Scalable oxide encapsulation of CVD-grown WS2 and WSe2 for photonic applications

Two-dimensional transition metal dichalcogenides (TMDs) like WS2 and WSe2 demonstrate strong light-matter interactions, including exciton formation, pronounced photoluminescence, and enhanced second-harmonic generation. Van der Waals epitaxy allows for the growth of these semiconducting monolayers on diverse substrates with minimal lattice mismatch effects, enabling scalable integration into photonic integrated circuits (PICs). Encapsulation in tailored dielectrics not only optimizes optical coupling for atomically thin TMDs but also can shield them from environmental degradation. This study systematically investigates thin and scalable dielectric encapsulation for CVD-grown WS2 and WSe2, focusing on their impact on material structure, excitonic behavior, and nonlinear optical response. A range of oxide thin films (SiO2, Al2O3, and TiO2) and deposition methods (atomic layer deposition - ALD and physical vapor deposition - PVD) compatible with PIC foundry technology are explored. Oxide growth rates via ALD and PVD are higher on WS2 surfaces than WSe2, though faster growth can lead to reduced uniformity. While the thin oxide encapsulation has minimal effect on the phonon spectra of WS2 and WSe2, it enhances second-harmonic efficiency as oxide surface roughness increases. WSe2 exciton spectra remain largely unaffected, whereas WS2 excitonic peaks broaden and redshift with higher oxide dielectric constants. This study emphasizes the potential and key considerations for incorporating CVD-grown WS2 and WSe2 into foundry-ready thin dielectrics for photonic applications, confirming that these TMD materials remain stable and controllable under the applied processes.