Spectroscopic imaging ellipsometry for spatially resolved mapping of layer-by-layer oxidation in WSe₂

Tungsten diselenide is a prominent candidate for nanoscale electronics and complementary metal–oxide–semiconductor technology due to its controllable p-type and n-type doping that is achievable through selective oxidation. However, characterizing the resulting oxide quality often relies on destructive or slow metrology. In this work, we demonstrate spectroscopic imaging ellipsometry (SIE) as a noninvasive technique to characterize the oxidation state of two-dimensional materials, providing spatially resolved measurements of layer thickness and uniformity. We apply this method to compare few-layer WSe₂ after thermal or plasma-based oxidation treatments. We show that plasma oxidation enables a highly controllable conversion of individual WSe₂ layers into uniform sub-stoichiometric tungsten oxide (WO_x), whereas thermal oxidation in atmospheric conditions yields a rough interface characterized by non-uniform oxide formation and blistering. Using SIE, we quantify a layer-by-layer conversion ratio, where a single WSe₂ monolayer transforms into ∼1.6 nm of WO_x during plasma oxidation. These findings, corroborated by atomic force microscopy, highlight the capability of SIE to resolve dielectric evolution and interface quality in layered semiconductors.