Correlated Intrinsic Electrical and Chemical Properties of Epitaxial WS2 via Combined C-AFM and ToF-SIMS Characterization

Abstract

Atomically thin, 2D semiconductors, such as transition metal dichalcogenides, complement silicon in ultra-scaled nano-electronic devices. However, the semiconductor and its interfaces become increasingly more difficult to characterize chemically and electrically. Conventional methodologies, including scanning probe microscopies, fail to capture insight into the chemical and electronic nature of the semiconductor, albeit vital to understand its impact on the semiconductor performance. Therefore, this work presents a unique and universal in situ approach combining time-of-flight secondary ion mass spectrometry and atomic force microscopy to map chemical differences between regions of different electrical conductivity in epitaxially deposited tungsten disulfide (WS2) on sapphire substrates. Surprisingly, WS2 regions of lower electrical conductivity possess a larger amount of sulfur compared to regions with higher conductivity, for which oxygen is also detected. Such difference in chemical composition likely roots from the non-homogeneously terminated sapphire starting surface, altering the WS2 nucleation behavior and associated defect formation between neighboring sapphire terraces. These resulting sapphire terrace-dependent doping effects in the WS2 hamper its electrical conductivity. Thus, accurate chemical assignment at a sub-micrometer lateral resolution of atomically thin 2D semiconductors is vital to achieve a more detailed understanding on how the growth behavior affects the electrical properties.