport with electronic relaxation in electrodes: Landauer-type formulas derived from the driven Liouville–von Neumann approach

We derive the exact steady-state solutions for the simplest model systems of resonant tunneling and tunneling with destructive quantum interference from the driven Liouville-von Neumann (DLvN) approach. Under the finite-state lead condition (the two electrodes have finite states), we analyze the asymptotic behavior of the steady-state current in the two limits of electronic relaxation. Under the infinite-state lead condition, the steady-state solutions of the two model systems can be cast as Landauer-type current formulas. According to the formulas, we show that the transmission functions near the resonant peak and the antiresonant dip can be significantly influenced by electronic relaxation in the electrodes. Moreover, under intermediate and strong electronic relaxation conditions, we analytically show that the steady-state current of the DLvN approach dramatically deviates from the Landauer current when destructive quantum interference occurs. In the regime of zero electronic relaxation, our results are reduced to the Landauer formula, indicating that the DLvN approach is equivalent to the Landauer approach when the leads have infinite states without any electronic relaxation.

Enhanced thermoelectric efficiency in nanocrystalline bismuth telluride nanotubes

We report on the thermal and thermoelectric properties of individual nanocrystalline Bi2 Te3 nanotubes synthesized by the solution phase method using 3ω method and a microfabricated testbench. Measurements show that the nanotubes offer improved ZT compared to bulk Bi2Te3 near room temperature due to an enhanced Seebeck coefficient and suppressed thermal conductivity. This improvement in ZT originates from the nanocrystalline nature and low dimensionality of the nanotubes. Domain boundary filtering of low-energy electrons provides an enhanced Seebeck coefficient. The scattering of phonons at the surface of the nanotube leads to suppressed thermal conductivity. These have been theoretically analyzed using the Boltzmann equation based on the relaxation time approximation and Landauer approach. This work clearly demonstrates the possibility of achieving enhancement in thermoelectric efficiency by combining nanocrystalline and low-dimensional systems.

Modeling of the temperature effects in filamentary-type resistive switching memories using quantum point-contact theory

Electron transport in filamentary-type resistive switching memories is modeled using quantum point-contact theory. The filament is represented by a parabolic-shaped tube-like constriction in which the first quantized subband behaves as a one-dimensional tunneling barrier. Computation of the current flowing through the atomic-sized structure is carried out by means of the finite-bias Landauer approach. Different approximations for the barrier transmission coefficient are assessed with the aim of determining the role played by the temperature of the charge reservoirs. In order to corroborate the proposed model, current-voltage measurements in electroformed Ni/HfO2/Si devices operating in the non-linear transport regime were performed in the temperature range from −40 C to 200 C. Obtained results using inverse modeling indicate that a temperature-induced barrier lowering effect explains the experimental observations. Finally, the model proposed to calculate the device current including the temperature dependence is developed.

Numerical Simulation of Thermal Conductivity of SiNW–SiGe0.3 Composite for Thermoelectric Applications

The electron band structure and phonon energy dispersion of the silicon nanowires (SiNWs) embedded in SiGe0.3 (SiNW–SiGe0.3 composite) are simulated by using the effective mass Schrodinger equation and the elastodynamic wave equation, respectively. Then, the TE properties of the SiNW–SiGe0.3 composite are investigated by the Landauer approach. The simulation shows the contribution from electrons/holes on both electrical conductance and thermal conductance increases few times by introducing SiNWs, but on the other hand, lattice thermal conductance reduces around two orders. These results are consistent with the experimental measurement and indicates that much lower lattice thermal conductance dominates the TE performance of the SiNW–SiGe0.3 composite.

Switching related activation field for polarization-reversal and for polarization-saturation in PVA based NaNO2–CsNO3 mixed system composite films fabricated at moderate elevated temperature

We investigated switching behavior in PVA based NaNO2-CsNO3 mixed system composite films fabricated at moderate elevated temperature. The switching transients in ‘ceramic-polymer mixed system’ exhibiting best ferroelectric response optimized with CsNO3 mole% 0.09 have been analyzed by two different approaches. Following Landauer approach, an activation field is calculated by measuring the rate at which ‘polarization switching’ takes place; it is calculated by measuring average polarization current under the switching transient over the switching time. In second approach, another kind of activation field is calculated by measuring maximum value of polarization current attained in the transient. The calculation of the activation field in the former approach gives the value of minimum field required to take the sample into its saturated state of polarization during the life time of the experiment whereas the one calculated from later approach helps to find the activation field which is just sufficient to initiate the state of polarization reversal.