Hybrid van der Waals p-n Heterojunctions based on SnO and 2D MoS2

Anti-Ambipolar Heterojunctions: Materials, Devices, and Circuits

Anti-ambipolar heterojunctions are vital in constructing high-frequency oscillators, fast switches, and multivalued logic (MVL) devices, which hold promising potential for next-generation integrated circuit chips and telecommunication technologies. Thanks to the strategic material design and device integration, anti-ambipolar heterojunctions have demonstrated unparalleled device and circuit performance that surpasses other semiconducting material systems. This review aims to provide a comprehensive summary of the achievements in the field of anti-ambipolar heterojunctions. First, the fundamental operating mechanisms of anti-ambipolar devices are discussed. After that, potential materials used in anti-ambipolar devices are discussed with particular attention to 2D-based, 1D-based, and organic-based heterojunctions. Next, the primary device applications employing anti-ambipolar heterojunctions, including anti-ambipolar transistors (AATs), photodetectors, frequency doublers, and synaptic devices, are summarized. Furthermore, alongside the advancements in individual devices, the practical integration of these devices at the circuit level, including topics such as MVL circuits, complex logic gates, and spiking neuron circuits, is also discussed. Lastly, the present key challenges and future research directions concerning anti-ambipolar heterojunctions and their applications are also emphasized.

Progress of charge carrier dynamics and regulation strategies in 2D CxNy-based heterojunctions

Two-dimensional carbon nitrides (CxNy) have gained significant attention in various fields including hydrogen energy development, environmental remediation, optoelectronic devices, and energy storage owing to their extensive surface area, abundant raw materials, high chemical stability, and distinctive physical and chemical characteristics. One effective approach to address the challenges of limited visible light utilization and elevated carrier recombination rates is to establish heterojunctions for CxNy-based single materials (e.g. C2N3, g-C3N4, C3N4, C4N3, C2N, and C3N). The carrier generation, migration, and recombination of heterojunctions with different band alignments have been analyzed starting from the application of CxNy with metal oxides, transition metal sulfides (selenides), conductive carbon, and Cx'Ny' heterojunctions. Additionally, we have explored diverse strategies to enhance heterojunction performance from the perspective of carrier dynamics. In conclusion, we present some overarching observations and insights into the challenges and opportunities associated with the development of advanced CxNy-based heterojunctions.

Emerging 2D Metal Oxides: From Synthesis to Device Integration

2D metal oxides have aroused increasing attention in the field of electronics and optoelectronics due to their intriguing physical properties. In this review, an overview of recent advances on synthesis of 2D metal oxides and their electronic applications is presented. First, the tunable physical properties of 2D metal oxides that relate to the structure (various oxidation-state forms, polymorphism, etc.), crystallinity and defects (anisotropy, point defects, and grain boundary), and thickness (quantum confinement effect, interfacial effect, etc.) are discussed. Then, advanced synthesis methods for 2D metal oxides besides mechanical exfoliation are introduced and classified into solution process, vapor-phase deposition, and native oxidation on a metal source. Later, the various roles of 2D metal oxides in widespread applications, i.e., transistors, inverters, photodetectors, piezotronics, memristors, and potential applications (solar cell, spintronics, and superconducting devices) are discussed. Finally, an outlook of existing challenges and future opportunities in 2D metal oxides is proposed.

Mixed-Dimensional Anti-ambipolar Phototransistors Based on 1D GaAsSb/2D MoS2 Heterojunctions

