Schottky Barriers in Bilayer Phosphorene Transistors

Chemically tuned intermediate band states in atomically thin CuxGeSe/SnS quantum material for photovoltaic applications

A new generation of quantum material derived from intercalating zerovalent atoms such as Cu into the intrinsic van der Waals gap at the interface of atomically thin two-dimensional GeSe/SnS heterostructure is designed, and their optoelectronic features are explored for next-generation photovoltaic applications. Advanced ab initio modeling reveals that many-body effects induce intermediate band (IB) states, with subband gaps (~0.78 and 1.26 electron volts) ideal for next-generation solar devices, which promise efficiency greater than the Shockley-Queisser limit of ~32%. The charge carriers across the heterojunction are both energetically and spontaneously spatially confined, reducing nonradiative recombination and boosting quantum efficiency. Using this IB material in a solar cell prototype enhances absorption and carrier generation in the near-infrared to visible light range. Tuning the active layer's thickness increases optical activity at wavelengths greater than 600 nm, achieving ~190% external quantum efficiency over a broad solar wavelength range, underscoring its potential in advanced photovoltaic technology.

Efficient Modulation of Schottky to Ohmic Contact in MoSi2N4/M3C2 (M = Zn, Cd, Hg) van der Waals Heterostructures

A strong Fermi level pinning (FLP) effect can induce a large Schottky barrier in metal/semiconductor contacts; reducing the Schottky barrier height (SBH) to form an Ohmic contact (OhC) is a critical problem in designing high-performance electronic devices. Herein, we report the interfacial electronic features and efficient modulation of the Schottky contact (ShC) to OhC for MoSi2N4/M3C2 (M = Zn, Cd, Hg) van der Waals heterostructures (vdWHs). We find that the MoSi2N4/M3C2 vdWHs can form a p-type ShC with small SBH with the calculated pinning factor S ≈ 0.8 for MoSi2N4/M3C2 contacts. These results indicate that the FLP effect can be effectively suppressed in MoSi2N4 contact with M3C2. Moreover, the interfacial properties and SBH of MoSi2N4/Zn3C2 vdWHs can be effectively modulated by a perpendicular electric field and biaxial strain. In particular, an efficient OhC can be achieved in MoSi2N4/Zn3C2 vdWHs by applying a positive electric field of 0.5 V/Å and strain of ±8%.

Interfacial Properties of Anisotropic Monolayer SiAs Transistors

The newly prepared monolayer (ML) SiAs is expected to be a candidate channel material for next-generation nano-electronic devices in virtue of its proper bandgap, high carrier mobility, and anisotropic properties. The interfacial properties in ML SiAs field-effect transistors are comprehensively studied with electrodes (graphene, V2CO2, Au, Ag, and Cu) by using ab initio electronic structure calculations and quantum transport simulation. It is found that ML SiAs forms a weak van der Waals interaction with graphene and V2CO2, while it forms a strong interaction with bulk metals (Au, Ag, and Cu). Although ML SiAs has strong anisotropy, it is not reflected in the contact property. Based on the quantum transport simulation, ML SiAs forms n-type lateral Schottky contact with Au, Ag, and Cu electrodes with the Schottky barrier height (SBH) of 0.28 (0.27), 0.40 (0.47), and 0.45 (0.33) eV along the a (b) direction, respectively, while it forms p-type lateral Schottky contact with a graphene electrode with a SBH of 0.34 (0.28) eV. Fortunately, ML SiAs forms an ideal Ohmic contact with the V2CO2 electrode. This study not only gives a deep understanding of the interfacial properties of ML SiAs with electrodes but also provides a guide for the design of ML SiAs devices.

