The use of a foreign metallic cold source (CS) has recently been proposed as a promising approach toward the steep-slope field-effect-transistor (FET). In addition to the selection of source material with desired density of states-energy relation (D(E)), engineering the source: channel interface for gate-tunable channel-barrier is crucial to a CS-FET. However, conventional metal: semiconductor (MS)-interfaces generally suffer from strong Fermi-level-pinning due to the inevitable chemical disorder and defect-induced gap states, precluding the gate-tunability of the barriers. By comprehensive materials and device modeling at the atomic-scale, we report that the two-dimensional (2D)-van der Waals (vdW)-MS-interfaces, with their atomic sharpness and cleanness, can be considered as general ingredients for CS-FETs. As test cases, InSe-based n-type FETs are studied. It is found that graphene can be spontaneously p-type doped along with slightly opened bandgap around the Dirac-point by interfacing with InSe, resulting in super-exponentially decaying hot carrier density with increasing n-type channel-barrier. Moreover, the D(E) relations suggest that 2D-transition-metal-dichalcogenides and 2D-transition-metal-carbides are rich libraries of CS materials. Both graphene and H-TaTe2 CSs lead to subthreshold swing below 60 mV/decade. This work broadens the application potentials of 2D-vdW-MS-heterostructures and serves as a springboard for more studies on low-power electronics based on 2D materials.
A ferroelectric junction transistor memory made from switchable van der Waals p-n heterojunctions
Zhejiang University（浙江大学）；King Abdullah University of Science and Technology（阿卜杜拉国王科技大学）；Wenzhou University（温州大学）；Chinese Academy of Sciences（中国科学院）；South China Normal University（华南师范大学）；Xidian University（西安电子科技大学）
A general self-aligned fabrication scheme is reported here for a diverse class of electronic devices based on van der Waals materials and heterojunctions. In particular, self-alignment enables the fabrication of source-gated transistors in monolayer MoS2 with near-ideal current saturation characteristics and channel lengths down to 135 nm. Furthermore, self-alignment of van der Waals p-n heterojunction diodes achieves complete electrostatic control of both the p-type and n-type constituent semiconductors in a dual-gated geometry, resulting in gate-tunable mean and variance of anti-ambipolar Ga…
Spin transistors (whose on-off operation is achieved by electric-field-controlled spin orientation 1), if realized, can revolutionize modern electronics through the implementation of a faster and a more energy-efficient performance as well as non-volatile data storage 2, 3. The original proposal by Datta and Das 1 that relies on electric-field-controlled spin precession in a semiconductor channel faces significant challenges including inefficient spin injection, spin relaxation and spread of the spin precession angle 4, 5. Recent demonstration of electric-field switching of magnetic order 6-8 …
Steep-slope $β$-Ga$_2$O$_3$ nano-membrane negative capacitance field-effect transistors (NC-FETs) are demonstrated with ferroelectric hafnium zirconium oxide in gate dielectric stack. Subthreshold slope less than 60 mV/dec at room temperature is obtained for both forward and reverse gate voltage sweeps with a minimum value of 34.3 mV/dec at reverse gate voltage sweep and 53.1 mV/dec at forward gate voltage sweep at $V_{DS}$=0.5 V. Enhancement-mode operation with threshold voltage ~0.4 V is achieved by tuning the thickness of $β$-Ga$_2$O$_3$ membrane. Low hysteresis of less than 0.1 V is obtain…
Light-emitting electronic devices are ubiquitous in key areas of current technology, such as data communications, solid-state lighting, displays, and optical interconnects. Controlling the spectrum of the emitted light electrically, by simply acting on the device bias conditions, is an important goal with potential technological repercussions. However, identifying a material platform enabling broad electrical tuning of the spectrum of electroluminescent devices remains challenging. Here, we propose light-emitting field-effect transistors based on van der Waals interfaces of atomically thin sem…
Room-temperature Fermi-Dirac electron thermal excitation in conventional three-dimensional (3D) or two-dimensional (2D) semiconductors generates hot electrons with a relatively long thermal tail in energy distribution. These hot electrons set a fundamental obstacle known as the "Boltzmann tyranny" that limits the subthreshold swing (SS) and therefore the minimum power consumption of 3D and 2D field-effect transistors (FETs). Here, we investigated a novel graphene (Gr)-enabled cold electron injection where the Gr acts as the Dirac source to provide the cold electrons with a localized electron d…
