Jiuxiang Dai

Ferroelectric and analog resistive switching in an ultrathin, epitaxial nitride ferroelectric heterostructure is demonstrated, showing high ON/OFF ratios, high uniformity, and good retention that enables multistate operation and linear analog computing with high accuracy. The results pave the way for constructing advanced memory/computing architectures based on emerging nitride ferroelectrics and promote homo and hybrid integrated functional edge devices beyond silicon.

Abstract

Computing in the analog regime using nonlinear ferroelectric resistive memory arrays can potentially alleviate the energy constraints and complexity/footprint challenges imposed by digital von Neumann systems. Yet the current ferroelectric resistive memories suffer from either low ON/OFF ratios/imprint or limited compatibility with mainstream semiconductors. Here, for the first time, ferroelectric and analog resistive switching in an epitaxial nitride heterojunction comprising ultrathin (≈5 nm) nitride ferroelectrics, i.e., ScAlN, with potentiality to bridge the gap between performance and compatibility is demonstrated. High ON/OFF ratios (up to 10⁵), high uniformity, good retention, (<20% variation after > 10⁵ s) and cycling endurance (>10⁴) are simultaneously demonstrated in a metal/oxide/nitride ferroelectric junction. It is further demonstrated that the memristor can provide programmability to enable multistate operation and linear analogue computing as well as image processing with high accuracy. Neural network simulations based on the weight update characteristics of the nitride memory yielded an image recognition accuracy of 92.9% (baseline 96.2%) on the images from Modified National Institute of Standards and Technology. The non-volatile multi-level programmability and analog computing capability provide first-hand and landmark evidence for constructing advanced memory/computing architectures based on emerging nitride ferroelectrics, and promote homo and hybrid integrated functional edge devices beyond silicon.

Wafer-scale tellurene is synthesized under room temperature by exploiting pulsed-laser deposition. On this basis, tellurene-based photodetector arrays are constructed, manifesting outstanding photosensitivity with the optimal responsivity, external quantum efficiency, and detectivity of 2.7 × 10⁷ A W⁻¹, 8.2 × 10⁹%, and 4.5 × 10¹⁵ Jones. Moreover, proof-of-concept ultrabroadband optical imaging is realized.

Abstract

High-resolution imaging is at the heart of the revolutionary breakthroughs of intelligent technologies, and it is established as an important approach toward high-sensitivity information extraction/storage. However, due to the incompatibility between non-silicon optoelectronic materials and traditional integrated circuits as well as the lack of competent photosensitive semiconductors in the infrared region, the development of ultrabroadband imaging is severely impeded. Herein, the monolithic integration of wafer-scale tellurene photoelectric functional units by exploiting room-temperature pulsed-laser deposition is realized. Taking advantage of the surface plasmon polaritons of tellurene, which results in the thermal perturbation promoted exciton separation, in situ formation of out-of-plane homojunction and negative expansion promoted carrier transport, as well as the band bending promoted electron–hole pair separation enabled by the unique interconnected nanostrip morphology, the tellurene photodetectors demonstrate wide-spectrum photoresponse from 370.6 to 2240 nm and unprecedented photosensitivity with the optimized responsivity, external quantum efficiency and detectivity of 2.7 × 10⁷ A W⁻¹, 8.2 × 10⁹% and 4.5 × 10¹⁵ Jones. An ultrabroadband imager is demonstrated and high-resolution photoelectric imaging is realized. The proof-of-concept wafer-scale tellurene-based ultrabroadband photoelectric imaging system depicts a fascinating paradigm for the development of an advanced 2D imaging platform toward next-generation intelligent equipment.

The nonvolatile dynamic random access memory (DRAM) using an optimized indium-gallium-zinc-oxide (IGZO) transistor with record-high threshold voltage beyond 1.78 V and on-current >40 µA µm⁻¹ is demonstrated for the first time. The DRAM shows a record retention time exceeding 25 h with an ultra-fast write speed of 10 ns under power interruption, along with a 3-bit multilevel operation with a wide margin and excellent linearity.

