Energy-Level Alignment at Interfaces Between Degenerately Doped and Nominally Undoped Transition-Metal Dichalcogenides Studied by Kelvin Probe Force Microscopy

Degenerately hole-doped transition-metal dichalcogenides (TMDs) have been shown to be excellent drain/source contact materials for p-type TMD field-effect transistors, enabling low-resistance interfaces with nominally undoped TMD channels. This performance arises primarily from their relatively high work functions and the formation of van der Waals interfaces. In this work, we present a systematic study of energy-level alignment between degenerately hole-doped and nominally undoped TMDs of varying thicknesses using Kelvin probe force microscopy (KPFM) to elucidate the underlying contact mechanisms. Doped/undoped TMD heterostructures were fabricated via a dry-transfer method and placed on thin graphite flakes supported by Au substrates for KPFM measurements. The work functions of undoped few-layer WSe₂ and MoS₂ are found to be appreciably higher than those of the underlying degenerately hole-doped layers (Nb-doped WSe₂ and Nb-doped MoS₂), suggesting that factors beyond simple interfacial charge transfer contribute to this behavior.