Van der Waals polarity-engineered 3D integration of 2D complementary logic.

Vertical three-dimensional integration of two-dimensional (2D) semiconductors holds great promise, as it offers the possibility to scale up logic layers in the z axis1-3. Indeed, vertical complementary field-effect transistors (CFETs) built with such mixed-dimensional heterostructures4,5, as well as hetero-2D layers with different carrier types6-8, have been demonstrated recently. However, so far, the lack of a controllable doping scheme (especially p-doped WSe2 (refs. 9-17) and MoS2 (refs. 11,18-28)) in 2D semiconductors, preferably in a stable and non-destructive manner, has greatly impeded the bottom-up scaling of complementary logic circuitries. Here we show that, by bringing transition metal dichalcogenides, such as MoS2, atop a van der Waals (vdW) antiferromagnetic insulator chromium oxychloride (CrOCl), the carrier polarity in MoS2 can be readily reconfigured from n- to p-type via strong vdW interfacial coupling. The consequential band alignment yields transistors with room-temperature hole mobilities up to approximately 425 cm2 V-1 s-1, on/off ratios reaching 106 and air-stable performance for over one year. Based on this approach, vertically constructed complementary logic, including inverters with 6 vdW layers, NANDs with 14 vdW layers and SRAMs with 14 vdW layers, are further demonstrated. Our findings of polarity-engineered p- and n-type 2D semiconductor channels with and without vdW intercalation are robust and universal to various materials and thus may throw light on future three-dimensional vertically integrated circuits based on 2D logic gates.

Large-Scale Vertically Interconnected Complementary Field-Effect Transistors Based on Thermal Evaporation.

With the rapid development of integrated circuits, there is an increasing need to boost transistor density. In addition to shrinking the device size to the atomic scale, vertically stacked interlayer interconnection technology is also an effective solution. However, realizing large-scale vertically interconnected complementary field-effect transistors (CFETs) has never been easy. Currently-used semiconductor channel synthesis and doping technologies often suffer from complex fabrication processes, poor vertical integration, low device yield, and inability to large-scale production. Here, a method to prepare large-scale vertically interconnected CFETs based on a thermal evaporation process is reported. Thermally-evaporated etching-free Te and Bi2S3 serve as p-type and n-type semiconductor channels and exhibit FET on-off ratios of 103 and 105, respectively. The vertically interconnected CFET inverter exhibits a clear switching behavior with a voltage gain of 17 at a 4 V supply voltage and a device yield of 100%. Based on the ability of thermal evaporation to prepare large-scale uniform semiconductor channels on arbitrary surfaces, repeated upward manufacturing can realize multi-level interlayer interconnection integrated circuits.

Controlling the 2D Magnetism of CrBr3 by van der Waals Stacking Engineering.

The manipulation of two-dimensional (2D) magnetic order is of significant importance to facilitate future 2D magnets for low-power and high-speed spintronic devices. van der Waals stacking engineering makes promises for controllable magnetism via interlayer magnetic coupling. However, directly examining the stacking order changes accompanying magnetic order transitions at the atomic scale and preparing device-ready 2D magnets with controllable magnetic orders remain elusive. Here, we demonstrate the effective control of interlayer stacking in exfoliated CrBr3 via thermally assisted strain engineering. The stable interlayer ferromagnetic (FM), antiferromagnetic (AFM), and FM-AFM coexistent ground states confirmed by the magnetic circular dichroism measurements are realized. Combined with the first-principles calculations, the atomically resolved imaging technique reveals the correlation between magnetic order and interlayer stacking order in CrBr3 flakes unambiguously. A tunable exchange bias effect is obtained in the mixed phase of FM and AFM states. This work will introduce new magnetic properties by controlling the stacking order and sequence of 2D magnets, providing ample opportunities for their application in spintronic devices.

Multi-state data storage in a two-dimensional stripy antiferromagnet implemented by magnetoelectric effect.

A promising approach to the next generation of low-power, functional, and energy-efficient electronics relies on novel materials with coupled magnetic and electric degrees of freedom. In particular, stripy antiferromagnets often exhibit broken crystal and magnetic symmetries, which may bring about the magnetoelectric (ME) effect and enable the manipulation of intriguing properties and functionalities by electrical means. The demand for expanding the boundaries of data storage and processing technologies has led to the development of spintronics toward two-dimensional (2D) platforms. This work reports the ME effect in the 2D stripy antiferromagnetic insulator CrOCl down to a single layer. By measuring the tunneling resistance of CrOCl on the parameter space of temperature, magnetic field, and applied voltage, we verified the ME coupling down to the 2D limit and probed its mechanism. Utilizing the multi-stable states and ME coupling at magnetic phase transitions, we realize multi-state data storage in the tunneling devices. Our work not only advances the fundamental understanding of spin-charge coupling, but also demonstrates the great potential of 2D antiferromagnetic materials to deliver devices and circuits beyond the traditional binary operations.