Abstract

Severe power consumption in the continuous scaling of Silicon-based dynamic random access memory (DRAM) technology quests for a transistor technology with a much lower off-state leakage current. Wide bandgap amorphous oxide semiconductors, especially indium-gallium-zinc-oxide (IGZO) exhibit many orders of magnitude lower off-state leakage. However, they are typically heavily n-doped and require negative gate voltage to turn off, which prevents them from true nonvolatile operation. The efforts on doping density reduction typically result in mobility degradation and high Schottky barriers at contacts, causing severe degradation of on-current and operation speed of the DRAM cells. Here, high-speed true nonvolatile DRAM cells are successfully demonstrated by deep suppression of doping density in the IGZO channel using in situ oxygen ion beam treatment and ohmic contact engineering by inserting a thin In-rich indium-tin-oxide (ITO) at contact regions. A record high on-current of 40 µA µm⁻¹ at a large positive threshold voltage of 1.78 V enables the first true nonvolatile DRAM with the fastest write speed of 10 ns and data retention up to 25 h under power interruption, five orders of magnitude higher than the previously projected values.

Reducing the electrode area and increasing the specific capacitance of hybrid supercapacitors remain challenging. Herein, a MoCl₅ Precursor-assisted Ultrafast Laser Carbonization method to fabricate symmetric hybrid supercapacitors with improved capacitance and reduced size is proposed. The specific area capacitance of the fabricated hybrid supercapacitor is 11.85 mF cm⁻², 9.2 times higher than that of the laser-induced carbon supercapacitor without precursor.

Abstract

Hybrid supercapacitors use electric double-layer capacitance and Faradaic pseudocapacitance as energy storage mechanisms. This type of supercapacitor is becoming a prime candidate for next-generation energy storage devices, with advantages in terms of energy density, specific capacitance, and life cycle. However, reducing the electrode area and increasing the specific capacitance of hybrid supercapacitors remain challenging. In this study, a MoCl₅ Precursor-assisted Ultrafast Laser Carbonization (MPAULC) method to fabricate symmetric hybrid supercapacitors with improved capacitance and reduced size is proposed. The method uses an ultrafast laser to induce the formation of carbon/MoO₃ composite with the assistance of the MoCl5 precursor. This ultrafast laser carbonization method exhibited high processing precision. The role of the precursor in laser processing is studied using time-resolved imaging and temperature calculations. The specific area capacitance of the C/MoO₃ hybrid supercapacitor is 11.85 mF cm⁻², 9.2 times higher than that of the laser-induced carbon supercapacitor without precursor. The MPAULC method provides a reliable pathway for fabricating miniaturized hybrid supercapacitors with carbon/metal oxide composite electrodes on polymer substrates.

A reformative mechanical exfoliation approach combining tape and polydimethylsiloxane exfoliations is developed to fabricate high-quality, narrow, and directed black phosphorus nanoribbons (PNRs) with smooth edges. The PNRs have widths from a dozen to hundreds of nanometers (down to 15 nm). The investigation finds the PNRs can align along a same direction. The PNR-based diode and transistor exhibit good performance.

Abstract

Black phosphorus nanoribbons (PNRs) are ideal candidates for constructing electronic and optoelectronic devices owing to their unique structure and high bandgap tunability. However, the preparation of high-quality narrow PNRs aligned along the same direction is very challenging. Here, a reformative mechanical exfoliation approach combining tape and polydimethylsiloxane (PDMS) exfoliations to fabricate high-quality, narrow, and directed PNRs with smooth edges for the first time is developed. In this method, partially-exfoliated PNRs are first formed on thick black phosphorus (BP) flakes via the tape exfoliation and further peeled off to obtain separated PNRs via the PDMS exfoliation. The prepared PNRs have widths from a dozen to hundreds of nanometers (down to 15 nm) and a mean length of 18 µm. It is found that the PNRs can align along a same direction and the length directions of directed PNRs are along the zigzag direction. The formation of PNRs is attributed to that the BP prefers to be unzipped along the zigzag direction and has an appropriate magnitude of interaction force with the PDMS substrate. The fabricated PNR/MoS₂ heterojunction diode and PNR field-effect transistor exhibit good device performance. This work provides a new pathway to achieve high-quality, narrow, and directed PNRs for electronic and optoelectronic applications.