Tunable ohmic van der Waals-type contacts in monolayer C3N field-effect transistors

Monolayer (ML) C3N, a novel two-dimensional flat crystalline material with a suitable bandgap and excellent carrier mobility, is a prospective channel material candidate for next-generation field-effect transistors (FETs). The contact properties of ML C3N-metal interfaces based on FETs have been comprehensively investigated with metal electrodes (graphene, Ti2C(OH/F)2, Zr2C(OH/F)2, Au, Ni, Pd, and Pt) by employing ab initio electronic structure calculations and quantum transport simulations. The contact properties of ML C3N are isotropic along the armchair and zigzag directions except for the case of Au. ML C3N establishes vertical van der Waals-type ohmic contacts with all the calculated metals except for Zr2CF2. The ML C3N-graphene, -Zr2CF2, -Ti2CF2, -Pt, -Pd, and -Ni interfaces form p-type lateral ohmic contacts, while the ML C3N-Ti2C(OH)2 and -Zr2C(OH)2 interfaces form n-type lateral ohmic contacts. The ohmic contact polarity can be regulated by changing the functional groups of the 2D MXene electrodes. These results provide theoretical insights into the characteristics of ML C3N-metal interfaces, which are important for choosing suitable electrodes and the design of ML C3N devices.

Directed electron regulation promoted sandwich-like CoO@FeBTC/NF with p-n heterojunctions by gel electrodeposition for oxygen evolution reaction

Metal organic framework (MOF) is currently-one of the key catalysts for oxygen evolution reaction (OER), but its catalytic performance is severely limited by electronic configuration. In this study, cobalt oxide (CoO) on nickel foam (NF) was first prepared, which then wrapped it with FeBTC synthesized by ligating isophthalic acid (BTC) with iron ions by electrodeposition to obtain CoO@FeBTC/NF p-n heterojunction structure. The catalyst requires only 255 mV overpotential to reach a current density of 100 mA cm^-2, and can maintain 100 h long time stability at 500 mA cm^-2 high current density. The catalytic properties are mainly related to the strong induced modulation of electrons in FeBTC by holes in the p-type CoO, which results in stronger bonding and faster electron transfer between FeBTC and hydroxide. At the same time, the uncoordinated BTC at the solid-liquid interface ionizes acidic radicals which form hydrogen bonds with the hydroxyl radicals in solution, capturing them onto the catalyst surface for the catalytic reaction. In addition, CoO@FeBTC/NF also has strong application prospects in alkaline electrolyzers, which only needs 1.78 V to reach a current density of 1 A cm^-2, and it can maintain long-term stability for 12 h at this current. This study provides a new convenient and efficient approach for the control design of the electronic structure of MOF, leading to a more efficient electrocatalytic process.

Suppressed Fermi Level Pinning and Wide-Range Tunable Schottky Barrier in CrX3 (X = I, Br)/2D Metal Contacts

CrX₃ (X = I, Br) monolayers exhibit outstanding performance in spintronic devices. However, the Schottky barrier at the CrX₃/electrode interface severely impedes the charge injection efficiency. Herein, we propose two-dimensional (2D) metals as electrodes to form van der Waals (vdW) contact with CrX₃ monolayers and systematically explore the contact properties of CrX₃/metal by density functional theory (DFT) calculations. The results demonstrate that the strongly suppressed Fermi level pinning (FLP) effect and the wide-range tunable Schottky barrier can be achieved in CrX₃/metal contacts. Specifically, the n-type and the p-type Schottky contacts can be realized in CrX₃/metal contacts by choosing 2D metal electrodes with different work functions. Importantly, the pinning factors for CrX₃/metal contacts are exceptionally larger than other commonly studied 2D semiconductors, indicating the strongly suppressed FLP in CrX₃/metal contacts, which leads to the wide-range tunable Schottky barrier. Our findings provide guidance to the choice of electrodes and promote the development of CrX₃-based spin devices.

