Mengkun Tian

Thermal atomic layer deposition (ALD) of BaTiO3 (BTO) is of interest for next-generation microelectronic devices, including RAM and NAND memory. Herein, we investigate the influence of deposition temperature on the microstructure and electronic properties of 50 nm BTO thin films fabricated by ALD at 200, 220, 240, and 280 °C using bis-(1,2,4 triisopropylcyclopentadienyl)-Ba [Ba(iPr3Cp2)], titanium isopropoxide, and water as precursors, and highlight the significance of void formation in films based on this chemistry. Self-limiting ALD behavior is observed, such that the deposition rate and Ba:Ti ratio are independent of prolonged Ba dose times. XRD shows that films deposited at 220 and 240 °C are significantly more crystalline than films deposited at 280 °C after annealing. Scanning electron microscopy reveals a difference in film microstructure depending on deposition temperature, indicative of different “structural zones”. Cross-sectional transmission electron microscopy reveals that annealed, crystallized BTO films are susceptible to void formation, especially when lower ALD deposition temperatures are used. This void formation appears to result from a combination of the microstructure of the as-deposited film and the easily hydrated barium constituent and is identified as an important structural defect that will need to be avoided to achieve desired dielectric performance. Electrical properties, such as capacitance–voltage (CV), dielectric loss, and DC current–voltage (IV), are closely correlated with microstructure. Fifty-nanometer BTO thin films deposited at 220, 240, and 280 °C have relative permittivity values (εr) of 73, 60, and 33, respectively, at 10 kHz after annealing. Dielectric loss (tanδ) is as low as 0.001. A modest dispersion in capacitance is observed across varying sampling frequencies. Future research to further understand and mitigate void formation while retaining proper stoichiometry for this ALD chemistry is recommended to further improve dielectric performance.

Rapid differential scanning calorimetry (DSC) measurements were performed on Ti, TiN, TiAl, and AlN multilayers that were magnetron sputtered at ambient temperature and had a similar overall composition to MAX-phase Ti2AlN. The DSC yielded thermal scans with exotherms indicating diffusion and crystallization processes during the annealing process, including the formation of Ti2AlN. Multilayer Ti/AlN yielded a distinctly large exotherm in the range of 500–700 °C, in contrast to the other films, and was estimated to have the highest cumulative enthalpy during annealing up to 700 °C. X-ray diffraction measurements and annealing treatments also indicated that all TiAlN films yielded Ti2AlN by 600 °C, with STEM imaging revealing the onset of Ti2AlN-like structural ordering in Ti/AlN by 475 °C. These data collectively show that multilayer films with Ti/AlN chemistry are distinctly reactive, experiencing phase transformation events that facilitate MAX-phase Ti2AlN synthesis at low substrate temperatures.

Space weathering of lunar minerals, due to bombardment from solar wind (SW) particles and micrometeoroid impacts, modifies the mineralogy within tens of nanometers of the surface, i.e., the rim. Spectroscopic signatures of these modifications, observed via remote sensing, have long been used to gauge surface exposure times on the Moon. However, the relative contributions of SW and micrometeoroids in the creation of rim features are still debated, particularly for the nanometer-scale clusters known as nanophase iron (npFe ⁰ ), which commonly form in ferrous minerals. We address this issue in the laboratory, using deuterium ions and low-energy electrons as a synthetic solar wind plasma to irradiate ilmenite (FeTiO ₃ ), a common lunar mineral. Characterization by high-resolution scanning transmission electron microscopy and electron energy-loss spectroscopy shows that the SW alone creates rims with all the main characteristics of lunar samples. We conclusively identify npFe ⁰ and quantify its distribution as a function of depth and fluence, allowing us to estimate the SW exposure of Apollo soil 71501. Our results confirm that small npFe ⁰ particles (<10 nm in diameter) form from SW irradiation. Such experiments provide microscopic details of space weathering, improving the link between surface modification processes and macroscopic remote-sensing data.

Lunar mare basalts are often rich in titanium, hosted predominantly within the mineral ilmenite (Fe²⁺Ti⁴⁺O₃). Here, we examine ilmenite in a ~ 3.8 billion-year-old mare basalt (Apollo rock 75035) using high-resolution electron microscopy and electron energy loss spectroscopy. A key finding is that 75035 ilmenite is itself enriched in Ti, beyond the end member of the conventional solid solution series. Using energy loss near-edge spectroscopy, we determine that the excess Ti is trivalent, with Ti³⁺ accounting for 13% of the total Ti content. This discovery confirms the presence of trivalent Ti in lunar ilmenite, long hypothesized based on the Moon’s reducing environment. Accounting for the change in implied stoichiometry, a review of literature data suggests that Ti³⁺ may be present in ilmenite across a wide range of lunar samples. We extrapolate known relationships from the literature to connect Ti³⁺ to redox conditions, estimating the oxygen fugacity during crystallization of 75035 to be below the iron-wüstite buffer, ΔIW≤ − 1.6. Further quantifying the relationship between Ti valence state and oxygen fugacity would allow Ti³⁺-bearing ilmenite to serve as an oxybarometer able to access the reducing conditions found on many planetary bodies.

