NEGF Formalism Research Articles

DFT+NEGF Insights on Boosting the Thermoelectric Figure of Merit of 2D GeTe/arsenene vdW Heterostructure Device: Interface Engineering

Stacking two-dimensional materials into van der Waal heterostructures (vdWH) is a promising choice for thermoelectric devices. Applying the first-principles method with NEGF formalism, we examined the structural, electronic study, and thermoelectrical transport features of the GeTe/arsenene 2D vdW heterostructure and its constituent monolayer. Heterostructures are constructed by combining GeTe with arsenene (AS) monolayers through vdW stacking using the Coincident site lattice (CSL) theory. The stability of these vdWH is proven by the parameters of interlayer distances, binding energies, and dynamical stability. The stable heterostructure exhibits an indirect bandgap (0.41 eV) with a band-type-II profile. A two-probe device model of the armchair (AC_vdWH) and zigzag (ZZ_vdWH) oriented systems are constructed to examine the thermoelectric transport properties. The thermoelectric transport properties are discussed in terms of different temperatures and chemical potentials. The negative Seebeck coefficient of the systems depicts an n-type character. Thermoelectric studies indicate that the AC_vdWH improves performance due to a reduction in phonon transmittance (26.83 nW/K) and a high Seebeck coefficient (-384.7 μV/K) at room temperature. Our findings predict that n-type AC_GeTe/arsenene vdWH with a high ZT value of 4.4 at 400 K, will be the promising choice for low-temperature thermoelectric device applications.

The effect of carbon nanotube electrodes on electron transport properties of nanowire phase change material Ge[formula omitted]Sb[formula omitted]Te[formula omitted]

The present work studied the scalability of phase change material (PCM) down to a single-digit nm with single wall metallic carbon nanotubes (SWCNT) electrodes. For this purpose, the low bias electron transport properties of Ge2Sb2Te5 (GST) are investigated using a DFT–NEGF formalism. The amorphous Ge2Sb2Te5 (a-GST) is obtained using DFT-based molecular dynamic simulation. It has been seen that while 6 nm crystalline Ge2Sb2Te5 (c-GST) has a large transmission near the Fermi level, a-GST exhibits a clear band-gap, and by further reduction of its length to 12 Å, the band-gap disappears. The electrical conductance of nanowire a-GST is predominantly due to coherent electron transport via acceptor-like and donor-like traps. The c-GST/a-GST conductance (Gc/Ga) ratio as a function of the device length is calculated, which shows good agreement compared with experimental works. We show that the ultimate limit for downscaling the nanowire GST sandwiched between SWCNT electrodes is about 24 Å . This can be attributed to the overlapping metal-induced gap states from electrodes that lead to the disappearance of the band-gap of the amorphous phase and a sharp decrease in the Gc/Ga ratio in shorter channel length. The On/Off ratio of 12 Å GST sharply drops below 10, and the reliable read procedure is not possible on this size scale. We have also investigated the effect of interfacial stress between the electrode and GST and show that it reduces the Gc/Ga ratio and hurts the switch-ability of the device.

A Ballistic Transport Nanodevice Based on Graphene Nanoribbon FET by Enhanced Productivity for Both Low-Voltage and Radio-Frequency Scopes

The top performance in both the low-voltage and radio frequency (RF) scopes has been nominated for an unique nanodevice made from a graphene nanoribbon with an extremely short gate length (7.5 nm) in this study. Two distinct material engineering options were used, yielding some interesting outcomes. Due to the use of an ultrascale GNRFET in this study, the band structure non-linearity in the Dirac point and the energy-position dependent effective mass model for dual material gate architectures were examined for the first time. The NEGF formalism is used to carry out both the low-voltage and RF research using a three-dimensional (3D) Poisson equation. Low-voltage high performance has been validated by monitoring the key parameters in the terms of on current (Ion), off current (Ioff), Ion to Ioff ratio, subthreshold swing, and drain induced barrier lowering (DIBL) for the proposed device as compared to other structures under the study. The RF performance is examined by evaluation of essential parameters in the cases of parasitic gate capacitance, intrinsic cut-off frequency, intrinsic delay time, and transconductance. Indeed, a device with a higher source side gate work function than it does on the drain side is proposed will shift the energy band from the device’s half to the drain electrode, altering carrier outflow. In addition, the performance of non-linearity and RF intermodulation distortion has been analyzed for all devices under investigation in order to attain the best attitude toward the suggested device in comparison to the other devices under investigation in this work.