Interlayer Doping of Cu on Bilayer Black Phosphorus for Enhanced Charge Transfer and Transport Properties

Metal doping between black phosphorus (BP) layers has great advantages in modulating electronic properties. Here, the effects of Cu intercalation on charge transfer and carrier dynamics are investigated by theoretical calculations. Relative to the pristine bilayer BP, Cu suppresses the nonradiative electron-hole recombination, reducing the major pathways of energy and current loss. Furthermore, we investigate a novel pn homogeneous junction based on the Cu-doped bilayer BP, which shows enhanced transport properties and Ohmic contact characteristics. This is because doping leads to the transformation of BP from p-type to n-type, charge accumulation on conduction bands allows electrons to be easily transferred to the p-type bilayer BP, and associated electrical properties can be modulated by the doping concentration. This study has fundamental importance for understanding structure-property relationships in metal intercalation, which is an important guidance for integration and interlayer engineering for two-dimensional materials.

Can Carbon Nanotube Transistors Be Scaled Down to the Sub-5 nm Gate Length?

Single-walled carbon nanotubes (CNTs) have been considered as a promising semiconductor to construct transistors and integrated circuits in the future owing to their ultrathin channel thickness and ultrahigh injection velocity. Although a 5 nm gate-length CNT field-effect transistor (FET) has already been experimentally fabricated and demonstrates excellent device performance, the potential or constraint factors on performance have not been explored or revealed. Based on the benchmark of the device performance between the experimental and simulated 5 nm gate-length CNT FETs, we use the first-principles quantum transport approach to explore the performance limit of CNT FETs based on the gate-all-around (GAA) device geometry for the first time. It is found that the GAA CNT FETs can fulfill the ITRS 2028 high-performance target in the 2 nm gate-length node in terms of the on-state current, delay time, and power consumption. We also find that the energy-delay product of the CNT FETs is superior to those of the high-performance 2D materials and Si Fin FETs at the sub-5 nm gate length due to its unique electrical property. Though theoretically the gate length of CNT FETs can be potentially scaled to 2 nm, considering the tradeoff between the performance and power consumption, 5 nm is the ultimate scaled limit.

Schottky barrier heights in two-dimensional field-effect transistors: from theory to experiment

Over the past decade, two-dimensional semiconductors (2DSCs) have aroused wide interest due to their extraordinary electronic, magnetic, optical, mechanical, and thermal properties, which hold potential in electronic, optoelectronic, thermoelectric applications, and so forth. The field-effect transistor (FET), a semiconductor gated with at least three terminals, is pervasively exploited as the device geometry for these applications. For lack of effective and stable substitutional doping techniques, direct metal contact is often used in 2DSC FETs to inject carriers. A Schottky barrier (SB) generally exists in the metal-2DSC junction, which significantly affects and even dominates the performance of most 2DSC FETs. Therefore, low SB or Ohmic contact is highly preferred for approaching the intrinsic characteristics of the 2DSC channel. In this review, we systematically introduce the recent progress made in theoretical prediction of the SB height (SBH) in the 2DSC FETs and the efforts made both in theory and experiments to achieve low SB contacts. From the comparison between the theoretical and experimentally observed SBHs, the emerging first-principles quantum transport simulation turns out to be the most powerful theoretical tool to calculate the SBH of a 2DSC FET. Finally, we conclude this review from the viewpoints of state-of-the-art electrode designs for 2DSC FETs.

Anisotropic interfacial properties of monolayer C2N field effect transistors

Monolayer C2N is promising for next-generation electronic and optoelectronic applications due to its appropriate band gap and high carrier efficiency. However, relative studies have been held back due to the lack of high-quality electrode contacts. Here, we comprehensively study the electronic and transport properties of monolayer C2N with a series of electrode materials (Al, Ti, Ni, Cu, Ag, Pt, V2C, Cr2C and graphene) by using the nonequilibrium Green's function (NEGF) method combined with density functional theory (DFT). The monolayer C2N forms Ohmic contacts with the Ti/Cu/Ag electrode material in both armchair and zigzag directions, whereas Ohmic contact is only formed in the zigzag direction of the C2N-Al field effect transistor. However, the C2N-Ni, -Pt, -V2C, -Mo2C, -graphene contact systems form n-type Schottky contacts in either the armchair or zigzag direction owing to the relatively strong Fermi level pinning (the pinning factor S = 0.32 in the armchair direction and S = 0.26 in the zigzag direction). By insertion of BN or graphene between the C2N and Pt electrode in the armchair direction of contact systems, the Fermi level pinning can be effectively weakened due to the suppression of metal-induced gap states. Conspicuously, an Ohmic contact is realized in the C2N field effect transistors with the BN-Pt electrode, suggesting a possible approach to fabricating high-performance devices. Our study is conducive to selecting appropriate electrode materials for C2N-based field effect transistors.