Two-dimensional atomic crystals are extensively studied in recent years due to their exciting physics and device applications. However, a molecular counterpart, with scalable processability and competitive device performance, is still challenging. Here, we demonstrate that high-quality few-layer dioctylbenzothienobenzothiophene molecular crystals can be grown on graphene or boron nitride substrate via van der Waals epitaxy, with precisely controlled thickness down to monolayer, large-area single crystal, low process temperature and patterning capability. The crystalline layers are atomically s…
The so-called Boltzmann Tyranny defines the fundamental thermionic limit of the subthreshold slope (SS) of a metal-oxide-semiconductor field-effect transistor (MOSFET) at 60 mV/dec at room temperature and, therefore, precludes the lowering of the supply voltage and the overall power consumption. Adding a ferroelectric negative capacitor to the gate stack of a MOSFET may offer a promising solution to bypassing this fundamental barrier. Meanwhile, two-dimensional (2D) semiconductors, such as atomically thin transition metal dichalcogenides (TMDs) due to their low dielectric constant, and ease of…
The device concept of ferroelectric-based negative capacitance (NC) transistors offers a promising route for achieving energy-efficient logic applications that can outperform the conventional semiconductor technology, while viable operation mechanisms remain a central topic of debate. In this work, we report steep slope switching in MoS$_2$ transistors back-gated by single-layer polycrystalline PbZr$_{0.35}$Ti$_{0.65}$O$_3$. The devices exhibit current on/off ratios up to 8$\times$10$^6$ within an ultra-low gate voltage window of $V_g$ = $\pm$0.5 V and subthreshold swing (SS) as low as 9.7 mV …
Pristine van der Waals (vdW) interfaces between two-dimensional (2D) and other materials are core to emerging optical and electronic devices. Their direct fabrication is, however, challenged as the vdW forces are weak and cannot be tuned to accommodate integration of arbitrary layers without solvents, sacrificial-layers or high-temperatures, steps that can introduce damage. To address these limitations, we introduce a single-step 2D material-to-device integration approach in which forces promoting transfer are decoupled from the vdW forces at the interface of interest. We use this adhesive mat…
Stacked van der Waals (vdW) heterostructures where semi-conducting two-dimensional (2D) materials are contacted by overlayed graphene electrodes enable atomically-thin, flexible electronics. We use first-principles quantum transport simulations of graphene-contacted MoS2 devices to show how the transistor effect critically depends on the stacking configuration relative to the gate electrode. We can trace this behavior to the stacking-dependent response of the contact region to the capacitive electric field induced by the gate. The contact resistance is a central parameter and our observation e…
Efficient and Robust p-type Transistor based on Ultra-wide-bandgap Semiconductor
Macau University of Science and Technology（澳门科技大学）；Zhuhai MUST Science and Technology Research Institute（珠海澳门科技大学研究院）；School of Physics and Astronomy, Monash University（蒙纳士大学物理与天文学院）；Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University（南洋理工大学物理与数学科学学院物理与应用物理部）；Department of Materials Science & Engineering, Monash University（蒙纳士大学材料科学与工程系）；Faculty of Materials Science and Engineering, Kunming University of Science and Technology（昆明理工大学材料科学与工程学院）；Department of Electrical and Electronic Engineering, The University of Melbourne（墨尔本大学电气与电子工程系）；School of Science, RMIT University（皇家墨尔本理工大学理学院）；Princeton Plasma Physics Laboratory（普林斯顿等离子体物理实验室）；Bio21 Institute, The University of Melbourne（墨尔本大学生物21研究所）；Research School of Chemistry, Australian National University（澳大利亚国立大学化学研究学院）；Shanghai Key Laboratory of High Temperature Superconductors, Department of Physics, Shanghai University（上海大学物理系高温超导重点实验室）；Centre for Materials Science, School of Chemistry and Physics, Queensland University of Technology（昆士兰科技大学化学与物理学院材料科学中心）；Macao Institute of Materials Science and Engineering (MIMSE), Faculty of Innovation Engineering, Macau University of Science and Technology（澳门科技大学创新工程学院澳门材料科学与工程研究所）；School of Electrical and Electronic Engineering, Nanyang Technological University（南洋理工大学电气与电子工程学院）
Identifying intrinsic low-dimensional ferromagnets with high transition temperature and electrically tunable magnetism is crucial for the development of miniaturized spintronics and magnetoelectrics. Recently long-range 2D ferromagnetism was observed in van der Waals crystals CrI$_3$ and Cr$_2$Ge$_2$Te$_6$, however their Curie temperature is significantly lowered when reducing down to monolayer/few layers. Herein, using renormalized spin-wave theory and first-principles electronic structure theory, we present a theoretical study of electrically tunable 2D ferromagnetism in van der Waals layere…
Scalable, universal and conformal direct electrodes microprinting for high-performance van der Waals-integrated two-dimensional electronics and flexible applications
