Jiuxiang Dai

The contact properties and performance limit of short-channel field-effect transistors based on M₃C₂T₂ (M = Ti, Zr, Hf; T = O, F, OH) MXenes and blue-phosphorene-phase group IV monochalcogenides and performance limit of short-channel transistors with M₃C₂(OH)₂ electrode are systematically investigated by using first-principles calculations.

Abstract

Realizing Ohmic contact is essential but challenging in the development of high-performance 2D-material-based electronics. Here, using first-principles calculations, the interfacial properties between 2D metallic M₃C₂T₂ (M = Ti, Zr, Hf; T = O, F, OH) and 2D semiconductors are comprehensively investigated, taking group ΙV monochalcogenides in blue-phosphorene phase as example, and the performance of short-channel field-effect transistors (FETs) with M₃C₂(OH)₂ electrode is studied. The band alignments and the interface dipole analysis demonstrate that the M₃C₂(OH)₂ and Ti₃C₂O₂ electrodes form van der Waals interaction with the group ΙV monochalcogenides, conductive to weakening the Fermi level pinning and forming desired Ohmic contacts. Moreover, owing to the large work function of the Ti₃C₂O₂, it realizes p-type Ohmic contacts with 2D semiconductors. In addition, due to the favorable Ohmic contacts, the on/off ratio of the 5 nm gate-length Zr₃C₂(OH)₂–GeTe FET is around 10⁵. Moreover, for the 3 nm gate-length Zr₃C₂(OH)₂–SnTe FET after adopting optimizing strategies, the on/off ratio increases from 10² to 10⁶, and the subthreshold slope decreases from 125 mV dec^-1 to 58 mV dec^-1 below the thermionic limit (60 mV dec^-1). The results provide a theoretical guidance for achieving the intrinsic n-type and p-type Ohmic contacts and high-performance FETs in 2D nanoelectronics.

This review summarizes the recent development of dielectrics for high-performance 2D devices. Adapting integration protocols of conventional high-κ dielectrics and emerging dielectrics with inert surface can significantly improve the dielectric film and interface quality. The advancement of novel dielectrics boosts overall performance of 2D devices and paves way toward next-generation nanoelectronics.

Abstract

2D semiconductors have emerged both as an ideal platform for fundamental studies and as promising channel materials in beyond-silicon field-effect-transistors due to their outstanding electrical properties and exceptional tunability via external field. However, the lack of proper dielectrics for 2D semiconductors has become a major roadblock for their further development toward practical applications. The prominent issues between conventional 3D dielectrics and 2D semiconductors arise from the integration and interface quality, where defect states and imperfections lead to dramatic deterioration of device performance. In this review article, the root causes of such issues are briefly analyzed and recent advances on some possible solutions, including various approaches of adapting conventional dielectrics to 2D semiconductors, and the development of novel dielectrics with van der Waals surface toward high-performance 2D electronics are summarized. Then, in the perspective, the requirements of ideal dielectrics for state-of-the-art 2D devices are outlined and an outlook for their future development is provided.

Semiconductors

In article number 2202529, Tongbo Wei, Zhongfan Liu, Peng Gao, Zhiqiang Liu, and co-workers find that geometry matched WS₂ can provide a proper lattice potential field for nitrides epitaxial growth. By using a transferred WS₂ buffer layer, a single-crystalline GaN epilayer can be obtained on an amorphous quartz glass by heterogenous epitaxy.

Exploring the interplay between magnetic and optoelectronic properties and developing spin-optoelectronic devices are potential research strategies for further studies of 2D materials and their applications. A great exchange bias, intrinsic negative magnetoresistance, and broadband photodetection with fast response time and high responsivity are presented in the newly synthesized magnetic Fe_0.75Ta_0.5S₂, demonstrating great potential for advanced magneto-optoelectronic applications.

