Ab Initio Quantum Transport Simulations Research Articles

Ab initio quantum transport simulations of monolayer GeS nanoribbons

Monolayer germanium monosulfide (GeS) was recently identified as one of the most promising 2D materials for ultra-scaled FETs. While sub-10 nm monolayer GeS FETs were studied by quantum transport, very little is known about GeS nanoribbons (GeSNRs) or GeSNR FET performance. In this work, we employ quantum transport and Hamiltonians with an orbital resolution to study the electronic, transport, and ballistic device properties of sub-4 nm-wide and ∼15 nm-long GeSNRs. While ultra-scaled GeSNR FETs exhibit ION/IOFF of at least ∼7×105, indicating good switching performance, they also offer modest ballistic ION values of up to ∼1.2 mA/μm.

Robust cryogenic ab-initio quantum transport simulation for LG = 10 nm nanowire

In this paper, we propose a simulation methodology for robust and accurate ab-initio quantum transport simulation down to 3 K of an n-type Si nanowire. This is important to understand the subthreshold swing (SS) at cryogenic temperature. We show that for LG = 10 nm, the SS is fully dominated by direct tunneling at cryogenic temperature, which is the first time to be demonstrated using ab-initio simulation, to the best of our knowledge. We propose a method to achieve more than 2x speed up in the simulation time and achieve convergence at high gate biases. It is also shown that from the leakage perspective, there is no advantage in operating LG = 10 nm transistor below 77 K.

Lifting on-state currents for GeS-based tunneling field-effect transistors with electrode optimization

Tunneling field-effect transistors (TFETs) arouse great enthusiasm for energy-efficient switching devices attributed to their steep subthreshold swing (SS), yet their low on-state currents (Ion) block the practical application. We study the possibility of lifting Ion for the case of GeS-based TFETs with electrode optimization using the ab initio quantum transport simulation. An apparent promotion on Ion to an amplitude of 270–410% is achieved for the optimal GeS HetJ-TFETs compared with the unstrained ML GeS TFET, where 23–58% promotion is contributed from the vdWH electrode and the left 42–77% is from the induced strain. Ion of 463 and 878 μA/μm of the optimal p-type 10-nm-Lg GeS HetJ-TFETs almost meet the International Roadmap for Device and Systems (IRDS) goals for both low-power (LP) and high-performance (HP) devices for the year 2028. Specifically, the p-type 10-nm-Lg GeS HetJ-TFETs possess the highest Ion(LP) and Ion(HP) when Ileak < 10−6 μA/μm compared with other 2D TFETs despite the material type and device architecture and own superior energy efficiency compared with 2D MOSFETs. The remarkable promotion of Ion attributes to the narrower bandgap and larger density of states of the vdWH electrode. Our findings will promote future development for 2D HetJ-TFETs.

Ion Migration in Monolayer MoS2 Memristors

Memristors hold great promise as building blocks for future computing architectures where memory and logic are combined at the hardware level. However, scaling down the dimensions of memristive devices has been limited by high leakage currents, thus inhibiting further progress. Recent studies have demonstrated memristors with monolayers of ${\mathrm{Mo}\mathrm{S}}_{2}$ and large high-to-low resistance ratios. Defects combined with metallic ion migrations are often seen as a possible explanation for this behavior. A detailed understanding of the switching mechanisms, in particular the role of metal ion diffusion into vacancy sites and crystal defects, remains elusive. Here we investigate how defect densities affect the performance of monolayer ${\mathrm{Mo}\mathrm{S}}_{2}$ memristors. We experimentally demonstrate that the resistive switching ratio becomes larger if the defect density in ${\mathrm{Mo}\mathrm{S}}_{2}$ is increased. Furthermore, by means of ab initio quantum transport simulations, we reveal the existence of an optimum range of defect densities and explore the theoretical limits of monolayer ${\mathrm{Mo}\mathrm{S}}_{2}$ memristors. Our results highlight the importance of defect engineering and control in transition metal dichalcogenides memristors.

