Ab Initio Quantum Transport Simulations Research Articles

Tuning Schottky barriers for monolayer GaSe FETs by exploiting a weak Fermi level pinning effect.

Monolayer gallium selenide (GaSe), an emerging two-dimensional semiconductor, holds great promise for electronics and optoelectronics. In reality, the contact properties between a semiconductor and the electrodes play a vital role in device performance. Thus, selecting suitable electrode materials with zero contact resistance is critical and challenging. Here, we explore the contact properties and electronic transport of monolayer GaSe with a series of electrode materials (including transition metals and MXenes) by ab initio electronic calculations and quantum transport simulations. Bare MXenes (V2C, Mo2C, Nb2C) and transition metals (Ni, Cu, Pt, Au) form strong chemical bonds with monolayer GaSe and lead to n-type contacts with Schottky barriers of 0-0.53 eV. The O-terminated MXenes (V2CO2 and Nb2CO2) establish van der Waals interactions with the GaSe sheet and form p-type contacts with vanishing hole Schottky barriers but large tunneling barriers. Impressively, Ohmic contact is achieved by the V2C and Mo2C electrodes, which is further demonstrated by the linear I-V curves. These contact systems exhibit a weak Fermi level pinning effect, with Schottky barriers in both the vertical and lateral directions controllable by the work function of the electrodes. To promote the carrier injection process, we propose to select electrode materials with a strong surface polarization and low work function (<5.0 eV) for GaSe-based devices.

Electrical Contacts in Monolayer Arsenene Devices.

Arsenene, arsenic analogue of graphene, as an emerging member of two-dimensional semiconductors (2DSCs), is quite promising in next-generation electronic and optoelectronic applications. The metal electrical contacts play a vital role in the charge transport and photoresponse processes of nanoscale 2DSC devices and even can mask the intrinsic properties of 2DSCs. Here, we present a first comprehensive study of the electrical contact properties of monolayer (ML) arsenene with different electrodes by using ab initio electronic calculations and quantum transport simulations. Schottky barrier is always formed with bulk metal contacts owing to the Fermi level pinning (pinning factor S = 0.33), with electron Schottky barrier height (SBH) of 0.12, 0.21, 0.25, 0.35, and 0.50 eV for Sc, Ti, Ag, Cu, and Au contacts and hole SBH of 0.75 and 0.78 eV for Pd and Pt contacts, respectively. However, by contact with 2D graphene, the Fermi level pinning effect can be reduced due to the suppression of metal-induced gap states. Remarkably, a barrier free hole injection is realized in ML arsenene device with graphene-Pt hybrid electrode, suggestive of a high device performance in such a ML arsenene device. Our study provides a theoretical foundation for the selection of favorable electrodes in future ML arsenene devices.

Tunneling field effect transistors based on in-plane and vertical layered phosphorus heterostructures**Project supported by the National Natural Science Foundation of China (Grant Nos. 11604019, 61574020, and 61376018), the Ministry of Science and Technology of China (Grant No. 2016YFA0301300), the Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications), China, and the Fundamental

Tunneling field effect transistors (TFETs) based on two-dimensional materials are promising contenders to the traditional metal oxide semiconductor field effect transistor, mainly due to potential applications in low power devices. Here, we investigate the TFETs based on two different integration types: in-plane and vertical heterostructures composed of two kinds of layered phosphorous (β-P and δ-P) by ab initio quantum transport simulations. NDR effects have been observed in both in-plane and vertical heterostructures, and the effects become significant with the highest peak-to-valley ratio (PVR) when the intrinsic region length is near zero. Compared with the in-plane TFET based on β-P and δ-P, better performance with a higher on/off current ratio of ~ 106 and a steeper subthreshold swing (SS) of ~ 23 mV/dec is achieved in the vertical TFET. Such differences in the NDR effects, on/off current ratio and SS are attributed to the distinct interaction nature of the β-P and δ-P layers in the in-plane and vertical heterostructures.