Abstract

The family of vanadium chalcogenides with variable stoichiometry and abundant crystallographic structures are promising platforms for realizing exotic emergent phenomena. Here, we report on a novel two-dimensional (2D) tetragonal structure of vanadium telluride (VTe) grown by molecular beam epitaxy. The atomic structures and electronic properties are revealed by scanning tunneling microscopy and first-principles calculations. Different from the hexagonal or trigonal lattices of 2D VTe₂, the 2D VTe with a V:Te ratio of 1:1 exhibits an uncommon square lattice. Non-zero differential conductivity at the Fermi energy detected by scanning tunneling spectroscopy reveals the metallic feature of VTe. Meanwhile, Friedel oscillations are observed near chiral point defects and domain walls, illustrating the itinerant nature of the electrons close to the Fermi energy. Our first-principles structure searches identify a 2D body-centered cubic (bcc)-like structure with a favorable formation energy to be the candidate of the metallic phase of the tetragonal VTe obtained experimentally. Based on our calculations the 2D bcc-like structure possesses a strong 2D antiferromagnetic order. Our work enriches the family of vanadium chalcogenides and provides a possible 2D antiferromagnetic material for fabricating advanced spintronic devices.

Nature Materials, Published online: 09 March 2023; doi:10.1038/s41563-023-01502-7

Large-size single-crystal van der Waals layered Bi2SeO5 has been synthesized with a high dielectric constant and high breakdown field strength for two-dimensional electronics applications.

Nature Physics, Published online: 09 March 2023; doi:10.1038/s41567-023-01983-y

Materials that simultaneously display ferroelectricity and magnetism, and are metallic, are very rare. Now, the two-dimensional electron gas in an oxide heterostructure brings all of this behaviour together.

Ultrabroadband photoconduction is an emerging method providing the subgap density of states (DoS) directly on thin-film transistors of amorphous oxide semiconductors. To suggest new routes to optimize transistor performance by controlling specific defect concentration, the sub-gap density of states is found to vary significantly with the active channel composition, hydrogen concentration, metal etchants and plasma post-processing methods.

Abstract

Under varying growth and device processing conditions, ultrabroadband photoconduction (UBPC) reveals strongly evolving trends in the defect density of states (DoS) for amorphous oxide semiconductor thin-film transistors (TFTs). Spanning the wide bandgap of amorphous InGaZnO _x (a-IGZO), UBPC identifies seven oxygen deep donor vacancy peaks that are independently confirmed by energetically matching to photoluminescence emission peaks. The subgap DoS from 15 different types of a-IGZO TFTs all yield similar DoS, except only back-channel etch TFTs can have a deep acceptor peak seen at 2.2 eV below the conduction band mobility edge. This deep acceptor is likely a zinc vacancy, evidenced by trap density which becomes 5-6× larger when TFT wet-etch methods are employed. Certain DoS peaks are strongly enhanced for TFTs with active channel processing damage caused from plasma exposure. While Ar implantation and He plasma processing damage are similar, Ar plasma yields more disorder showing a ≈2 × larger valence-band Urbach energy, and two orders of magnitude increase in the deep oxygen vacancy trap density. Changing the growth conditions of a-IGZO also impacts the DoS, with zinc-rich TFTs showing much poorer electrical performance compared to 1:1:1 molar ratio a-IGZO TFTs owing to the former having a ∼10 × larger oxygen vacancy trap density. Finally, hydrogen is found to behave as a donor in amorphous indium tin gallium zinc oxide TFTs.

The discovery of van der Waals magnets has provided a new platform for the electrical control of magnetism. Here, the realization of nonvolatile control of exchange bias and coercive fields in Fe₃GeTe₂/MgO heterostructures, and the gate voltage is as low as tens of mV which is two orders of magnitude smaller than those in previous experiments is presented.

