Multistate, Ultrathin, Back-End-of-Line-Compatible AlScN Ferroelectric Diodes.

The growth in data generation necessitates efficient data processing technologies to address the von Neumann bottleneck in conventional computer architecture. Memory-driven computing, which integrates nonvolatile memory (NVM) devices in a 3D stack, is gaining attention, with CMOS back-end-of-line (BEOL)-compatible ferroelectric (FE) diodes being ideal due to their two-terminal design and inherently selector-free nature, facilitating high-density crossbar arrays. Here, we demonstrate BEOL-compatible, high-performance FE diodes scaled to 5, 10, and 20 nm FE Al0.72Sc0.28N/Al0.64Sc0.36N films. Through interlayer (IL) engineering, we show substantial improvements in the on/off ratios (>166 times) and rectification ratios (>176 times) in these scaled devices. These characteristics also enable 5-bit multistate operation with a stable retention. We also experimentally and theoretically demonstrate the counterintuitive result that the inclusion of an IL can lead to a decrease in the ferroelectric switching voltage of the device. An in-depth analysis into the device transport mechanisms is performed, and our compact model aligns seamlessly with the experimental results. Our results suggest the possibility of using scaled AlxSc1-xN FE diodes for high-performance, low-power, embedded NVM.

Nonvolatile Control of Valley Polarized Emission in 2D WSe2-AlScN Heterostructures.

Achieving robust and electrically controlled valley polarization in monolayer transition metal dichalcogenides (ML-TMDs) is a frontier challenge for realistic valleytronic applications. Theoretical investigations show that the integration of 2D materials with ferroelectrics is a promising strategy; however, an experimental demonstration has remained elusive. Here, we fabricate ferroelectric field-effect transistors using a ML-WSe2 channel and an Al0.68Sc0.32N (AlScN) ferroelectric dielectric and experimentally demonstrate efficient tuning as well as non-volatile control of valley polarization. We measure a large array of transistors and obtain a maximum valley polarization of ∼27% at 80 K with stable retention up to 5400 s. The enhancement in the valley polarization is ascribed to the efficient exciton-to-trion (X-T) conversion and its coupling with an out-of-plane electric field, viz., the quantum-confined Stark effect. This changes the valley depolarization pathway from strong exchange interactions to slow spin-flip intervalley scattering. Our research demonstrates a promising approach for achieving non-volatile control over valley polarization for practical valleytronic device applications.

Comparative Effectiveness of Personal Sound Amplification Products Versus Hearing Aids for Unilateral Hearing Loss: A Prospective Randomized Crossover Trial.

BACKGROUND: This study compared hearing outcomes with use of personal sound amplification products (PSAPs) and hearing aids (HAs) in patients with moderate to moderately severe unilateral hearing loss. METHODS: Thirty-nine participants were prospectively enrolled, and randomly assigned to use either one HA (basic or premium type) or one PSAP (basic or high-end type) for the first 8 weeks and then the other device for the following 8 weeks. Participants underwent a battery of examinations at three visits, including sound-field audiometry, word recognition score (WRS), speech perception in quiet and in noise, real-ear measurement, and self-report questionnaires. RESULTS: Functional gain was significantly higher with HAs across all frequencies (P < 0.001). While both PSAPs and HAs improved WRS from the unaided condition, HAs were superior to PSAPs. The speech recognition threshold in quiet conditions and signal-to-noise ratio in noisy conditions were significantly lower in the HA-aided condition than in the PSAP-aided condition, and in the PSAP-aided condition than in the unaided condition. Subjective satisfaction also favored HAs than PSAPs in questionnaires, Abbreviated Profile of Hearing Aid Benefit, International Outcome Inventory for Hearing Aids, and Host Institutional Questionnaire. CONCLUSION: While PSAPs provide some benefit for moderate to moderately severe unilateral hearing loss, HAs are more effective. This underscores the potential role of PSAPs as an accessible, affordable first-line intervention in hearing rehabilitation, particularly for individuals facing challenges in accessing conventional HAs.

Tuning Polarity in WSe2/AlScN FeFETs via Contact Engineering.

Recent advancements in ferroelectric field-effect transistors (FeFETs) using two-dimensional (2D) semiconductor channels and ferroelectric Al0.68Sc0.32N (AlScN) allow high-performance nonvolatile devices with exceptional ON-state currents, large ON/OFF current ratios, and large memory windows (MW). However, previous studies have solely focused on n-type FeFETs, leaving a crucial gap in the development of p-type and ambipolar FeFETs, which are essential for expanding their applicability to a wide range of circuit-level applications. Here, we present a comprehensive demonstration of n-type, p-type, and ambipolar FeFETs on an array scale using AlScN and multilayer/monolayer WSe2. The dominant injected carrier type is modulated through contact engineering at the metal-semiconductor junction, resulting in the realization of all three types of FeFETs. The effect of contact engineering on the carrier injection is further investigated through technology-computer-aided design simulations. Moreover, our 2D WSe2/AlScN FeFETs achieve high electron and hole current densities of ∼20 and ∼10 µA/µm, respectively, with a high ON/OFF ratio surpassing ∼107 and a large MW of >6 V (0.14 V/nm).