Three-layer phosphorene-metal interfaces

Phosphorene has attracted much attention recently as an alternative channel material in nanoscale electronic and optoelectronic devices due to its high carrier mobility and tunable direct bandgap. Compared with monolayer (ML) phosphorene, few-layer (FL) phosphorene is easier to prepare, is more stable in experiments, and is expected to form a smaller Schottky barrier height (SBH) at the phosphorene-metal interface. Using ab initio electronic structure calculations and quantum transport simulations, we perform a systematic study of the interfacial properties of three-layer (3L) phosphorene field effect transistors (FETs) contacted with several common metals (Al, Ag, Au, Cu, Ti, Cr, Ni, and Pd) for the first time. The SBHs obtained in the vertical direction from projecting the band structures of the 3L phosphorene-metal systems to the left bilayer (2L) phosphorenes are comparable with those obtained in the lateral direction from the quantum transport simulations for 2L phosphorene FETs. The quantum transport simulations for the 3L phosphorene FETs show that 3L phosphorene forms n-type Schottky contacts with electron SBHs of 0.16 and 0.28 eV in the lateral direction, when Ag and Cu are used as electrodes, respectively, and p-type Schottky contacts with hole SBHs of 0.05, 0.11, 0.20, 0.30, 0.30, and 0.31 eV in the lateral direction when Cr, Pd, Ni, Ti, Al, and Au are used as electrodes, respectively. The calculated polarity and SBHs of the 3L phosphorene FETs are generally in agreement with the available experiments.

A computational study of monolayer hexagonal WTe2 to metal interfaces

Monolayer (ML) hexagonal (2H) WTe2 is predicted to be the best channel material of tunnel field effect transistor (TFET) and metal–oxide–semiconductor field effect transistor (MOSFET) among ML transition‐metal dichalcogenides. Actual devices based on 2H WTe2 typically have a contact with metal. We explore for the first time the interfacial properties between ML 2H WTe2 and Sc, Ti, Pd, Pt, Ag, and Au by using ab initio electronic structure calculation and ab initio quantum transport simulations. The energy bands of ML 2H WTe2 on Sc, Ti, Pd, and Pt substrates are destroyed strongly due to strong adhesion of ML 2H WTe2 with metal substrates, and ML 2H WTe2–Sc, −Ti, −Pd, and −Pt systems are regarded as new metallic materials. Weak adhesion is formed between ML 2H WTe2 and the Ag and Au surfaces, with the electronic energy band of ML 2H WTe2 being identifiable. Ag and Au form n‐type Schottky contact with ML 2H WTe2 at the vertical direction with electron Schottky barrier height (SBH) of 0.24 and 0.49 eV, respectively. In contrast, Pd, Pt, and Ti form p‐type Schottky contact with ML 2H WTe2 in the lateral direction with hole SBH of 0.26, 0.40, and 0.63 eV, respectively. Our study not only presents a theoretical insight into the ML 2H WTe2–metal interfaces but also help in ML 2H WTe2 based device design.

Schottky Barriers in Bilayer Phosphorene Transistors.

