Non‐Invasive Optical Inspection of Thermally Driven Interface State Evolution at Buried Metal–MoS 2 Contacts

Van der Waals (vdW) metal contacts to two‐dimensional semiconductors promise efficient carrier extraction alongside long photocarrier lifetimes, yet how the buried interface evolves under back‐end‐of‐line thermal budgets remains difficult to track. Here, annealing‐dependent transient reflection spectroscopy is established as a device‐free optical readout of interface state evolution for transferred vdW Au, graphene‐buffered Au, and deposited semimetal Sb contacts on multilayer MoS 2 up to 400°C, correlated with cross‐sectional electron microscopy, photoelectron spectroscopy, and first‐principles calculations. As the Au–S separation contracts from 3.47 Å to 2.16 Å (into the bonding range) two distinct optical signatures emerge: reduction of the early‐time excitonic response from the first annealing step, consistent with strengthening extraction, and a picosecond capture channel that appears at 200°C, following thermally activated chemisorption whose hybridization‐induced gap states straddle the pinned Fermi level. At matched separation, the Sb interface forms comparable gap states that lie occupied below its Fermi level, explaining its nearly invariant response, whereas graphene delays the transformation by holding the metal away from the semiconductor. These results frame contact selection as a navigable trade‐off between carrier extraction and photocarrier lifetime and identify interlayer engineering and semimetal contacts as complementary routes to thermally robust 2D interfaces.