Electron transport in nitrogen functionalized graphene: A case of arsenic detection

Abstract The Nitrogen (N) functionalized graphene (NG) has been investigated to confirm its suitability for the detection of toxic Arsenic (As) present in the water, causing health hazards. The calculations have been performed using density functional theory and NEGF formalism. Here, a monolayer graphene has been modeled and functionalized by substituting its carbon with nitrogen. The computed binding energy for the adsorption of As reveals a strong interaction between the adsorbate and adsorbent. The band structure and current-voltage relationship has been analysed to examine the detection ability of the modeled NG towards the Arsenic. Band opening is the resultant of introducing As near to NG, resulted in to decrement of current in I-V characteristics in comparison to its pristine counterpart. To confirm the suitability of proposed NG as a promising candidate for detection of heavy metal Arsenic in the water, it has been observed that that the NG has sufficient binding energy, along with variation in electronic and transport properties with a reasonably short recovery time of 5.5 ms and 21.3 ms in vacuum and aqueous environment, respectively.

Insights on Si doping on PNRs for NDR with high PVR and diode behaviour with a high rectification ratio

This work uncovers the physics and application of APNRs by doping Silicon and Sulfur as P-type and N-type dopants. Structurally optimized 8APNRs chains were used throughout the work where the electrical properties were calculated using DFT technique. It was found that heavy doping by either P-type or N-type atoms in 8APNRs device produces NDR behaviour but only p-doped APNRs provides higher Peak to Valley Ratio (PVR). Consequently, if both the impurities were doped in an equal proportion to form a PN junction in 8APNRs, NDR occurrence was suppressed by the added opponent dopants and turned to possess rectifying behaviour. Electrical transport characteristics of 8APNRs based PN Junction diode was analyzed using NEGF formalism where a very high rectification ratio (RR) of the order of 1012 was observed which was much higher than what achieved in N doped AGNRs device (RR ~ 102). This observation benchmarks the utilization of PNRs diodes as an ideal rectifier over GNRs.

Asymmetric lateral graphene/h-BCN heterojunctions: A new method for separation of carriers in graphene nanoribbon photodetectors

Separation of photo-excited carriers (electrons and holes) is one of the most basic mechanisms needed in any photodetector and implies the efficiency of the photodetector. Applying electric field in the longitudinal direction of the photodetector channel is the traditional way to this end. The necessary electric field can be applied externally by a voltage bias or internally by a p-n junction, a Schottky barrier, etc. However, recently it is shown that any factor that causes any asymmetry in the channel of the photodetector can induce carrier separation. Asymmetric potential barriers can be used to this end. In this paper, a new method will be introduced. Recently, new 2D hybrid structures of carbon and boron-nitride, h-BCN, have been introduced. Different mole fractions of carbon, boron, and nitrogen atoms in these structures can lead to different band-gaps and electron affinities. This property can be used to construct different heterostructures. In this work, we use these heterostructures to produce asymmetric potential barriers for separation of photo-excited carriers. The structures are computed with a TB model and NEGF formalism in this paper and the results show that they can produce carrier separation.

Different Dielectrics for Source, Drain and Channel Regions of 1D Nanowire MOSFETs and TFETs

This work compares the changes in performance of a carbon nanotube CNT-based n-i-n metal-oxide-semiconductor MOSFET and p-i-n tunnel field effect transistor TFET based on different dielectric values of gate κc and source/drain regions κsd. The results prove that at a constant κsd a higher κc improves transconductance gm, output conductance gd, ON-state current, subthreshold swing SS and may lower the threshold voltage VT. However, at a constant κc for an n-i-n MOSFET the performance does not change significantly with κsd, but gd increases. For the p-i-n TFET gm, gd and ON-state current degrade prominently while the minimum current declines and SS may improve. Also two different dielectrics for source κs and drain κd could provide TFET with a lower power-higher performance characteristic. The results are given based on certified NEGF formalism and are explained based on semiclassical discussion with respect to energy band diagram concept. This method simplifies the realization of device's performance optimization. Due to 1D transport properties of CNTs the results could be applied to any device with 1D electronic transport characteristics.

Quantum Monte Carlo simulation of dissipative transport using Bohmian trajectories

In this paper, a Monte Carlo method is proposed, which utilizes Bohmian trajectories to simulate dissipative transport in one-dimensional quantum devices. The proposed method, similar to the classical Monte Carlo method, is capable of simulating both elastic and inelastic scattering effects, with the distinction that quantum effects such as tunneling are also included. At first, the Bohmian trajectories for the wave packets injected from the right and the left contacts are obtained by solving the time-dependent Schrodinger equation, and then scattering effects are included via stochastic changes applied on the electron trajectories. We have shown that the results of the proposed model agree well with those of NEGF formalism.