Scalable CMOS back-end-of-line-compatible AlScN/two-dimensional channel ferroelectric field-effect transistors.

Three-dimensional monolithic integration of memory devices with logic transistors is a frontier challenge in computer hardware. This integration is essential for augmenting computational power concurrent with enhanced energy efficiency in big data applications such as artificial intelligence. Despite decades of efforts, there remains an urgent need for reliable, compact, fast, energy-efficient and scalable memory devices. Ferroelectric field-effect transistors (FE-FETs) are a promising candidate, but requisite scalability and performance in a back-end-of-line process have proven challenging. Here we present back-end-of-line-compatible FE-FETs using two-dimensional MoS2 channels and AlScN ferroelectric materials, all grown via wafer-scalable processes. A large array of FE-FETs with memory windows larger than 7.8 V, ON/OFF ratios greater than 107 and ON-current density greater than 250 µA um-1, all at ~80 nm channel length are demonstrated. The FE-FETs show stable retention up to 10 years by extension, and endurance greater than 104 cycles in addition to 4-bit pulse-programmable memory features, thereby opening a path towards the three-dimensional heterointegration of a two-dimensional semiconductor memory with silicon complementary metal-oxide-semiconductor logic.

Wurtzite and fluorite ferroelectric materials for electronic memory.

Ferroelectric materials, the charge equivalent of magnets, have been the subject of continued research interest since their discovery more than 100 years ago. The spontaneous electric polarization in these crystals, which is non-volatile and programmable, is appealing for a range of information technologies. However, while magnets have found their way into various types of modern information technology hardware, applications of ferroelectric materials that use their ferroelectric properties are still limited. Recent advances in ferroelectric materials with wurtzite and fluorite structure have renewed enthusiasm and offered new opportunities for their deployment in commercial-scale devices in microelectronics hardware. This Review focuses on the most recent and emerging wurtzite-structured ferroelectric materials and emphasizes their applications in memory and storage-based microelectronic hardware. Relevant comparisons with existing fluorite-structured ferroelectric materials are made and a detailed outlook on ferroelectric materials and devices applications is provided.

Microwave Facilitated Covalent Organic Framework/Transition Metal Dichalcogenide Heterostructures.

Organic/inorganic heterostructures present a versatile platform for creating materials with new functionalities and hybrid properties. In particular, junctions between two dimensional materials have demonstrated utility in next generation electronic, optical, and optoelectronic devices. This work pioneers a microwave facilitated synthesis process to readily incorporate few-layer covalent organic framework (COF) films onto monolayer transition metal dichalcogenides (TMDC). Preferential microwave excitation of the monolayer TMDC flakes result in selective attachment of COFs onto the van der Waals surface with film thicknesses between 1 and 4 nm. The flexible process is extended to multiple TMDCs (MoS2, MoSe2, MoSSe) and several well-known COFs (TAPA-PDA COF, TPT-TFA-COF, and COF-5). Photoluminescence studies reveal a power-dependent defect formation in the TMDC layer, which facilitates electronic coupling between the materials at higher TMDC defect densities. This coupling results in a shift in the A-exciton peak location of MoSe2, with a red or blue shift of 50 or 19 meV, respectively, depending upon the electron donating character of the few-layer COF films. Moreover, optoelectronic devices fabricated from the COF-5/TMDC heterostructure present an opportunity to tune the PL intensity and control the interaction dynamics within inorganic/organic heterostructures.

High-Density, Localized Quantum Emitters in Strained 2D Semiconductors.

Two-dimensional chalcogenide semiconductors have recently emerged as a host material for quantum emitters of single photons. While several reports on defect- and strain-induced single-photon emission from 2D chalcogenides exist, a bottom-up, lithography-free approach to producing a high density of emitters remains elusive. Further, the physical properties of quantum emission in the case of strained 2D semiconductors are far from being understood. Here, we demonstrate a bottom-up, scalable, and lithography-free approach for creating large areas of localized emitters with high density (∼150 emitters/um2) in a WSe2 monolayer. We induce strain inside the WSe2 monolayer with high spatial density by conformally placing the WSe2 monolayer over a uniform array of Pt nanoparticles with a size of 10 nm. Cryogenic, time-resolved, and gate-tunable luminescence measurements combined with near-field luminescence spectroscopy suggest the formation of localized states in strained regions that emit single photons with a high spatial density. Our approach of using a metal nanoparticle array to generate a high density of strained quantum emitters will be applied to scalable, tunable, and versatile quantum light sources.