The incapability of modulating the photoresponse of assembled heterostructure devices has remained a challenge for the development of optoelectronics with multifunctionality. Here, a gate-tunable and anti-ambipolar phototransistor is reported based on 1D GaAsSb nanowire/2D MoS₂ nanoflake mixed-dimensional van der Waals heterojunctions. The resulting heterojunction shows apparently asymmetric control over the anti-ambipolar transfer characteristics, possessing potential to implement electronic functions in logic circuits. Meanwhile, such an anti-ambipolar device allows the synchronous adjustment of band slope and depletion regions by gating in both components, thereby giving rise to the gate-tunability of the photoresponse. Coupled with the synergistic effect of the materials in different dimensionality, the hybrid heterojunction can be readily modulated by the external gate to achieve a high-performance photodetector exhibiting a large on/off current ratio of 4 × 10⁴, fast response of 50 μs, and high detectivity of 1.64 × 10¹¹ Jones. Due to the formation of type-II band alignment and strong interfacial coupling, a prominent photovoltaic response is explored in the heterojunction as well. Finally, a visible image sensor based on this hybrid device is demonstrated with good imaging capability, suggesting the promising application prospect in future optoelectronic systems.

Carrier-Transport Mechanism in Organic Antiambipolar Transistors Unveiled by Operando Photoemission Electron Microscopy

Organic antiambipolar transistors (AATs) have partially overlapped p-n junctions. At room temperature, this p-n junction induces a negative differential transconductance in an AAT. However, the detailed carrier-transport mechanism remains unclear. Herein, an operando photoemission electron microscopy is used to tackle this issue owing to the technique's ability to visualize conductive electrons in real time during transistor operation. Notably, it is observed that when the AAT is on, a depletion layer forms at the lateral p-n junction. The visualized depletion layer shows that both p- and n-type channels have pinch-off states in the gate voltage range when the AAT is in on state. The steep potential gradient at the lateral p-n interface enhances the electron conduction from n-type to p-type semiconductor. Another significant finding is that most electrons are considered to recombine with the accumulated holes in the p-type semiconductor, affording the reduction of photoemission intensity by ≈80%. This technique provides a thorough understanding of carrier transport in AATs, further improving the device performance.

Bias-controlled multi-functional transport properties of InSe/BP van der Waals heterostructures

Van der Waals (vdW) heterostructures, consisting of a variety of low-dimensional materials, have great potential use in the design of a wide range of functional devices thanks to their atomically thin body and strong electrostatic tunability. Here, we demonstrate multi-functional indium selenide (InSe)/black phosphorous (BP) heterostructures encapsulated by hexagonal boron nitride. At a positive drain bias (V_D), applied on the BP while the InSe is grounded, our heterostructures show an intermediate gate voltage (V_BG) regime where the current hardly changes, working as a ternary transistor. By contrast, at a negative V_D, the device shows strong negative differential transconductance characteristics; the peak current increases up to ~5 μA and the peak-to-valley current ratio reaches 1600 at V_D = −2 V. Four-terminal measurements were performed on each layer, allowing us to separate the contributions of contact resistances and channel resistance. Moreover, multiple devices with different device structures and contacts were investigated, providing insight into the operation principle and performance optimization. We systematically investigated the influence of contact resistances, heterojunction resistance, channel resistance, and the thickness of BP on the detailed operational characteristics at different V_D and V_BG regimes.

NIR self-powered photodetection and gate tunable rectification behavior in 2D GeSe/MoSe2 heterojunction diode

Two-dimensional (2D) heterostructure with atomically sharp interface holds promise for future electronics and optoelectronics because of their multi-functionalities. Here we demonstrate gate-tunable rectifying behavior and self-powered photovoltaic characteristics of novel p-GeSe/n-MoSe2 van der waal heterojunction (vdW HJ). A substantial increase in rectification behavior was observed when the devices were subjected to gate bias. The highest rectification of ~ 1 × 104 was obtained at Vg = - 40 V. Remarkable rectification behavior of the p-n diode is solely attributed to the sharp interface between metal and GeSe/MoSe2. The device exhibits a high photoresponse towards NIR (850 nm). A high photoresponsivity of 465 mAW-1, an excellent EQE of 670%, a fast rise time of 180 ms, and a decay time of 360 ms were obtained. Furthermore, the diode exhibits detectivity (D) of 7.3 × 109 Jones, the normalized photocurrent to the dark current ratio (NPDR) of 1.9 × 1010 W-1, and the noise equivalent power (NEP) of 1.22 × 10-13 WHz-1/2. The strong light-matter interaction stipulates that the GeSe/MoSe2 diode may open new realms in multi-functional electronics and optoelectronics applications.