It is unreliable to evaluate the Schottky barrier height (SBH) in monolayer (ML) 2D material field effect transistors (FETs) with strongly interacted electrode from the work function approximation (WFA) because of existence of the Fermi-level pinning. Here, we report the first systematical study of bilayer (BL) phosphorene FETs in contact with a series of metals with a wide work function range (Al, Ag, Cu, Au, Cr, Ti, Ni, and Pd) by using both ab initio electronic band calculations and quantum transport simulation (QTS). Different from only one type of Schottky barrier (SB) identified in the ML phosphorene FETs, two types of SBs are identified in BL phosphorene FETs: the vertical SB between the metallized and the intact phosphorene layer, whose height is determined from the energy band analysis (EBA); the lateral SB between the metallized and the channel BL phosphorene, whose height is determined from the QTS. The vertical SBHs show a better consistency with the lateral SBHs of the ML phosphorene FETs from the QTS compared than that of the popular WFA. Therefore, we develop a better and more general method than the WFA to estimate the lateral SBHs of ML semiconductor transistors with strongly interacted electrodes based on the EBA for its BL counterpart. In terms of the QTS, n-type lateral Schottky contacts are formed between BL phosphorene and Cr, Al, and Cu electrodes with electron SBH of 0.27, 0.31, and 0.32 eV, respectively, while p-type lateral Schottky contacts are formed between BL phosphorene and Pd, Ti, Ni, Ag, and Au electrodes with hole SBH of 0.11, 0.18, 0.19, 0.20, and 0.21 eV, respectively. The theoretical polarity and SBHs are in good agreement with available experiments. Our study provides an insight into the BL phosphorene-metal interfaces that are crucial for designing the BL phosphorene device.

Many-body Effect, Carrier Mobility, and Device Performance of Hexagonal Arsenene and Antimonene

Two-dimensional (2D) semiconductors are very promising channel materials in next-generation field effect transistors (FETs) due to the enhanced gate electrostatics and smooth surface. Two new 2D materials, arsenene and antimonene (As and Sb analogues of graphene), have been fabricated very recently. Here, we provide the first investigation of the many-body effect, carrier mobility, and device performance of monolayer (ML) hexagonal arsenene and antimonene based on accurate ab initio methods. The quasi-particle band gaps of ML arsenene and antimonene by using the GW approximation are 2.47 and 2.38 eV, respectively. The optical band gaps of ML arsenene and antimonene from the GW-Bethe–Salpeter equation are 1.6 and 1.5 eV, with exciton binding energies of 0.9 and 0.8 eV, respectively. The carrier mobility is found to be considerably low in ML arsenene (21/66 cm2/V·s for electron/hole) and moderate in ML antimonene (150/510 cm2/V·s for electron/hole). In terms of the ab initio quantum transport simulations, t...

Can a Black Phosphorus Schottky Barrier Transistor Be Good Enough?

Experimental two-dimensional (2D) black phosphorus (BP) transistors typically appear in the form of Schottky barrier field effect transistors (SBFETs), but their performance limit remains open. We investigate the performance limit of monolayer BP SBFETs in the sub-10 nm scale by using ab initio quantum transport simulations. The devices with 2D graphene electrodes are apparently superior to those with bulk Ti electrodes due to their smaller and tunable Schottky barrier heights and the absence of metal induced gap states in the channels. With graphene electrodes, the performance limit of the sub-10 nm monolayer BP SBFETs outperforms the monolayer MoS2, carbon nanotube, and advanced silicon transistors and even can meet the requirements of both high performance and low power logic applications of the next decade in the latest International Technology Roadmap for Semiconductors. It appears that the ML BP SBFETs have the best intrinsic device performance among the reported sub-10 nm 2D material SBFETs.

Interfacial Properties of Monolayer MoSe2–Metal Contacts

Monolayer (ML) transition-metal dichalcogenides are considered as promising channel materials in next-generation transistors. Using ab initio energy band calculations and more reliable ab initio quantum transport simulations, we study the interfacial properties of ML MoSe2–metal interfaces (metals = Al, Ag, Pt, Cr, Ni, and Ti). Weak or medium adsorption is found between ML MoSe2 and the Al, Ag, and Pt surfaces with the band structure of ML MoSe2 preserved, while strong adsorption is found between ML MoSe2 and the Ni, Ti, and Cr surfaces with the band structure of ML MoSe2 destroyed. The two methods give similar polarity and height of Schottky barriers for ML MoSe2 with Al, Ag, Pt, and Ti electrodes. ML MoSe2 forms an n-type Schottky contact with Ag, Ti, and Al electrodes with electron Schottky barrier heights (SBH) of 0.25, 0.29, and 0.56 eV, respectively, and a p-type Schottky contact with Pt electrode with hole SBH of 0.78 eV according to ab initio quantum transport simulations. Our study offers a guida...

