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Ultra-thin (UT) oxide semiconductors are promising candidates for back-end-of-line (BEOL) compatible transistors and monolithic three-dimensional integration. Experimentally, UT indium oxide (In$_2$O$_3$) field-effect transistors (FETs) with thicknesses down to 0.4 nm exhibits extremely high drain current (10000 $\mu$A/$\mu$m) and transconductance (4000 $\mu$S/$\mu$m). Here, we employ the ab initio quantum transport simulation to investigate the performance limit of sub-5-nm gate length (Lg) UT In$_2$O$_3$ FET. Based on the International Technology Roadmap for Semiconductors (ITRS) criteria for high-performance (HP) devices, the scaling limit of UT In$_2$O$_3$ FETs can reach 2 nm in terms of on-state current, delay time, and power dissipation. The wide bandgap nature of UT In$_2$O$_3$ (3.15 eV) renders it a suitable candidate for ITRS low-power (LP) electronics with Lg down to 3 nm. Both the HP and LP UT In$_2$O$_3$ FETs exhibit superior energy-delay products as compared to other common 2D semiconductors such as monolayer MoS2 and MoTe2. Our study unveils the immense promise of UT In$_2$O$_3$ for both HP and LP device applications., Comment: 16 pages, 7 figures

Gate-all-around (GAA) nanowire (NW) field-effect transistor (FET) is a promising device architecture due to its superior gate controllability than that of the conventional FinFET architecture. The significantly higher electron mobility of indium phosphide (InP) NW than silicon NW makes it particularly well-suited for high-performance (HP) electronics applications. In this work, we perform an ab initio quantum transport simulation to investigate the performance limit of sub-5-nm gate length (Lg) GAA InP NW FETs. The GAA InP NW FETs with Lg of 4 nm can meet the International Technology Roadmap for Semiconductors (ITRS) requirements for HP devices from the perspective of on-state current, delay time, and power dissipation. We also investigate the impact of strain on 3-nm-Lg GAA InP NW FETs. The application of tensile strain results in a remarkable increase of over 60% in the on-state current. These results highlight the potential of GAA InP NW FETs for HP applications in the sub-5-nm Lg region., Comment: 22 pages, 7 figures

Formation of low-resistance metal contacts is the biggest challenge that masks the intrinsic exceptional electronic properties of 2D WSe2 devices. We present the first comparative study of the interfacial properties between ML/BL WSe2 and Sc, Al, Ag, Au, Pd, and Pt contacts by using ab initio energy band calculations with inclusion of the spin-orbital coupling (SOC) effects and quantum transport simulations. The interlayer coupling tends to reduce both the electron and hole Schottky barrier heights (SBHs) and alters the polarity for WSe2-Au contact, while the SOC chiefly reduces the hole SBH. In the absence of the SOC, Pd contact has the smallest hole SBH with a value no less than 0.22 eV. Dramatically, Pt contact surpasses Pd contact and becomes p-type Ohmic or quasi-Ohmic contact with inclusion of the SOC. Our study provides a theoretical foundation for the selection of favorable metal electrodes in ML/BL WSe2 devices.

Recently, phosphorene electronic and optoelectronic prototype devices have been fabricated with various metal electrodes. We systematically explore for the first time the contact properties of monolayer (ML) phosphorene with a series of commonly used metals (Al, Ag. Cu, Au, Cr, Ni, Ti, and Pd) via both ab initio electronic structure calculations and more reliable quantum transport simulations. Strong interactions are found between all the checked metals, with the energy band structure of ML phosphorene destroyed. In terms of the quantum transport simulations, ML phosphorene forms a n-type Schottky contact with Au, Cu, Cr, Al, and Ag electrodes, with electron Schottky barrier heights (SBHs) of 0.30, 0.34, 0.37, 0.51, and 0.52 eV, respectively, and p-type Schottky contact with Ti, Ni, and Pd electrodes, with hole SBHs of 0.30, 0.26, and 0.16 eV, respectively. These results are in good agreement with available experimental data. Our findings not only provide an insight into the ML phosphorene-metal interfaces but also help in ML phosphorene based device design.

