Selectors are required to unlock the potential scalability of memory crossbar arrays and achieve three-dimensional stacking. However, the development of a two-terminal, stackable selector that meets key performance metrics—including large selectivity, high endurance, fast response speed, negligible cycling variation, minimal temperature dependence and good interdevice uniformity—remains a challenge. Here we report tunnel-junction selectors that are based on five vertically stacked van der Waals layers. The selectors are made of graphene/molybdenum disulfide/primary tunnel barrier/molybdenum disulfide/graphene heterostructures. We develop a theoretical model to predict the characteristics of selectors with a graded tunnel barrier, and use it to guide the design of two structures with hexagonal boron nitride and gallium sulfide as the primary tunnel barriers. The device with hexagonal boron nitride exhibits a nonlinearity above 107, an endurance of more than 1012 cycles, a switching speed of less than 20 ns, minimal temperature dependence and low device-to-device variability. The device with gallium sulfide can operate at a voltage of only 2.5 V and exhibit a nonlinearity above 106. We show that the selectors can be integrated into one-selector–one-resistor and one-selector–one-capacitor memory cells for both volatile and non-volatile memory technologies. Using five vertically stacked van der Waals layers, a tunnel-junction selector can be created that exhibits a nonlinearity above 107, an endurance of more than 1012 cycles, a switching speed of less than 20 ns, minimal temperature dependence and low device-to-device variability.
By driving a magnetic film with several microwave tones, a magnonic frequency comb with thousands of spin-wave comb lines can be created, providing a potential route to chip-scale microwave computing.
A layer of gallium oxide that is only a half-unit-cell thick can be converted via strain engineering into a zincblende phase with switchable ferroelectricity and a coercive voltage below 1 V.
Oxide-based ferroelectric materials are expected to have a key role in the future of semiconductor devices. Tim Böscke, Ulrich Böttger and Uwe Schroeder recount how ferroelectricity in hafnium oxide thin films was first discovered.
Atomic-scale ferroelectric films could be used to build high-density, low-power devices. However, ferroelectric materials typically suffer from stability and voltage-scaling challenges at extreme thinness. Here we report ferroelectricity in single-crystalline two-dimensional gallium oxide (Ga2O3), a wide-bandgap semiconductor. The material is just 6 Å thick and offers both high retention and thermal stability. We show that epitaxial β-Ga2O3 can be exfoliated down to a half-unit-cell thickness via a self-limiting exfoliation mechanism, which leads to a biaxial-strain-induced phase transition into a ferroelectric layered structure. Strain modulation reduces the polarization switching voltage to 0.8 V, meeting the voltage-scaling requirements of complementary metal–oxide–semiconductor technology. Theoretical calculations show that switching is driven by covalent bond reconstruction, effectively countering depolarization and enhancing stability. We also integrate ferroelectric two-dimensional Ga2O3 onto silicon using a low-temperature, back-end-of-line-compatible process. When β-Ga2O3 is thinned to the half-unit-cell limit, it can undergo a strain-induced phase transition that leads to intrinsic ferroelectricity with a polarization switching voltage of 0.8 V, meeting the voltage-scaling requirements of complementary metal–oxide–semiconductor technology.
Magnonic frequency combs—the magnetic analogue of optical frequency combs—consist of evenly spaced spin-wave spectral lines and are of potential use in applications such as metrology, spectroscopy and information processing. However, their narrow bandwidths and small number of comb lines limit practical implementation. Here we report an on-chip broadband magnonic frequency comb based on multi-tone excitation. The octave-spanning magnonic frequency comb is generated at low power thresholds in a continuous yttrium iron garnet thin film. It is driven by spin-wave modulation instability stemming from four-magnon scattering, a nonlinear process validated by micromagnetic simulations. Our magnonic frequency combs exhibit long decay lengths, reaching up to hundreds of micrometres, and offer broad tunability in both the number of comb lines and their frequency spacing. We demonstrate, in particular, a comb with over 2,100 comb lines and densities of up to 200 lines per kilohertz. We also create a magnonic ruler that uses this finely structured spectra to detect microwave frequencies with high precision. A multi-tone excitation technique can be used to generate octave-spanning magnonic frequency combs with thousands of lines in continuous yttrium iron garnet thin films.
The emergence of two-dimensional ferroelectrics has challenged established theories of ferroelectricity and offered routes to novel devices. However, distinguishing ferroelectric behaviour from artefacts remains an experimental challenge.
Robots are increasingly required to work closely with different people in diverse tasks. The success of such human–robot collaboration will hinge on shared alignment across capabilities, tasks and timing — and communication will be key to achieving this.
Inorganic colloidal quantum dot light-emitting diodes could be used to build next-generation electroluminescent displays due to their colour properties and electrical stability. However, to create high-resolution and large-area displays, a pixel integration method is required, which can deposit quantum dot arrays on an active-matrix backplane and maintain uniformity and precision, without colour cross-contamination. Here we report a cracking-assisted transfer printing technology that can be used to pattern high-resolution full-colour pixel arrays over large areas. The technology uses a controlled cracking process to fracture interparticle cohesive bonds between quantum dots. This facilitates subsequent pick-up and transfer to a thin-film transistor backplane with high precision. With the technology, we achieve pixels down to a size of 600 nm with electroluminescent emission and uniform pixelization over areas up to 4 inches. We create a cadmium-free full-colour active-matrix display with a resolution of 341 pixels per inch, as well as a blue active-matrix display with a flexible form factor. Furthermore, the cracking-assisted transfer printing can improve electroluminescence performance—with higher maximum luminance and operational lifetime than other quantum dot patterning techniques—through precise nano-interface control and high quantum dot packing density. Quantum dot pixel arrays can be transfer printed over areas up to 4 inches, and with pixel sizes down to 600 nm, using a cracking step to fracture the cohesive bonds between dots before pick-up and transfer.
By breaking the symmetry of a Ruddlesden–Popper ferroelectric material, an out-of-plane dielectric material with high tunability and low loss can be fabricated.
