Highly Sensitive Phototransistors Based on Partially Suspended Monolayer WS₂

Two-dimensional (2D) materials have attracted tremendous interests because of various advantages, such as high carrier mobility, favorable band gap, strong light-matter interaction, and flat dangling-bond-free surface. However, the photodetection performance of the pristine monolayer 2D materials is not competitive with that of their bulk counterpart due to the atomic thickness. Here, we demonstrated the direct growth of monolayer WS₂ on the Si₃N₄ substrate by chemical vapor deposition. The porous morphology is formed at the surface of the Si₃N₄ substrate during the growth process, and therefore, the as-grown WS₂ is partially suspended on porous Si₃N₄. The phototransistor based on as-grown monolayer WS₂ exhibits a high responsivity of 1.58 × 10⁵ A/W and a fast response speed of 40 ms. Because of light resonant scattering of the porous Si₃N₄ substrate, the absorption coefficient of monolayer WS₂ is enhanced from finite element analysis. In addition, biaxial tensile strain distribution in the partially suspended WS₂ induces a built-in in-plane electric field and less carrier mass from first-principles calculations. Therefore, we attributed the high performance of the phototransistor to the hybrid structure of partially suspended monolayer WS₂ on the porous substrate. Our present work paves a way to achieve high-performance monolayer photodetector from substrate morphology engineering.