Sang Woon Lee

Alternative interconnect materials with short electron mean free paths can mitigate the resistivity size effect beyond Cu. Single-element, binary, ternary, and topological candidates are discussed with key integration challenges.

Light-driven quantum tunneling offers an attractive platform for coherent electron control at ultrafast time scales; however, sustainable operation remains challenging due to the high electric fields required for quantum tunneling, often leading to thermal damage caused by Joule heating. Here, we experimentally investigate how quantum barrier engineering influences the nonlinear terahertz (THz) quantum tunneling response across tailored nanometer-scale junctions. By integrating lower-band-gap oxide materials such as titanium dioxide (TiO₂) into metal-insulator-metal junctions using atomic layer lithography, we substantially reduce the onset field of Fowler-Nordheim tunneling, 13 kV cm^-1, which is 4 times lower than that in existing higher-band-gap aluminum oxide junctions. At an incident field amplitude of 34 kV cm^-1, we achieved the enhanced field of 14 MV cm^-1 (1.4 V nm^-1) at the TiO₂ nanogap, allowing up to 60% modulation of THz transmission. Beyond the expected barrier-height dependence, we identify the thermal conductivity of the tunnel barrier as a critical factor for reproducibility. TiO₂ combines a lower barrier with higher thermal conductivity, enabling stable operation over 1000 reversible cycles and revealing thermal transport in THz tunneling stability. We offer a comprehensive theoretical analysis that confirms the experimental findings, emphasizing the importance of both barrier material selection and thermal management. We expect our findings to advance the development of robust, low-field THz quantum plasmonic devices for ultrafast, energy-efficient optoelectronic applications.

Copper (Cu) is currently the dominant interconnect material in back-end-of-line processes due to its low bulk resistivity (ρ₀). Unfortunately, its resistivity increases significantly at small dimensions due to a long electron mean free path (EMFP, λ) of 39 nm, which leads to enhanced electron scattering at grain boundaries and surfaces, thereby limiting high-density integration. In this study, we demonstrate that single-phase hexagonal-close-packed (HCP) nickel-cobalt (NiCo) alloy thin films exhibit a significantly reduced resistivity size effect, outperforming Cu interconnect material for thicknesses below 8 nm (19.83 μΩ·cm at 4.9 nm). First-principles calculations predicted a lower ρ₀·λ of 5.68 × 10^-16 Ω m² for HCP NiCo (xx) than to Cu (6.73 × 10^-16 Ω m²), due to a significant contribution from the short EMFP of approximately 5 nm in the HCP NiCo alloy. To date, the growth of single-phase HCP NiCo film had not been achieved, as both HCP and face-centered-cubic (FCC) phases coexist at Co concentration of 50-80 atom %. However, single-phase HCP NiCo films were successfully grown by using an HCP Co seed layer, which reduced the lattice mismatch with the sapphire (0001) substrate. Moreover, NiCo thin films can be successfully dry etched, providing a significant advantage in device fabrication, since it does not require a damascene process unlike Cu. The NiCo thin films also exhibited high thermal stability above 500 °C. Therefore, HCP NiCo is considered a promising alternative to overcome the scaling limitations of Cu interconnects.

2D electron gas field‐effect transistors (2DEG‐FETs), employing 2DEG formed at an interface of ultrathin (≈6 nm) Al ₂ O ₃ /ZnO heterostructure as the active channel, exhibit outstanding drive current (≈215 µA), subthreshold swing (≈132 mV dec ⁻¹ ), and field effect mobility (≈49.6 cm ² V ⁻¹ s ⁻¹ ) with a high on/off current ratio of ≈10 ⁷ . It is demonstrated that the Al ₂ O ₃ upper layer in Al ₂ O ₃ /ZnO heterostructure acts as the source/drain resistance component during transistor operations, and the applied potential to the 2DEG channel is successfully modulated by Al ₂ O ₃ thickness variations so that the threshold voltage (V _th ) is effectively tuned. Remarkably, double‐stacked 2DEG‐FETs consisting of two Al ₂ O ₃ /ZnO heterostructured 2DEG channels with a single gate exhibit multiple V _th , enabling a ternary logic state in a single device. By inducing a voltage difference between the stacked channels, a sequential operation of the upper and lower FETs is achieved, successfully realizing a stable ternary logic operation.

