Jun-Kei Seo

Layered transition metal dichalcogenides (TMDs) draw much attention as the key semiconducting material for two-dimensional electrical, optoelectronic, and spintronic devices. For most of these applications, both n- and p-type materials are needed to form junctions and support bipolar carrier conduction. However, typically only one type of doping is stable for a particular TMD. For example, molybdenum disulfide (MoS2) is natively an n-type presumably due to omnipresent electron-donating sulfur va

Abstract Doping of traditional semiconductors has enabled technological applications in modern electronics by tailoring their chemical, optical and electronic properties. However, substitutional doping in two-dimensional semiconductors is at a comparatively early stage, and the resultant effects are less explored. In this work, we report unusual effects of degenerate doping with Nb on structural, electronic and optical characteristics of MoS 2 crystals. The doping readily induces a structural tr

Simultaneous increases in electrical conductivity (up to 200%) and thermopower (up to 70%) are demonstrated by introducing native defects in Bi2 Te3 films, leading to a high power factor of 3.4 × 10(-3) W m(-1) K(-2). The maximum enhancement of the power factor occurs when the native defects act beneficially both as electron donors and energy filters to mobile electrons. They also act as effective phonon scatterers.

To implement artificial neural networks (ANNs) based on memristor devices, it is essential to secure the linearity and symmetry in weight update characteristics of the memristor, and reliability in the cycle-to-cycle and device-to-device variations. This study experimentally demonstrated and compared the filamentary and interface-type resistive switching (RS) behaviors of tantalum oxide (Ta₂O₅ and TaO₂)-based devices grown by atomic layer deposition (ALD) to prop

Two-dimensional materials and their heterostructures have thus far been identified as leading candidates for nanoelectronics owing to the near-atom thickness, superior electrostatic control, and adjustable device architecture. These characteristics are indeed advantageous for neuro-inspired computing hardware where precise programming is strongly required. However, its successful demonstration fully utilizing all of the given benefits remains to be further developed. Herein, we present van der W

Two-dimensional electron gas (2DEG) coexists with topological states on the surface of topological insulators (TIs), while the origin of the 2DEG remains elusive. In this work, electron density in TI thin films (${\text{Bi}}_{2}$${\text{Se}}_{3}$, ${\text{Bi}}_{2}$${\text{Te}}_{3}$, and their alloys) were manipulated by controlling the density of electronically active native defects with particle irradiation. The measured electron concentration increases with irradiation dose but saturates at di

Scalable production and integration techniques for van der Waals (vdW) layered materials are vital for their implementation in next-generation nanoelectronics. Among available approaches, perhaps the most well-received is atomic layer deposition (ALD) due to its self-limiting layer-by-layer growth mode. However, ALD-grown vdW materials generally require high processing temperatures and/or additional postdeposition annealing steps for crystallization. Also, the collection of ALD-producible vdW ma

Phase-change memory (PCM), a non-volatile memory technology, is considered the most promising candidate for storage class memory and neuro-inspired devices. It is generally fabricated based on GeTe–Sb2Te3 pseudo-binary alloys. However, natively, it has technical limitations, such as noise and drift in electrical resistance and high current in operation for real-world device applications. Recently, heterogeneously structured PCMs (HET-PCMs), where phase-change materials are hetero-assembled with

Electrical and thermal conductivities of epitaxial, high-quality Ta-doped TiO2 (Ta:TiO2) thin films were experimentally investigated in the temperature range of 35–375 K. Structurally identified as the anatase phase, degenerate Ta doping leads to high electrical conductivity in TiO2, reaching >105 (Ω-m)−1 at 5 at. % of Ta, making it a potential candidate for indium-free transparent conducting oxides. In stark contrast, Ta doping suppresses the thermal conductivity of TiO2 via strong phono

Abstract Following an initial nucleation stage at the flake level, atomically thin film growth of a van der Waals material is promoted by ultrafast lateral growth and prohibited vertical growth. To produce these highly anisotropic films, synthetic or post‐synthetic modifications are required, or even a combination of both, to ensure large‐area, pure‐phase, and low‐temperature deposition. A set of synthetic strategies is hereby presented to selectively produce wafer‐scale tin selenides, SnSe x (b

With reduced dimensionality and a high surface area-to-volume ratio, two-dimensional (2D) semiconductors exhibit intriguing electronic properties that are exceptionally sensitive to surrounding environments, including directly interfacing gate dielectrics. These influences are tightly correlated to their inherent behavior, making it critical to examine when extrinsic charge carriers are intentionally introduced to the channel for complementary functionality. This study explores the physical orig

Abstract Recent advancements in phase‐change memory (PCM) technology have predominantly stemmed from material‐level designs, which have led to fast and durable device performances. However, there remains a pressing need to address the enormous energy consumption through device‐level electrothermal solutions. Thus, the concept of a 3D heater‐all‐around (HAA) PCM fabricated along the vertical nanoscale hole of dielectric/metal/dielectric stacks is proposed. The embedded thin metallic heater comple

Currently, analog in-memory computing, employing memristors into a crossbar array architecture (CAA), is the leading system among available neuromorphic hardware. This study presents a highly tunable synaptic weight update based on a multiterminal memtransistor device as a solution for nonlinear synaptic operations and crosstalk issues in CAA memristors, which are long-standing challenges in neuromorphic hardware applications. To explore an effective device structure for tunable weight update pr

Copper (Cu) oxide compounds (CuxO), which include cupric (CuO) and cuprous (Cu2O) oxide, have been recognized as a promising p-channel material with useful photovoltaic properties and superior thermal conductivity. Typically, deposition methods or thermal oxidation can be used to obtain CuxO. However, these processes are difficult to apply to flexible substrates because plastics have a comparatively low glass transition temperature. Also, additional patterning steps are needed to fabricate appli