Phase Transition-Induced Temperature-Dependent Phonon Shifts in Molybdenum Disulfide Monolayers Interfaced with a Vanadium Dioxide Film

We report the optical phonon shifts induced by phase transition effects of vanadium dioxide (VO₂) in monolayer molybdenum disulfide (MoS₂) when interfacing with a VO₂ film showing a metal-insulator transition coupled with structural phase transition (SPT). To this end, the monolayer MoS₂ directly synthesized on a SiO₂/Si substrate by chemical vapor deposition was first transferred onto a VO₂/c-Al₂O₃ substrate in which the VO₂ film was prepared by a sputtering method. We compared the MoS₂ interfaced with the VO₂ film with the as-synthesized MoS₂ by using Raman spectroscopy. The temperature-dependent Raman scattering characteristics exhibited the distinct phonon behaviors of the E_2g¹ and A_1g modes in the monolayer MoS₂. Specifically, for the as-synthesized MoS₂, there were no Raman shifts for each mode, but the enhancement in the Raman intensities of E_2g¹ and A_1g modes was clearly observed with increasing temperature, which could be interpreted by the significant contribution of the interface optical interference effect. In contrast, the red-shifts of both the E_2g¹ and A_1g modes for the MoS₂ transferred onto VO₂ were clearly observed across the phase transition of VO₂, which could be explained in terms of the in-plane tensile strain effect induced by the SPT and the enhancement of electron-phonon interactions due to an increased electron density at the MoS₂/VO₂ interface through the electronic phase transition. This study provides further insights into the influence of interfacial hybridization for the heterogeneous integration of 2D transition-metal dichalcogenides and strongly correlated materials.

The first-principles study of nH-VSn complex: impurity effects on p-type SnO monolayer

As a rare typical p-channel layered oxide semiconductor, two-dimensional tin monoxide has attracted great attention due to its wide promising applications in nano-electronics. Using the first-principles calculation, we studied the effects of multi-hydrogen-tin/oxygen vacancy complex impurities on the electronic properties of the p-type monolayer SnO. The calculation results indicated that O vacancy (V_O) is a donor and Sn vacancy (V_Sn) acts as a double acceptor. V_Sn should be the source of p-type in undoped SnO in an O-rich environment. When hydrogen is introduced, the more stable nH-V_Sn (n = 1, 2, and 3) complex defects can be formed. These complex impurities can affect the p-type SnO monolayer in the following three main ways: (i) the p-type H-V_Sn compensates the deeper acceptor level of V_Sn and enhances the majority carrier mobility. (ii) The more stable 2H-V_Sn neutralizes the p-type dopant nature of V_Sn and H-V_Sn. (iii) The 3H-V_Sn converts the defect to be an n-type dopant. Our results indicated that limitation of hydrogen is necessary for the preparation of high-quality p-type two-dimensional SnO, as a small amount of hydrogen produces positive effect on p-type SnO; however, the higher concentration of hydrogen is destructive to the p-type character of monolayer SnO.

Emerging Opportunities for Electrostatic Control in Atomically Thin Devices

Electrostatic control of charge carrier concentration underlies the field-effect transistor (FET), which is among the most ubiquitous devices in the modern world. As transistors and related electronic devices have been miniaturized to the nanometer scale, electrostatics have become increasingly important, leading to progressively sophisticated device geometries such as the finFET. With the advent of atomically thin materials in which dielectric screening lengths are greater than device physical dimensions, qualitatively different opportunities emerge for electrostatic control. In this Review, recent demonstrations of unconventional electrostatic modulation in atomically thin materials and devices are discussed. By combining low dielectric screening with the other characteristics of atomically thin materials such as relaxed requirements for lattice matching, quantum confinement of charge carriers, and mechanical flexibility, high degrees of electrostatic spatial inhomogeneity can be achieved, which enables a diverse range of gate-tunable properties that are useful in logic, memory, neuromorphic, and optoelectronic technologies.