Abstract

The discovery of van der Waals magnets has provided a new platform for the electrical control of magnetism. Recent experiments have demonstrated that the magnetic properties of van der Waals magnets can be tuned by various gate modulations, although most of them are volatile and require gate voltages no lower than several volts. Here, the realization of nonvolatile control of exchange bias and coercive fields in Fe₃GeTe₂/MgO heterostructures, and the gate voltage is as low as tens of mV which is two orders of magnitude smaller than those in previous experiments is presented. The discovery of an ionic-irradiated phase formed in Fe₃GeTe₂ by MgO sputtering revealed that an exchange bias effect can be obtained in this heterostructure and tuned from ≈700 to 0 Oe through voltages ranging from 5 to 20 mV. Owing to the high stability of oxidized Fe₃GeTe₂, the voltage-driven oxygen incorporated into Fe₃GeTe₂ from the irradiated phase induces a nonvolatile magnetism modulation that can be retained after turning off the gate voltage. These findings demonstrate a methodology to modulate the magnetism of van der Waals magnets, opening new opportunities to fabricate all-solid, long-retention, and low-dissipation nano-electronic devices using van der Waals materials.

Journal of Applied Physics, Volume 133, Issue 9, March 2023.
Ferroelectricity in thin films of HfO2 has been the subject of extensive studies in materials science as well as device applications. The emergence of ferroelectricity is attributable to the orthorhombic phase (Pca21) of HfO2, stabilized in the films by metal-element doping, strains from substrates and electrode films, and oxygen deficiency. Recently, ferroelectricity has been reported in nanolaminates of HfO2 with other oxides such as ZrO2 and Al2O3, implying that nanolaminates are another effective way to bring about ferroelectricity in HfO2. However, the mechanism of orthorhombic phase stabilization in nanolaminates is not fully understood. In this study, we demonstrated that ferroelectricity emerges in nanolaminates consisting of undoped HfO2 and perovskite SrHfO3 deposited on Sn-doped In2O3 bottom electrodes, when the thickness of HfO2 layers was ≥6 nm. For nanolaminates in which the thickness of the HfO2 layers was ≤5 nm, ferroelectricity was remarkably suppressed due to Sr-incorporation into the HfO2 layers at the interface. In those nanolaminates, the crystal orientations of HfO2 grains were well aligned throughout the HfO2 layers, indicating that the HfO2 layers grew in a pseudo-coherent manner. This study aids to understand the stabilization of the ferroelectric orthorhombic phase in nanolaminates in terms of their structural properties.

Journal of Applied Physics, Volume 133, Issue 9, March 2023.
Ion beam fabrication of metastable polymorphs of Ga2O3, assisted by the controllable accumulation of the disorder in the lattice, is an interesting alternative to conventional deposition techniques. However, the adjustability of the electrical properties in such films is unexplored. In this work, we investigated two strategies for tuning the electron concentration in the ion beam created metastable κ-polymorph: adding silicon donors by ion implantation and adding hydrogen via plasma treatments. Importantly, all heat treatments were limited to ≤600 °C, set by the thermal stability of the ion beam fabricated polymorph. Under these conditions, silicon doping did not change the high resistive state caused by the iron acceptors in the initial wafer and residual defects accumulated upon the implants. Conversely, treating samples in a hydrogen plasma converted the ion beam fabricated κ-polymorph to n-type, with a net donor density in the low 1012 cm−3 range and dominating deep traps near 0.6 eV below the conduction band. The mechanism explaining this n-type conductivity change may be due to hydrogen forming shallow donor complexes with gallium vacancies and/or possibly passivating a fraction of the iron acceptors responsible for the high resistivity in the initial wafers.