Role of chemical termination in edge contact to graphene

Edge contacts to graphene can offer excellent contact properties. Role of different chemical terminations is examined by using ab initio density functional theory and quantum transport simulations. It is found that edge termination by group VI elements O and S offers considerably lower contact resistance compared to H and group VII element F. The results can be understood by significantly larger binding energy and shorter binding distance between the metal contact and these group VI elements, which results in considerably lower interface potential barrier and larger transmission. The qualitative conclusion applies to a variety of contact metal materials.

Enhancement of band-to-band tunneling in mono-layer transition metal dichalcogenides two-dimensional materials by vacancy defects

The band-to-band tunneling of monolayer transition metal dichalcogenides nano-junction is investigated using atomistic ab initio quantum transport simulations. From the simulation, it is found that the transition metal vacancy defect in the two-dimensional MX2 (M = Mo,W; X = S,Se) band-to-band tunneling diode can dramatically boost the on-state current up to 10 times while maintaining the device sub-threshold swing. The performance enhancement mechanism is discussed in detail by examining partial density of states of the system. It is found that the transition metal vacancy induces band-gap states, which reduce the effective length of the tunneling transition region.

Sub-10 nm Gate Length Graphene Transistors: Operating at Terahertz Frequencies with Current Saturation

Radio-frequency application of graphene transistors is attracting much recent attention due to the high carrier mobility of graphene. The measured intrinsic cut-off frequency (fT) of graphene transistor generally increases with the reduced gate length (Lgate) till Lgate = 40 nm, and the maximum measured fT has reached 300 GHz. Using ab initio quantum transport simulation, we reveal for the first time that fT of a graphene transistor still increases with the reduced Lgate when Lgate scales down to a few nm and reaches astonishing a few tens of THz. We observe a clear drain current saturation when a band gap is opened in graphene, with the maximum intrinsic voltage gain increased by a factor of 20. Our simulation strongly suggests it is possible to design a graphene transistor with an extraordinary high fT and drain current saturation by continuously shortening Lgate and opening a band gap.

Tunable and sizable band gap in silicene by surface adsorption.

Opening a sizable band gap without degrading its high carrier mobility is as vital for silicene as for graphene to its application as a high-performance field effect transistor (FET). Our density functional theory calculations predict that a band gap is opened in silicene by single-side adsorption of alkali atom as a result of sublattice or bond symmetry breaking. The band gap size is controllable by changing the adsorption coverage, with an impressive maximum band gap up to 0.50 eV. The ab initio quantum transport simulation of a bottom-gated FET based on a sodium-covered silicene reveals a transport gap, which is consistent with the band gap, and the resulting on/off current ratio is up to 108. Therefore, a way is paved for silicene as the channel of a high-performance FET.

HALF-METALLIC SILICENE AND GERMANENE NANORIBBONS: TOWARDS HIGH-PERFORMANCE SPINTRONICS DEVICE

By using first-principles calculations, we predict that an in-plane homogenous electrical field can induce half-metallicity in hydrogen-terminated zigzag silicene and germanene nanoribbons ( ZSiNRs and ZGeNRs ). A dual-gated finite ZSiNR device reveals a nearly perfect spin-filter efficiency (SFE) of up to 99% while a quadruple-gated finite ZSiNR device serves as an effective spin field effect transistor (FET) with an on/off current ratio of over 100 from ab initio quantum transport simulation. This discovery opens up novel prospect of silicene and germanene in spintronics.

