A team at Pennsylvania State University led by Jake A. DeChiara and Jon-Paul Maria reports in the Journal of Materials Science a surprisingly elegant solution to one of the field’s most stubborn bottlenecks: the sputtering target. Physical vapor deposition techniques such as sputtering and pulsed laser deposition need dense, chemically stable, mechanically robust source targets to deliver uniform films with stable deposition rates and minimal particulate generation. For calcium oxide, the conventional options are unattractive. High-purity CaO ceramics are difficult to procure, expensive even at 99.9 percent purity, prone to slow conversion back to hydroxide, and hard to sinter without additives. Calcium metal targets, though commercially available, drift in composition as they take up oxygen even under oxygen-lean sputter conditions, producing inconsistent plasma energetics and deposition instability, and they often carry residual manufacturing impurities that are difficult to remove.

The Penn State researchers’ insight was to sidestep calcium oxide entirely and work with its hydrated cousin, calcium hydroxide. Ca(OH)2 is already produced at extreme chemical purity through precipitation methods because it serves as a precursor for deep-ultraviolet CaF2 optical components, making it an ideal calcium source for quantum-grade materials that demand stringent defect control. During sputter deposition, any residual hydroxide simply dissociates in the plasma; since calcium hydroxide is thermodynamically unstable above about 525 degrees Celsius even at 100 Torr of water vapor pressure, the residual water is pumped away from the vacuum chamber rather than incorporated into the growing film. The catch was that nobody had ever sintered dense polycrystalline calcium hydroxide by conventional means, because solid-state sintering typically requires heating a material to roughly two-thirds of its melting temperature, and calcium hydroxide decomposes into water vapor and CaO long before that point.

The solution was hydrothermally assisted sintering, better known as cold sintering. In this technique, a small amount of a transport phase, here ultrapure deionized water, partially dissolves the ceramic particles under modest temperature and uniaxial pressure, unlocking densification mechanisms that include pressure-induced dissolution and precipitation, solution creep, surface diffusion, and plastic deformation. Because the metal die forms a semi-open system, the water can partially escape during densification. The only additive is water, and the sintering temperatures stay below 300 degrees Celsius, so the precursor’s exceptional purity is preserved in the final ceramic. In the experiments, 99.999 percent pure calcium hydroxide powder was mixed with 5 or 10 weight percent water, dispersed mechanically, loaded into a stainless steel die, and pressed at roughly 393 megapascals while heated to temperatures between 100 and 300 degrees Celsius for one hour.

The results were striking. Cold-sintered ceramics consistently reached higher densities than dry hot-pressed counterparts across the entire temperature range, and increasing the water content from 5 to 10 weight percent pushed the maximum relative density to 98 percent at intermediate temperatures, far exceeding the 81 percent achieved in earlier hot-pressing studies. At 300 degrees Celsius, densities dropped, consistent with accelerated evaporation of the free water flux. X-ray diffraction confirmed that the trigonal brucite phase was preserved at all sintering temperatures, with only small fractions of calcium carbonate inherited from unavoidable atmospheric exposure of the precursor powder, and thermogravimetric analysis with mass spectrometry showed that less than 1 weight percent of mass was lost between 80 and 400 degrees Celsius, indicating the finished ceramics are dry, mechanically robust, and vacuum compatible.

Scanning electron microscopy of fracture cross sections revealed how the densification actually happens. Dry compaction alone produced a green body of roughly 80 percent density, hot pressing at 150 degrees Celsius without water reached about 90 percent, and adding 10 weight percent water at or above 150 degrees Celsius produced ceramics approaching the highest densities. The water-containing samples pressed at 250 degrees Celsius showed extensive lamellar features that the team associates with slip traces, evidence of plastic deformation. This makes physical sense: the hexagonal brucite structure possesses a (0001) slip system thanks to comparatively weak hydrogen bonding between its primarily ionic layers, so under heat, water, and pressure the grains can literally slide past one another and pack more tightly. Deformation twinning and grain coarsening through coalescence were also observed, pointing to a combination of plastic deformation and pressure solution creep.

That deformation leaves a fingerprint in the crystallography as well. Pole-figure measurements and Lotgering factor analysis showed a strong preferred orientation of the (001) planes out of the pressing direction, with Lotgering factors of 0.84 for the cold-sintered ceramic versus 0.81 for the hot-pressed one, while grains remained randomly oriented around the c-axis. The deformed grains were especially pronounced near the die wall where pressure gradients and shear stresses are greatest. Together, these observations demonstrate that cold sintering activates densification mechanisms that conventional hot pressing cannot, driving the ceramic to final-stage sintering while the brucite chemistry remains intact.

