Dong-Ho Shin

Multilayer thin film structuring offers a versatile strategy to tailor nanostructures through interface engineering. Atomic layer deposition (ALD) provides precise control of composition and structure via supercycle approaches that enable the multiple metal species introduction. However, the resulting growth behavior is often interpreted without fully accounting for interfacial effects that govern nucleation and nanostructure evolution. In this work, we demonstrate that introducing SnSe2 layers into Sb2Te3/SnSe2 multilayer films using ALD supercycle significantly enhances Sb2Te3 nucleation and accelerates out-of-plane growth, leading to an increase in growth per cycle (GPC) from 0.18 to 0.69 Å/cycle. First-principles atomistic simulations of precursor deposition chemistry and extremely surface-sensitive low-energy ion scattering (LEIS) analysis confirm the promoted growth of Sb2Te3 in Sb2Te3/SnSe2 multilayer structures, which is also accompanied by elemental exchange reactions between Te and Se atoms. The resulting Sb2Te3/SnSe2 multilayer thin films exhibit increased charge carrier and phonon scattering, leading to a significant reduction in lattice thermal conductivity from 1.09 to 0.25 W/mK at room temperature due to the modulated nanostructure. This study establishes a robust analytical framework for understanding and engineering interfacial reactions and nucleation phenomena in ALD-based multilayer systems.

Quantum confinement in artificial superlattices enables the engineering of electronic and optical properties that exceed bulk limitations. Despite the unrivaled precision in thickness control and scalability of atomic layer deposition (ALD), the experimental demonstration of confinement effects in the superlattice geometry has remained elusive, especially for nonoxide systems. In this study, we report the experimental demonstration of distinct quantum confinement in the chalcogenide-based heteromorphic superlattices via an ALD supercycle approach. Polycrystalline PbS and amorphous SnS₂ are alternately deposited with subnanometer thickness control, resulting in the formation of a well-defined heteromorphic superlattice structure with sharp, strain-relieved, and defect-passivated interfaces. Raman spectroscopy also revealed the activation of low-frequency vibrational modes, indicating strong interlayer coupling of strain-free layers within the superlattice. A systematic reduction in the PbS sublayer thickness below its excitonic Bohr radius enables a substantial and controllable widening of the bandgap, from 1.74 eV for PbS (14 nm)/SnS₂ (5 nm) to 2.51 eV for PbS (3 nm)/SnS₂ (5 nm), compared to 1.54 eV for individually grown PbS films. This marked bandgap modulation unambiguously demonstrates the strong quantum confinement of charge carriers within the strain-relaxed PbS layers. Density functional theory (DFT) calculations confirm the experimental observations, revealing the emergence of both lateral and vertical quantum confinement and elucidating the role of the superlattice architecture in shaping the electronic structure. Together, these results establish ALD as an effective platform for quantum superlattice engineering, enabling precise control of confinement effects in complex chalcogenide heterostructures and their integration into next-generation optoelectronics and quantum devices.

Atomic layer deposition of cobalt oxide thin films is crucial for various applications in microelectronics, energy storage, and catalysis. However, challenges such as slow deposition rates and island growth have limited its widespread adoption. This work investigates a novel supercycle approach combining thermal and plasma-enhanced atomic layer deposition to address these issues and optimize cobalt oxide film growth. This study explores the effects of varying thermal-to-plasma ratios on film morphology, composition, and growth characteristics. The results demonstrate that increasing plasma exposure leads to higher growth rates, thicker films, and changes in surface roughness and porosity. X-ray photoemission spectroscopy reveals the presence of multiple cobalt species, including oxides and hydroxides, whose ratios vary with plasma exposure. Surprisingly, carbon content in the films increases with higher plasma amounts despite the use of ozone as an oxidizer. The study also observes a logarithmic decrease in the cobalt-to-oxygen ratio with increasing plasma exposure, attributed to the potential sputtering effects from argon in the plasma mixture. These findings provide unique insights into the complex interplay between plasma parameters and film characteristics in atomic layer deposition processes, offering new avenues for tailoring cobalt oxide thin films for specific applications.