High-performance 5.1 nm in-plane Janus WSeTe Schottky barrier field effect transistors

Using ab initio quantum-transport simulations, we studied the intrinsic transfer characteristics and benchmarks of the ballistic performance of 5.1 nm double-gated Schottky-barrier field effect transistors (SBFETs) consisting of in-plane (IP) heterojunctions of metallic-phase (1T or 1T') MTe2 (M = Ti, Zr, Hf, Cr, Mo, W) and semiconducting-phase (2H) WSe2, WTe2 and Janus WSeTe. The 2H-phase Janus WSeTe is a semiconductor with an indirect bandgap (1.26 eV), which is less than the bandgap of 2H-phase WSe2 (1.64 eV) and is greater than the bandgap of 2H-phase WTe2 (1.02 eV). The band alignments show that all IP 1T/2H contacts are Schottky-barrier contacts with the Fermi levels of 1T or 1T' MTe2 (M = Ti, Zr, Hf, Cr, Mo, W) located within the bandgaps of 2H WSe2, WTe2 and Janus WSeTe. Although double-gated IP WSe2-SBFETs can satisfy the OFF current requirement, their ON currents all fall below the requirements of the high performance transistor outlined by the ITRS (International Technology Roadmap for Semiconductors, 2013 version) for the production year 2028. Double-gated IP WTe2-SBFETs cannot overcome the short channel effect leading to minimum drain currents all beyond the OFF current requirement of ITRS (2013 version) for the production year 2028. Fortunately, double-gated IP WSeTe-SBFETs with 1T MoTe2 or 1T' WTe2 electrodes can overcome the short channel effect and satisfy the requirements of the high-performance transistor outlined by the ITRS (2013 version) for the production year 2028.

Sub-5 nm monolayer germanium selenide (GeSe) MOSFETs: towards a high performance and stable device

Two-dimensional (2D) black phosphorene (BP) field-effect transistors (FETs) show excellent device performance but suffer from serious instability under ambient conditions. Isoelectronic 2D germanium selenide (GeSe) shares many similar properties with 2D BP, such as high carrier mobility and anisotropy, but is stable under ambient conditions. Herein, we explore the quantum transport properties of sub-5 nm ML GeSe MOSFETs using first-principles quantum transport simulation. A p-type (zigzag-directed) device is superior to other types (n- and p-type armchair-directed and n-type zigzag-directed). The on-state current of p-type devices (zigzag-directed), even at a 1 nm gate-length, can fulfill the requirements of high-performance applications for the next decade in the International Technology Roadmap for Semiconductors (ITRS, 2013 version). To the best of our knowledge, these ML GeSe MOSFETs have the smallest gate-length that can fulfill the ITRS HP on-state current requirements among reported 2D material FETs.

Monolayer Hexagonal Boron Nitride Tunnel Barrier Contact for Low-Power Black Phosphorus Heterojunction Tunnel Field-Effect Transistors

Transistor downscaling by Moore's law has facilitated drastic improvements in information technology, but this trend cannot continue because power consumption issues have pushed Moore's law to its limit. Tunnel field-effect transistors (TFETs) have been suggested to address these issues; however, so far they have not achieved the essential criteria for fast, low-power switches, i.e., an average subthreshold swing over four decades of current (SS_{ave_4dec}) below 60 mV/dec and a current of 1-10 μA/μm where the SS is 60 mV/dec (I₆₀). Here, we report a black phosphorus (BP) heterojunction (HJ) TFET that exhibits a record high I₆₀ of 19.5 μA/μm and subthermionic SS_{ave_4dec} of 37.6 mV/dec at 300 K, using a key material factor, monolayer hexagonal boron nitride tunnel barrier for the drain contact. This work, demonstrating the influence of the tunnel barrier contact on device performance, paves the way for the development of ultrafast low-power logic circuits beyond CMOS capabilities.