The real payoff came when the process was scaled up. Using 12 grams of powder with 10 weight percent water in a 2-inch die wrapped in a heating jacket, the team produced full-sized sputtering targets exceeding 90 percent relative density. From these targets they reactively RF-sputtered calcium oxide thin films onto r-plane sapphire substrates held at 875 degrees Celsius in a mixture of argon and oxygen at 10 millitorr. The films adopted a single (002) out-of-plane orientation, and phi scans at the CaO (113) and sapphire (122) in-plane peaks confirmed true epitaxy. The deposition rate was a stable and linear 1.2 nanometers per minute, verified by X-ray reflectivity and cross-sectional electron microscopy. Because energetic bombardment during growth proved substantial, the researchers employed an off-axis cathode geometry to protect the growing film.

Film quality metrics were impressive for a first demonstration. Rocking curves around the CaO (002) reflection yielded full widths at half maximum of 0.3 degrees for 100-nanometer films and 0.2 degrees for 300-nanometer films, indicating improving crystallinity with thickness. Atomic force microscopy measured root-mean-square surface roughness of about 0.11 nanometers for the 100-nanometer films and 0.47 nanometers for the 300-nanometer films, and X-ray photoelectron spectroscopy showed carbon confined to the outermost surface region with the calcium-to-oxygen ratio approaching stoichiometric CaO beneath. Survey scans detected no additional elements after consolidation, indicating that the cold-sintering process introduced no detectable contamination. A protective amorphous boron nitride capping layer deposited at 500 degrees Celsius shields the films from atmospheric degradation.

For the quantum materials community, the significance extends well beyond calcium chemistry. Cold sintering offers a general laboratory-scale route to consolidate sputter targets at temperatures low enough to preserve the original powder chemistry, sidestepping the contaminant diffusion that plagues high-temperature sintering of other systems. If CaO lives up to the theoretical predictions and hosts clock-like spin defects with long coherence times, the path from commercially available ultra-pure powder to epitaxial quantum-grade film now runs through a press, a heating jacket, and a splash of water, a recipe far simpler than anything the field has managed before. The work was supported by the Air Force Office of Scientific Research, and with the process already demonstrated at the 2-inch scale, the barrier between a promising theoretical prediction and a working quantum device has just gotten measurably lower.

Subject of Research: Cold sintering of calcium hydroxide sputtering targets for quantum-grade calcium oxide thin films

Article Title: Hydrothermally assisted sintering of calcium hydroxide sputtering targets: a route to quantum-grade CaO thin films

Article References: DeChiara, J. A., Carvalho, T. C., Marin, A. H., Almishal, S. S. I., & Maria, J.-P. (2026). Hydrothermally assisted sintering of calcium hydroxide sputtering targets: a route to quantum-grade CaO thin films. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13788-4

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13788-4

Keywords: cold sintering, calcium hydroxide, calcium oxide, sputtering targets, thin films, quantum materials, qubits, epitaxy, ceramics, hydrothermal sintering, wide-bandgap insulators, defect qubits

The trick behind this new class of electronics is called sliding ferroelectricity, a phenomenon that emerged from the study of atomically thin layered crystals. In conventional ferroelectrics, such as the oxides used in some sensors and emerging memories, an electric field shifts charged atoms within the crystal lattice to flip the direction of an internal polarization. In sliding ferroelectrics, the polarization is instead controlled by nudging entire atomic layers sideways relative to one another. Because the layers are held together only by weak van der Waals forces, they can slide with remarkably little resistance, which means the switching can be extremely fast and should, in principle, endure billions of cycles without the lattice damage that gradually kills conventional ferroelectric devices.

Rhombohedral boron nitride is particularly attractive for this purpose. In its common hexagonal form, hBN, the atomic layers stack in an alternating pattern that cancels out any net polarization. In the rhombohedral form, rBN, the layers stack in a repeating ABC sequence, so that the small electric dipoles formed by each boron-nitrogen pair add up in the same direction, producing a vertical polarization that can be reversed by sliding the layers. The problem has been making rBN at all. The rhombohedral phase is thermodynamically less stable than the hexagonal phase, and earlier efforts produced only small crystals or films whose stacking order drifted between regions, which is fatal for devices that depend on a uniform polarization direction.

The research team, led by Kaihui Liu of Peking University along with colleagues at several Chinese institutions, solved this problem with what they call a step-templated interfacial epitaxy strategy. They began with sapphire wafers cut along the m-plane orientation, then annealed them so that the surface reorganized into a staircase of atomic-scale steps. Onto these stepped templates they sputtered films of a nickel-boron alloy. The surface steps of the alloy act as a template: as boron nitride crystallizes out of the film, the steps force the growing layers into a single, unidirectional ABC stacking order rather than the thermodynamically favored alternating arrangement. Control experiments showed the importance of this geometry, because films grown on step-free c-plane and a-plane sapphire templates yielded predominantly the ordinary hexagonal phase, while growth at elevated temperatures also pushed the material toward the wrong polymorph.