Silicene, as the silicon analog of graphene, has been successfully fabricated by epitaxial growing on various substrates. Similar to free-standing graphene, free-standing silicene possesses a honeycomb structure and Dirac-cone-shaped energy band, resulting in many fascinating properties such as high carrier mobility, quantum spin Hall effect, quantum anomalous Hall effect, and quantum valley Hall effect. The maintenance of the honeycomb crystal structure and the Dirac cone of silicene is crucial for observation of its intrinsic properties. In this review, we systematically discuss the substrate effects on the atomic structure and electronic properties of silicene from a theoretical point of view, especially focusing on the changes of the Dirac cone., Comment: 33 pages, 13 figures, 2 tables

Spintronics involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. The fascinating spin-resolved properties of graphene motivate numerous researchers into the studies of spintronics in graphene and other two-dimensional (2D) materials. Silicene, silicon analog of graphene, is considered as a promising material for spintronics. Here, we present a review on the theoretical advances about the spin-dependent properties including the electric field and exchange field tunable topological properties of silicene and the corresponding spintronic device simulations., Comment: review article; 17 figures

Similar to graphene, zero band gap limits the application of silicene in nanoelectronics despite of its high carrier mobility. By using first-principles calculations, we reveal that a band gap is opened in silicene nanomesh (SNM) when the width W of the wall between the neighboring holes is even. The size of the band gap increases with the reduced W and has a simple relation with the ratio of the removed Si atom and the total Si atom numbers of silicene. Quantum transport simulation reveals that the sub-10 nm single-gated SNM field effect transistors show excellent performance at zero temperature but such a performance is greatly degraded at room temperature.

Although many prototype devices based on two-dimensional (2D) MoS2 have been fabricated and wafer scale growth of 2D MoS2 has been realized, the fundamental nature of 2D MoS2-metal contacts has not been well understood yet. We provide a comprehensive ab initio study of the interfacial properties of a series of monolayer (ML) and bilayer (BL) MoS2-metal contacts (metal = Sc, Ti, Ag, Pt, Ni, and Au). A comparison between the calculated and observed Schottky barrier heights (SBHs) suggests that many-electron effects are strongly suppressed in channel 2D MoS2 due to a charge transfer. The extensively adopted energy band calculation scheme fails to reproduce the observed SBHs in 2D MoS2-Sc interface. By contrast, an ab initio quantum transport device simulation better reproduces the observed SBH in the two types of contacts and highlights the importance of a higher level theoretical approach beyond the energy band calculation in the interface study. BL MoS2-metal contacts have a reduced SBH than ML MoS2-metal contacts due to the interlayer coupling and thus have a higher electron injection efficiency., Comment: 36 pages, 13 figures, 3 tables

Graphdiyne is prepared on metal surface, and making devices out of it also inevitably involves contact with metals. Using density functional theory with dispersion correction, we systematically studied for the first time the interfacial properties of graphdiyne contacting with a series of metals (Al, Ag, Cu, Au, Ir, Pt, Ni, and Pd). Graphdiyne is in an n-type Ohmic or quasi-Ohmic contact with Al, Ag, and Cu, while it is in a Schottky contact with Au (at source/drain interface), Pd, Pt, Ni, and Ir (at source/drain-channel interface), with high Schottky barrier heights of 0.39, 0.21 (n-type), 0.30, 0.41, and 0.45 (p-type) eV, respectively. A graphdiyne field effect transistor (FET) with Al electrodes is simulated by using quantum transport calculations. This device exhibits an on-off ratio up to 104 and a very large on-state current of 1.3 * 104 mA/mm in a 10 nm channel length. Thus, a new prospect is opened up for graphdiyne in high performance nanoscale devices., Comment: 27 pages, 9 figures

It is a persisting pursuit to use metal as a channel material in a field effect transistor. All metallic transistor can be fabricated from pristine semimetallic Dirac materials (such as graphene, silicene, and germanene), but the on/off current ratio is very low. In a vertical heterostructure composed by two Dirac materials, the Dirac cones of the two materials survive the weak interlayer van der Waals interaction based on density functional theory method, and electron transport from the Dirac cone of one material to the one of the other material is therefore forbidden without assistance of phonon because of momentum mismatch. First-principles quantum transport simulations of the all-metallic vertical Dirac material heterostructure devices confirm the existence of a transport gap of over 0.4 eV, accompanied by a switching ratio of over 104. Such a striking behavior is robust against the relative rotation between the two Dirac materials and can be extended to twisted bilayer graphene. Therefore, all-metallic junction can be a semiconductor and novel avenue is opened up for Dirac material vertical structures in high-performance devices without opening their band gaps., Comment: 7 figures

In this Letter, a tunable valley polarization is investigated for honeycomb systems with broken inversion symmetry such as transition-metal dichalcogenide MX2 (M = Mo, W; X = S, Se) monolayers through elliptical pumping. As compared to circular pumping, elliptical pumping is more universal and effective method to create coherent valley polarization. When two valleys of MX2 monolayers are doped or polarized, a novel anomalous valley orbital magnetic moment Hall effect driven by opposite Berry curvatures at different valleys is predicted and can generate orbital magnetic moment current without the accompaniment of a charge current, opening a new avenue for exploration of valleytronics and orbitronics. Valley orbital magnetic moment Hall effect is expected to obscure spin Hall effect and is tunable under elliptical pumping.

