New‑Generation Ferroelectric AlScN Materials

Ferroelectrics have great potential in the field of nonvolatile memory due to programmable polarization states by external electric field in nonvolatile manner.However,complementary metal oxide semiconductor compatibility and uniformity of ferroelectric performance after size scaling have always been two thorny issues hindering practical application of ferroelectric memory devices.The emerging ferroelectricity of wurtzite structure nitride offers opportunities to circumvent the dilemma.This review covers the mechanism of ferroelectricity and domain dynamics in ferroelectric AlScN films.The performance optimization of AlScN films grown by different techniques is summarized and their applications for memories and emerging in-memory computing are illustrated.Finally,the challenges and perspectives regarding the commercial avenue of ferroelectric AlScN are discussed.

First-principles study on the alkali chalcogenide secondary compounds in Cu(In,Ga)Se_2 and Cu_2ZnSn(S,Se)_4 thin film solar cells

The beneficial effect of the alkali metals such as Na and K on the Cu（In.Ga）Se2 （CIGS） and Cu2ZnSn（S,Se）4 （CZTSSe） solar cells has been extensively investigated in the past two decades, however, in most of the studies the alkali metals were treated as dopants. Several recent studies have showed that the alkali metals may not only act as dopants but also form secondary phases in the absorber layer or on the surfaces of the films. Using the first-principles calculations, we screened out the most probable secondary phases of Na and K in CIGS and CZTSSe, and studied their electronic structures and optical properties. We found that all these alkali chalcogenide compounds have larger band gaps and lower VBM levels than CIGS and CZTSSe, because the existence of strong p-d coupling in CIS and CZTS pushes the valence band maximum （VBM） level up and reduces the band-gaps, while there is no such p-d coupling in these alkali chalcogenides. This band alignment repels the photo-generated holes from the secondary phases and prevents the electron-hole recombination. Moreover, the study on the optical properties of the secondary phases showed that the absorption coefficients of these alkali chalcogenides are much lower than those of CIGS and CZTSSe in the energy range of 0-3.4eV, which means that the alkali chalcogenides may not influence the absorption of solar light. Since the alkali metal dopants can passivate the grain boundaries and increase the hole carrier concentration, and meanwhile their related secondary phases have innocuous effect on the optical absorption and band alignment, we can understand why the alkali metal dopants can improve the CIGS and CZTSSe solar cell performance.

Ferroelectric materials for neuroinspired computing applications

In recent years,the emergence of numerous applications of artificial intelligence(AI)has sparked a new technological revolution.These applications include facial recognition,autonomous driving,intelligent robotics,and image restoration.However,the data processing and storage procedures in the conventional von Neumann architecture are discrete,which leads to the“memory wall”problem.As a result,such architecture is incompatible with AI requirements for efficient and sustainable processing.Exploring new computing architectures and material bases is therefore imperative.Inspired by neurobiological systems,in-memory and in-sensor computing techniques provide a new means of overcoming the limitations inherent in the von Neumann architecture.The basis of neural morphological computation is a crossbar array of high-density,high-efficiency non-volatile memory devices.Among the numerous candidate memory devices,ferroelectric memory devices with non-volatile polarization states,low power consumption and strong endurance are expected to be ideal candidates for neuromorphic computing.Further research on the complementary metal-oxide-semiconductor(CMOS)compatibility for these devices is underway and has yielded favorable results.Herein,we first introduce the development of ferroelectric materials as well as their mechanisms of polarization reversal and detail the applications of ferroelectric synaptic devices in artificial neural networks.Subsequently,we introduce the latest developments in ferroelectrics-based in-memory and in-sensor computing.Finally,we review recent works on hafnium-based ferroelectric memory devices with CMOS process compatibility and give a perspective for future developments.

Retinomorphic hardware for in-sensor computing

Rapid developments in the Internet of Things and Artificial Intelligence trigger higher requirements for image perception and learning of external environments through visual systems.However,limited by von Neumann's bottleneck,the physical separation of sense,memory,and processing units in a conventional personal computer-based vision system tend to consume a significant amount of energy,time latency,and additional hardware costs.By integrating computational tasks of multiple functionalities into the sensors themselves,the emerging bio-inspired neuromorphic visual systems provide an opportunity to overcome these limitations.With high speed,ultralow power and strong adaptability,it is highly desirable to develop a neuromorphic vision system that is based on highly precise in-sensor computing devices,namely retinomorphic devices.We here present a timely review of retinomorphic devices for visual in-sensor computing.We begin with several types of physical mechanisms of photoelectric sensors that can be constructed for artificial vision.The potential applications of retinomorphic hardware are,thereafter,thoroughly summarized.We also highlight the possible strategies to existing challenges and give a brief perspective of retinomorphic architecture for in-sensor computing.

Research progress on 2D ferroelectric and ferrovalley materials and their neuromorphic application

Two-dimensional(2D)ferroelectric and ferrovalley materials have recently received extensive attention due to their significant advantages for modern electronic devices,such as miniaturization,low-dissipation,non-volatility,and multi-functionality.More interestingly,the couplings between the ferroic orders in these materials have enriched the development of intelligent devices,especially in neuromorphic computing.In this paper,the research progress of 2D ferroelectric and ferrovalley materials is introduced and the coupling effects between them are also described.Then,we briefly introduce recent neuromorphic computing reports based on 2D ferroelectric materials and give perspectives on ferrovalley neuromorphic devices.

