Large-Grain Thin Films of Biaxial Hyperbolic α-MoO₃ Using Alkali Metal Compound-Assisted Vapor Transport Deposition

As the application space of mid-IR technologies continues to expand, materials with new and advanced properties become of interest. α-MoO₃ is a transparent, layered semiconductor that can exhibit biaxial hyperbolicity of the dielectric function, where the optical properties can be switched from out-of-plane hyperbolic to in-plane hyperbolic by varying the mid-IR wavelength. This opens up opportunities for planar emitters, non-reciprocal plasmon waveguides, and directional collimation of energy within nanomaterials via long-lifetime hyperbolic phonon polaritons. However, research on these properties has been predominantly performed on samples of exfoliated flakes of α-MoO₃, with thin film fabrication techniques being relatively undeveloped. This is in part due to the difficulty of producing the large grains necessary for many polaritonic experiments of in-plane anisotropic crystals. Therefore, the experimental flexibility of α-MoO₃ studies has been historically limited and it is of great interest to repeatably produce large single crystals in thin film form. In this work, a physical vapor transport technique is developed which uses alkali metal compounds to promote grains possessing lateral dimensions of hundreds of µm to few mm. We observed that this technique overcomes the poor film texturing and morphology that is observed in α-MoO₃ grown on most substrates. Data will be presented on the film morphology and crystallinity as functions of different growth parameters including substrate composition and preparation, growth temperature, and presence of alkali metal compounds. Additionally, the applicability of this synthesis technique to the nanophotonics community will be demonstrated by characterizing the optical and hyperbolic phonon polariton properties of the synthesized films. This synthesis technique opens up new capabilities for controllable fabrication of large-area α-MoO₃crystals for nanophotonics and polaritonics research.