Unconventional correlated insulator in CrOCl-interfaced Bernal bilayer graphene.

The realization of graphene gapped states with large on/off ratios over wide doping ranges remains challenging. Here, we investigate heterostructures based on Bernal-stacked bilayer graphene (BLG) atop few-layered CrOCl, exhibiting an over-1-GΩ-resistance insulating state in a widely accessible gate voltage range. The insulating state could be switched into a metallic state with an on/off ratio up to 107 by applying an in-plane electric field, heating, or gating. We tentatively associate the observed behavior to the formation of a surface state in CrOCl under vertical electric fields, promoting electron-electron (e-e) interactions in BLG via long-range Coulomb coupling. Consequently, at the charge neutrality point, a crossover from single particle insulating behavior to an unconventional correlated insulator is enabled, below an onset temperature. We demonstrate the application of the insulating state for the realization of a logic inverter operating at low temperatures. Our findings pave the way for future engineering of quantum electronic states based on interfacial charge coupling.

Quantum Hall phase in graphene engineered by interfacial charge coupling.

The quantum Hall effect can be substantially affected by interfacial coupling between the host two-dimensional electron gases and the substrate, and has been predicted to give rise to exotic topological states. Yet the understanding of the underlying physics and the controllable engineering of this interaction remains challenging. Here we demonstrate the observation of an unusual quantum Hall effect, which differs markedly from that of the known picture, in graphene samples in contact with an antiferromagnetic insulator CrOCl equipped with dual gates. Two distinct quantum Hall phases are developed, with the Landau levels in monolayer graphene remaining intact at the conventional phase, but largely distorted for the interfacial-coupling phase. The latter quantum Hall phase is even present close to the absence of a magnetic field, with the consequential Landau quantization following a parabolic relation between the displacement field and the magnetic field. This characteristic prevails up to 100 K in a wide effective doping range from 0 to 1013 cm-2.

Electrically Pumped Polarized Exciton-Polaritons in a Halide Perovskite Microcavity.

Recently, exciton-polaritons in lead halide perovskite microcavities have been extensively investigated to address striking phenomena such as polariton condensation and quantum emulation. However, a critical step in advancing these findings into practical applications, i.e., realizing electrically pumped perovskite polariton light-emitting devices, has not yet been presented. Here, we devise a new method to combine the device with a microcavity and report the first halide perovskite polariton light-emitting device. Specifically, the device is based on a CsPbBr3 capacitive structure, which can inject the electrons and holes from the same electrode, conducive to the formation of excitons and simultaneously maintaining the high quality of the microcavity. In addition, highly polarized polariton emissions have been demonstrated due to the optical birefringence in the CsPbBr3 microplate. This work paves the way for realizing practical polaritonic devices such as high-speed light-emitting devices for information communications and inversionless electrically pumped lasers based on perovskites.

2D FeOCl: A Highly In-Plane Anisotropic Antiferromagnetic Semiconductor Synthesized via Temperature-Oscillation Chemical Vapor Transport.

2D van der Waals (vdW) transition-metal oxyhalides with low symmetry, novel magnetism, and good stability provide a versatile platform for conducting fundamental research and developing spintronics. Antiferromagnetic FeOCl has attracted significant interest owing to its unique semiconductor properties and relatively high Néel temperature. Herein, good-quality centimeter-scale FeOCl single crystals are controllably synthesized using the universal temperature-oscillation chemical vapor transport (TO-CVT) method. The crystal structure, bandgap, and anisotropic behavior of the 2D FeOCl are explored in detail. The absorption spectrum and electrical measurements reveal that 2D FeOCl is a semiconductor with an optical bandgap of ≈2.1 eV and a resistivity of ≈10-1  Ω m at 295 K, and the bandgap increases with decreasing thickness. Strong in-plane optical and electrical anisotropies are observed in 2D FeOCl flakes, and the maximum resistance anisotropic ratio reaches 2.66 at 295 K. Additionally, the lattice vibration modes are studied through temperature-dependent Raman spectra and first-principles density functional calculations. A significant decrease in the Raman frequencies below the Néel temperature is observed, which results from the strong spin-phonon coupling effect in 2D FeOCl. This study provides a high-quality low-symmetry vdW magnetic candidate for miniaturized spintronics.

Layer-Number-Dependent Antiferromagnetic and Ferromagnetic Behavior in MnSb_{2}Te_{4}.