Abstract

Understanding the interplay between magnetic and optoelectronic properties and developing spin-optoelectronic devices are promising research strategies to further study 2D materials and advance their applications. Here, the broadband photoresponse in the newly synthesized magnetic Fe_0.75Ta_0.5S₂ single crystals is reported. Because the uncompensated magnetic moment of the spin glass state is pinned by the moment of the antiferromagnetic state, a large exchange bias field of ≈1.98 T is found at 2 K when cooled down at a field of 7 T. The as-prepared samples show a large negative magnetoresistance (nMR). The field dependence of nMRs displays a similar trend up to 50 K, which is likely to originate from the significant dependence of the localization length on magnetic field. In addition, a photodetector prepared using Fe_0.75Ta_0.5S₂ flakes exhibits a fast response time (121.7 ms), good stability, high responsivity of 26.1 A W⁻¹, and broadband photodetection, showing application potentials in spin-optoelectronics.

Nature Photonics, Published online: 13 October 2022; doi:10.1038/s41566-022-01085-w

In this study, a lateral p–n junction diode is formed in WSe₂ using a combination of edge and surface contacts with extremely high performance. A rectification ratio of 10³ and the ideality factor of 1.11 are achieved by the WSe₂ single-channel device. This should be advantageous for the implementation of more complicated applications such as inverters, photodetectors, and light-emitting diodes.

Abstract

Owing to their practical applications, two-dimensional semiconductor p–n diodes have attracted enormous attention. Over the past decade, various methods, such as chemical doping, heterojunction structures, and metallization using metals with different work functions, have been reported for fabrication of such devices. In this study, a lateral p–n junction diode is formed in tungsten diselenide (WSe₂) using a combination of edge and surface contacts. The appearance of amorphous tungsten oxide at etched WSe₂, and the formation of a junction near the edge contact, are verified by high-resolution transmission electron microscopy. The device demonstrates high on/off ratio for both the edge and surface contacts, with respective values of 10⁷ and 10⁸. The diode can achieve extremely high mobility of up to 168 cm² V⁻¹ s⁻¹ and a rectification ratio of 10³. The ideality factor is 1.11 at a back gate voltage V _G  = 60 V at 300 K. The devices with encapsulation of hexagonal boron nitride exhibit good stability to atmospheric exposure, over a measured period of 2 months. In addition, the architecture of the contacts, which is based on a single-channel device, should be advantageous for the implementation of more complicated applications such as inverters, photodetectors, and light-emitting diodes.

A de-wrinkling technique for 2D materials that utilizes electron beam of a transmission electron microscope (TEM) for ironing out the surface corrugations (wrinkles and ripples) of black phosphorus flake is demonstrated. Experiments show the ordered black phosphorus lattice with the removal of wrinkles and the associated de-stressing of lattice under optimal exposure to electron beam fluence.

Abstract

Large area 2D nanomaterials are susceptible to the formation of surface corrugations during synthesis, transfer, and handling of samples and their physicochemical properties are extraordinarily affected by the formation of surface corrugations. Even though several strategies have been devised by researchers for smoothing the 2D flakes, the issue is far from resolved. Here, the straightening of black phosphorus (BP) flakes using electron beam irradiation that enables the removal of ripples, disclination, and line defects from lattice are reported. The crystallinity and buckling of the flake are controlled by varying the electron fluence rate and irradiation time in a high-resolution transmission electron microscopy set-up. Experimental results show that the optimal electron beam exposure (20 to 30 min of exposure at fluence rate = 1.02 × 10²⁹ m⁻² s⁻¹) de-stresses/relaxes the lattice and the maximum ordering of lattice planes is achieved; beyond which, the stress in lattice rises again and lattice planes start buckling. Thus, straightening the 2D flakes using an electron beam ensures the removal of surface corrugations with nanoscale precision and allows for real-time monitoring of the process.

MoSe₂/MoSe₂ homo-stack transistors are fabricated for n-type memory devices whose nonvolatile memory behavior originates from stack interface-induced traps via surface oxidation of bottom MoSe₂, Here, it is found that long-term oxidation surprisingly enables ambipolar nonvolatile memory behavior due to nm-thin MoOx embedded between upper and lower MoSe₂. Moreover, alternating V _GS pulses reversibly convert the channel polarity of such homo-stack transistors.