Device simulation of GeSe homojunction and vdW GeSe/GeTe heterojunction TFETs for high-performance application

Compared with a two-dimensional (2D) homogeneous channel, the introduction of a 2D/2D homojunction or heterojunction is a promising method to improve the performance of a tunnel field-effect transistor (TFET), mainly by controlling the tunneling barrier. We simulate 10-nm-Lg double-gated GeSe homojunction TFETs and van der Waals (vdW) GeSe/GeTe heterojunction TFETs based on a ballistic quasi-static ab initio quantum transport simulation. Two device configurations are considered for both the homojunction and heterojunction TFETs by placing the bilayer (BL) GeSe or vdW GeSe/GeTe heterojunction as the source or drain, while the channel and the remaining drain or source use monolayer (ML) GeSe. The on-state current (Ion) values of the optimal n-type BL GeSe source homojunction TFET and the optimal p-type vdW GeSe/GeTe drain heterojunction TFET are 2320 and 2387 μA μm−1, respectively, which are 50% and 64% larger than Ion of the ML GeSe homogeneous TFET. Notably, the device performance (Ion, intrinsic delay time τ, and power dissipation PDP) of both the optimal n-type GeSe homojunction and p-type vdW GeSe/GeTe heterojunction TFETs meets the requirements of the International Roadmap for Devices and Systems for high-performance devices for the year 2034 (2020 version).

Scaling Behavior of Magnetoresistance with the Layer Number in CrI3 Magnetic Tunnel Junctions

Two-dimensional (2D) van der Waals (vdW) magnetic semiconductors have attracted wide interest for their promising application in next-generation spintronic devices. We investigate the scaling behavior of the tunnel magnetoresistance (TMR) of the $\mathrm{Ag}/$n-layer ${\mathrm{Cr}\mathrm{I}}_{3}/\mathrm{Ag}$ and graphite/n-layer ${\mathrm{Cr}\mathrm{I}}_{3}$/graphite magnetic tunnel junctions (MTJs) by using ab initio quantum-transport simulations. The calculated monotonic increasing TMR of the graphite/n-layer ${\mathrm{Cr}\mathrm{I}}_{3}$/graphite MTJ with n = 2--4 at zero bias agrees with the experimental value. The TMR of the $\mathrm{Ag}/$n-layer ${\mathrm{Cr}\mathrm{I}}_{3}/\mathrm{Ag}$ MTJ generally increases with the tunnel-barrier layer number, n, that is, from 200% (2-layer ${\mathrm{Cr}\mathrm{I}}_{3}$) to a record ${10}^{9}$% value (12-layer ${\mathrm{Cr}\mathrm{I}}_{3}$) at zero bias but has an odd-even oscillation when n 7. When we apply a bias voltage to the $\mathrm{Ag}/$2-layer ${\mathrm{Cr}\mathrm{I}}_{3}/\mathrm{Ag}$ MTJ, the TMR first decreases slightly and then increases, followed by a monotonic decrease. The noncollinear magnetization direction of the ${\mathrm{Cr}\mathrm{I}}_{3}$ layers also changes the TMR value of the graphite/n-layer ${\mathrm{Cr}\mathrm{I}}_{3}$/graphite MTJ relative to the collinear case, a result in agreement with experiments.

Effect of crystallographic orientations on transport properties of methylthiol-terminated permethyloligosilane molecular junction

