Transition metal dichalcogenide phase-change materials for infrared photonics

Transition metal dichalcogenides (TMD) exist in 2H (usually semiconducting) and 1T’ (semi-metallic) polymorphs. Switching between these polymorphs offers a new paradigm for controlling light. However, energy required for phase-change (PC) in pure phases is large. Alloying 2H and 1T’ materials, for example MoS2 and TiS2, could offer low-energy PC. This low energy transformation would especially be useful for infrared (IR) integrated photonics.
We present a combined experimental and theoretical study of IR properties of MoS2, TiS2 and alloys thereof. Our density functional theory (DFT) calculations predict a large refractive index contrast in the 1-1.5um spectral range, between 2H and 1T’ phases, for optimized alloys. We experimentally test our DFT predictions using spectroscopic ellipsometry and Fourier-transform IR spectroscopy (FTIR). We measure the complex dielectric constants of bulk crystals, and of synthesized (many-layer) thin films. We characterize the quality of our samples – including wafer-scale combinatorial composition spreads - using Raman and X-ray diffraction mapping. Our work lays the foundation for thin-film TMD PC active IR materials that can be switched optically, electrically, or mechanically.