Room Temperature <i>Cmcm</i> Phase of Ca_<i>x</i>Sn_1–<i>x</i>Se for Thermoelectric Energy Conversion

While SnSe has been known as a high figure of merit (ZT) thermoelectric material at high temperatures, it has a low ZT at room temperature. SnSe has the β (Cmcm) phase at high temperature but the α (Pnma) phase at room temperature. In the present work, we first investigate the phase-transition temperature Tc between the α and β phases of SnSe based on density functional theory calculations and obtain 740 K, which is close to the experimental value of about 800 K. We then consider Ca alloying in SnSe and calculate Tc between the α and β phases of CaxSn1–xSe. It is found that the Ca alloying lowers Tc down to 220 K as the Ca content x increases up to x = 0.1875. For x > 0.14, CaxSn1–xSe is obtained to have the β phase at room temperature, allowing it to be suggested as a room-temperature high figure of merit thermoelectric material

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