NAND flash forms the core of modern solid-state storage, which is critical for data-intensive AI applications, yet charge-trap NAND suffers rapid threshold-voltage (V_th) degradation under ionizing radiation, causing reliability challenges for space and defense applications. Here we show that ferroelectric field-effect transistors (FeFETs) with laminated gate stacks offer a promising route to achieving radiation resilience in vertical NAND technology. We demonstrate that large-memory-window, vertical NAND-compatible laminated poly-silicon-channel FeFETs with an 8 nm Hf_0.5Zr_0.5O₂/3 nm Al₂O₃/8 nm Hf_0.5Zr_0.5O₂ stack retain a full memory window and robust switching up to 10 Mrad(air) of the total ionizing dose (TID). Programmed and erased states show negligible TID-induced drift after 1 Mrad(air), while only the erased state degrades by ∼2 V at 10 Mrad(air). Technology computer-aided design (TCAD) modeling attributes these asymmetric shifts to state-dependent traps. Compared to charge-trap NAND, laminated FeFETs exhibit ∼30-fold lower V_th degradation per unit dose, positioning them as superior radiation-resilient storage candidates.

Breaking the memory wall in advanced computing architectures will require complex 3D integration of emerging memory materials such as ferroelectricseither within the back-end-of-line (BEOL) of CMOS front-end processes or through advanced 3D packaging technologies. Achieving this integration demands that memory materials exhibit high thermal resilience, with the capability to operate reliably at elevated temperatures, such as 125°C, due to the substantial heat generated by front-end transistors. However, silicon-compatible HfO₂-based ferroelectrics tend to exhibit antiferroelectric-like behavior in this temperature range, accompanied by a more pronounced wake-up effect, posing significant challenges to their thermal reliability. Here, we report that by introducing a thin tungsten oxide (WO_3–x ) layerknown as an oxygen reservoirand carefully tuning its oxygen content, ultrathin Hf_0.5Zr_0.5O₂ (5 nm) films can be made robust against the ferroelectric-to-antiferroelectric transition at elevated temperatures. This approach not only minimizes polarization loss in the pristine state but also effectively suppresses the wake-up effect, reducing the required wake-up cycles from 10⁵ to only 10 at 125°C, a qualifying temperature for back-end memory integrated with front-end logic, as defined by the JEDEC standard. First-principles density functional theory (DFT) calculations reveal that WO₃ enhances the stability of the ferroelectric orthorhombic phase (o-phase) at elevated temperatures by increasing the tetragonal-to-orthorhombic phase energy gap and promoting favorable phonon mode evolution, thereby supporting o-phase formation under both thermodynamic and kinetic constraints.

Advanced logic transistors require gate dielectrics that achieve subnanometer equivalent oxide thickness (EOT), suppress leakage, and satisfy three key requirements: (i) compatibility with RMG-like high-temperature processing, (ii) sufficient V_th tunability for multi-V_th design, and (iii) high device reliability. However, meeting all of these requirements at once has been difficult with conventional high-κ systems. In this work, we demonstrate that our Hf/Zr-based gate stacks quantitatively satisfy these conditions. (i) After a 700 °C N₂ anneal, the HZH superlattice achieves EOT = 7.3 Å, lower than conventional HfO₂-only stacks (8.5 Å) while maintaining comparable leakage. (ii) Embedding a 3 Å Al₂O₃ dipole within the HfO₂/ZrO₂/HfO₂ superlattice (HZHA) breaks the conventional dipole trade-off, achieving an 8.4 Å EOT─lower than the 9.0 Å of a standard HfO₂/Al₂O₃ stack─while providing a > 200 mV V_FB shift, thereby enabling multi-V_th tuning without compromising scaling. (iii) Furthermore, under -2 V negative-bias temperature stress at 125 °C for 100 s, HZHA and HA exhibit comparable V_FB drifts of 87 mV and 97 mV, respectively, confirming that strong V_th tunability and subnanometer EOT can be achieved without compromising stability. In addition to these quantitative advances, this study reveals previously unreported physical insights into the dipole behavior and interfacial diffusion in ultrathin Hf/Zr multilayers. These results establish HZHA as an RMG-compatible, V_th-tunable, low-EOT dielectric platform capable of supporting logic scaling beyond the 1 nm frontier.

Surface reconstruction by formation of inert phases in Ni‐rich cathodes is widely viewed as a degradation mechanism for batteries. Herein, this seemingly undesirable phase is leveraged to stabilize Ni‐rich cathodes. Density functional theory reveals a reduction in Ni 3d–O 2p hybridization in NiO compared to LiNiO ₂ (LNO), suggesting its potential as a protective layer. Guided by theory, variable temperature X‐ray diffraction is used to identify optimal conditions for introducing oxygen vacancies on the surface of LiNi _0.8 Mn _0.1 Co _0.1 O ₂ (NMC811) particles, which triggers a phase transformation from layered to rock‐salt NiO on the surface, creating a core–shell structure as evidenced by X‐ray photoelectron spectroscopy and scanning transmission electron microscopy (STEM). Electrochemical methods such as constant‐current long‐term cycling, cyclic voltammetry, and electrochemical impedance spectroscopy reveal improved capacity, higher Li ⁺ diffusivity, and lower resistance during cycling. X‐ray absorption spectroscopy confirms that the bulk‐averaged oxidation state remains unchanged after modification, and STEM imaging confirm reduced structural heterogeneity. By reframing surface NiO as a controllable design principle, a materials‐intrinsic, scalable route to extend the durability of Ni‐rich cathodes is offered.