Giant tunnelling electroresistance in atomic-scale ferroelectric tunnel junctions.

Ferroelectric tunnel junctions are promising towards high-reliability and low-power non-volatile memories and computing devices. Yet it is challenging to maintain a high tunnelling electroresistance when the ferroelectric layer is thinned down towards atomic scale because of the ferroelectric structural instability and large depolarization field. Here we report ferroelectric tunnel junctions based on samarium-substituted layered bismuth oxide, which can maintain tunnelling electroresistance of 7 × 105 with the samarium-substituted bismuth oxide film down to one nanometer, three orders of magnitude higher than previous reports with such thickness, owing to efficient barrier modulation by the large ferroelectric polarization. These ferroelectric tunnel junctions demonstrate up to 32 resistance states without any write-verify technique, high endurance (over 5 × 109), high linearity of conductance modulation, and long retention time (10 years). Furthermore, tunnelling electroresistance over 109 is achieved in ferroelectric tunnel junctions with 4.6-nanometer samarium-substituted bismuth oxide layer, which is higher than commercial flash memories. The results show high potential towards multi-level and reliable non-volatile memories.

Probing Linear to Nonlinear Damping in 2D Semiconductor Nanoelectromechanical Resonators toward a Unified Quality Factor Model.

In resonant nanoelectromechanical systems (NEMS), the quality (Q) factor is essential for sensing, communication, and computing applications. While a large vibrational amplitude is useful for increasing the signal-to-noise ratio, the damping in this regime is more complex because both linear and nonlinear damping are important, and an accurate model for Q has not been fully explored. Here, we demonstrate that by combining the time-domain ringdown and frequency-domain resonance measurements, we extract the accurate Q for two-dimensional (2D) MoS2 and MoTe2 NEMS resonators at different vibration amplitudes. In particular, in the transition region between linear and nonlinear damping, Q can be precisely extracted by fitting to the ringdown characteristics. By varying AC driving, we tune the Q by ΔQ/Q = 269% and extract the nonlinear damping coefficient. We develop the dissipation model that well captures the linear to nonlinear damping, providing important insights for accurately modeling and optimizing Q in 2D NEMS resonators.

Monolithic 3D integration of 2D transistors and vertical RRAMs in 1T-4R structure for high-density memory.

Emerging data-intensive computation has driven the advanced packaging and vertical stacking of integrated circuits, for minimized latency and energy consumption. Yet a monolithic three-dimensional (3D) integrated structure with interleaved logic and high-density memory layers has been difficult to achieve due to challenges in managing the thermal budget. Here we experimentally demonstrate a monolithic 3D integration of atomically-thin molybdenum disulfide (MoS2) transistors and 3D vertical resistive random-access memories (VRRAMs), with the MoS2 transistors stacked between the bottom-plane and top-plane VRRAMs. The whole fabrication process is integration-friendly (below 300 °C), and the measurement results confirm that the top-plane fabrication does not affect the bottom-plane devices. The MoS2 transistor can drive each layer of VRRAM into four resistance states. Circuit-level modeling of the monolithic 3D structure demonstrates smaller area, faster data transfer, and lower energy consumption than a planar memory. Such platform holds a high potential for energy-efficient 3D on-chip memory systems.

Ferroelectricity in layered bismuth oxide down to 1 nanometer.

Atomic-scale ferroelectrics are of great interest for high-density electronics, particularly field-effect transistors, low-power logic, and nonvolatile memories. We devised a film with a layered structure of bismuth oxide that can stabilize the ferroelectric state down to 1 nanometer through samarium bondage. This film can be grown on a variety of substrates with a cost-effective chemical solution deposition. We observed a standard ferroelectric hysteresis loop down to a thickness of ~1 nanometer. The thin films with thicknesses that range from 1 to 4.56 nanometers possess a relatively large remanent polarization from 17 to 50 microcoulombs per square centimeter. We verified the structure with first-principles calculations, which also pointed to the material being a lone pair-driven ferroelectric material. The structure design of the ultrathin ferroelectric films has great potential for the manufacturing of atomic-scale electronic devices.

Strain-Modulated Dissipation in Two-Dimensional Molybdenum Disulfide Nanoelectromechanical Resonators.

Resonant nanoelectromechanical systems (NEMS) based on two-dimensional (2D) materials such as molybdenum disulfide (MoS2) are interesting for highly sensitive mass, force, photon, or inertial transducers, as well as for fundamental research approaching the quantum limit, by leveraging the mechanical degree of freedom in these atomically thin materials. For these mechanical resonators, the quality factor (Q) is essential, yet the mechanism and tuning methods for energy dissipation in 2D NEMS resonators have not been fully explored. Here, we demonstrate that by tuning static strain and vibration-induced strain in suspended MoS2 using gate voltages, we can effectively tune the Q in 2D MoS2 NEMS resonators. We further show that for doubly clamped resonators, the Q increases with larger DC gate voltage, while fully clamped drumhead resonators show the opposite trend. Using DC gate voltages, we can tune the Q by ΔQ/Q = 448% for fully clamped resonators, and by ΔQ/Q = 369% for doubly clamped resonators. We develop the strain-modulated dissipation model for these 2D NEMS resonators, which is verified against our measurement data for 8 fully clamped resonators and 7 doubly clamped resonators. We find that static tensile strain decreases dissipation while vibration-induced strain increases dissipation, and the actual dependence of Q on DC gate voltage depends on the competition between these two effects, which is related to the device boundary condition. Such strain dependence of Q is useful for optimizing the resonance linewidth in 2D NEMS resonators toward low-power, ultrasensitive, and frequency-selective devices for sensing and signal processing.

Analytical Transient Responses and Gain-Bandwidth Products of Low-Dimensional High-Gain Photodetectors.

Low-dimensional photodetectors, in particular those in photoconductive mode, often have extraordinarily high photogain. However, high gain always comes along with a slow frequency response. The gain-bandwidth product (GBP) is a figure of merit to evaluate the performance of a photodetector. Whether the high-gain photoconductors can outperform standard PIN photodiodes in terms of GBP remains an open question. In this article, we derived the analytical transient photoresponses of nanowire photoconductors which were validated with the simulations and experiments. Surprisingly, the fall transients do not follow a simple time-dependent exponential function except for some special cases. Given the analytical photogains that were established previously, we derived the theoretical GBP of high-gain nanowire photoconductors. Analysis of the analytical GBP indicates that nanoscale photoconductors, although having extremely high gain, will never outperform typical PIN photodiodes in terms of GBP.