The understanding of the influence of electrode characteristics on charge transport is essential in the field of molecular electronics. In this work, we investigate the electronic transport properties of molecular junctions comprising methylthiol-terminated permethyloligosilanes and face-centered crystal Au/Ag electrodes with crystallographic orientations of (111) and (100), based on the ab initio quantum transport simulations. The calculations reveal that the molecular junction conductance is dominated by the electronic coupling between two interfacial metal–S bonding states, which can be tuned by varying the molecular length, metal material of the electrodes, and crystallographic orientation. As the permethyloligosilane backbone elongates, although the σ conjugation increases, the decreasing of coupling induced by the increasing number of central Si atoms reduces the junction conductance. The molecular junction conductance of methylthiol-terminated permethyloligosilanes with Au electrodes is higher than that with Ag electrodes with a crystallographic orientation of (111). However, the conductance trend is reversed when the electrode crystallographic orientation varies from (111) to (100), which can be ascribed to the reversal of interfacial coupling between two metal–S interfacial states. These findings are conducive to elucidating the mechanism of molecular junctions and improving the transport properties of molecular devices by adjusting the electrode characteristics.

First principles studies on the electronic and contact properties of single layer 2H-MoS2/1T'-MX2 heterojunctions.

Constructed via in-plane heterojunction contacts between the semiconducting 2H phase (as the channel) and the metallic 1T' phase (as the electrode), two-dimensional (2D) transition metal dichalcogenide (TMD) field-effect transistors (FETs) have received much recent attention because they significantly reduce contact resistance. In this paper, ab initio quantum transport simulation is done to study and predict the electronic states and contact properties of the 2H-MoS2/1T'-MX2 (WS2, TaSe2, NbSe2, MoSe2, TaS2, and NbS2) in-plane heterojunctions. It is found that the interfacial states are not obvious and the fluctuation of the average electron density at the 1T'/2H phase boundary is small for all 2H-MoS2/1T'-MX2 heterojunctions. The average electrostatic potential differences (ΔV) are all negative, which is beneficial to promote the charge transfer from 1T'-MX2 to 2H-MoS2. Moreover, the p-type Schottky contact of the 2H-MoS2/1T'-MX2 heterojunctions is formed and the ΦSB,P values are 0.609 eV, 0.625 eV, 0.641 eV, 0.617 eV, 0.469 eV and 0.477 eV for 1T'-WS2, 1T'-TaSe2, 1T'-NbSe2, 1T'-MoSe2, 1T'-TaS2, and 1T'-NbS2, respectively. The results provide theoretical guidance for designing two-dimensional material devices.

Quantum transport simulation of the two-dimensional GaSb transistors

Owing to the high carrier mobility, two-dimensional (2D) gallium antimonite (GaSb) is a promising channel material for field-effect transistors (FETs) in the post-silicon era. We investigated the ballistic performance of the 2D GaSb metal–oxide–semiconductor FETs with a 10 nm-gate-length by the ab initio quantum transport simulation. Because of the wider bandgap and better gate-control ability, the performance of the 10-nm monolayer (ML) GaSb FETs is generally superior to the bilayer counterparts, including the three-to-four orders of magnitude larger on-current. Via hydrogenation, the delay-time and power consumption can be further enhanced with magnitude up to 35% and 57%, respectively, thanks to the expanded bandgap. The 10-nm ML GaSb FETs can almost meet the International Technology Roadmap for Semiconductors (ITRS) for high-performance demands in terms of the on-state current, intrinsic delay time, and power-delay product.

Electronic Structure and Quantum Transport Properties of 2D SiP: A First-Principles Study

With the rapid evolution of microelectronics, the field of integrated circuits is facing unprecedented challenges. Traditional silicon-based transistors cannot maintain the advantages of high performance during the process of further ultra-scaling due to severe short-channel effects. Two-dimensional (2D) materials are potential channel materials that can replace silicon. Herein, 2D SiP is predicted to have a band gap of 1.49 eV with anisotropic electronic properties by means of first-principles calculations, which is suitable as a channel candidate of transistors. Hence, we investigate the ballistic transport properties of 2D SiP double-gate metal oxide semiconductor field-effect transistors (MOSFETs) by using ab initio quantum transport simulations. Despite anisotropic electronic properties of 2D SiP, the performances of monolayer SiP MOSFETs have weak directional dependence due to high valley degeneracy. The n-MOSFETs with 10-nm gate length can fulfill the high-performance requirements of the International Roadmap for Devices and Systems 2020 Edition (IRDS 2020). However, the p-MOSFETs cannot fulfill the demands of IRDS 2020 because of heavy hole effective masses. Considering the appropriate on-current of 1292 μA/μm for SiP n-MOSFETs, 2D SiP could be utilized as a potential channel material in the next-generation FETs.Graphic abstract