Journal of Applied Physics, Volume 133, Issue 9, March 2023.
Instrumented indentation experiments at elevated temperatures require careful attention to a myriad of experimental details. Not the least of these is the choice of the indenter tip material. Traditional room-temperature indenters, e.g., diamond and sapphire, can break down, react, and wear excessively at elevated temperatures. In this work, rf-induction heating float-zone and high-temperature solution single-crystal growth techniques have been used to prepare a suite of bulk refractory carbide specimens (i.e., ZrC, VC0.86, NbC, TiC0.95, WC). These potential indenter tip materials were subsequently characterized using nanoindentation testing techniques to determine their single-crystal elastic modulus, hardness, and fracture toughness in order to evaluate their potential for use as elevated-temperature nanoindentation tips. Additionally, subject carbide crystal characteristics were compared to those of single-crystal sapphire and polycrystalline WC-Co. The cumulative results show that single-crystal WC is a promising candidate for indenter tip material based on a combination of its high elastic modulus, hardness, and resistance to cracking—in addition to being crystallographically favorable for fabrication in the frequently used three-sided pyramidal indenter tip geometries.

Applied Physics Letters, Volume 122, Issue 10, March 2023.
Wide bandgap oxide semiconductors have gained significant attention in the fields from flat panel displays to solar cells, but their uses have been limited by the lack of high mobility p-type oxide semiconductors. Recently, β-phase TeO2 has been identified as a promising p-type oxide semiconductor with exceptional device performance. In this Letter, we report on the electronic structure of β-TeO2 studied by a combination of high-resolution x-ray spectroscopy and hybrid density functional theory calculations. The bulk bandgap of β-TeO2 is determined to be 3.7 eV. Direct comparisons between experimental and computational results demonstrate that the top of a valence band (VB) of β-TeO2 is composed of the hybridized Te 5s, Te 5p, and O 2p states, whereas a conduction band (CB) is dominated by unoccupied Te 5p states. The hybridization between spatially dispersive Te 5s2 states and O 2p orbitals helps us to alleviate the strong localization in the VB, leading to small hole effective mass and high hole mobility in β-TeO2. The Te 5p states provide stabilizing effect to the hybridized Te 5s-O 2p states, which is enabled by structural distortions of a β-TeO2 lattice. The multiple advantages of large bandgap, high hole mobility, two-dimensional structure, and excellent stability make β-TeO2 a highly competitive material for next-generation opto-electronic devices.

Applied Physics Letters, Volume 122, Issue 10, March 2023.
Tuning the Gilbert damping of ferromagnetic (FM) metals via a nonvolatile way is of importance to exploit and design next-generation novel spintronic devices. Through systematical first-principles calculations, we study the magnetic properties of the van der Waals heterostructure of two-dimensional FM metal CrTe2 and ferroelectric (FE) In2Te3 monolayers. The ferromagnetism of CrTe2 is maintained in CrTe2/In2Te3 and its magnetic easy axis can be switched from in-plane to out-of-plane by reversing the FE polarization of In2Te3. Excitingly, we find that the Gilbert damping of CrTe2 is tunable when the FE polarization of In2Te3 is reversed from upward to downward. By analyzing the k-dependent contributions to the Gilbert damping, we unravel that such tunability results from the changed intersections between the bands of CrTe2 and Fermi level on the reversal of the FE polarizations of In2Te3 in CrTe2/In2Te3. Our work provides an appealing way to electrically tailor Gilbert dampings of two-dimensional FM metals by contacting them with ferroelectrics.

Abstract

Broadband light detection and sensing are widely applied in modern technology. As a promising candidate for next-generation two-dimensional (2D) optoelectronic material, bismuth oxyselenide (Bi₂O₂Se) nanoplates exhibit many prospects in the application of visible light detection due to their peculiar properties. In this work, we report the photodetection performance of single-crystal 2D Bi₂O₂Se nanoplates grown on SiO₂ based on a ternary-alloy growth model by utilizing chemical vapor deposition (CVD). The Bi₂O₂Se nanoplates were found to have an even and uniform square shape with side lengths up to 15 µm and an approximate thickness of 15 nm. A visible-light photodetector was fabricated based on a CVD-grown Bi₂O₂Se nanoplate, and characterized by a set of illumination experiments using a 400 nm laser at temperatures ranging from 77 to 370 K. The device exhibited superior performance at the temperature of 77 K, with a responsivity of 523 A/W, a specific detectivity of 1.37 × 10¹¹ Jones, a response time of 0.2175 ms, and an external quantum efficiency of 162,119.44%, resulting in high-quality and full-color imaging in the visible spectrum. These results indicate that the single-crystalline Bi₂O₂Se nanoplates have excellent potential in broadband photodetection and non-cryogenic imaging.

