Geoffrey Pourtois

Ru is a promising candidate for future post-Cu interconnect metallization because of its short electron mean free path and barrier-free reliability. Here, we demonstrate epitaxial Ru thin films on CMOS-compatible AlN (0001)/Si (111) grown by physical vapor deposition. X-ray diffraction, Rutherford backscattering ion channeling, and transmission electron microscopy confirm the single-crystal nature of the Ru films with a honeycomb epitaxial relationship. The films exhibit low resistivity of 7.8 μΩ cm at 30 nm thickness and a high residual resistivity ratio of 11, approaching bulk Ru values and outperforming polycrystalline Ru and Cu in the ultrathin film regime. Additional epitaxial growth on 300 mm wafers establishes epitaxial Ru on AlN (0001)/Si (111) as a scalable, CMOS-compatible platform for low-resistivity interconnect metallization.

We use first-principles electronic-structure and lattice-dynamics calculations to identify dynamically stable configurations of silicon-compatible superlattices with Si/O or Si/Ge monolayers and Si/2Ge bilayers that provide a materials-level handle for engineering in-plane transport anisotropy in silicon channels. Across controlled periodicities, we quantify pronounced, tunable in-plane anisotropy in transport-relevant band curvature: in Si/O stacks, the electron effective mass reaches ≈0.26m* along [1̄10] while reaching 1.6 m* along [110], whereas Si/Ge preserves in-plane symmetry and Si/2Ge favors transport along the Y axis. Bandgaps are concurrently adjustable by interface chemistry and period, with tunability ranging from ∼50 meV (Si/Ge) to ∼430 meV (Si/2Ge), enabling threshold-voltage control alongside directional transport design. Acoustic deformation potentials increase with interface density (≈11 to 24 eV), indicating stronger electron–phonon coupling and a practical trade-off between anisotropy and mobility. Interface-specific vibrational features, including high-frequency oxygen modes (20–26 THz) and tunable phonon gaps in Ge-containing stacks, further differentiate thermal transport. Together, these results provide practical guidelines based on periodicity and interface chemistry for engineering leakage-suppressing, directionally selective silicon channel materials that are directly relevant to advanced nanosheet MOSFET technologies.

Topological insulators (TIs) are materials with an insulating bulk and topologically protected conductive surface states. Many potential applications can be found in the literature for this class of materials. However, the small bandgap and the poor stability of the TIs extensively studied in the literature, such as Bi2Se3 and Bi2Te3, restrict their potential. One of the proposed alternatives is barium bismuthate (BBO), an oxide perovskite that is predicted to be a TI with a relatively large bandgap of about 0.7 eV. Despite the potential of this material, accessing its TI phase requires increasing the Fermi energy by ∼2 eV, and, to our knowledge, this has not yet been achieved experimentally. One proposed way to shift the Fermi energy is to substitute a part of the oxygen atoms in its structure with fluorine. In this study, we investigated the low-temperature fluorination of MBE-grown BBO thin films using SF6 plasma as a fluorine source. We began our study by using XRD and AFM to identify the processing conditions that allowed us to preserve the quality of the film. Then, we confirmed the incorporation of fluorine into the exposed thin film by XPS and SIMS. Since oxygen deficient films are expected to be easier to alloy, we also compared these results with the fluorination of oxygen-deficient BBO3−x films. Finally, we used DFT to demonstrate that the fluorination of BBO3 is an energetically favorable process, supporting our observations.

Extreme ultraviolet (EUV) lithography has revolutionized the high-volume manufacturing of nanoscale components. The use of EUV light leads to ionization-driven chemistry in the imaging materials of lithography, the photoresists. The complex interplay of ionization, generation of primary/secondary electrons, and the subsequent chemical mechanisms that lead to image formation in photoresists has been notoriously difficult to study. This is in particular true for the radiochemical transformations occurring during exposure. In this work, we deploy table-top EUV photoemission spectroscopy to observe in situ chemical changes occurring during exposure in a model chemically amplified photoresist and discover a surprising chemical reaction pathway, the EUV-induced breakdown of a perfluoroalkyl substance (PFAS) photoacid generator (PAG). This previously unobserved breakdown of the PFAS PAG, a critical component in the EUV exposure mechanism, manifests as changes in the intensity of the valence band peaks of the EUV photoemission spectrum, which are linked to degradation of the PFAS PAG via an advanced atomistic simulation framework. Our combined experimental and theoretical approach shows that EUV photoemission can simultaneously resolve chemical dynamics and the production of primary and secondary electrons, giving unique insights into the radiochemical transformation of photoresist materials. More generally, our approach also shows that EUV photoemission spectroscopy can provide a unique platform for tracking degradation pathways of PFAS molecules in thin films, owing to the high ionization cross section of fluorine at EUV wavelengths. Our results pave the way for utilizing accessible, table-top EUV spectroscopy systems for observing EUV photoresist chemical dynamics, with the potential for time-resolved measurements of photoemission processes in the future.

Transition metal dichalcogenides (TMDCs) are promising candidates for future nano-transistor channels due to their outstanding intrinsic transport properties. However, their electron mobility is highly sensitive to the surrounding dielectric, often falling well below theoretical expectations. In this work, we explore how a stacked Al₂O₃ dielectric affects electron mobility in monolayer WS₂ using first-principles quantum transport simulations. We identify that fluctuations in the electrostatic potential, arising from the disordered structure of Al₂O₃, significantly degrade mobility, especially when WS₂ interfaces with under-coordinated aluminum atoms. Our calculated mobilities (≃1–30 cm²/(V ⋅ s)) align with experimental observations and remain far from the ideal limit (≃300 cm²/(V ⋅ s)). We further demonstrate that encapsulating WS₂ with hexagonal boron nitride (hBN) or employing a crystalline oxide can recover high mobility values. However, these strategies introduce trade-offs in electrostatic control and fabrication complexity, underlining the need for careful dielectric engineering in TMDC-based devices.