Achieving good quality Ohmic contacts to van der Waals materials is a challenge, since at the interface between metal and van der Waals material, different conditions can occur, ranging from the presence of a large energy barrier between the two materials to the metallization of the layered material below the contacts. In black phosphorus (bP), a further challenge is its high reactivity to oxygen and moisture, since the presence of uncontrolled oxidation can substantially change the behavior of the contacts. In this study, we investigate the influence of the metal used for the contacts to bP a…
We report a new class of large-gap quantum spin Hall insulators in two-dimensional transition metal dichalcogenides, namely, MX$_2$ with M=(Mo, W) and X=(S, Se, and Te), whose topological electronic properties are highly tunable by external electric field. We propose a novel topological field effect transistor made of these atomic layer materials and their van der Waals heterostructures. Our device exhibits parametrically enhanced charge-spin conductance through topologically protected transport channels, and can be rapidly switched off by electric field through topological phase transition in…
Van der Waals heterostructures are the ideal material platform for tunnel field effect transistors (TFETs) because a band-to-band tunneling (BTBT) dominant current is feasible at room temperature (RT) due to ideal, dangling bond free heterointerfaces. However, achieving subthreshold swing (SS) values lower than 60 mVdec-1 of the Boltzmann limit is still challenging. In this work, we systematically studied the band alignment and heterointerface quality in n-MoS2 channel heterostructure TFETs. By selecting a p+-MoS2 source with a sufficiently high doping level, stable gate modulation to a type I…
Van der Waals heterostructures (vdWH) are made of different two-dimensional (2D) layers stacked on top of each other, forming a single material with unique properties that differ from those of the individual 2D constituent layers, and that can be modulated through the interlayer interaction. These hetero-materials can be artificially made by mechanical stamping, solution processing or epitaxial growth. Alternatively, franckeite has been recently described as an example of a naturally-occurring vdWH that can be exfoliated down to nanometer thicknesses. Research on vdWHs has so far been limited …
We report on the results of the low-frequency (1/f, where f is frequency) noise measurements in MoS2 field-effect transistors revealing the relative contributions of the MoS2 channel and Ti/Au contacts to the overall noise level. The investigation of the 1/f noise was performed for both as fabricated and aged transistors. It was established that the McWhorter model of the carrier number fluctuations describes well the 1/f noise in MoS2 transistors, in contrast to what is observed in graphene devices. The trap densities extracted from the 1/f noise data for MoS2 transistors, are 1.5 x 10^19 eV-…
The physics of two-dimensional (2D) materials and heterostructures based on such crystals has been developing extremely fast. With new 2D materials, truly 2D physics has started to appear (e.g. absence of long-range order, 2D excitons, commensurate-incommensurate transition, etc). Novel heterostructure devices are also starting to appear - tunneling transistors, resonant tunneling diodes, light emitting diodes, etc. Composed from individual 2D crystals, such devices utilize the properties of those crystals to create functionalities that are not accessible to us in other heterostructures. We re…
The recently emerged organohalide perovskites (e.g., CH3NH3PbI3) have drawn intense attention for high efficiency solar cells. However, with a considerable solubility in many solvents, these perovskites are not typically compatible with conventional lithography processes for more complicated device fabrications that are important for both fundamental studies and technological applications. Here we report the creation of novel heterojunction devices based on perovskites and two-dimensional (2D) crystals by taking advantage of the layered characteristic of lead iodide (PbI2) and vapor phase inte…
We demonstrate a graphene-MoS2 architecture integrating multiple field-effect transistors and we independently probe and correlate the conducting properties of van der Waals coupled graphene-MoS2 contacts with the ones of the MoS2 channels. Devices are fabricated starting from high-quality single-crystal monolayers grown by chemical vapor deposition and characterized by scanning Raman and photoluminescence spectroscopies. Transconductance curves of MoS2 are compared with the current-voltage characteristics of graphene contact stripes, revealing a significant suppression of transport on the n-s…