Gate-controlled reversible rectifying behavior investigated in a two-dimensional MoS2 diode

By using density functional theory and ab initio quantum-transport simulation, we study the Schottky barrier and the rectifying behavior of diodes consisting of the two-dimensional metal phase $1T\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$ and semiconductor phase 2H-${\mathrm{MoS}}_{2}$. The results show that the Schottky barrier of the out-of-plane (OP) contacted ${\mathrm{MoS}}_{2}$ heterostructure diode is a little different from that of the in-plane (IP) contacted ${\mathrm{MoS}}_{2}$ heterostructure diode. The current-voltage characteristics show that the OP diode has the better rectifying behavior compared to the IP diode under the zero gate voltage. The corresponding maximum rectifier ratio of the OP Schottky barrier diode is close to ${10}^{7}$ at 0.9 V bias voltage. More interestingly, we find that the gate voltage can be used to effectively control the rectifying behavior of the two diodes. The positive gate voltages can increase the current value of two Schottky barrier diodes, but weaken their rectification ratios. The negative gate voltages can reverse the rectifying direction of two Schottky barrier diodes. The above results provide good theoretical guidance for the designing of diode devices based on two-dimensional materials in the future.

Van der waals BP/InSe heterojunction for tunneling field-effect transistors

Introducing heterogeneous architecture is a prospective way to improve tunneling field-effect transistors (TFETs). We investigate the van der Waals (vdW) heterojunction based on monolayer black phosphorene and indium selenide (BP/InSe heterojunction) and the double-gated 10-nm TFETs based on the vdW BP/InSe heterojunction with the contact length and position by using the ab-initio quantum transport simulations. The vdW BP/InSe heterojunction shows a type-II band edge alignment. The optimal vdW BP/InSe heterojunction TFETs have a 1-nm-length BP/InSe heterojunction at the channel’s left and right sites (1L and 1R for short). Novelty, the BP/InSe heterojunction TFETs with 1L and 1R configurations are n- and p-type devices, respectively, and corresponding high on-currents of 240 and 408 μA/μm are obtained for high-performance application (off-current: 0.1 μA/μm) at a very low supply voltage (0.3 V).

Bilayer Tellurene: A Potential p‐Type Channel Material for Sub‐10 nm Transistors

AbstractThe emerging two‐dimensional (2D) tellurium (tellurene) has attracted much attention due to its high carrier transportability and prominent air stability. Micrometer‐scale bilayer (BL) tellurene field‐effect transistors (FETs) are successfully fabricated with a large on/off current ratio of about 105. Here, the transport properties of both the n‐ and p‐type double‐gated sub‐10 nm BL tellurene metal‐oxide‐semiconductor FETs (MOSFETs) are systematically explored by using ab initio quantum transport simulation. The optimized 5, 7, and 9 nm gate‐length p‐type x‐ and y‐directed BL tellurene MOSFETs with a proper underlap and negative capacitance dielectric can meet or nearly meet the on‐state current, delay time, power dissipation, and energy‐delay product requirements of the International Technology Roadmap for Semiconductors for the 2022–2028 horizons for high‐performance applications. This renders BL tellurene to join the air‐stable channel candidate for the p‐type sub‐10 nm transistors.

Can ultra-thin Si FinFETs work well in the sub-10 nm gate-length region?