Nature Electronics, Published online: 06 March 2023; doi:10.1038/s41928-023-00931-1

Magnetic hysteresis in multiferroic heterostructures formed from the two-dimensional magnetic insulator chromium germanium telluride and a thin ferroelectric polymer can be electrically controlled with voltages of around 5 V.

Nature Communications, Published online: 04 March 2023; doi:10.1038/s41467-023-36833-1

Well-defined metastable phase nanostructures are a core issue for catalyst design. Here, the authors report metastable monoclinic phase IrO2 nanoribbons obtained via a molten-alkali mechanochemical method, which exhibit intrinsic high performance towards the acidic oxygen evolution reaction.

Novel 2M-phase transition metal dichalcogenide (TMD) materials are built from quantum spin Hall 1T'-TMD layers. A systematic investigation combining angle-resolved photoemission spectroscopy and first-principles calculations reveals a topology hierarchy: 2M-WSe₂, MoS₂ and MoSe₂ are weak topological insulators, whereas 2M-WS₂ is a strong topological insulator. They can serve as parent compounds of various exotic phases and promise potentials in quantum electronics.

Abstract

The evolution of the physical properties of 2D material from monolayer limit to the bulk reveals unique consequences from dimension confinement and provides a distinct tuning knob for applications. Monolayer 1T'-phase transition metal dichalcogenides (1T'-TMDs) with ubiquitous quantum spin Hall (QSH) states are ideal 2D building blocks of various 3D topological phases. However, the stacking geometry has been previously limited to the bulk 1T'-WTe₂ type. Here, the novel 2M-TMDs consisting of translationally stacked 1T'-monolayers are introduced as promising material platforms with tunable inverted bandgaps and interlayer coupling. By performing advanced polarization-dependent angle-resolved photoemission spectroscopy as well as first-principles calculations on the electronic structure of 2M-TMDs, a topology hierarchy is revealed: 2M-WSe₂, MoS_2, and MoSe₂ are weak topological insulators (WTIs), whereas 2M-WS₂ is a strong topological insulator (STI). Further demonstration of topological phase transitions by tunning interlayer distance indicates that band inversion amplitude and interlayer coupling jointly determine different topological states in 2M-TMDs. It is proposed that 2M-TMDs are parent compounds of various exotic phases including topological superconductors and promise great application potentials in quantum electronics due to their flexibility in patterning with 2D materials.

Electrochemical capacitors and rechargeable batteries are considered to be enticing next-generation energy storage technologies for large-scale applications. This perspective examines the most recent developments and offers some distinctive mechanism insights on energy storage systems in order to develop better energy storage devices. The schematic representation of 2D hybrid systems based electrode depicts ion/charge transport characteristics in energy storage systems.

Abstract

To address the global energy and environmental crisis, advanced energy storage systems with their superior electrochemical performances have been growing exponentially. The electrochemical properties of the selected electrode materials have a direct impact on the performance of these energy storage technologies. 2D structures are considered promising materials for energy storage systems because of their unique and outstanding electrochemical characteristics. Therefore, hybridization of 2D nanosheets with other low-dimensional materials can improve storage capacity by modulating and exploiting the synergy between the same. This comprehensive review highlights energy storage devices, their mechanisms, and key problems, with a focus on electrodes made of new generation 2D hybrids. Following that, the strategies that enable face-to-point, face-to-line, and face-to-face heterointerfaces are discussed in details. In addition, a design approach is provided for synergistically coupling 2D with other quantum materials for designing efficient and long-lasting storage systems. It is anticipated that this review article will serve as helpful motivation for developing excellent energy storage devices with improved electrochemical capability.