Laser-reconfigured MoS2/ZnO van der Waals synapse

Inspired by biological neural systems, neuromorphic devices may lead to new computing paradigms for exploring cognition, learning and limits of parallel computation. Synapses form the basis of neuromorphic computing and have attracted significant research interest in recent years. Herein, a three-terminal transistor based on a transition metal sulfide and zinc oxide heterojunction is proposed for emulating biological synapses. The transistor exhibits an ON/OFF ratio (10⁴) and significant rectifying behavior with forward-to-reverse bias current ratios of 10⁴. The device demonstrates the essential synaptic behaviors, such as excitatory postsynaptic current, modulation of synaptic weight and paired-pulse facilitation. Furthermore, we show that the hysteretic effect of the transfer curves and the post-synapse current triggered by the presynaptic pulses can be modulated by illumination, and the current under illumination conditions is about 10 times greater than that in the dark. These synapses combine photonic with electric neuromorphic functions, thus showing the application prospects of the optoelectronic interfaces for integrated photonic circuits based on mixed-mode electro-optical operation. Hence, this work offers a new landscape for 2D-material electronics and encourages future research on neuro-electronics.

Stannous oxide promoted charge separation in rationally designed heterojunction photocatalysts with a controllable mechanism

Due to the sluggish mobility of holes, the low charge-separation rate remains an intrinsic issue that limits further increase of the photocatalytic conversion efficiency. Herein, we proposed an in situ hydrothermal method to expedite the charge transfer with enhanced photocatalytic H2 evolution rate and photodegradation activities via introducing SnO microplates into TiO2. As compared to bare TiO2, the SnO/TiO2 heterojunction achieves remarkable 470% and 150% higher efficiency for the photocatalytic H2 evolution rate and photodegradation of rhodamine B, respectively. In particular, it is demonstrated that the charge transfer mechanism of SnO/TiO2 can be switched from the Z-scheme to type II by Pt loading, leading to a significant enhancement of photocatalytic performances. Furthermore, the photocatalytic H2 evolution activities of ZnO and C3N4 can also be improved by introducing SnO via simple mechanical mixing. This work provides not only a new versatile stimulant for enhancing photocatalytic activities but also in-depth understanding of the charge transfer mechanism of heterointerfaces of semiconductors.

Understanding of MoS2/GaN Heterojunction Diode and its Photodetection Properties

Fabrication of heterojunction between 2D molybdenum disulfide (MoS₂) and gallium nitride (GaN) and its photodetection properties have been reported in the present work. Surface potential mapping at the MoS₂/GaN heterojunction is done using Kelvin Probe Force Microscopy to measure the conduction band offset. Current-voltage measurements show a diode like behavior of the heterojunction. The origin of diode like behavior is attributed to unique type II band alignment of the heterojunction. The photocurrent, photoresponsivity and detectivity of the heterojunction are found to be dependent on power density of the light. Photoresponse investigations reveal that the heterojunction is highly sensitive to 405 nm laser with very high responsivity up to 10⁵ A/W. The heterojunction also shows very high detectivity of the order of 10¹⁴ Jones. Moreover, the device shows photoresponse in UV region also. These observations suggest that MoS₂/GaN heterojunction can have great potential for photodetection applications.

Multi-Valued Logic Circuits Based on Organic Anti-ambipolar Transistors

Multivalued logic circuits, which can handle more information than conventional binary logic circuits, have attracted much attention as a promising way to improve the data-processing capabilities of integrated circuits. In this study, we developed a ternary inverter based on organic field-effect transistors (OFET) as a potential component of high-performance and flexible integrated circuits. Key elements are anti-ambipolar and n-type OFETs connected in series. First, we demonstrate an organic ternary inverter that exhibits three distinct logic states. Second, the operating voltage was greatly reduced by taking advantage of an Al₂O₃ gate dielectric. Finally, the operating voltage was finely tuned by the designing of the device geometry. These results are achievable owing to the flexible controllability of the device configuration, suggesting that the organic ternary inverter plays an important role with regard to high-performance organic integrated circuits.