Fin field-effect transistors (FinFETs) dominate the present Si FETs. However, when the gate length is scaled down to the sub-10 nm region, the experimental Si FinFETs suffer from poor performance due to a large fin width (the minimum value is 3 nm). In this paper, an ultra-thin Si FinFET with a width of 0.8 nm is investigated for the first time by utilizing ab initio quantum transport simulations. Remarkably, even with the gate length down to 5 nm, the on-state current, delay time, power dissipation, and energy-delay product of the optimized perfect ultra-thin Si FinFET still meet the high-performance applications' requirements of the International Technology Roadmap for Semiconductors in the next decade. The overall performance of the simulated ultra-thin Si FinFET is even comparable with that of the typical two-dimensional FETs. Such a good performance can be significantly degraded by the defect. Hence, Si FinFETs have the potential to be scaled down to the sub-10 nm gate length as long as the width is scaled down while keeping a perfect structure.

High‐Performance Spin Filters and Spin Field Effect Transistors Based on Bilayer VSe2

AbstractRecently, two‐dimensional (2D) magnetic van der Waals materials have drawn great attention as they are remarkably promising in numerous vital areas, such as data storage and information processing. Theoretically, bilayer (BL) 2H‐VSe2 has been predicted to be an A‐type 2D antiferromagnetic van der Waals crystal (intralayer ferromagnetism and interlayer antiferromagnetism) and to have electrically‐induced half‐metallicity. Herein, by using ab initio quantum transport simulations, BL 2H‐VSe2 spin devices are designed and spin‐resolved transport properties are investigated for the first time. The spin‐filter efficiency (SFE) of the dual‐gated BL 2H‐VSe2 spin filter reaches up to 99%, and the conductance on‐off ratio of this device is up to 106. Also, the conductance on‐off ratio of the quadruple‐gated BL 2H‐VSe2 spin field effect transistor can reach 4 × 103 after reversing the spin direction. This work shows the high performance of the BL 2H‐VSe2 in spintronic devices and will motivate further studies of the spin devices based on this kind of material.

Asymmetric transport in boron intratube p-i-n junction via gas storage for diode applications

Boron nanotubes (BNTs) possess higher conductivities than carbon nanotubes (CNTs), making them potential candidates for nanoscale p-i-n diode applications. Herein, we designed a novel boron intratube p-i-n junction via gas storage, by using ab initio quantum transport simulations. NO and NH3 are the optimal oxide and hydride to inject holes and electrons into (5,5) buckled triangular BNT (BT-BNT), respectively. The device performance in (5,5) BT-BNT FET indicates that the gas storage can rationally change the majority carrier type and concentration, by which (5,5) BT-BNT-based p-i-n junction with large negative rectification can be constructed. We further investigated the dependence of asymmetric transport on the intrinsic region length in the formed p-i-n junction. It is found that the insertion of intrinsic region can act as a buffer layer to reduce the carrier concentration difference on p-type and n-type regions, thus weakening the transport asymmetry. A short intrinsic region with length of 6.22 Å, i.e., n = 2, contributes to improve the rectifying ratio. Our results demonstrate the significance of gas storage combined with the intrinsic region in the formation of p-i-n junction for diode applications based on one-dimensional nanotubes.

Sub-5-nm Monolayer Silicane Transistor: A First-Principles Quantum Transport Simulation

As one of the thinnest forms of semiconducting silicon, monolayer (ML) silicane has not only excellent gate electrostatics and carrier transport ability, but also compatibility with well-established silicon-based technology. We explore the device performance limits of sub-5-nm ML silicane metal-oxide-semiconductor field-effect transistors (MOSFETs) by applying ab initio quantum transport simulations. The on-state current, effective delay time, and power-delay product of the optimized n-type and p-type ML silicane MOSFETs can well or nearly meet the high-performance device requirements of the International Technology Roadmap for Semiconductors (ITRS) at a gate length of 5 nm. Those of the optimized n-type ML silicane MOSFETs at a gate length of 3 nm and the p-type ML silicane MOSFETs at a gate length of 5 nm can meet the low-power-device demands of the ITRS. Thus, ML silicane as channel materials can scale the Moore's law down to 5 nm.