Performance Limit of Monolayer WSe2 Transistors; Significantly Outperform Their MoS2 Counterpart

With the scaling limits of silicon-based MOS technology, the critical and challenging issue is to explore more and more alternative materials to improve the performance of devices. Two-dimensional (2D) semiconductor WSe₂ with a proper band gap and inherent stability under ambient conditions makes it a potential channel material for realizing new generation field-effect transistors (FETs). In light of the low on-state current of the experimental sub-10 nm 2D MoS₂ FETs, we explore the limitation of the monolayer (ML) WSe₂ device performance by using accurate ab initio quantum transport simulation. We find that the sub-10 nm 2D WSe₂ FETs apparently outperform their MoS₂ counterpart. The on-state current of the optimized p-type ML WSe₂ FETs can satisfy the criteria of the International Technology Roadmap for Semiconductors (ITRS) on both the high-performance (HP) and low-power (LP) devices until the gate length is scaled down to 2 and 3 nm, respectively. By the aid of the negative capacitance effect, even the 1 nm gate-length WSe₂ MOSFETs can satisfy both the HP and LP requirements in the ITRS 2028 completely. Remarkably, the ML WSe₂ MOSFET has the highest theoretical on-current in LP application among the examined 2D MOSFETs at the 5 nm gate length to the best of our knowledge.

Two-Dimensional Pnictogen for Field-Effect Transistors

Two-dimensional (2D) layered materials hold great promise for various future electronic and optoelectronic devices that traditional semiconductors cannot afford. 2D pnictogen, group-VA atomic sheet (including phosphorene, arsenene, antimonene, and bismuthene) is believed to be a competitive candidate for next-generation logic devices. This is due to their intriguing physical and chemical properties, such as tunable midrange bandgap and controllable stability. Since the first black phosphorus field-effect transistor (FET) demo in 2014, there has been abundant exciting research advancement on the fundamental properties, preparation methods, and related electronic applications of 2D pnictogen. Herein, we review the recent progress in both material and device aspects of 2D pnictogen FETs. This includes a brief survey on the crystal structure, electronic properties and synthesis, or growth experiments. With more device orientation, this review emphasizes experimental fabrication, performance enhancing approaches, and configuration engineering of 2D pnictogen FETs. At the end, this review outlines current challenges and prospects for 2D pnictogen FETs as a potential platform for novel nanoelectronics.

First-principles simulation of monolayer hydrogen passivated Bi2O2S2–metal interfaces

Lateral SBH and Fermi level change in the hydrogen-passivated Bi₂O₂S₂ FET.

Sub-10 nm tunneling field-effect transistors based on monolayer group IV mono-chalcogenides

The development of air-stable channels with a high on-state current (I_on) is in high demand for the feasible application of TFETs. Monolayer group IV mono-chalcogenides (i.e., GeS, GeSe, SnS, and SnSe), as emerging air-stable atomic-thin semiconductors, are potential channels for sub-10 nm tunneling field-effect transistors due to their high carrier mobility and anisotropic electronic properties. Herein, we investigated the performances of sub-10 nm monolayer (ML) group IV mono-chalcogenide TFETs using ab initio quantum transport simulation. The ML GeSe TFET exhibited the best performance with regards to both high I_on and low leakage current (I_leak) among the four devices, followed by the ML SnSe TFET with a high I_on. The I_on of the optimal ML GeSe TFET with a physical gate length of L_g = 10 nm surpasses the International Technology Roadmap for Semiconductors (ITRS, 2013 Edition) requirements for high-performance (HP) and low-power (LP) devices, simultaneously, and that of the ML SnSe TFET with L_g = 10 nm surpasses the requirement of ITRS HP devices. In combination with our former works, we suggest an E_g of 0.77-1.19 eV and of 0.11-0.15m₀ to search for competitive 2D channels with a high I_on for HP application in TFET devices with a planar homogeneous p-i-n architecture.