Carrier Transport and Photoresponse in GeSe/MoS2 Heterojunction p-n Diodes

Simple stacking of thin van der Waals 2D materials with different physical properties enables one to create heterojunctions (HJs) with novel functionalities and new potential applications. Here, a 2D material p-n HJ of GeSe/MoS₂ is fabricated and its vertical and horizontal carrier transport and photoresponse properties are studied. Substantial rectification with a very high contrast (>10⁴ ) through the potential barrier in the vertical-direction tunneling of HJs is observed. The negative differential transconductance with high peak-to-valley ratio (>10⁵ ) due to the series resistance change of GeSe, MoS₂ , and HJs at different gate voltages is observed. Moreover, strong and broad-band photoresponse via the photoconductive effect are also demonstrated. The explored multifunctional properties of the GeSe/MoS₂ HJs are expected to be important for understanding the carrier transport and photoresponse of 2D-material HJs for achieving their use in various new applications in the electronics and optoelectronics fields.

Oxide Thin-Film Electronics using All-MXene Electrical Contacts

2D MXenes have shown great promise in electrochemical and electromagnetic shielding applications. However, their potential use in electronic devices is significantly less explored. The unique combination of metallic conductivity and hydrophilic surface suggests that MXenes can also be promising in electronics and sensing applications. Here, it is shown that metallic Ti₃ C₂ MXene with work function of 4.60 eV can make good electrical contact with both zinc oxide (ZnO) and tin monoxide (SnO) semiconductors, with negligible band offsets. Consequently, both n-type ZnO and p-type SnO thin-film transistors (TFTs) have been fabricated entirely using large-area MXene (Ti₃ C₂ ) electrical contacts, including gate, source, and drain. The n- and p-type TFTs show balanced performance, including field-effect mobilities of 2.61 and 2.01 cm² V^-1 s^-1 and switching ratios of 3.6 × 10⁶ and 1.1 × 10³ , respectively. Further, complementary metal oxide semiconductor (CMOS) inverters are demonstrated. The CMOS inverters show large voltage gain of 80 and excellent noise margin of 3.54 V, which is 70.8% of the ideal value. Moreover, the operation of CMOS inverters is shown to be very stable under a 100 Hz square waveform input. The current results suggest that MXene (Ti₃ C₂ ) can play an important role as contact material in nanoelectronics.

Vertical MoSe2-MoO x p-n heterojunction and its application in optoelectronics

The hybrid n-type 2D transition-metal dichalcogenide (TMD)/p-type oxide van der Waals (vdW) heterojunction nanosheets consist of 2D layered MoSe₂ (the n-type 2D material) and MoO _x (the p-type oxide) which are grown on SiO₂/Si substrates for the first time via chemical vapor deposition technique, displaying the regular hexagon structures with the average length dimension of sides of ∼8 μm. Vertical MoSe₂-MoO _x p-n heterojunctions demonstrate obviously current-rectifying characteristic, and it can be tuned via gate voltage. What is more, the photodetector based on vertical MoSe₂-MoO _x heterojunctions displays optimal photoresponse behavior, generating the responsivity, detectivity, and external quantum efficiency to 3.4 A W^-1, 0.85 × 10⁸ Jones, and 1665.6%, respectively, at V _ds = 5 V with the light wavelength of 254 nm under 0.29 mW cm^-2. These results furnish a building block on investigating the flexible and transparent properties of vdW and further optimizing the structure of the devices for better optoelectronic and electronic performance.

