Thickness-Dependent Structural and Electrical Properties of WS Nanosheets Obtained via the ALD-Grown WO Sulfurization Technique as a Channel Material for Field-Effect Transistors.

Ultrathin WS films are promising functional materials for electronic and optoelectronic devices. Therefore, their synthesis over a large area, allowing control over their thickness and structure, is an essential task. In this work, we investigated the influence of atomic layer deposition (ALD)-grown WO seed-film thickness on the structural and electrical properties of WS nanosheets obtained via a sulfurization technique. Transmission electron microscopy indicated that the thinnest (1.9 nm) film contains rather big (up to 50 nm) WS grains in the amorphous matrix. The signs of incomplete sulfurization, namely, oxysulfide phase presence, were found by X-ray photoemission spectroscopy analysis. The increase in the seed-film thickness of up to 4.7 nm resulted in a visible grain size decrease down to 15-20 nm, which was accompanied by defect suppression. The observed structural evolution affected the film resistivity, which was found to decrease from ∼10 to 10 (μΩ·cm) within the investigated thickness range. These results show that the thickness of the ALD-grown seed layer may strongly affect the resultant WS structure and properties. Most valuably, it was shown that the growth of the thinnest WS film (3-4 monolayers) is most challenging due to the amorphous intergrain phase formation, and further investigations focused on preventing the intergrain phase formation should be conducted.