Bulk gallium oxide (Ga2O3) has been widely used in lasers, dielectric coatings for solar cells, deep-ultraviolet transistor applications due to the large band gap over 4.5 eV. With the miniaturization of electronic devices, atomically thin Ga2O3 monolayer has been unveiled recently, which features an asymmetric configuration with a quintuple-layer atomic structure. The superior stability, the strain-tunable electronic properties, high carrier mobility and optical absorption indicate the promising applications in the electronic and photoelectronic devices. However, the strict investigation of l…
Demonstrate of High-Performance Top-Gate ALD Crystalline In2O3 Transistor Enabled by Lattice-Matched HfO2 and In2O3 Heterostructure
National Key Laboratory of Advanced Micro and Nano Manufacture Technology and School of Information Science and Electronic Engineering, Shanghai Jiao Tong University（上海交通大学信息科学与工程学院及先进微纳制造技术国家重点实验室）
The reliable production of two-dimensional crystals is essential for the development of new technologies based on 2D materials. However, current synthesis methods suffer from a variety of drawbacks, including limitations in crystal size and stability. Here, we report the fabrication of large-area, high-quality 2D tellurium (tellurene) using a substrate-free solution process. Our approach can create crystals with a process-tunable thickness, from monolayer to tens of nanometres, and with lateral sizes of up to 100 um. The chiral-chain van der Waals structure of tellurene gives rise to strong in…
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 ph…
Using first-principles calculations, we investigate six transition-metal nitride halides (TMNHs): HfNBr, HfNCl, TiNBr, TiNCl, ZrNBr, and ZrNCl as potential van der Waals (vdW) dielectrics for transition metal dichalcogenide (TMD) channel transistors. We calculate the exfoliation energies and bulk phonon energies and find that the six TMNHs are exfoliable and thermodynamically stable. We calculate both the optical and static dielectric constants in the in-plane and out-of-plane directions for both monolayer and bulk TMNHs. In monolayers, the out-of-plane static dielectric constant ranges from 5…
Determining the electronic properties of nanoscopic, low-dimensional materials free of external influences is key to discovery and understanding of new physical phenomena. An example is the suspension of graphene, which has allowed access to their intrinsic charge transport properties. Furthermore, suspending thin films enables their application as membranes, sensors, or resonators, as has been explored extensively. While the suspension of covalently-bound, electronically-active thin films is well established, semiconducting thin films composed of functional molecules only held together by van…
Developing van der Waals (vdW) homojunction devices requires materials with narrow bandgaps and simultaneously high hole and electron mobilities for bipolar transport, as well as methods to image and study spatial variations in carrier type and associated conductivity with nanometer spatial resolution. Here we demonstrate the general capability of near-field scanning microwave microscopy (SMM) to image and study the local carrier type and associated conductivity in operando by studying ambiploar field effect transistors (FETs) of the 1D vdW material tellurium in 2D form. To quantitatively unde…
Controlling the flow of charge neutral interlayer exciton (IX) quasiparticles can potentially lead to low loss excitonic circuits. Here, we report unidirectional transport of IXs along nanoscale electrostatically defined channels in an MoSe$_2$-WSe$_2$ heterostructure. These results are enabled by a lithographically defined triangular etch in a graphene gate to create a potential energy ''slide''. By performing spatially and temporally resolved photoluminescence measurements, we measure smoothly varying IX energy along the structure and high-speed exciton flow with a drift velocity up to 2 * 1…
In 2D-semiconductor-based field-effect transistors and optoelectronic devices, metal-semiconductor junctions are one of the crucial factors determining device performance. The Fermi-level (FL) pinning effect, which commonly caused by interfacial gap states, severely limits the tunability of junction characteristics, including barrier height and contact resistance. A tunneling contact scheme has been suggested to address the FL pinning issue in metal-2D-semiconductor junctions, whereas the experimental realization is still elusive. Here, we show that an oxidized-monolayer-enabled tunneling barr…
Atomically thin chromium triiodide (CrI3) has recently been identified as a layered antiferromagnetic insulator, in which adjacent ferromagnetic monolayers are antiferromagnetically coupled. This unusual magnetic structure naturally comprises a series of anti-aligned spin filters which can be utilized to make spin-filter magnetic tunnel junctions with very large tunneling magnetoresistance (TMR). Here we report voltage control of TMR formed by four-layer CrI3 sandwiched by monolayer graphene contacts in a dual-gated structure. By varying the gate voltages at fixed magnetic field, the device ca…