Reduction of Fermi level pinning at Cu-BP interfaces by atomic passivation

Black phosphorus (BP) is a semiconducting material with a direct finite band gap in its monolayer, attracting intense attention for its application in field-effect transistors. However, strong Fermi level pinning (FLP) has been observed for contacts between BP and high work function metals, e.g., Cu. Such FLP presents an undesirable hurdle preventing the achievement of high performance field-effect devices. In this regard, there is a crucial need to understand the FLP occurring at the metal-BP interfaces and explore the possibility to reduce it. The present work studied atomic passivation in reducing FLP for the Cu-BP system using density functional theory calculations. The passivation by H, N, F, S, and Cl atoms on the Cu(111) surface has been considered. The results showed that the passivated atoms can shield the direct contact between Cu(111) and BP, thus reducing FLP at Cu-BP interfaces. In particular, S and Cl atoms were found to be highly effective agents to achieve a significant reduction of FLP, leading to Cu-BP contacts with ultralow Schottky barrier height (SBH) and suggesting the possibility of ohmic contact formation. Our findings demonstrate surface passivation as an effective method towards depinning the Fermi level at the metal-BP interface and subsequently controlling the SBH for BP-based electronic devices.

Toward barrier free contact to MoSe2/WSe2 heterojunctions using two-dimensional metal electrodes

In the design of electronic devices based on two-dimensional heterojunctions, the contact between electrodes and different surfaces of two-dimensional heterojunctions may produce different effects. Furthermore, metal-semiconductor contact plays an important role in modern devices. However, due to the Fermi level pinning effect (FLPE), it is difficult to tune the Schottky barrier height between common metals (e.g. Au, Ag, and Cu) and semiconductors. Fortunately, the FLPE becomes weak at the contact between the 2D metal and 2D semiconductor, due to the suppression of metal-induced gap states. Here, we choose monolayer NbS₂ as the electrode to be in contact with the MoSe₂/WSe₂ bilayer. The interfacial properties as well as the stacking dependence are discussed based on the density functional theory, combined with the nonequilibrium Green's functions. Two configurations are considered, i.e. the WSe₂/MoSe₂/NbS₂ and MoSe₂/WSe₂/NbS₂ stacking sequences. Our results show that barrier free contact can be formed in these 2D metal-semiconductor junctions (MSJs). In addition, the transport properties of the proposed devices are sensitive to the stacking sequence. The drain-source current versus bias voltage (I-V) curve exhibits a linear relationship for the WSe₂/MoSe₂/NbS₂ system and its resistance is much lower than the MoSe₂/WSe₂/NbS₂ MSJ. Detailed analysis reveals that the transport properties are governed by the electronic coupling between specific interlayer states. In WSe₂/MoSe₂/NbS₂ configuration, large overlapping states are observed, which facilitate charge transfer and result in good ohmic contact. Our work may provide a theoretical guidance for the designing of next-generation ultrathin and flexible devices.

Sub-5 nm monolayer black phosphorene tunneling transistors

The successful fabrication of sub-5 nm 2D MoS₂ field-effect transistors (FETs) announces the approaching post-silicon era. It is possible for tunneling field-effect transistors (TFETs) based on monolayer black phosphorene (ML BP) to work well in the sub-5 nm region because of its moderate direct band gap, anisotropic electronic properties and high carrier mobility. We simulate the device performance limit of the ML BP TFETs at the sub-5 nm scale using ab initio quantum transport calculations. We predict that the on-state currents (I _on) of the sub-5 nm ML BP TFETs will exceed those of the ML WTe₂ TFETs, which possess the highest I _on among the transition-metal dichalcogenide family. In particular, the I _on of the ML BP TFETs can fulfill the 2028 requirements of the international technology roadmap for semiconductors (ITRS) for the high-performance (HP) devices until the gate length is scaled down to 4 nm, while the delay times and power dissipations always surpass the 2028 requirements of the ITRS HP devices significantly in the whole sub-5 nm region.

