arXiv subjects

Han Uk Lee

Publications and source records attributed to Han Uk Lee.

3 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↗

arXiv2026

Oxygen stoichiometry directs rutile-anatase phase selection through kinetic control of nucleation

Synthesis of a target polymorph remains more empirical than predictive because crystallization often selects the most accessible nucleation pathway rather than the thermodynamically most stable phase. Here, we show that oxygen stoichiometry converts this empirical synthesis variable into a kinetic control parameter for anatase-rutile selection in TiO$_{2-x}$. Enhanced-sampling simulations reveal that oxygen content alters the nucleation-barrier landscape, switching the relative accessibility of anatase and rutile, even while rutile remains thermodynamically favored. Molecular dynamics simulations show the presence of a diffuse intermediate shell around the nucleus, where oxygen deficiency alters Ti-O coordination and connectivity and drives shell-local motif evolution from anatase-like toward rutile-like environments. A coupled-flux model that integrates barrier competition with shell-mediated attachment/exchange yields a relative nucleation-rate map consistent with reported oxygen-dependent synthesis trends. These results establish stoichiometry-controlled intermediate-shell motif evolution as a kinetic origin of polymorph selection and provide a framework for predicting target phases in composition-coupled crystallization.

cond-mat.mtrl-sci↗

arXiv2026

Precursor-Dependent Energetics as a Predictive Principle for Polymorph Selection in Thin Films

Vapor deposition allows for the synthesis of metastable polymorphs with unique properties, yet polymorph selection remains largely empirical due to the lack of predictive guidelines bridging thermodynamics, kinetics, and synthesis conditions. Here, we show that precursor chemistry can control metastable polymorph selection by modulating the reaction driving force governing nucleation. By integrating first-principles reaction energetics and substrate interactions into classical nucleation theory, we establish a quantitative framework that connects precursor-dependent reaction energetics to polymorph accessibility during vapor deposition. Using Ga2O3 as a model system, we demonstrate that highly reactive precursors with large reaction driving forces kinetically stabilize the metastable α phase, whereas low-driving-force precursors permit thermodynamic relaxation to the stable \b{eta} phase. Furthermore, precursor flow rates amplify supersaturation, expanding the kinetic window for stabilizing the elusive \k{appa} phase. The predictive capability of this approach is further validated in the TiO2 system, where precursor-dependent reaction energetics correctly capture the competitive nucleation between rutile and anatase. These results establish precursor chemistry as a tunable chemical lever for controlling nucleation kinetics and provide a predictive design principle for metastable polymorph synthesis in vapor deposition.

cond-mat.mtrl-sci↗