Quantitative ToF-SIMS Assessment of In-Plane and Out-of-Plane Nb Doping Uniformity in CVT-Grown MoS₂ Crystals

Substitutional doping in transition metal dichalcogenides (TMDs) has recently garnered renewed attention due to increasing high demands for ultra-low contact resistance. However, understanding and controlling the spatial distribution of dopants in 2D TMDs is key for precise modulation of their electronic properties for next-generation electronics. In this study, chemical vapor transport (CVT) grown Nb-doped MoS₂ is used as a model system to quantitatively assess impurity distribution via time-of-flight secondary ion mass spectrometry (ToF-SIMS) and photoluminescence (PL) mapping. In-plane PL mapping of monolayers reveals spatially uniform Nb incorporation within the lateral resolution limit of ≈1 µm, whereas bulk crystals exhibit variation in Nb content, suggesting limited out-of-plane dopant diffusion. Moreover, a non-destructive method is established for estimating Nb concentration in individual monolayers by leveraging the linear correlation between PL peak position and Nb content, enabling selective use of monolayers with desired doping levels for device applications. The integration of these techniques offers an effective framework for spatially characterizing and fine-tuning doping in 2D TMDs, paving the way for their reliable implementation into future electronic devices.