Two-dimensional (2D) materials are highly promising for tunnel field effect transistors (TFETs) with low subthreshold swing and high drive current because the shorter tunnel distance and strong gate controllability can be expected from the van der Waals gap distance and the atomically sharp heterointerface formed independently of lattice matching. However, the common problem for 2D-2D TFETs is the lack of highly doped 2D materials with the high process stability as the sources. In this study, we have found that p+-WSe2 doped by charge transfer from a WOx surface oxide layer can be stabilized b…
We report a new strategy for fabricating 2D/2D low-resistance ohmic contacts for a variety of transition metal dichalcogenides (TMDs) using van der Waals assembly of substitutionally doped TMDs as drain/source contacts and TMDs with no intentional doping as channel materials. We demonstrate that few-layer WSe2 field-effect transistors (FETs) with 2D/2D contacts exhibit low contact resistances of ~ 0.3 k ohm.um, high on/off ratios up to > 109, and high drive currents exceeding 320 uA um-1. These favorable characteristics are combined with a two-terminal field-effect hole mobility ~ 2x102 cm2 V-…
The celebrated electronic properties of graphene have opened way for materials just one-atom-thick to be used in the post-silicon electronic era. An important milestone was the creation of heterostructures based on graphene and other two-dimensional (2D) crystals, which can be assembled in 3D stacks with atomic layer precision. These layered structures have already led to a range of fascinating physical phenomena, and also have been used in demonstrating a prototype field effect tunnelling transistor - a candidate for post-CMOS technology. The range of possible materials which could be incorpo…
The single particle tunneling in a vertical stack consisting of monolayers of two-dimensional semiconductors is studied theoretically and its application to a novel Two-dimensional Heterojunction Interlayer Tunneling Field Effect Transistor (Thin-TFET) is proposed and described. The tunneling current is calculated by using a formalism based on the Bardeen's transfer Hamiltonian, and including a semi-classical treatment of scattering and energy broadening effects. The misalignment between the two 2D materials is also studied and found to influence the magnitude of the tunneling current, but hav…
Tellurium can form nanowires of helical atomic chains. Given their unique one-dimensional van der Waals structure, these nanowires are expected to show remarkably different physical and electronic properties than bulk tellurium. Here we show that few-chain and single-chain van der Waals tellurium nanowires can be isolated using carbon nanotube and boron nitride nanotube encapsulation. With the approach, the number of atomic chains can be controlled by the inner diameter of the nanotube. The Raman response of the structures suggests that the interaction between a single-atomic tellurium chain a…
Molecular-beam epitaxy (MBE) provides a simple but powerful way to synthesize large-area high-quality thin films and heterostructures of a wide variety of materials including accomplished group III-V and II-VI semiconductors as well as newly-developing oxides and chalcogenides, leading to major discoveries in condensed-matter physics. For two-dimensional (2D) materials, however, main fabrication routes have been mechanical exfoliation and chemical-vapor deposition by making good use of weak van der Waals bonding nature between neighboring layers, and MBE growth of 2D materials, in particular o…
Heterostructure materials form the basis of much of modern electronics, from transistors to lasers and light-emitting diodes. Recent years have seen a renewed focus on creating heterostructures through the vertical integration of two-dimensional materials, including graphene, hexagonal boron nitride, and transition metal dichalcogenides (TMDCs). However, fundamental challenges associated with materials processing have limited material quality and impeded scalability. We demonstrate a method to convert sub-nanometer metal films deposited on silicon and sapphire into TMDC heterostructures throug…
Van der Waals heterostructures formed by stacking different types of 2D materials are attracting increasing attention due to new emergent physical properties such as interlayer excitons. Recently synthesized atomically thin indium selenide (InSe) and antimony (Sb) individually exhibit interesting electronic properties such as high electron mobility in the former and high hole mobility in the latter. In this work, we present a first-principles investigation on the stability and electronic properties of ultrathin bilayer heterostructures composed of InSe and Sb single layers. The calculated elec…
