arXiv subjects

Jouko Lahtinen

Publications and source records attributed to Jouko Lahtinen.

4 recordsLinked to original sources

arXiv2026

Topological-Insulator Heterophase Gate Stacks for Transistor Electrostatics

Conventional gate-stack scaling reduces dielectric thickness and increases permittivity while largely treating the position and electronic character of the gate-side screening boundary as fixed. As equivalent oxide thickness is reduced, however, finite interfacial responses can increasingly constrain gate control [1-5]. Here we show that this screening boundary can itself be engineered by converting the surface of the topological insulator Bi2Se3 into insulating high-kappa$ BiF3. Position-resolved calculations reveal a gap-opened immediate amorphous-BiF3/crystalline-Bi2Se3 interface and a reconstructed gap-closed Bi2Se3-derived state in the adjacent subinterface layer, accompanied by a localized interfacial dipole. Independently, capacitor measurements resolve a finite series response consistent with the electronic compressibility of this buried boundary, which reduces rather than enhances the nominal stack capacitance. Despite this capacitance penalty, MoS2 transistors with closely matched BiF3 thicknesses and a common BiF3/MoS2 channel-side material interface exhibit near-thermionic switching, negligible hysteresis, and approximately sevenfold lower drain-induced barrier lowering than BiF3-only controls. These results identify the position and electronic character of the gate-side screening boundary as additional design variables for transistor electrostatics beyond nominal dielectric capacitance.

physics.app-ph↗

arXiv2020

Electronic and magnetic characterization of epitaxial CrBr$_3$ monolayers

The ability to imprint a given material property to another through proximity effect in layered two-dimensional materials has opened the way to the creation of designer materials. Here, we use molecular-beam epitaxy (MBE) for a direct synthesis of a superconductor-magnet hybrid heterostructure by combining superconducting niobium diselenide (NbSe$_2$) with the monolayer ferromagnetic chromium tribromide (CrBr$_3$). Using different characterization techniques and density-functional theory (DFT) calculations, we have confirmed that the CrBr$_3$ monolayer retains its ferromagnetic ordering with a magnetocrystalline anisotropy favoring an out-of-plane spin orientation. Low-temperature scanning tunneling microscopy (STM) measurements show a slight reduction of the superconducting gap of NbSe$_2$ and the formation of a vortex lattice on the CrBr$_3$ layer in experiments under an external magnetic field. Our results contribute to the broader framework of exploiting proximity effects to realize novel phenomena in 2D heterostructures.

cond-mat.mtrl-sci↗

arXiv2013

The Structure and Local Variations of the Graphene Moiré on Ir(111)

We have studied the incommensurate moiré structure of epitaxial graphene grown on iridium(111) by dynamic low energy electron diffraction [LEED-I(V)] and non-contact atomic force microscopy (AFM) with a CO terminated tip. Our LEED-I(V) results yield the average positions of all the atoms in the surface unit cell and are in qualitative agreement with the structure obtained from density functional theory (DFT). The AFM experiments reveal local variations of the moiré structure: the corrugation varies smoothly over several moiré unit cells between 42 and 56 pm. We attribute these variations to the varying registry between the moiré symmetry sites and the underlying substrate. We also observe isolated outliers, where the moiré top sites can be offset by an additional 10 pm. This study demonstrates that AFM imaging can be used to directly yield the local surface topography with pm accuracy even on incommensurate 2D structures with varying chemical reactivity.

cond-mat.mtrl-sci↗

arXiv2011

Topographic and electronic contrast of the graphene moiré on Ir(111) probed by scanning tunneling microscopy and non-contact atomic force microscopy

Epitaxial graphene grown on transition metal surfaces typically exhibits a moiré pattern due to the lattice mismatch between graphene and the underlying metal surface. We use both scanning tunneling microscopy (STM) and atomic force microscopy (AFM) experiments to probe the electronic and topographic contrast of the graphene moiré on the Ir(111) surface. While STM topography is influenced by the local density of states close to the Fermi energy and the local tunneling barrier height, AFM is capable of yielding the 'true' surface topography once the background force arising from the van der Waals (vdW) interaction between the tip and the substrate is taken into account. We observe a moiré corrugation of 35$\pm$10 pm, where the graphene-Ir(111) distance is the smallest in the areas where the graphene honeycomb is atop the underlying iridium atoms and larger on the fcc or hcp threefold hollow sites.

cond-mat.mtrl-sci↗