Interface Engineering for Controlling Device Properties of Organic Antiambipolar Transistors

The main purpose of this study is to establish a guideline for controlling the device properties of organic antiambipolar transistors. Our key strategy is to use interface engineering to promote carrier injection at channel/electrode interfaces and carrier accumulation at a channel/dielectric interface. The effective use of carrier injection interlayers and an insulator layer with a high dielectric constant (high-k) enabled the fine tuning of device parameters and, in particular, the onset (V_on) and offset (V_off) voltages. A well-matched combination of the interlayers and a high-k dielectric layer achieved a low peak voltage (0.25 V) and a narrow on-state bias range (2.2 V), indicating that organic antiambipolar transistors have high potential as negative differential resistance devices for multivalued logic circuits.

Stability and electronic properties of hybrid SnO bilayers: SnO/graphene and SnO/BN

Van der Waals structures based on two-dimensional materials have been considered as promising structures for novel nanoscale electronic devices. Two-dimensional SnO films which display intrinsic p-type semiconducting properties were fabricated recently. In this paper, we consider vertically stacked heterostructures consisting of a SnO monolayer with graphene or a BN monolayer to investigate their stability, electronic and transport properties using density functional theory. The calculated results find that the properties of the constituent monolayers are retained in these SnO-based heterostructures, and a p-type Schottky barrier is formed in the SnO/graphene heterostructure. Additionally, the Schottky barrier can be effectively controlled with an external electric field, which is useful characteristic for the van der Waals heterostructure-based electronic devices. In the SnO/BN heterostructure, the electronic properties of SnO are least affected by the insulating monolayer suggesting that the BN monolayer would be an ideal substrate for SnO-based nanoscale devices.

Interfacing 2D Semiconductors with Functional Oxides: Fundamentals, Properties, and Applications

Two-dimensional semiconductors, such as transition-metal dichalcogenides (TMDs) and black phosphorous (BP), have found various potential applications in electronic and opto-electronic devices. However, several problems including low carrier mobility and low photoluminescence efficiencies still limit the performance of these devices. Interfacing 2D semiconductors with functional oxides provides a way to address the problems by overcoming the intrinsic limitations of 2D semiconductors and offering them multiple functionalities with various mechanisms. In this review, we first focus on the physical effects of various types of functional oxides on 2D semiconductors, mostly on MoS2 and BP as they are the intensively studied 2D semiconductors. Insulating, semiconducting, conventional piezoelectric, strongly correlated, and magnetic oxides are discussed. Then we introduce the applications of these 2D semiconductors/functional oxides systems in field-effect devices, nonvolatile memory, and photosensing. Finally, we discuss the perspectives and challenges within this research field. Our review provides a comprehensive understanding of 2D semiconductors/functional oxide heterostructures, and could inspire novel ideas in interface engineering to improve the performance of 2D semiconductor devices.

Diverse Functionalities of Vertically Stacked Graphene/Single layer n-MoS2/SiO2/p-GaN Heterostructures

Integrating different dimentional materials on vertically stacked p-n hetero-junctions have facinated a considerable scrunity and can open up excellent feasibility with various functionalities in opto-electronic devices. Here, we demonstrate that vertically stacked p-GaN/SiO₂/n-MoS₂/Graphene heterostructures enable to exhibit prominent dual opto-electronic characteristics, including efficient photo-detection and light emission, which represents the emergence of a new class of devices. The photoresponsivity was found to achieve as high as ~10.4 AW⁻¹ and the detectivity and external quantum efficiency were estimated to be 1.1 × 10¹⁰ Jones and ~30%, respectively. These values are superier than most reported hererojunction devices. In addition, this device exhibits as a self-powered photodetector, showing a high responsivity and fast response speed. Moreover, the device demonstrates the light emission with low turn-on voltage (~1.0 V) which can be realized by electron injection from graphene electrode and holes from GaN film into monolayer MoS₂ layer. These results indicate that with a suitable choice of band alignment, the vertical stacking of materials with different dimentionalities could be significant potential for integration of highly efficient heterostructures and open up feasible pathways towards integrated nanoscale multi-functional optoelectronic devices for a variety of applications.