Changes in transconductance(gm) and Ion/Ioff with high-K dielectrics in MX2 monolayer 10 nm channel double gate n-MOSFET

We investigate monolayer Transition Metal Dichalcogenides (TMDs) of type MX2 (MoS2, MoSe2, MoTe2, WS2 and WSe2) 10 nm n-channel Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) with different dielectrics (SiO2, Al2O3, HfO2 and TiO2) using DFT and NEGF formalism. Results suggest that increasing dielectric constant increases both transconductance (gm) and Ion/Ioff for all type of MX2 channels. Ion/Ioff and gm increases by one order with increase in dielectric constant K. Among all type of MX2 channels considered, WS2 channels results into highest values of gm (∼36.29 uS) and Ion/Ioff (4.3*107) with TiO2 dielectric. Variation in Subthreshold Slope (SS) with increase in dielectric constants are negligibly small, SS value of 58.52 mV/dec is obtained with Al2O3 dielectric for WS2 channel. However, a SS value of ∼60.02 mV/dec is obtained with TiO2 dielectric for all MX2 channels. The results suggest that TiO2 dielectric with WS2 channel can be used for High Performance (HP) and Low Power (LP) devices since it shows large Ion/Ioff (∼107) and SS around 60 mV/dec.

Electron transport through 8-oxoG: NEGF/DFT study

We present a first-principles study of the conductance of Guanine and 8-Oxoguanine (8-oxoG) attached to Au(111) electrodes. Cellular levels of 8-oxoG have been found in larger concentrations in cancer patients. The current through the structure was calculated using a DFT–NEGF formalism. We have compared flat and pyramidal electrode geometries and show that there is a measurable difference between the I–V characteristics of the pristine molecule and the 8-oxoG. For a flat electrode geometry, 8-oxoG produces a 2.57 (18.3) times increase in current than the corresponding counterpart at 3 V with a bond separation of 1.2 Å (2.4 Å). This can be attributed to molecular orbital energies shifting at the junction. Overall the flat geometry produces larger currents. We have also investigated the sensitivity of the current to the electrode molecule separation. For the flat geometry, the current dropped approximately 80% (97%) for 8-oxoG (pristine Guanine) with the doubling of the electrode separation.

Strain and deformations engineered germanene bilayer double gate-field effect transistor by first principles

Germanene, silicene, stanene, phosphorene and graphene are some of single atomic materials with novel properties. In this paper, we explored bilayer germanene-based Double Gate-Field Effect Transistor (DG-FET) with various strains and deformations using Density Functional Theory (DFT) and Green’s approach by first-principle calculations. The DG-FET of 1.6nm width, 6nm channel length (Lch) and HfO2 as gate dielectric has been modeled. For intrinsic deformation of germanene bilayer, we have enforced minute mechanical deformation of wrap and twist (5°) and ripple (0.5Å) on germanene bilayer channel material. By using NEGF formalism, I–V Characteristics of various strains and deformation tailored DG-FET was calculated. Our results show that rough edge and single vacancy (5–9) in bilayer germanene diminishes the current around 47% and 58% respectively as compared with pristine bilayer germanene. In case of strain tailored bilayer DG-FET, multiple NDR regions were observed which can be utilized in building stable multiple logic states in digital circuits and high frequency oscillators using negative resistive techniques.

Random phonon model of dissipative electron transport in nanowire MOSFETs

A method for quantum transport simulations of nanowire (NW) field-effect transistors (FETs) with inelastic electron---phonon scattering processes incorporated is presented in this paper. The microscopic device Hamiltonian with realistic phonon spectrum and electron---phonon interaction is transformed into an equivalent low-dimensional transport model with discrete random phonon modes. The electron---phonon coupling constants are optimized in order to reproduce the inelastic scattering effects. Small size of the model and special form of the inelastic self-energy terms in the NEGF formalism make it a powerful tool to study dissipative transport in realistic NW transistors. The utility of the method is demonstrated by computing inelastic transport characteristics in Si NW FETs.

Study of Nitrogen terminated doped zigzag GNR FET exhibiting negative differential resistance

This paper presents the study of Gallium and Aluminum doped Nitrogen terminated zigzag Graphene Nano Ribbon (GNR) FET with high-k dielectric. The GNR FET structure has been designed and simulated using Quantumwise Atomistix Toolkit software package. The presented GNR FET with n-type (Nitrogen doped) electrodes and p-type (Gallium or Aluminum doped) scattering region are simulated and analyzed using Density Functional Theory combined with NEGF formalism and Device Density of States (DDOS). The device shows a negative differential resistance phenomenon which can be controlled by the gate of the zigzag GNR FET. It is found that doping of Gallium and Aluminum in scattering region provides higher drain current, higher ION/IOFF and IP/IV ratios as compared to that of Boron doped zigzag GNR FET. The potential applications of the device are in logical, high frequency, and memory devices.