MnBi_{2}Te_{4}, an intrinsic magnetic topological insulator, has shown layer-number-correlated magnetic and topological phases. More interestingly, in the isostructural material MnSb_{2}Te_{4}, the antiferromagnetic (AFM) and ferromagnetic (FM) states have been both observed in the bulk counterparts, which are also predicted to be topologically nontrivial. Revealing the layer-number-dependent magnetic properties of MnSb_{2}Te_{4} down to a single septuple layer (SL) is of great significance for exploring the topological phenomena. However, this is still elusive. Here, using the polar reflective magnetic circular dichroism spectroscopy, both the A-type AFM and FM behaviors are observed and comprehensively studied in MnSb_{2}Te_{4} down to a single SL limit. In A-type AFM MnSb_{2}Te_{4} flakes, an obvious odd-even layer-number effect is observed. An additional surface spin-flop (SSF) transition occurs in even-SL flakes with the number of layers larger than 2. With the AFM linear-chain model, we identify that the even-SL flakes stabilize in a collinear state between the SSF transition and the spin-flop transition due to their appropriate energy ratio between the magnetic-field-scale anisotropy and interlayer interaction. In FM MnSb_{2}Te_{4} flakes, we observe very different magnetic behaviors with an abrupt spin-flipping transition and very small saturation fields, indicating a weakened interlayer interaction. By revealing the rich magnetic states of few-SL MnSb_{2}Te_{4} on the parameter space of the number of layers, external magnetic field, and temperature, our findings pave the way for further quantum transport studies of few-SL MnSb_{2}Te_{4}.

Magnetic Phase Transitions and Magnetoelastic Coupling in a Two-Dimensional Stripy Antiferromagnet.

Materials with a quasi-one-dimensional stripy magnetic order often exhibit low crystal and magnetic symmetries, thus allowing the presence of various energy coupling terms and giving rise to macroscopic interplay between spin, charge, and phonon. In this work, we performed optical, electrical and magnetic characterizations combined with first-principles calculations on a van der Waals antiferromagnetic insulator chromium oxychloride (CrOCl). We detected the subtle phase transition behaviors of exfoliated CrOCl under varying temperature and magnetic field and clarified its controversial spin structures. We found that the antiferromagnetism and its air stability persist down to few-layer samples, making it a promising candidate for future 2D spintronic devices. Additionally, we verified the magnetoelastic coupling effect in CrOCl, allowing for the potential manipulation of the magnetic states via electric field or strain. These virtues of CrOCl provide us with an ideal platform for fundamental research on spin-charge, spin-phonon coupling, and spin-interactions.

Light helicity detector based on 2D magnetic semiconductor CrI3.

Two-dimensional magnetic semiconductors provide a platform for studying physical phenomena at atomically thin limit, and promise magneto-optoelectronic devices application. Here, we report light helicity detectors based on graphene-CrI3-graphene vdW heterostructures. We investigate the circularly polarized light excited current and reflective magnetic circular dichroism (RMCD) under various magnetic fields in both monolayer and multilayer CrI3 devices. The devices exhibit clear helicity-selective photoresponse behavior determined by the magnetic state of CrI3. We also find abnormal negative photocurrents at higher bias in both monolayer and multilayer CrI3. A possible explanation is proposed for this phenomenon. Our work reveals the interplay between magnetic and optoelectronic properties in CrI3 and paves the way to developing spin-optoelectronic devices.

Seeded 2D epitaxy of large-area single-crystal films of the van der Waals semiconductor 2H MoTe2.

The integration of two-dimensional (2D) van der Waals semiconductors into silicon electronics technology will require the production of large-scale, uniform, and highly crystalline films. We report a route for synthesizing wafer-scale single-crystalline 2H molybdenum ditelluride (MoTe2) semiconductors on an amorphous insulating substrate. In-plane 2D-epitaxy growth by tellurizing was triggered from a deliberately implanted single seed crystal. The resulting single-crystalline film completely covered a 2.5-centimeter wafer with excellent uniformity. The 2H MoTe2 2D single-crystalline film can use itself as a template for further rapid epitaxy in a vertical manner. Transistor arrays fabricated with the as-prepared 2H MoTe2 single crystals exhibited high electrical performance, with excellent uniformity and 100% device yield.

Photoluminescent Quantum Interference in a van der Waals Magnet Preserved by Symmetry Breaking.

Quantum interference gives rise to the asymmetric Fano resonance line shape when the final states of an electronic transition follow within a continuum of states and a discrete state, which has significant applications in optical switching and sensing. The resonant optical phenomena associated with the Fano resonance have been observed by absorption spectra, Raman spectra, transmission spectra, etc., but have rarely been reported in photoluminescence (PL) spectroscopy. In this work, we performed spectroscopic studies on layered chromium thiophosphate (CrPS4), a promising ternary antiferromagnetic semiconductor with PL in the near-infrared wavelength region and observed a Fano resonance when CrPS4 experiences phase transition into the antiferromagnetic state below the Néel temperature (38 K). The photoluminescence of the continuum states results from the d band transitions localized at Cr3+ ions, whereas the discrete state is formed by an impurity level, the electronic transition of which is enabled by symmetry breaking. Our findings provide insights into the photon-emitting coherent electronic transitions of CrPS4 and their connection to the magnetism-related broken symmetry.