Designing and Processing Transition Metal Dichalcogenide Alloys for Photonic Integrated Circuit Applications

Transition metal dichalcogenides (TMDs) exist in several polymorphs including 2H (usually semiconducting), and 1T/1T’ (usually semi-metallic). Our theoretical calculations and experimental measurements show that TMDs are optically-dense and can feature a difference in refractive index greater than unity between phases ([endif]--&gt;) in the near-infrared (NIR). Fast, non-thermal martensitic phase-change behavior has been observed in TMDs, and transformation strains are expected to be low due to the layered crystal structure. For these reasons, we suggest that TMDs may be useful as active materials to control the phase of light in integrated photonic circuits, surpassing the performance of traditional phase-change materials [1].<br/>Sulfide TMDs are attractive for application to photonics because they present lower optical loss and higher material stability than selenides or tellurides. However, sulfide TMDs also have higher transformation energy barriers. We address this problem by designing sulfide TMD alloys that are thermodynamically-adjacent to phase boundaries. We use density-functional theory (DFT) and the quasi-harmonic approximation to calculate free energy-composition diagrams at finite temperature for polymorphs in the MoS₂-TiS₂-ZrS₂ system. Our calculations predict that the free energy difference between phases can be reduced to near-zero through alloy design. We then synthesize sulfide TMD thin films through a two-step process of metal sputtering, followed by sulfurization in H₂S in a chemical vapor deposition (CVD) furnace. This two-step method, combined with combinatorial sputtering, enables rapid exploration of composition space and phase boundaries. By controlling the CVD furnace conditions, and in particular the trace oxygen concentration, we are able to lower the processing temperature for large-area alloy thin films to 500 °C and below; this is important for yielding good film morphology and for integration with photonic integrated circuit (PIC) manufacturing. We use transmission electron microscopy (TEM) and synchrotron nanodiffraction and nanoflourescence to characterize the competing processes of alloy stabilization and secondary phase formation. Finally, we demonstrate a test concept to study phase-change behavior with electrical stimulation and optical readout for a composition spread across a wafer, and we characterize the phase-change behavior of sulfide TMD alloys by Raman spectroscopy, spectroscopic ellipsometry, and infrared spectroscopy.<br/>[1] A. Singh, S. S. Jo, Y. Li, C. Wu, M. Li, and R. Jaramillo, <i>Refractive Uses of Layered and Two-Dimensional Materials for Integrated Photonics</i>, ACS Photonics <b>7</b>, 3270 (2020).