Transport properties of a single-molecular diode with one backbone, and two backbones in parallel: Frontier orbital analysis and NEGF-DFT study

The conductance and electronic transport properties of a single-molecular diode with one backbone (1), and two backbones in parallel (2) have been investigated using frontier orbital analysis, and the NEGF formalism combined with DFT. The frontier orbital analysis results demonstrate that the electron transport from one end of the studied molecules to other end is symmetrically allowed and the conductance of the molecule with two parallel backbones is more than the molecule with a single backbone. Transmission spectra study based on the NEGF-DFT of the selected molecules sandwiched between two gold (1 1 1) electrodes showed that, due to a higher coupling between the two electrodes and the molecule 2, the zero-bias conductance is more than twice that of the other molecular junction. Transmission spectra under different biases showed that the maximum constructive interference exists at the bias voltage 0.2, while in some of the biases destructive effects are observed. I-V curves showed that the rectifying directions of molecular junctions 1 and 2 are opposite.

Effect of edge states on the transport properties of pentacene–graphene nanojunctions

We investigate the effect of edge states on the transport properties of pentacene–graphene nanojunctions on the basis of DFT and NEGF formalism. The calculations reveal that strong interaction between pentacene and zigzag GNR leads to edge-induced transmission channels at the Fermi region which controls the low-bias current. Effects of substitution by electron withdrawing and donating groups on the transport properties of molecular pentacene have also been discussed, some of which show spin resolved transport properties with negative differential resistance behavior which may have potential application in spintronics devices.

Nitrogen-Terminated Semiconducting Zigzag GNR FET With Negative Differential Resistance

We investigate the effects of nitrogen passivation on band structure and density of states in zigzag graphene nanoribbon (zzGNR) using first principle quantum mechanical simulations. The results show that nitrogen edge termination of zzGNR produces a bandgap (~0.7eV) around the Fermi level. We analyze the Bloch functions and projected density of states for understanding the origin of the bandgap. Based on these findings, we propose a nitrogen-passivated zzGNR FET structure having n-type electrodes and p-type scattering region using nitrogen and boron doping, respectively. We simulate and analyze its current-voltage (I-V ) characteristics using DFT combined with NEGF formalism and device density of states (DDOS). We observe a new negative differential resistance phenomenon in GNR FET, which can be controlled by the variation of the potential applied at gate of the zzGNR FET. This device has potential applications in logic, high frequency, and memory devices.

Investigation of localized versus uniform strain as a performance booster in InAs Tunnel-FETs

We investigate the effect of spatially localized versus uniform strain on the performance of n-type InAs nanowire Tunnel FETs. To this purpose we make use of a simulator based on the NEGF formalism and employing an eight-band k·p Hamiltonian, describing the strain implicitly as a modification of the band-structure. Our results show that, when the uniform strain degrades the subthreshold slope because of an increased band-to-band-tunneling at the drain, a localized strain at the source side permits to obtain a better tradeoff between on-current and subthreshold slope than a uniform strain configuration.

Electrical transport of zig-zag and folded graphene nanoribbons

ABSTRACTIn recent years, there has been much interest in modelling graphene nanoribbons as they have great potential for use in molecular electronics. We have employed the NEGF formalism to determine the conductivity of graphene nanoribbons in various configurations. The electronic structure calculations were performed within the framework of the Extended Huckel Approximation. Both zigzag and armchair nanoribbons have been considered. In addition, we have also computed the transmission and conductance using the non-equilibrium Greens function formalism for these structures. We also investigated the effect of defects by considering a zigzag nanoribbon with six carbon atoms removed. Finally, the effect of embedding boron nitride aromatic molecules in the nanoribbon has been considered. The results of our calculations are compared with that obtained from recent work carried out using tight-binding model Hamiltonians.

Emission and absorption of optical phonons in Multigate Silicon Nanowire MOSFETs

In this paper we study the effect of emission and absorption processes due to inelastic optical phonons in multigate silicon nanowire transistors. We show that low-energy optical phonons reduce drain current through both phonon emission/absorption processes while high-energy phonons redistribute current spectrum inside the nanowire merely by phonon emission process without reducing the current drive significantly. A three-dimensional quantum mechanical device simulator based on the NEGF formalism in uncoupled-mode space has been developed to model quantities of interest in the presence of electron-phonon interactions. Electron-phonon scattering is accounted for by adopting the self-consistent Born approximation and using the deformation potential theory.

Quantum transport in nano MOSFETs

In this work, a general method for describing quantum electron transport will be introduced. These formulations are based on Schrödinger equation, Pauli master equation, density matrix, Wigner function and the Green function. Here, we describe the microscopic quantum theory of electron transport in silicon devices based on the NEGF formalism. We review the NEGF formalism and derive its key equations. We include the electron–phonon interactions and other scattering mechanisms such as the impurity scattering and the surface roughness scattering. For the electron–electron interactions, we used the assumption that each electron moves independently and sees only the average field generated by all the other electrons.