Amorphous oxide semiconductor transistors control the illuminance of pixels in an ecosystem of displays from largescreen TVs to wearable devices. To satisfy application-specific requirements, oxide semiconductor transistors of various cation compositions have been explored. However, a comprehensive study has not been carried out where the influence of cation composition, oxygen, and hydrogen on device characteristics and stability is systematically quantified, using commercial-grade process technology. In this study, we fabricate self-aligned topgate structure thin-film transistors with three oxide semiconductor materials, InGaZnO (In/Ga/Zn = 1:1:1), In-rich InGaZnO, and InZnO, having mobility values of 10, 27, and 40 cm 2 /V•s, respectively. Combinations of varied amounts of oxygen and hydrogen are incorporated into each transistor by controlling the fabrication process to study the effect of these gaseous elements on the physical nature of the channel material. Electrons can be captured by peroxy linkage (O 2 2− ) or undercoordinated In (In* to become In + ), which are manifested in the extracted subgap density-of-states profile and first-principles calculations. Energy difference between electron-trapped In + and O 2 2− σ* is the smallest for IGZO, and In + −O 2 2− annihilation occurs by electron excitation from the subgap In + state to the O 2 2− σ*. Furthermore, characteristic time constants during positive bias stress and recovery reveal the various microscopic physical phenomena within the transistor structure between different cation compositions.

Edge atomic and electronic structures of S-saturated Mo-edge triangular MoS nanoclusters are investigated using density functional theory calculations. The edge electrons described by the S-p pπ* (S-Π ) and Mo-d orbitals are found to interplay to pin the S-Π Fermi wavenumber at k = 2/5 as the nanocluster size increases, and correspondingly, the ×5 Peierls edge S interdimer spacing modulation is induced. For the particular sizes of N = 5 n - 2 and 5 n, where N is the number of Mo atoms at one edge representing the nanocluster size and n is a positive integer, the effective ×5 interdimer spacing modulation stabilizes the nanoclusters, which are identified here to be the magic S-saturated Mo-edge triangular MoS nanoclusters. With the S-Π Peierls gap, the MoS nanoclusters become far-edge S-Π semiconducting and subedge Mo-d metallic as N → ∞.

Amorphous oxide semiconductors have been applied to thin-film electronics on the backplanes of organic light-emitting diode (OLED) displays. In mobile and high-refresh-rate display applications, demands have been increasing for both low power consumption and high operation speed of the electronics. Here, based on ab initio calculations, we suggest that density engineering of amorphous InGaZnO 4 semiconductors can improve electrical properties. The density of an amorphous material is typically variable over a wide range through process control as well as device design and mechanical operation. It is shown here that increasing the density (up to 6.4 g/cm 3 ) of amorphous InGaZnO 4 semiconductors with respect to the conventional density (5.8 g/cm 3 ) widens the electronic energy gap by +3.8% and reduces the effective mass of electrons by −4.3%, simultaneously. In a wide range of 3.6−7.8 g/cm 3 , the electrical properties are found to vary nonmonotonically, of which the physical mechanisms combined with the microstructures are investigated in depth. Density optimization can ultimately lead to both a reduction of off-state current and an enhancement of electron mobility in amorphous InGaZnO 4 -based thin-film transistors.

We report a method to precisely control the atomic defects at grain boundaries (GBs) of monolayer MoS 2 by vapor−liquid−solid (VLS) growth using sodium molybdate liquid alloys, which serve as growth catalysts to guide the formations of the thermodynamically most stable GB structure. The Mo-rich chemical environment of the alloys results in Mopolar 5|7 defects with a yield exceeding 95%. The photoluminescence (PL) intensity of VLS-grown polycrystalline MoS 2 films markedly exceeds that of the films, exhibiting abundant S 5|7 defects, which are kinetically driven by vapor−solid−solid growths. Density functional theory calculations indicate that the enhanced PL intensity is due to the suppression of nonradiative recombination of charged excitons with donor-type defects of adsorbed Na elements on S 5|7 defects. Catalytic liquid alloys can aid in determining a type of atomic defect even in various polycrystalline 2D films, which accordingly provides a technical clue to engineer their properties.

Two-dimensional (2D)-layered material tantalum disulfide (2H-TaS 2 ) is known to be a van der Waals conductor at room temperature. Here, 2D-layered TaS 2 has been partially oxidized by utraviolet-ozone (UV-O 3 ) annealing to form a 12nm-thin TaO X on conducting TaS 2 , so that the TaO X /2H-TaS 2 structure might be self-assembled. Utilizing the TaO X /2H-TaS 2 structure as a platform, each device of a β-Ga 2 O 3 channel MOSFET and a TaO X memristor has been successfully fabricated. An insulator structure of Pt/TaO X /2H-TaS 2 shows good a dielectric constant (k ∼ 21) and strength (∼3 MV/cm) of achieved TaO X , which is enough to support a β-Ga 2 O 3 transistor channel. Based on the quality of TaO X and low trap density of the TaO X /β-Ga 2 O 3 interface, which is achieved via another UV-O 3 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 TaO X /2H-TaS 2 structure, the TaO X acts as a memristor operating around ∼2 V for nonvolatile bipolar and unipolar mode memories. The functionalities of the TaO X /2H-TaS 2 platform become more distinguished finally when the Cu/TaO X /2H-TaS 2 memristor and β-Ga 2 O 3 MOSFET are integrated to form a resistive memory switching circuit. The circuit nicely demonstrates the multilevel memory functions. KEYWORDS: β-Ga 2 O 3 , vdW, platform, memristor, transistor

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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 T c 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 T c between the α and β phases of Ca x Sn 1−x Se. It is found that the Ca alloying lowers T c down to 220 K as the Ca content x increases up to x = 0.1875. For x > 0.14, Ca x Sn 1−x Se 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.