First-principles investigations on MXene-blue phosphorene and MXene-MoS2 transistors

In the semiconductor industry, one of the most important steps in the development of electronic devices is the discovery of electrode materials that are suitable for ohmic contact. As a newly found type of 2D materials, MXenes have been explored as materials for use in field effect transistors (FETs) with promising performances, which urges for the underlying mechanisms to be understood. In this work, the behaviors of the 5–10 nm device models for monolayer blue phosphorene (BlueP) and MoS2 with a MXene electrode are investigated using ab initio quantum transport simulations. Firstly, the interfacial properties of BlueP and MoS2 in contact with M3C2T2 (M = Ti, Zr, or Hf; T = F, OH, or O) MXene are studied. The results show that OH and some of the F functionalized MXenes form an n-type ohmic contact with BlueP or MoS2, while the O functionalized MXenes form a p-type ohmic contact with BlueP and MoS2. Accordingly, when the FET model is built with M3C2(OH)2 electrodes, these FETs exhibit high on-currents due to the ohmic contacts with subthreshold swing between 100 ∼ 200 mV/decade, and high on/off ratios up to 106 at a bias voltage of 0.5 V. Our results imply that a FET with a sub-10 nm channel length can satisfy the requirements of both high performance and low power logic applications. The results from this study indicate that MXenes may act as the appropriate electrode for high-performance BlueP and MoS2 FETs, which may provide new clues to guide the application of various 2D materials in electronics.

Unit cell restricted Bloch functions basis for first-principle transport models: Theory and application

We present the theory and the application of a first-principle transport model employing a basis set obtained directly from the ab initio Bloch functions. We use a plane-wave density functional theory Hamiltonian and show that a judicious choice of the reduced basis set can effectively suppress the potentially thorny problem of the unphysical solutions. Our methodology enables ab initio transport simulations with a huge reduction of the size of the problem compared to the original ab initio formulation. Moreover, the approach can also be used for local and nonlocal empirical pseudopotential Hamiltonians, thus promising a wide range of possible applications. We report results for ab initio simulations of ${\mathrm{MoS}}_{2}$ field effect transistors, where the transport and electrostatics equations are solved self-consistently for channel lengths up to about 20 nanometers. The simulation results rapidly converge with the size of the basis set, so that the blocks of the Hamiltonian matrix can be reduced to a size below 100. Our methodology is a viable approach for ab initio and semiempirical quantum transport simulations and, in particular, it offers an alternative to the use of maximally localized Wannier functions.

Ohmic contacts of monolayer Tl2O field-effect transistors

Monolayer (ML) Tl2O, as a newly discovered metal-shrouded two-dimensional semiconductor with an appropriate bandgap and high carrier mobility, is a promising candidate as the channel materials for the next-generation field-effect transistors (FETs). Using ab initio electronic structure calculation and quantum transport simulation, the contact properties of ML Tl2O–metal interfaces based on FET are comprehensively investigated with Au, Sc, Tl, Ni, graphene, Ti2C, Ti2CF2, and Ti2C(OH)2 electrodes. ML Tl2O undergoes metallization with Au, Sc, Tl, Ni, and Ti2C, while it forms van der Waals-type contact with graphene, Ti2CF2, and Ti2C(OH)2. An n-type lateral Schottky contact is formed with Ni, Au, Sc, and Ti2C electrodes with the electron Schottky barrier height (SBH) of 0.25, 0.27, 0.27, and 0.36 eV, respectively, while a p-type lateral Schottky contact is formed with graphene electrode with the hole SBH of 0.10 eV. Surprisingly, a desired n-type Ohmic contact arises with Tl and Ti2C(OH)2 electrodes and a desired p-type Ohmic contact arises with Ti2CF2 electrode. The study not only provides a deep understanding of the interfacial properties of the ML Tl2O FETs but also reveals a versatile approach to realize both n- and p-type Ohmic contact for the ML Tl2O FETs.