Comprehensive structural characterization confirmed the success of the approach. Electron backscatter diffraction and low-energy electron diffraction patterns collected at randomly selected positions across the wafers showed large-scale single crystallinity, and cross-sectional electron microscopy revealed the perfect ABC stacking order maintained throughout the film thickness. Second-harmonic generation, an optical technique sensitive to the symmetry of non-centrosymmetric crystals, produced signals roughly two thousand times stronger than those of a single layer, and intensity mappings across the wafers were uniform, indicating a consistent crystal configuration from edge to edge. Crucially for any future manufacturing effort, ten different batches of wafers grown under identical conditions showed congruent quality across spectroscopic and microscopic measurements, addressing the batch-to-batch variability that has plagued attempts to industrialize two-dimensional materials.

With the wafers in hand, the researchers built ferroelectric field-effect transistors, or FeFETs, in which the sliding polarization of the rBN layer gates a semiconductor channel and encodes memory as a shift in the transistor’s switching voltage. Measurements on rBN devices with graphene channels verified the intrinsic ferroelectricity, with polarization-electric field loops showing a remnant polarization of about 1.3 microcoulombs per square centimeter and clear switching current peaks, while identical devices built with ordinary hBN showed no such hysteresis, ruling out artifacts from the device architecture itself. Piezoresponse force microscopy confirmed that reversing the applied bias reversed the local polarization, and cross-sectional electron microscopy of oppositely poled regions captured the structural signature of the layer sliding.

The performance figures are striking. The devices switched at nanosecond speeds, survived more than two billion read-write cycles without degradation, and retained their state for timescales consistent with ten-year non-volatility. Even when the transistor channels were shrunk to an ultrashort length of thirty nanometers, the devices maintained large memory windows of four volts, a parameter that determines how reliably stored bits can be distinguished, and remained stable at temperatures above 470 kelvin. The films themselves proved to be excellent insulators, with a breakdown field of roughly ten megavolts per centimeter and minuscule off-state currents, properties that matter when billions of such devices must sit side by side without leaking charge into their neighbors.

Scaling from single devices to arrays is where most exotic memory concepts stumble, but the wafer format allowed the team to take that step. They constructed FeFET arrays across the rBN wafers using molybdenum disulfide channels, achieving high integration density with on/off current ratios of around one million, large enough for robust readout in dense circuits. The thickness of the rBN layer provided an additional design knob: thicker layers produced larger memory windows, with devices using 10.6-nanometer films showing windows of 1.7 volts and those using 40.3-nanometer films reaching 4.4 volts. That tunability, combined with the thermal robustness, suggests the material could serve not only as standalone non-volatile memory but also as the basis for artificial synapses in in-memory computing architectures, where the same devices that store weights also perform the multiply-and-accumulate operations that dominate neural network workloads.

What makes the result resonate beyond the laboratory is the manufacturing story. Two-dimensional materials have dazzled physicists for two decades, yet almost none have made the leap from millimeter-scale flakes to the four-inch, six-inch, and larger wafers that fabs require. By demonstrating batch production with consistent quality, the researchers have converted sliding ferroelectricity from a curiosity assembled under tweezers into a platform compatible with wafer-scale processing. The step-templated epitaxy itself is a generalizable idea, hinting that surface geometry can be used to stabilize other metastable stacking orders in layered materials, potentially unlocking ferroelectric versions of semiconductors and metals as well. Challenges certainly remain, including integration with existing silicon processes, contact engineering, and scaling the growth to larger substrates, but the combination of nanosecond switching, billion-cycle endurance, decade-long retention, and high-temperature stability in a wafer-scale format marks a genuine milestone. If the momentum continues, the humble boron nitride layer that once served only as a passive substrate may find itself at the active heart of the memory chips that power the next generation of artificial intelligence.

Subject of Research: Wafer-scale synthesis of single-crystal rhombohedral boron nitride for sliding ferroelectric memory devices

Article Title: Single-crystal rhombohedral boron nitride wafers for integrated sliding ferroelectric memory

Article References: Qi, J., Gu, T., Tu, J., Yang, Q., Guo, Q., Guo, L., Wu, P., Chen, L., Zhang, M., Zhao, C., Tian, E., Yang, Q., Xie, K., Yang, F., Song, B., Zhang, L., Zheng, X., Lu, X., Wang, E., … Liu, K. (2026). Single-crystal rhombohedral boron nitride wafers for integrated sliding ferroelectric memory. Nature Nanotechnology. https://doi.org/10.1038/s41565-026-02280-4

Image Credits: AI Generated

DOI: 10.1038/s41565-026-02280-4

Keywords: rhombohedral boron nitride, sliding ferroelectricity, two-dimensional materials, ferroelectric memory, FeFET, epitaxy, non-volatile memory, nanosecond switching, wafer-scale synthesis, in-memory computing, sapphire template, Nature Nanotechnology