Two-dimensional black phosphorus: its fabrication, functionalization and applications

Phosphorus, one of the most abundant elements in the Earth (∼0.1%), has attracted much attention in the last five years since the rediscovery of two-dimensional (2D) black phosphorus (BP) in 2014. The successful scaling down of BP endows this 'old material' with new vitality, resulting from the intriguing semiconducting properties in the atomic scale limit, i.e. layer-dependent bandgap that covers from the visible light to mid-infrared light spectrum as well as hole-dominated ambipolar transport characteristics. Intensive research effort has been devoted to the fabrication, characterization, functionalization and application of BP and other phosphorus allotropes. In this review article, we summarize the fundamental properties and fabrication techniques of BP, with particular emphasis on the recent progress in molecular beam epitaxy growth of 2D phosphorus. Subsequently, we highlight recent progress in BP (opto)electronic device applications achieved via customized manipulation methods, such as interface, defect and bandgap engineering as well as forming Lego-like stacked heterostructures.

Facile Synthesis of Highly Dispersed Co3O4 Nanoparticles on Expanded, Thin Black Phosphorus for a ppb-Level NO x Gas Sensor

Expanded few-layer black phosphorus nanosheets (FL-BP NSs) were functionalized by branched polyethylenimine (PEI) using a simple noncovalent assembly to form air-stable overlayers (BP-PEI), and a Co₃O₄@BP-PEI composite was designed and synthesized using a hydrothermal method. The size of the highly dispersed Co₃O₄ nanoparticles (NPs) on the FL-BP NSs can be controlled. The BP-C5 (190 °C for 5 h) sensor, with 4-6 nm Co₃O₄ NPs on the FL-BP NSs, exhibited an ultrahigh sensitivity of 8.38 and a fast response of 0.67 s to 100 ppm of NO _x at room temperature in air, which is 4 times faster than the response of the FL-BP NS sensor, and the lower detection limit reached 10 ppb. This study points to a promising method for tuning properties of BP-based composites by forming air-stable overlayers and highly dispersed metal oxide NPs for use in high-performance gas sensors.

Sub-5 nm Monolayer Arsenene and Antimonene Transistors

Novel two-dimensional (2D) semiconductors arsenene and antimonene are promising channel materials for next-generation field effect transistors (FETs) because of the high carrier mobility and stability under ambient conditions. Stimulated by the recent experimental development of sub-5 nm 2D MoS₂ FETs, we investigate the device performance of monolayer (ML) arsenene and antimonene in the sub-5 nm region by using accurate ab initio quantum transport simulation. We reveal that the optimized sub-5 nm double-gate (DG) ML arsenene and antimonene metal-oxide-semiconductor FETs (MOSFETs) can fulfill the low power requirements of the International Technology Roadmap for Semiconductors in 2028 until the gate length is scaled down to 4 nm. When the gate length is scaled down to 1 nm, the performances of the DG ML arsenene and antimonene MOSFETs are superior to that of the DG ML MoS₂ MOSFETs in terms of the on-current. Therefore, 2D arsenene and antimonene are probably more suitable for ultrascaled FETs than 2D MoS₂ in the post-silicon era.