The two-dimensional electron gas (2DEG) is a group of electrons that can move freely in horizontal dimensions but are confined in the third direction. It has been reported that atomic layer deposition (ALD) of Al2O3 on various reducible n-type oxides can lead to the formation of 2DEG at the heterojunction interfaces, among which ZnO is known to provide promising properties. In this study, we have performed a theoretical analysis using density functional theory calculations combined with experimental investigations to elucidate the surface reactions of Al2O3 ALD on low-index nonpolar ZnO surfaces, specifically focusing on the formation of oxygen vacancies (VO). The trimethylaluminum precursor was observed to undergo sequential dissociation of CH3 ligands, leading to the removal of surface oxygen of ZnO in the form of dimethyl ether. In addition, by examining the electronic structure after the removal of oxygen, the localization of the charge density at the surface was confirmed. Experimentally, the carrier density of the 2DEG at the Al2O3/ZnO interface showed a strong dependence on the ALD process temperature of Al2O3, confirming the endothermic nature of the formation of the 2DEG. By examining the characteristics of the 2DEG induced by VO, insights into the fundamental comprehension of oxide-based 2DEG systems are provided.

We demonstrated an ultra‐sensitive terahertz virus detection method combined with virus‐sized gold nanogaps filled with Al ₂ O ₃ . Large‐area high‐density 20 nm‐gap rectangular loop structures, containing a resonant frequency in the terahertz range, were fabricated on a 4‐inch wafer using atomic layer lithography. When target viruses with a 60 nm diameter were located on the nanogaps, we observed a significant redshift of the resonant peak already with an average number of about 100 viruses per unit loop due to the strong field confinement and enhancement near the gap. Furthermore, when the virus was tightly attached to an etched gap like a bridge connecting metals, its sensitivity is doubled compared to the unetched gap, which resulted in 400% more resonance frequency shift per single virus particle than our previous work. Full‐wave simulations and theoretical calculations based on modal expansions were in good agreement with the experiments, revealing that the resonant transmission spectrum was mostly determined by the change in refractive index in a two‐dimensional‐like optical hotspot near the nanogap. A further step could be taken to increase sensitivity by tuning nanogap‐loops to the absorption frequencies associated with the intermolecular vibrational modes of the viruses and fingerprinting them as well.

Dynamic random access memories (DRAMs) are currently used as the core memory in computing systems because of their high speed and density. Their demand should continue to grow owing to increased data usage. A unit cell in a DRAM consists of one transistor and one capacitor, and the data are stored in the capacitor. As the size of the unit cell decreases to improve the memory density by aggressive scaling, it is important to secure sufficient capacitance in the capacitor. In this regard, technological advances in the fabrication of capacitors are of great importance; accordingly, the materials and processing of high‐k thin films require developmental innovations. Besides, it is necessary to develop electrode materials that optimize the function of high‐k thin films. In this review, recent advances in achieving sufficient capacitance in DRAM capacitors are summarized from structural and material/process perspectives, and the future direction of DRAM capacitor development is discussed. Atomic layer deposition (ALD) is a key technique that enables the growth of functional thin films for DRAM capacitors; thus, recent advances in the deposition of high‐k and electrode thin films grown using the ALD technique are addressed.

Metal thin films have been widely used as conductors in semiconductor devices for several decades. However, the resistivity of metal thin films such as Cu and TiN increases substantially (>1000%) as they become thinner (2AlC films grown on sapphire substrates exhibited a significantly weaker dependence on the film thickness than conventional metal films that resulted in a resistivity increase of only 30%, as the V₂AlC film thickness decreased from approximately 45 to 5 nm. The resistivity was almost identical for film thicknesses of 10–50 nm. The small change in the resistivity of V₂AlC films with decreasing film thickness originated from the highly ordered crystalline quality and a small electron mean free path (11–13.6 nm). Thus, MAX-phase thin films have great potential for advanced metal technology applications to overcome the current scaling limitations of semiconductor devices.