Abstract

2D semiconductor devices have been studied due to their unique potential in architecture and properties. As one of the unique devices approaches, 2D hetero-stack channel field-effect transistors (FETs) have recently been reported, but homo-stack FETs are rare to find. Here, MoSe₂/MoSe₂ homo-stack transistors are rather fabricated for study. Unlike the equivalently-thick single MoSe₂ FET, homo-stack FETs show n-type memory behavior that originates from stack interface-induced traps. Particularly, when their stack interfaces are engineered by surface oxidation of bottom MoSe₂, more stable nonvolatile memory behavior turns out. Short-term ultraviolet ozone (UVO)-induced oxidation only results in n-type memory, but 15 min-long oxidation surprisingly enables both n- and p-type nonvolatile memory behavior due to nm-thin MoO_x embedded between upper and lower MoSe₂. Furthermore, by alternating gate voltage pulse to the 15 min-long UVO-treated FETs, channel polarity conversion appears reversible in a small gate voltage (V_GS) sweep range, which means that the channel type of a transistor can be reversibly modulated via stack interface engineering. It is believed that homo-stack interface engineering must be one of the approaches to maximize the potential of 2D devices.

The latest advances and developments in the field of chemical sensors based on van der Waals heterostructures of 2D materials, with specific insights into sensing mechanisms, are reviewed and future directions, challenges, and opportunities for the next generation of (bio)chemical sensors with potential impact in environmental sciences and biomedical applications are discussed.

Abstract

During the last 15 years, 2D materials have revolutionized the field of materials science. Moreover, because of their highest surface-to-volume ratio and properties extremely susceptible to their interaction with the local environment they became powerful active components for the development the high-performance chemical sensors. By combining different 2D materials to form van der Waals heterostructures (VDWHs) it is possible to overcome the drawback of individual materials (such as inertness and zero-bandgap of pristine graphene and less environmental stability of transition metal dichalcogenides). Meanwhile, VDWHs possess unprecedented and fascinating properties arising from the intimate interaction between the components, which can yield superior sensitivities, higher selectivity, and stability when employed to detect gases, biomolecules, and other organic/inorganic molecules. Herein, the latest developments and advances in the field of chemical sensors based on VDWH of 2D materials, with specific insight into the sensing mechanisms, are reviewed and future directions, challenges, and opportunities for the development of the next generation of (bio)chemical sensors with potential impact in environmental sciences and biomedical applications, and more specifically in (bio)chemical defense, industrial safety, food, and environmental surveillance, and medical (early) diagnostics, are discussed.

Compared to the other types of gas sensors, this 1T′/2H/1T′ device possesses distinctive advantages. First, the polycrystalline 2H MoTe₂ provides numerous defects, such as vacancies, edges, and grain boundaries, which play crucial roles in enhancing the response rate and sensitivity of a gas sensor. Second, the low-resistance planar heterostructure ensures a microamp scale current output even at larger channel aspect ratios.

Abstract

2D materials, with their extraordinary physical and chemical properties, have gained extensive interest for physical, chemical and biological sensing applications. However, 2D material-based devices, such as field effect transistors (FETs) often show high contact resistance and low output signals, which severely affect their sensing performance. In this study, a new strategy is developed to combine metallic and semiconducting polymorphs of transition-metal dichalcogenides (TMDCs) to solve this critical issue. Such a phase engineering methodology to integrate large-scale and spatially assembled multilayers of 2H MoTe₂ FETs with coplanar metallic 1T′ MoTe₂ contacts is applied. Such in-plane heterophase-based FETs exhibit an ohmic contact behavior with an extremely low contact resistance due to the coplanar and seamless connections between 2H and 1T′ phases of MoTe₂. These 1T′/2H/1T′ based FETs are successfully demonstrated for detecting NH₃ with high current outputs increased up to microamp levels without using any conventional interdigital electrodes, which is compatible with the current CMOS circuits for practical applications. Furthermore, the as-fabricated sensors can detect NH₃ gas concentrations down to 5 ppm at room temperature. This study demonstrates a new strategy of applying the heterophase MoTe₂-based nanoelectronics for high-performance sensing applications.