In-situ artificial retina with all-in-one reconfigurable photomemristor networks

Despite that in-sensor processing has been proposed to remove the latency and energy consumption during the inevitable data transfer between spatial-separated sensors,memories and processors in traditional computer vision,its hardware implementation for artificial neural networks(ANNs)with all-in-one device arrays remains a challenge,especially for organic-based ANNs.With the advantages of biocompatibility,low cost,easy fabrication and flexibility,here we implement a self-powered in-sensor ANN using molecular ferroelectric(MF)-based photomemristor arrays.Tunable ferroelectric depolarization was intentionally introduced into the ANN,which enables reconfigurable conductance and photoresponse.Treating photoresponsivity as synaptic weight,the MFbased in-sensor ANN can operate analog convolutional computation,and successfully conduct perception and recognition of white-light letter images in experiments,with low processing energy consumption.Handwritten Chinese digits are also recognized and regressed by a large-scale array,demonstrating its scalability and potential for low-power processing and the applications in MF-based in-situ artificial retina.

Molecular ferroelectric/semiconductor interfacial memristors for artificial synapses

With the burgeoning developments in artificial intelligence,hardware implementation of artificial neural network is also gaining pace.In this pursuit,ferroelectric devices(i.e.,tunneling junctions and transistors)with voltage thresholds were recently proposed as suitable candidates.However,their development is hindered by the inherent integration issues of inorganic ferroelectrics,as well as poor properties of conventional organic ferroelectrics.In contrast to the conventional ferroelectric synapses,here we demonstrated a two-terminal ferroelectric synaptic device using a molecular ferroelectric(MF)/semiconductor interface.The interfacial resistance can be tuned via the polarization-controlled blocking effect of the semiconductor,owing to the high ferroelectricity and field amplification effect of the MF.Typical synaptic features including spike timing-dependent plasticity are substantiated.The introduction of the semiconductor also enables the attributes of optoelectronic synapse and in-sensor computing with high image recognition accuracies.Such interfaces may pave the way for the hardware implementation of multifunctional neuromorphic devices.

Nonreciprocal directional dichroism in multiferroics

Nonreciprocal directional dichroism in multiferroics,namely magnetoelectric coupling in the dynamic regime,is endowed with rich physics and promising applications,which are entangled with fundamental physical components,such as spin,orbital,lattice,charge,and topology.Such a linear nonreciprocal response behavior in the GHz-THz frequency range,represented by optical magnetoelectric effect and magnetochiral dichroism,occurs ubiquitously in material systems with the spontaneous breaking of space-time symmetry,and is subject to Onsager’s reciprocal theorem in the thermodynamic limit.Microscopically,these nonreciprocal responses are usually encoded by toroidization(chirality)and electromagnon(quasiparticle),thus establishing a comprehensive understanding of magnetoelectric coupling and irreversible dynamics.Herein,the basic mechanisms and emergent nonreciprocal directional dichroism in single-phase multiferroics are summarized.We expect that the present review will stimulate diverse possibilities toward nonreciprocal directional dichroism within and beyond multiferroics.

Stimulated emission at 1.54 μm from erbium/oxygen-doped silicon-based light-emitting diodes

Silicon-based light sources, including light-emitting diodes(LEDs) and laser diodes(LDs) for information transmission, are urgently needed for developing monolithic integrated silicon photonics. Silicon with erbium ions(Er^(3+)) doped by ion implantation is considered a promising approach, but it suffers from an extremely low quantum efficiency. Here we report an electrically pumped superlinear emission at 1.54 μm from Er/O-doped silicon planar LEDs, which are produced by applying a new deep cooling process. Stimulated emission at room temperature is realized with a low threshold current of ~6 mA(~0.8 A∕cm^(2)). Time-resolved photoluminescence and photocurrent results have revealed the complex carrier transfer dynamics by relaxing electrons from the Si conduction band to the Er^(3+) ion. This picture differs from the frequently assumed energy transfer via electron–hole pair recombination of the silicon host. Moreover, the amplified emission from the LEDs is likely due to a quasi-continuous Er/O-related donor band created by the deep cooling technique. This work paves the way for fabricating superluminescent diodes or efficient LEDs at communication wavelengths based on rare-earth-doped silicon.

Topology paving the way to controllable readout of ferroelectricity dself-assembled topologically confined domain walls become the key to ferroelectric memory

Ferroelectric materials are long believed to be useful in informationtechnology. As early as 1952, the concept of ferroelectricrandom-access memory (FeRAM) had already been proposed byMr. Dudley Allen Buck, who was by then only a graduate student[1]. However, after more than sixty years, the application of FeRAMis still limited. One major obstacle hindering the development ofFeRAM is the readout of ferroelectric polarization, which is generallydestructive, i.e. it requires a write-after-read architecture, justlike standard dynamic random-access memory (DRAM) capacitorcells.