Absence of the Dirac cone due to a strong band hybridization is revealed to be a common feature for epitaxial silicene on metal substrates according to our first-principles calculations for silicene on Ir, Cu, Mg, Au, Pt, Al, and Ag substrates. The destroyed Dirac cone of silicene, however, can be effectively restored with linear or parabolic dispersion by intercalating alkali metal atoms between silicene and the metal substrates, offering an opportunity to study the intriguing properties of silicene without further transfer of silicene from the metal substrates.

Opening a sizable band gap in the zero-gap germanene without heavy loss of carrier mobility is a key issue for its application in nanoelectronic devices such as high-performance field effect transistors (FETs) operating at room temperature. Using the first-principles calculations, we find a band gap is opened at the Dirac point in germanene by single-side adsorption of alkali metal (AM) atoms. This band gap is tunable by varying the coverage and the species of AM atoms, ranging from 0.02 to 0.31 eV, and the maximum global band gap is 0.26 eV. Since the effective masses of electrons and holes in germanene near the Dirac point after surface adsorption (ranging from 0.005 to 0.106 me) are small, the carrier mobility is expected not to degrade much. Therefore germanene is a potential candidate of effective FET channel operating at room temperature upon surface adsorption., Comment: 17 pages, 6 figures

Structural and electronic properties of silicene adsorbed by five kinds of transition metal atoms (Cu, Ag, Au, Pt, and Ir) are systematically studied by using first-principles calculations. We find that such adsorption can induce a band gap at the Dirac point of doped silicene. Doped silicene can reach a band gap up to 0.23 eV while keeping a relatively small effective mass of around 0.1 me, thus having high carrier mobility estimated to be 50000 cm2/Vs. P-type doping and neutral state is realized in silicene by Ir and Pt adsorption, respectively, while n-type doping is done by Cu, Ag, and Au adsorption. Based on the knowledge above, a silicene p-i-n tunneling field effect transistor (TFET) is proposed and simulated by both first-principles and semi-empirical approaches. Silicene TFET shows high performance with an on-off ratio of 10^3, a sub-threshold swing of 90 mV/dec, and an on-state current of 1 mA/{\mu}m. Such an on-state current is even larger than that of most other existing TFETs., Comment: 17pages, 1 table, 12 figures

By using first-principles calculations, we systematically investigated several observed phases of silicene on Ag(111) substrates and their electronic structures. We find that the original Dirac cone of silicene is about 1.5-1.7 eV deeply below the Fermi level and severely destroyed by the band hybridization between silicene and Ag in all the examined phases. Thus, silicene synthesized on Ag(111) substrates could not preserve its excellent electronic property and new method is needed to develop in synthesizing silicene with its Dirac cone surviving.

There is a tunable band gap in ABC-stacked few-layer graphene (FLG) via applying a vertical electric field, but the operation of FLG-based field effect transistor (FET) requires two gates to create a band gap and tune channel's conductance individually. Using first principle calculations, we propose an alternative scheme to open a band gap in ABC-stacked FLG namely via single-side adsorption. The band gap is generally proportional to the charge transfer density. The capability to open a band gap of metal adsorption decreases in this order: K/Al > Cu/Ag/Au > Pt. Moreover, we find that even the band gap of ABA-stacked FLG can be opened if the bond symmetry is broken. Finally, a single-gated FET based on Cu-adsorbed ABC-stacked trilayer graphene is simulated. A clear transmission gap is observed, which is comparable with the band gap. This renders metal-adsorbed FLG a promising channel in a single-gated FET 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 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.

One popular approach to prepare graphene is to grow them on transition metal substrates via chemical vapor deposition. By using the density functional theory with dispersion correction, we systematically investigate for the first time the interfacial properties of bilayer (BLG) and trilayer graphene (TLG) on metal substrates. Three categories of interfacial structures are revealed. The adsorption of B(T)LG on Al, Ag, Cu, Au, and Pt substrates is a weak physisorption, but a band gap can be opened. The adsorption of B(T)LG on Ti, Ni, and Co substrates is a strong chemisorption, and a stacking-insensitive band gap is opened for the two uncontacted layers of TLG. The adsorption of B(T)LG on Pd substrate is a weaker chemisorption, with a band gap opened for the uncontacted layers. This fundamental study also helps for B(T)LG device study due to inevitable graphene/metal contact., Comment: 1 table, 8 figures