This study shows an irreversible conductive filament (ICF) contact on a 2D channel passivated by hexagonal boron nitride. Using oxygen-plasma treatment, defects form permanent conductive paths by the migration of metal ions and vacancies, generating ICFs through the defective paths to form contacts with the embedded 2D channel layer. So ICF contact can be of immense potential for use in high-performance 2D electronic devices.

Abstract

2D materials with atomic-scale thickness have attracted immense interest owing to their intriguing properties, which can be useful for electronic devices. As ultrathin 2D materials are highly vulnerable to external conditions, passivation of 2D materials is required to maintain the stability of 2D electronic devices. However, 2D channels are embedded in passivation layers, making the formation of suitable contacts in passivated 2D devices challenging. Here, a novel method for fabricating irreversible conductive filament (ICF) contacts on a 2D channel passivated by hexagonal boron nitride (hBN) layers is demonstrated. Defective paths are formed in the top hBN layer of hBN-encapsulated graphene (or MoS₂) using oxygen-plasma treatment, along which ICFs are fabricated by applying repetitive bias. ICF contacts formed in the combined paths of migrated metal atoms and vacancies are stable during device operation, which is in contrast with that the filaments in hBN memristors are reversible. Field-effect transistors with ICF contacts exhibit a low contact resistance and high stability. This study shows a new contact method, which has great potential for high-performance 2D electronics devices.

A new method for the preparation of large 2D Ti₃C₂T _x MXene nanosheets is reported. The method is based on conventional etching followed by repetitive precipitation and vortex shaking process, which efficiently transfer the mechanical energy for exfoliation. Consequently, large defect-free sheets that show an excellent electromagnetic shielding performance are produced with high yields.

Abstract

Evaluating the delamination process in the synthesis of MXenes (2D transition metal carbides and nitrides) is critical for their development and applications. However, the preparation of large defect-free MXene flakes with high yields is challenging. Here, a power-focused delamination (PFD) strategy is demonstrated that can enhance both the delamination efficiency and yield of large Ti₃C₂T _x MXene nanosheets through repetitive precipitation and vortex shaking processes. Following this protocol, a colloidal concentration of 20.4 mg mL^–1 of the Ti₃C₂T _x MXene can be achieved after five PFD cycles, and the yield of the basal-plane-defect-free Ti₃C₂T _x nanosheets reaches 61.2%, which is 6.4-fold higher than that obtained using the sonication–exfoliation method. Both nanometer-thin devices and self-supporting films exhibit excellent electrical conductivities (≈25 000 and 8260 S cm^-1 for a 1.8 nm thick monolayer and 11 µm thick film, respectively). Hydrodynamic simulations reveal that the PFD method can efficiently concentrate the shear stress on the surface of the unexfoliated material, leading to the exfoliation of the nanosheets. The PFD-synthesized large MXene nanosheets exhibit superior electrical conductivities and electromagnetic shielding (shielding effectiveness per unit volume: 35 419 dB cm² g^–1). Therefore, the PFD strategy provides an efficient route for the preparation of high-performance single-layer MXene nanosheets with large areas and high yields.

This work uses epitaxial strain to tune the oxygen stoichiometry and magnetism in UO_{2+ x} thin films. The UO_{2+ x} films are hypostoichiometric (x<0) when in-plane lattice is tensile strained, while films are hyperstoichiometric (x>0) when in-plane lattice is compressively strained. Induced ferromagnetism is attributed to the epitaxial strain and strain relaxation-induced point defects such as oxygen vacancy and oxygen interstitials.

Abstract

Actinide materials have various applications that range from nuclear energy to quantum computing. Most current efforts have focused on bulk actinide materials. Tuning functional properties by using strain engineering in epitaxial thin films is largely lacking. Using uranium dioxide (UO₂) as a model system, in this work, the authors explore strain engineering in actinide epitaxial thin films and investigate the origin of induced ferromagnetism in an antiferromagnet UO₂. It is found that UO_{2+ x} thin films are hypostoichiometric (x<0) with in-plane tensile strain, while they are hyperstoichiometric (x>0) with in-plane compressive strain. Different from strain engineering in non-actinide oxide thin films, the epitaxial strain in UO₂ is accommodated by point defects such as vacancies and interstitials due to the low formation energy. Both epitaxial strain and strain relaxation induced point defects such as oxygen/uranium vacancies and oxygen/uranium interstitials can distort magnetic structure and result in magnetic moments. This work reveals the correlation among strain, point defects and ferromagnetism in strain engineered UO_{2+ x} thin films and the results offer new opportunities to understand the influence of coupled order parameters on the emergent properties of many other actinide thin films.

