Synthesis of Ultra-Thin Superionic Cu2Se and New Aspects of the Low-Temperature Crystal Configurations

Superionic conductors offer unique advantages for novel technological devices in various fields, such as energy storage and neuromorphic computing. Above 414 K, Cu2Se turns into a well-known superionic conductor via a phase transition, and it is demonstrated to exhibit peculiar electrical and thermoelectric properties in bulk. Here, we report a large-area synthesis of ultra-thin single crystalline Cu2Se using the chemical vapor deposition method. We demonstrate that Cu2Se crystals exhibit optically and electrically controllable robust phase reconfiguration below 414 K. Moreover, our results show that the mobility of the liquid-like Cu ion vacancies in Cu2Se causes macroscopic fluctuations in the Cu ordering. Consequently, phase variations are not dictated by the diffusive motion of the ions but by the local energy minima formed due to the interplay between the extrinsic and the intrinsic material parameters. As a result, long-range ordering of the crystal below 414 K is optically observable at a micrometer scale. Our results show that Cu2Se could find applications beyond thermoelectric such as smart optical coatings, optoelectronic switching, and ionic transistors

Reliable Rowhammer Attack and Mitigation Based on Reverse Engineering Memory Address Mapping Algorithms

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Atomic Layer Deposition Route to Scalable, Electronic-Grade van der Waals Te Thin Films

Scalable production and integration techniques for vander Waals(vdW) layered materials are vital for their implementation in next-generationnanoelectronics. Among available approaches, perhaps the most well-receivedis atomic layer deposition (ALD) due to its self-limiting layer-by-layergrowth mode. However, ALD-grown vdW materials generally require highprocessing temperatures and/or additional postdeposition annealingsteps for crystallization. Also, the collection of ALD-produciblevdW materials is rather limited by the lack of a material-specifictailored process design. Here, we report the annealing-free wafer-scalegrowth of monoelemental vdW tellurium (Te) thin films using a rationallydesigned ALD process at temperatures as low as 50 & DEG;C. They exhibitexceptional homogeneity/crystallinity, precise layer controllability,and 100% step coverage, all of which are enabled by introducing adual-function co-reactant and adopting a so-called repeating dosingtechnique. Electronically, vdW-coupled and mixed-dimensional verticalp-n heterojunctions with MoS2 and n-Si, respectively, aredemonstrated with well-defined current rectification as well as spatialuniformity. Additionally, we showcase an ALD-Te-based threshold switchingselector with fast switching time (& SIM;40 ns), selectivity (& SIM;10(4)), and low V (th) (& SIM;1.3 V).This synthetic strategy allows the low-thermal-budget production ofvdW semiconducting materials in a scalable fashion, thereby providinga promising approach for monolithic integration into arbitrary 3Ddevice architectures

Anomalous Temperature and Polarization Dependences of Photoluminescence of Metal-Organic Chemical Vapor Deposition-Grown GeSe2

Germanium diselenide (GeSe2) is a 2D semiconductor with air stability, a wide bandgap, and anisotropic optical properties. The absorption and photoluminescence (PL) of single-crystalline 2D GeSe2 grown by metal-organic chemical vapor deposition and their dependence on temperature and polarization are studied. The PL spectra exhibit peaks at 2.5 eV (peak A) and 1.8 eV (peak B); peak A displays a strongly polarized emission along the short axis of the crystal, and peak B displays a weak polarization perpendicular to that of peak A. With increasing temperature, peak B shows anomalous behaviors, i.e., an increasing PL energy and intensity. The excitation energy-dependent PL, time-resolved PL, and density functional theory calculations suggest that peak A corresponds to the band-edge transition, whereas peak B originates from the inter-band mid-gap states caused by selenium vacancies passivated by oxygen atoms. The comprehensive study on the PL of single-crystalline GeSe2 sheds light on the origins of light emission in terms of the band structure of anisotropic GeSe2, making it beneficial for the corresponding optoelectronic applications

Gas-Phase Alkali Metal-Assisted MOCVD Growth of 2D Transition Metal Dichalcogenides for Large-Scale Precise Nucleation Control

Advances in large-area and high-quality 2D transition metal dichalcogenides (TMDCs) growth are essential for semiconductor applications. Here, the gas-phase alkali metal-assisted metal-organic chemical vapor deposition (GAA-MOCVD) of 2D TMDCs is reported. It is determined that sodium propionate (SP) is an ideal gas-phase alkali-metal additive for nucleation control in the MOCVD of 2D TMDCs. The grain size of MoS2 in the GAA-MOCVD process is larger than that in the conventional MOCVD process. This method can be applied to the growth of various TMDCs (MoS2, MoSe2, WSe2, and WSe2) and the generation of large-scale continuous films. Furthermore, the growth behaviors of MoS2 under different SP and oxygen injection time conditions are systematically investigated to determine the effects of SP and oxygen on nucleation control in the GAA-MOCVD process. It is found that the combination of SP and oxygen increases the grain size and nucleation suppression of MoS2. Thus, the GAA-MOCVD with a precise and controllable supply of a gas-phase alkali metal and oxygen allows achievement of optimum growth conditions that maximizes the grain size of MoS2. It is expected that GAA-MOCVD can provide a way for batch fabrication of large-scale atomically thin electronic devices based on 2D semiconductors

Large Memory Window of van der Waals Heterostructure Devices Based on MOCVD-Grown 2D Layered Ge4Se9

Van der Waals (vdW) heterostructures have drawn much interest over the last decade owing to their absence of dangling bonds and their intriguing low-dimensional properties. The emergence of 2D materials has enabled the achievement of significant progress in both the discovery of physical phenomena and the realization of superior devices. In this work, the group IV metal chalcogenide 2D-layered Ge4Se9 is introduced as a new selection of insulating vdW material. 2D-layered Ge4Se9 is synthesized with a rectangular shape using the metalcorganic chemical vapor deposition system using a liquid germanium precursor at 240 degrees C. By stacking the Ge4Se9 and MoS2, vdW heterostructure devices are fabricated with a giant memory window of 129 V by sweeping back gate range of +/- 80 V. The gate-independent decay time reveals that the large hysteresis is induced by the interfacial charge transfer, which originates from the low band offset. Moreover, repeatable conductance changes are observed over the 2250 pulses with low non-linearity values of 0.26 and 0.95 for potentiation and depression curves, respectively. The energy consumption of the MoS2/Ge4Se9 device is about 15 fJ for operating energy and the learning accuracy of image classification reaches 88.3%, which further proves the great potential of artificial synapses