2, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
The realization of wafer-scale transition metal dichalcogenide monolayer films and the successful exploration of 2D TMDs library are two major milestones in the recent efforts on the synthesis of 2D materials. Interestingly, NaCl is used in both cases to reduce the nucleation density and melting points of oxides. However, the presence of alkali metals is detrimental in conventional silicon-based semiconductors due to high rates of ion diffusion through the bulk, leading to degradation in device performance and reliability. In 2D materials, like traditional dopants such as rhenium and niobium, alkali metals, such as potassium K+, results in degenerate doping of MoS2. Therefore, it is critical to comprehensively evaluate the impacts of NaCl on the properties of synthetic 2D materials. In this work, we elucidate NaCl-induced heterogeneities in synthesis, photonic response and electrical performance of MoS2 films, and demonstrate that salt-assisted synthesis of epitaxial MoS2 monolayers degrades electrical device performance. First, we develop the NaCl free large area (2×2 cm2) epitaxy of MoS2, evident from in-plane X-ray diffraction (XRD) and utilize it to compare to the growth with NaCl. Atomic force microscopy (AFM), scanning electron microscopy (SEM), Raman, and photoluminescence (PL) indicate that growth of monolayer films without NaCl are crystalline and uniform across sapphire substrates without island growth, while two distinct growth modes (island and layer-by-layer) and rates are identified when NaCl is utilized. The heterogeneous growth kinetics due to NaCl leads to photonic heterogeneities related to non-uniform monolayer strain. Moreover, utilizing NaCl introduces a 1.5x decrease in mobility, 2x increase in subthreshold slope, and 100x reduction in on/off ratio due to electronic trap states. Finally, comparison of transport properties between devices fabricated using as-grown and transferred films reveals that interface coupling (charge transfer, trap states) can dominate the device performance, indicating the importance of substrate engineering to realize the high performance, synthetic 2D based devices.
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