Toolbox of Advanced Atomic Layer Deposition Processes for Tailoring Large-Area MoS2 Thin Films at 150 °C

Ultralow-Resistance Contacts to Heavily Doped p-Type NbxW1-xSy Thin Films Grown by Atomic Layer Deposition

Transition metal dichalcogenides (TMDs) are an important class of materials for future microelectronics. Of particular interest are TMDs deposited by atomic layer deposition (ALD) since this technique allows both back-end-of-line (BEOL) compatible deposition and the ability to create heavily doped regions for contact formation. In this work, we characterize ∼3 nm-thick heavily doped NbxW1-xSy thin films grown by plasma-enhanced ALD using gated transfer-length measurement (TLM) structures. An analysis of films with different Nb concentrations, x, found that films with x = 0.22 had the lowest sheet resistivity of 86 kΩ/sq along with an ultrahigh carrier concentration of 4.2 × 1020 cm-3. The contact resistance, RC, of different metals to NbxW1-xSy thin films was also analyzed. Among Pd, Ni, and Ti contacts, Pd was found to have the lowest RC, whereas Ni (Ti) had an average RC that was 6× (20×) higher than Pd. Physical analysis of the films using Raman spectroscopy and transmission electron microscopy shows that the crystal quality degrades going from x = 0.08 to 0.33, while Kelvin probe force microscopy, complemented by density functional theory, is used to explain the Nb concentration of the extracted work function. The best TLM structures have an RC value as low as 0.30 ± 0.26 kΩ-μm and a mean specific contact resistivity, ρC, of 11 ± 27 nΩ-cm2. Even after accounting for experimental error, this value is lower than the other values reported for p-type TMD contacts in the literature. These results suggest that NbxW1-xSy can be a promising intermediate layer between metal contacts and monolayer WSe2 in future scaled-down TMD MOSFETs.

Empowering Green Energy Storage Systems with MXene for a Sustainable Future

Green energy storage systems play a vital role in enabling a sustainable future by facilitating the efficient integration and utilization of renewable energy sources. The main problems related to two-dimensional (2D) materials are their difficult synthesis process, high cost, and bulk production, which hamper their performance. In recent years, MXenes have emerged as highly promising materials for enhancing the performance of energy storage devices due to their unique properties, including their high surface area, excellent electrical and thermal conductivity, and exceptional chemical stability. This paper presents a comprehensive scientific approach that explores the potential of MXenes for empowering green energy storage systems. Which indicates the novelty of the article. The paper reviews the latest advances in MXene synthesis techniques. Furthermore, investigates the application of MXenes in various energy storage technologies, such as lithium-ion batteries, supercapacitors, and emerging energy storage devices. The utilization of MXenes as electrodes in flexible and transparent energy storage devices is also discussed. Moreover, the paper highlights the potential of MXenes in addressing key challenges in energy storage, including enhancing energy storage capacity, improving cycling stability, and promoting fast charging and discharging rates. Additionally, industrial application and cost estimation of MXenes are explored. As the output of the work, we analyzed that HF and modified acid (LiF and HCl) are the established methods for synthesis. Due to high electrical conductivity, MXene materials are showing extraordinary results in energy storage and related applications. Making a composite hydrothermal method is one of the established methods. This scientific paper underscores the significant contributions of MXenes in advancing green energy storage systems, paving the way for a sustainable future driven by renewable energy sources. To facilitate the research, this article includes technical challenges and future recommendations for further research gaps in the topic.

Structural Aspects of MoS x Prepared by Atomic Layer Deposition for Hydrogen Evolution Reaction

Molybdenum sulfides (MoS x ) in both crystalline and amorphous forms are promising earth-abundant electrocatalysts for hydrogen evolution reaction (HER) in acid. Plasma-enhanced atomic layer deposition was used to prepare thin films of both amorphous MoS x with adjustable S/Mo ratio (2.8-4.7) and crystalline MoS2 with tailored crystallinity, morphology, and electrical properties. All the amorphous MoS x films transform into highly HER-active amorphous MoS2 (overpotential 210-250 mV at 10 mA/cm2 in 0.5 M H2SO4) after electrochemical activation at approximately -0.3 V vs reversible hydrogen electrode. However, the initial film stoichiometry affects the structure and consequently the HER activity and stability. The material changes occurring during activation are studied using ex situ and quasi in situ X-ray photoelectron spectroscopy. Possible structures of as-deposited and activated catalysts are proposed. In contrast to amorphous MoS x , no changes in the structure of crystalline MoS2 catalysts are observed. The overpotentials of the crystalline films range from 300 to 520 mV at 10 mA/cm2, being the lowest for the most defective catalysts. This work provides a practical method for deposition of tailored MoS x HER electrocatalysts as well as new insights into their activity and structure.

Nb Doping and Alloying of 2D WS2 by Atomic Layer Deposition for 2D Transition Metal Dichalcogenide Transistors and HER Electrocatalysts

We utilize plasma-enhanced atomic layer deposition to synthesize two-dimensional Nb-doped WS2 and NbxW1-xSy alloys to expand the range of properties and improve the performance of 2D transition metal dichalcogenides for electronics and catalysis. Using a supercycle deposition process, films are prepared with compositions spanning the range from WS2 to NbS3. While the W-rich films form crystalline disulfides, the Nb-rich films form amorphous trisulfides. Through tuning the composition of the films, the electrical resistivity is reduced by 4 orders of magnitude compared to pure ALD-grown WS2. To produce Nb-doped WS2 films, we developed a separate ABC-type supercycle process in which a W precursor pulse precedes the Nb precursor pulse, thereby reducing the minimum Nb content of the film by a factor of 3 while maintaining a uniform distribution of the Nb dopant. Initial results are presented on the electrical and electrocatalytic performances of the films. Promisingly, the NbxW1-xSy films of 10 nm thickness and composition x ≈ 0.08 are p-type semiconductors and have a low contact resistivity of (8 ± 1) × 102 Ω cm to Pd/Au contacts, demonstrating their potential use in contact engineering of 2D TMD transistors.

Two-Dimensional Semiconductors and Transistors for Future Integrated Circuits

Silicon transistors are approaching their physical limit, calling for the emergence of a technological revolution. As the acknowledged ultimate version of transistor channels, 2D semiconductors are of interest for the development of post-Moore electronics due to their useful properties and all-in-one potentials. Here, the promise and current status of 2D semiconductors and transistors are reviewed, from materials and devices to integrated applications. First, we outline the evolution and challenges of silicon-based integrated circuits, followed by a detailed discussion on the properties and preparation strategies of 2D semiconductors and van der Waals heterostructures. Subsequently, the significant progress of 2D transistors, including device optimization, large-scale integration, and unconventional devices, are presented. We also examine 2D semiconductors for advanced heterogeneous and multifunctional integration beyond CMOS. Finally, the key technical challenges and potential strategies for 2D transistors and integrated circuits are also discussed. We envision that the field of 2D semiconductors and transistors could yield substantial progress in the upcoming years and hope this review will trigger the interest of scientists planning their next experiment.