High-Efficiency WSe2 Photovoltaic Devices with Electron-Selective Contacts.

A rapid surge in global energy consumption has led to a greater demand for renewable energy to overcome energy resource limitations and environmental problems. Recently, a number of van der Waals materials have been highlighted as efficient absorbers for very thin and highly efficient photovoltaic (PV) devices. Despite the predicted potential, achieving power conversion efficiencies (PCEs) above 5% in PV devices based on van der Waals materials has been challenging. Here, we demonstrate a vertical WSe2 PV device with a high PCE of 5.44% under one-sun AM1.5G illumination. We reveal the multifunctional nature of a tungsten oxide layer, which promotes a stronger internal electric field by overcoming limitations imposed by the Fermi-level pinning at WSe2 interfaces and acts as an electron-selective contact in combination with monolayer graphene. Together with the developed bottom contact scheme, this simple yet effective contact engineering method improves the PCE by more than five times.

High-specific-power flexible transition metal dichalcogenide solar cells.

Semiconducting transition metal dichalcogenides (TMDs) are promising for flexible high-specific-power photovoltaics due to their ultrahigh optical absorption coefficients, desirable band gaps and self-passivated surfaces. However, challenges such as Fermi-level pinning at the metal contact-TMD interface and the inapplicability of traditional doping schemes have prevented most TMD solar cells from exceeding 2% power conversion efficiency (PCE). In addition, fabrication on flexible substrates tends to contaminate or damage TMD interfaces, further reducing performance. Here, we address these fundamental issues by employing: (1) transparent graphene contacts to mitigate Fermi-level pinning, (2) MoOx capping for doping, passivation and anti-reflection, and (3) a clean, non-damaging direct transfer method to realize devices on lightweight flexible polyimide substrates. These lead to record PCE of 5.1% and record specific power of 4.4 W g-1 for flexible TMD (WSe2) solar cells, the latter on par with prevailing thin-film solar technologies cadmium telluride, copper indium gallium selenide, amorphous silicon and III-Vs. We further project that TMD solar cells could achieve specific power up to 46 W g-1, creating unprecedented opportunities in a broad range of industries from aerospace to wearable and implantable electronics.

Post-CMOS Compatible Aluminum Scandium Nitride/2D Channel Ferroelectric Field-Effect-Transistor Memory.

Recent advances in oxide ferroelectric (FE) materials have rejuvenated the field of low-power, nonvolatile memories and made FE memories a commercial reality. Despite these advances, progress on commercial FE-RAM based on lead zirconium titanate has stalled due to process challenges. The recent discovery of ferroelectricity in scandium-doped aluminum nitride (AlScN) presents new opportunities for direct memory integration with logic transistors due to the low temperature of AlScN deposition (approximately 350 °C), making it compatible with back end of the line integration on silicon logic. Here, we present a FE-FET device composed of an FE-AlScN dielectric layer integrated with a two-dimensional MoS2 channel. Our devices show an ON/OFF ratio of ∼106, concurrent with a normalized memory window of 0.3 V/nm. The devices also demonstrate stable memory states up to 104 cycles and state retention up to 105 s. Our results suggest that the FE-AlScN/2D combination is ideal for embedded memory and memory-based computing architectures.

Double Negative Differential Resistance Device Based on Hafnium Disulfide/Pentacene Hybrid Structure.

Recently, combinations of 2D van der Waals (2D vdW) materials and organic materials have attracted attention because they facilitate the formation of various heterojunctions with excellent interface quality owing to the absence of dangling bonds on their surface. In this work, a double negative differential resistance (D-NDR) characteristic of a hybrid 2D vdW/organic tunneling device consisting of a hafnium disulfide/pentacene heterojunction and a 3D pentacene resistor is reported. This D-NDR phenomenon is achieved by precisely controlling an NDR peak voltage with the pentacene resistor and then integrating two distinct NDR devices in parallel. Then, the operation of a controllable-gain amplifier configured with the D-NDR device and an n-channel transistor is demonstrated using the Cadence Spectre simulation platform. The proposed D-NDR device technology based on a hybrid 2D vdW/organic heterostructure provides a scientific foundation for various circuit applications that require the NDR phenomenon.

Negative differential transconductance device with a stepped gate dielectric for multi-valued logic circuits.

