Because the silicon-based electronics approaches its bodily limits imposed by Moore’s Regulation, conventional silicon-based units face extreme bottlenecks in downscaling and efficiency enhancement, which demand steady efforts to discover various semiconductors for next-generation built-in circuits. Among the many rising supplies, two-dimensional transition metallic dichalcogenides (2D-TMDs) have emerged as notably promising candidates for extending Moore’s Regulation and enabling next-generation electronics on account of their distinctive electrical, optical, and physicochemical properties [1], [2], [3]. At present, n-type 2D units, together with the field-effect transistors based mostly on MoS2 [4], [5], [6], [7], [8], [9], [10], InSe [11], [12], [13], and WS2 [14], [15], [16] et.al., have current wonderful electrical performances that method the targets outlined within the Worldwide Roadmap for Units and Techniques (IRDS), whereas the efficiency of p-type 2D units lags considerably behind their n-type counterparts [17]. This efficiency asymmetry creates a essential efficiency hole in complementary electronics which can be commonly-used in sensible business built-in circuits.
On this context, WSe2 has attracted in depth analysis consideration as a promising candidate for overcoming the efficiency bottlenecks of p-type units, owing to its excessive intrinsic service mobility and good environmental stability [18], [19], [20]. Nevertheless, the digital properties of monolayer WSe2 exhibit sturdy layer-dependent traits, resulting in the next limitations in sensible purposes: (1) In comparison with multilayer constructions, monolayer WSe2 is extra vulnerable to phonon scattering, which considerably impacts its intrinsic mobility; in sensible system purposes, efficiency is additional constrained by distant optical phonon scattering and extrinsic doping results [21], [22]; (2) The roughly 1.6 eV direct bandgap of monolayer WSe2 tends to advertise the formation of high-density interface states, leading to sturdy Fermi-level pinning close to the mid-gap area when contacting metallic electrodes. This results in excessive contact resistance and degraded cost transport efficiency [23], [24]. In distinction, few-layer WSe2 possesses a narrower bandgap and reveals considerably weakened Fermi-level pinning (i.e., the depinning impact), which facilitates the formation of decrease Schottky limitations and diminished contact resistance [23], [25]. Current research [1], [20], [23], [26] have highlighted the appreciable potential of bilayer WSe2 for high-performance transistors. Quantum transport simulations [27], [28] additional affirm that the bilayer construction can notably improve the on-state efficiency of units by lowering the bandgap of WSe2 and growing the cell cost density, thereby offering essential help for the event of ultra-scaled CMOS know-how. Though units based mostly on monolayer WSe2 have demonstrated promising efficiency, the controllable synthesis of large-area, uniform bilayer WSe2 stays a serious problem hindering its sensible utility.
Chemical vapor deposition (CVD) has been broadly used for synthesizing bilayer WSe2 because it permits scalable manufacturing of large-area, high-quality TMDs with managed thickness [4], [5], [6], [7], [8], [9], [10], [14], [15], [16], [29], [30], [31], [32]. The molten salt-assisted CVD technique, which includes the introduction of halide salts (e.g., NaCl, and many others.), not solely successfully reduces the melting level of the response precursors but in addition promotes the formation of risky tungsten oxyhalide intermediates resembling WO2Cl2 and WOCl4 in the course of the synthesis of WSe2. These intermediates considerably improve the mass transport effectivity of the precursors, thereby considerably growing the grain dimension of the synthesized materials. This technique has been demonstrated as an efficient technique for getting ready high-quality TMDs [33], [34], [35]. Nevertheless, making use of this technique to the managed progress of uniform bilayer WSe2 stays difficult, primarily on account of two main points [36]: (1) Difficulties of enabling new atom nucleation on the inert floor of as-grown monolayer WSe2, resulting in a powerful dependence on stochastic nucleation, excessive temperature or extreme molten salt, which is simple to trigger the decomposition of the underlying WSe2 template, deterioration of crystal high quality and degradation of digital efficiency; (2) The shortage of atomic-scale understanding of the vertical progress mechanism in molten salt-assisted processes, which significantly hinders exact management over layer stacking and limits reproducible uniform system constructions.
Prior research have indicated that energetic clusters with excessive diffusion limitations (resembling W1Se1) can induce nucleation on TMDC template surfaces [36], [37]. Nevertheless, the position of molten-salt-assisted intermediates (e.g., WO2Cl2) within the progress course of has not but been systematically elucidated. On this work, by combining density practical concept (DFT) calculations with experimental validation, we make clear the atomic-scale mechanism of vertical bilayer WSe2 progress in molten salt-assisted CVD, specializing in nucleation and kinetics. DFT outcomes reveal that the cooperative adsorption of WO2Cl2 atop the monolayer template facilities, along with the excessive diffusion barrier of W1Se1 clusters, energetically drives vertical stacking and promotes the formation of bilayer constructions. By exactly tuning the Se/WO₃ vapor ratio, we achieved epitaxial progress from a monolayer (ML) WSe2 to unequal-bilayer (UEB) WSe2 and at last to equal-bilayer (EB) WSe2. Atomic drive microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy (STEM) characterization of the ensuing EB-WSe2 samples reveal a clear, contamination-free uniform floor and excessive crystal high quality. Electrical measurements reveal considerably improved contact traits between EB-WSe2 and metallic electrodes. Efficiency comparability based mostly on back-gated field-effect transistors reveals that EB-WSe2 reveals superior electrical properties in comparison with ML-WSe2 (on/off ratio of 10⁶, gap mobility of three.4 cm²V⁻¹s⁻¹), with an on/off ratio of 10⁷ and a gap mobility of fifty.1 cm²V⁻¹s⁻¹.