Improving Performances of In-Plane Transition-Metal Dichalcogenide Schottky Barrier Field-Effect Transistors

Monolayer Schottky barrier (SB) field-effect transistors based on the in-plane heterojunction of 1T/1T'-phase (metallic) and 2H-phase (semiconducting) transition-metal dichalcogenides (TMDs) have been proposed following the recent experimental synthesis of such devices. By using density functional theory and ab initio simulations, intrinsic device performance, sub-10 nm scaling, and performance boosting of MoSe₂, MoTe₂, WSe₂, and WTe₂, SB field-effect transistors are systematically investigated. We find that the Schottky barrier heights (SBHs) of these in-plane 1T(1T')/2H contacts are proportional to their band gaps: the bigger band gap corresponds to bigger SBH. For four TMDs, the SBH of 1T/2H contact is always smaller than that of 1T'/2H contact. The WTe₂ SB field-effect transistor can provide the best performance and satisfy the requirement of the high-performance transistor outlined by the International Technology Roadmap for Semiconductors down to a 6 nm gate length. In addition, the replacement of suitable 1T-TMD on the source/drain regions can modulate conduction band SB, leading to the 8.8 nm WSe₂ SB field-effect transistor also satisfying the requirement. Moreover, the introduction of the underlap can increase the effective channel length and reduce the coupling between the source/drain and the channel, leading to the 5.1 nm WTe₂ SB field-effect transistor also satisfying the International Technology Roadmap for Semiconductors high-performance requirement. The underlying physical mechanisms are discussed, and it is concluded that the in-plane SB engineering is the key point to optimize such two-dimensional devices.

n-Type Ohmic contact and p-type Schottky contact of monolayer InSe transistors

We explore the contact properties of monolayer InSe transistors and obtain n-type Ohmic/p-type Schottky contacts.

Black phosphorus transistors with van der Waals-type electrical contacts

Contact engineering is a possible solution to decrease the pervasive Schottky barrier in a two dimensional (2D) material transistor with bulk metal electrodes. In this paper, two kinds of typical van der Waals (vdW)-type electrical contacts (a 2D metal contact and a 2D material/bulk metal hybrid contact) in monolayer (ML) black phosphorus (BP) transistors are investigated by ab initio energy band calculations and quantum transport simulations. Compared with the traditional bulk metal Ni contact, the gate electrostatic control is significantly improved by using both 2D graphene and borophene electrodes featuring a decrease of 30-50% in the subthreshold swing and an increase by a factor of 4-7 in the on-state current due to the depressed metal induced gap states and reduced screening of the 2D metal electrodes to the gate. In contrast, graphene insertion between the Ni electrode and ML BP shows only a slight improvement in the gate electrostatic control ability and BN insertion shows almost no improvement. The higher efficiency using the 2D metal contact than the 2D material/bulk metal hybrid contact in improving the ML BP FET device performance also provides helpful guidance in the selection of vdW-type electrical contacts of other 2D transistors.

Monolayer Bismuthene-Metal Contacts: A Theoretical Study

Bismuthene, a bismuth analogue of graphene, has a moderate band gap, has a high carrier mobility, has a topological nontriviality, has a high stability at room temperature, has an easy transferability, and is very attractive for electronics, optronics, and spintronics. The electrical contact plays a critical role in an actual device. The interfacial properties of monolayer (ML) bismuthene in contact with the metal electrodes spanning a wide work function range in a field-effect transistor configuration are systematically studied for the first time by using both first-principles electronic structure calculations and quantum transport simulations. The ML bismuthene always undergoes metallization upon contact with the six metal electrodes owing to a strong interaction. According to the quantum transport simulations, apparent metal-induced gap states (MIGSs) formed in the semiconductor-metal interface give rise to a strong Fermi-level pinning. As a result, the ML bismuthene forms an n-type Schottky contact with Ir/Ag/Ti electrodes with electron Schottky barrier heights (SBHs) of 0.17, 0.22, and 0.25 eV, respectively, and a p-type Schottky contact with Pt/Al/Au electrodes with hole SBHs of 0.09, 0.16, and 0.38 eV, respectively. The effective channel length of the ML bismuthene transistors is also significantly reduced by the MIGSs. However, the MIGSs are eliminated and the effective channel length is increased when ML graphene is used as an electrode, accompanied by a small hole SBH of 0.06 eV (quasi-Ohmic contact). Hence, an insight is provided into the interfacial properties of the ML bismuthene-metal composite systems and a guidance is provided for the choice of metal electrodes in ML bismuthene devices.