Multi-valued logic (MVL) technology is a promising approach for improving the data-handling capabilities and decreasing the power consumption of integrated circuits. This is especially attractive as conventional complementary metal-oxide-semiconductor technology is approaching its scaling and power density limits. Here, an ambipolar WSe2 field-effect transistor with two or more negative-differential-transconductance (NDT) regions in its transfer characteristic (NDTFET) is proposed for MVL applications of various radices. The operation and charge carrier transport mechanism of the NDTFET are studied first by Kelvin probe force microscopy, electrical, and capacitance-voltage measurements. Next, strategies for increasing the number of NDT regions and engineering the NDTFET transfer characteristic are discussed. Finally, the extensibility and tunability of our concept are demonstrated by adapting NDTFETs as core devices for ternary, quaternary, and quinary MVL inverters through simulations, where only WSe2 is employed as a channel material for all devices comprising the inverters. The MVL inverter operation principle and the mechanism of the multiple logic state formation are analyzed in detail. The proposed concept is practically verified by the fabrication of a ternary inverter.

A multiple negative differential resistance heterojunction device and its circuit application to ternary static random access memory.

For increasing the restricted bit-density in the conventional binary logic system, extensive research efforts have been directed toward implementing single devices with a two threshold voltage (VTH) characteristic via the single negative differential resistance (NDR) phenomenon. In particular, recent advances in forming van der Waals (vdW) heterostructures with two-dimensional crystals have opened up new possibilities for realizing such NDR-based tunneling devices. However, it has been challenging to exhibit three VTH through the multiple-NDR (m-NDR) phenomenon in a single device even by using vdW heterostructures. Here, we show the m-NDR device formed on a BP/(ReS2 + HfS2) type-III double-heterostructure. This m-NDR device is then integrated with a vdW transistor to demonstrate a ternary vdW latch circuit capable of storing three logic states. Finally, the ternary latch is extended toward ternary SRAM, and its high-speed write and read operations are theoretically verified.

Development and Evaluation of a New Chest Compression Technique for Cardiopulmonary Resuscitation in Infants.

We designed the newly developed flexed two-finger chest compression technique for cardiopulmonary resuscitation (CPR) in infants to increase the quality of chest compression by considering the advantages and disadvantages of the two-thumb encircling hand technique and conventional two-finger technique. The aim of the study is to compare the performance of the flexed two-finger technique and the currently used two-thumb technique or two-finger technique for infant CPR. A total of 42 doctors conducted 2-min single-rescuer CPR on a cardiac arrest infant model using the two-thumb technique followed, in a random order, by the two-finger technique and the flexed two-finger technique. Although the ratio of the adequate compression depth was highest in the two-thumb technique, followed by the flexed two-finger technique and two-finger technique (100% [98-100] vs. 99% [80-100] vs. 76% [42-95], respectively, P < 0.001), the hand-off time of the two-thumb technique was significantly longer than in the two-finger technique and flexed two-finger technique (31 s [28-35] vs. 29 s [27-32] vs. 29 s [26-32], respectively, P < 0.001). The number of total chest compressions of the two-thumb technique was significantly lower than in the two-finger technique and flexed two-finger technique (150 [148-159] vs. 159 [149-173] vs. 162 [150-172], respectively, P < 0.001). The newly developed chest compression technique could provide adequate compression depth without increasing the hand-off time during single-rescuer infant CPR.Trial registration: Clinical Research Information Service, KCT0002730.

Polarity control in a single transition metal dichalcogenide (TMD) transistor for homogeneous complementary logic circuits.

Recently, there have been various attempts to demonstrate the feasibility of transition metal dichalcogenide (TMD) transistors for digital logic circuits. A complementary inverter circuit, which is a basic building block of a logic circuit, was implemented in earlier works by heterogeneously integrating n- and p-channel transistors fabricated on different TMD materials. Subsequently, to simplify the circuit design and fabrication process, complementary inverters were constructed on single-TMD materials using ambipolar transistors. However, continuous transition from the electron-conduction to the hole-conduction state in the ambipolar devices led to the problem of a high leakage current. Here, we report a polarity-controllable TMD transistor that can operate as both an n- and a p-channel transistor with a low leakage current of a few picoamperes. The device polarity can be switched simply by converting the sign of the drain voltage. This is because a metal-like tungsten ditelluride (WTe2) with a low carrier concentration is used as a drain contact, which subsequently allows selective carrier injection at the palladium/tungsten diselenide (WSe2) junction. In addition, by using the operating principle of the polarity-controllable transistor, we demonstrate a complementary inverter circuit on a single TMD channel material (WSe2), which exhibits a very low static power consumption of a few hundred picowatts. Finally, we confirm the expandability of this polarity-controllable transistor toward more complex logic circuits by presenting the proper operation of a three-stage ring oscillator.