Interband Transitions in Monolayer and Few-Layer WSe₂ Probed Using Photoexcited Charge Collection Spectroscopy

Transition-metal dichalcogenides are currently under rigorous investigation because of their distinct layer-dependent physical properties originating from the corresponding evolution of the band structure. Here, we report the highly resolved probing of layer-dependent band structure evolution for WSe2 using photoexcited charge collection spectroscopy (PECCS). Monolayer, few-layer, and multilayer WSe2 can be probed in top-gate field-effect transistor platforms, and their interband transitions are efficiently observed. Our theoretical calculations show a great coincidence with the PECCS results, proving that the indirect Γ–K and Γ–Λ transitions as well as the direct K–K transition are clearly resolved in multilayer WSe2 by PECCS

Anisotropic Electron Mobility and Contact Resistance of β‑Ga₂O₃ Obtained via Radio Frequency Transmission Line Methods on Schottky Devices

Monoclinic semiconducting β-Ga2O3 has drawn attention, particularly because its thin film could be achieved via mechanical exfoliation from bulk crystals, which is analogous to van der Waals materials’ behavior. For the transistor devices with exfoliated β-Ga2O3, the channel direction becomes [010] for in-plane electron transport, which changes to vertical [100] near the source/drain (S/D) contact. Hence, anisotropic transport behavior is certainly worth to study but rarely reported. Here we achieve the vertical [100] direction electron mobility of 4.18 cm2/(V s) from Pt/β-Ga2O3 Schottky diodes with various thickness via radio frequency-transmission line method (RF-TLM), which is recently developed. The specific contact resistivity (ρc) could also be estimated from RF-TLM, to be 4.72 × 10–5 Ω cm2, which is quite similar to the value (5.25 × 10–5 Ω cm2) from conventional TLM proving the validity of RF-TLM. We also fabricate metal–semiconductor field-effect transistors (MESFETs) to study anisotropic transport behavior and contact resistance (RC). RC-free [010] in-plane mobility appears as high as maximum ∼67 cm2/(V s), extracted from total resistance in MESFETs

Self-Assembled TaO_X/2H-TaS₂ as a van der Waals Platform of a Multilevel Memristor Circuit Integrated with a β‑Ga₂O₃ Transistor

Two-dimensional (2D)-layered material tantalum disulfide (2H-TaS2) is known to be a van der Waals conductor at room temperature. Here, 2D-layered TaS2 has been partially oxidized by utraviolet-ozone (UV-O3) annealing to form a 12-nm-thin TaOX on conducting TaS2, so that the TaOX/2H-TaS2 structure might be self-assembled. Utilizing the TaOX/2H-TaS2 structure as a platform, each device of a β-Ga2O3 channel MOSFET and a TaOX memristor has been successfully fabricated. An insulator structure of Pt/TaOX/2H-TaS2 shows good a dielectric constant (k ∼ 21) and strength (∼3 MV/cm) of achieved TaOX, which is enough to support a β-Ga2O3 transistor channel. Based on the quality of TaOX and low trap density of the TaOX/β-Ga2O3 interface, which is achieved via another UV-O3 annealing, excellent device properties such as little hysteresis (<∼0.04 V), band-like transport, and a steep subthreshold swing of ∼85 mV/dec are achieved. With a Cu electrode on top of the TaOX/2H-TaS2 structure, the TaOX acts as a memristor operating around ∼2 V for nonvolatile bipolar and unipolar mode memories. The functionalities of the TaOX/2H-TaS2 platform become more distinguished finally when the Cu/TaOX/2H-TaS2 memristor and β-Ga2O3 MOSFET are integrated to form a resistive memory switching circuit. The circuit nicely demonstrates the multilevel memory functions

Electrical Transport Properties Driven by Unique Bonding Configuration in γ‑GeSe

Group IV monochalcogenides have recently shown great potential for their thermoelectric, ferroelectric, and other intriguing properties. The electrical properties of group IV monochalcogenides exhibit a strong dependence on the chalcogen type. For example, GeTe exhibits high doping concentration, whereas S/Se-based chalcogenides are semiconductors with sizable bandgaps. Here, we investigate the electrical and thermoelectric properties of γ-GeSe, a recently identified polymorph of GeSe. γ-GeSe exhibits high electrical conductivity (∼106 S/m) and a relatively low Seebeck coefficient (9.4 μV/K at room temperature) owing to its high p-doping level (5 × 1021 cm–3), which is in stark contrast to other known GeSe polymorphs. Elemental analysis and first-principles calculations confirm that the abundant formation of Ge vacancies leads to the high p-doping concentration. The magnetoresistance measurements also reveal weak antilocalization because of spin–orbit coupling in the crystal. Our results demonstrate that γ-GeSe is a unique polymorph in which the modified local bonding configuration leads to substantially different physical properties

Near-Infrared Self-Powered Linearly Polarized Photodetection and Digital Incoherent Holography Using WSe₂/ReSe₂ van der Waals Heterostructure

Polarization-sensitive photodetection has attracted considerable attention as an emerging technology for future optoelectronic applications such as three-dimensional (3D) imaging, quantum optics, and encryption. However, traditional photodetectors based on Si or III–V InGaAs semiconductors cannot directly detect polarized light without additional optical components. Herein, we demonstrate a self-powered linear-polarization-sensitive near-infrared (NIR) photodetector using a two-dimensional WSe2/ReSe2 van der Waals heterostructure. The WSe2/ReSe2 heterojunction photodiode with semivertical geometry exhibits excellent performance: an ideality factor of 1.67, a broad spectral photoresponse of 405–980 nm with a significant photovoltaic effect, outstanding linearity with a linear dynamic range wider than 100 dB, and rapid photoswitching behavior with a cutoff frequency up to 100 kHz. Strongly polarized excitonic transitions around the band edge in ReSe2 lead to significant 980 nm NIR linear-polarization-dependent photocurrent. This linear polarization sensitivity remains stable even after exposure to air for longer than five months. Furthermore, by leveraging the NIR (980 nm)-selective linear polarization detection of this photodiode under photovoltaic operation, we demonstrate digital incoherent holographic 3D imaging