Abstract

Thanks to its single-atomic-layer structure, high carrier transport, and low power dissipation, carbon nanotube electronics is a leading candidate towards beyond-silicon technologies. Its low temperature fabrication processes enable three-dimensional (3D) integration with logic and memory (static random access memory (SRAM), magnetic random access memory (MRAM), resistive random access memory (RRAM), etc.) to realize efficient near-memory computing. Importantly, carbon nanotube transistors require good thermal stability up to 400 °C processing temperature to be compatible with back-end-of-line (BEOL) process, which has not been previously addressed. In this work, we developed a robust wafer-scale process to build complementary carbon nanotube transistors with high thermal stability and good uniformity, where AIN was employed as electrostatic doping layer. The gate stack and passivation layer were optimized to realize high-quality interfaces. Specifically, we demonstrate 1-bit carbon nanotube full adders working under 250 °C with rail-to-rail outputs.

Highlights

• The vertically integrated electronic devices based on emerging semiconductor materials including organic, metal oxide, and two-dimensional materials are revisited.

• Comprehensive aspects of the device architecture, performance, and fabrication method of the vertically stacked electronics according to each semiconductor material are discussed.

• Recent advances in vertically integrated electronic devices for emerging applications such as advanced integrated circuits, sensors, and display systems are highlighted.

npj 2D Materials and Applications, Published online: 08 October 2022; doi:10.1038/s41699-022-00348-y

Using a precisely designed interface-decoupling strategy, quasi-suspended graphene can be grown directly on a 4 inch Si wafer. The obtained Gr/Si sample enables transfer-free fabrication of graphene-based field-effect transistor devices with high electronic performance and a carrier mobility up to 15 000 cm² V⁻¹ s⁻¹.

Abstract

The direct growth of graphene affording wafer-scale uniformity on insulators is paramount to electronic and optoelectronic applications; however, it remains a challenge to date, because it entails an entirely different growth mode than that over metals. Herein, the metal-catalyst-free growth of quasi-suspended graphene on a Si wafer is demonstrated using an interface-decoupling chemical vapor deposition strategy. The employment of lower-than-conventional H₂ dosage and concurrent introduction of methanol during growth can effectively weaken the interaction between the synthesized graphene and the underlying substrate. The growth mode can be thus fine-tuned, producing a predominantly monolayer graphene film with wafer-level homogeneity. Graphene thus grown on a 4 inch Si wafer enables the transfer-free fabrication of high-performance graphene-based field-effect transistor arrays that exhibit almost no shift in the charge neutral point, indicating a quasi-suspended feature of the graphene. Moreover, a carrier mobility up to 15 000 cm² V^-1 s^-1 can be attained. This study is anticipated to offer meaningful insights into the synthesis of wafer-scale high-quality graphene on dielectrics for practical graphene devices.

Abstract

Recently, the discovery of a variety of moiré-related properties in the twisted vertical stacking of two different monolayers has attracted considerable attention. The introduction of small twist angles in transition metal dichalcogenide (TMD) heterostructures leads to the emergence of moiré potentials, which provide a fascinating platform for the study of strong interactions of electrons. While there has been extensive research on moiré excitons in twisted bilayer superlattices, the capture and study of moiré excitons in homostructure superlattices with layer-coupling effects remain elusive. Here, we present the observation of moiré excitons in the twisted 1L-WSe₂/1L-WSe₂ and 1L-WSe₂/2L-WSe₂ homostructures with various layer-coupling interactions. The results reveal that the moiré potential increases (~ 260%) as the number of underlying layers decreases, indicating the effect of layer coupling on the modulation of the moiré potential. The effects of the temperature and laser power dependence as well as valley polarization on moiré excitons were further demonstrated, and the crucial spectral features observed were explained. Our findings pave the way for exploring quantum phenomena and related applications of quantum information.