Tuning Electronic Structure of Bilayer MoS2 by Vertical Electric Field: A First-Principles Investigation

Interest in the two-dimensional MoS2 material is consistently increasing because of its many potential applications, in particular in the next-generation nanoelectronic devices. By means of density functional theory computations, we systematically examined the effect of vertical electric field on the electronic structure of MoS2 bilayer. The bandgaps of the bilayer MoS2 monotonically decrease with an increasing vertical electric field. The critical electric fields, at which the semiconductor-to-metal transition occurs, are predicted to be in the range of 1.0–1.5 V/A depending on different stacked conformations. Ab initio quantum transport simulations of a dual-gated bilayer MoS2 channel clearly confirm that the vertical electric field continuously manipulates the transmission gap of bilayer MoS2.

Tunable and sizable band gap of single-layer graphene sandwiched between hexagonal boron nitride

Opening a tunable and sizable band gap in single-layer graphene (SLG) without degrading its structural integrity and carrier mobility is a significant challenge. Using density functional theory calculations, we show that the band gap of SLG can be opened to 0.16 eV (without an electric field) and 0.34 eV (with a strong electric field) when properly sandwiched between two hexagonal boron nitride single layers. The zero-field band gaps are increased by more than 50% when the many-body effects are included. The ab initio quantum transport simulation of a dual-gated field effect transistor (FET) made of such a sandwich structure reveals an electric-field-enhanced transport gap, and the on/off current ratio is increased by a factor of 8.0 compared with that of a pure SLG FET. The tunable and sizeable band gap and structural integrity render this sandwich structure a promising candidate for high-performance SLG FETs. Jing Lu and co-workers have revealed how to open up a tunable band gap in single-layer graphene, the one-atom-thick honeycomb carbon layer that has sparked much interest both in fundamental physics and in practical applications. Although graphene's excellent mechanical, thermal and electrical properties are very attractive, one major drawback is its lack of ‘band gap’ — the energy gap in the electronic structure of a material that enables to switch its conductivity on and off. Previous attempts to create such a gap in single-layer graphene have typically damaged its structure or conductivity. Through extensive calculations, the researchers have now examined the properties of single-layer graphene when sandwiched between two honeycomb boron nitride (BN) layers. They revealed that for a specific positioning of the layers, a sizable band gap can be opened and further tuned by applying an electric field without causing damage, making the sandwich structure a promising component for field-effect transistors. An electric field-enhanced transport gap is well established in a dual-gated field effect transistor (FET) based on the h-BN/single-layer graphene/h-BN sandwich structure, and the on/off current ratio is increased by a factor of 8.0 compared with pure single-layer graphene FET. The tunable and sizeable band gap and structural integrity render this sandwich structure a promising candidate for high-performance single-layer graphene FETs.

Ab initio quantum transport simulation of silicide-silicon contacts

Parasitic contact resistance increasingly limits modern CMOS performance. Atomistic scale features and quantum effects play an important role in determining silicide-silicon contact properties. We develop ab initio quantum transport simulation capabilities for silicide-silicon contacts, which provide first principle descriptions of interfacial structural, electronic, and transport properties. It is found that transmission through the CoSi2-Si contact of the primed valleys can be distinctively different from that of the unprimed valleys, due to the size of the CoSi2 Fermi surface. Interfacial atomistic doping can modulate the barrier height, but, at the same time, lower the transmission and induce localized bandgap states. The primary effect of strain is to modulate the barrier height instead of quantum transmission. The physical insights obtained here would not have been possible by tight binding simulations, where reliable interface binding parameters lack.

Tunable Bandgap in Silicene and Germanene

By using ab initio calculations, we predict that a vertical electric field is able to open a band gap in semimetallic single-layer buckled silicene and germanene. The sizes of the band gap in both silicene and germanene increase linearly with the electric field strength. Ab initio quantum transport simulation of a dual-gated silicene field effect transistor confirms that the vertical electric field opens a transport gap, and a significant switching effect by an applied gate voltage is also observed. Therefore, biased single-layer silicene and germanene can work effectively at room temperature as field effect transistors.