Homoepitaxial growth of CaWO4

Abstract

Rare-earth ion-doped dielectric crystals are a promising materials platform for quantum device applications due to their stable and highly coherent optical transitions. Recently, REIs in thin film form have become attractive because of their enhanced control of stoichiometry, lattice structure, and dimensionality. This flexibility provides a versatile host crystal environment. Control of surface and interface structures of host crystals at the atomic scale offers an avenue to further improve the optical properties of the system by mitigating defects, which can otherwise compromise the coherence time of quantum devices. In this work, we have investigated the impact of thermal annealing on the surface morphology of a promising host crystal, CaWO4. Our findings reveal that crystal miscut plays a significant role in determining the surface step-terrace structure at the atomic level. Additionally, by iterating an annealing-wet etch cycle, we have achieved atomically flat surfaces with a roughness of less than 0.5 Å rms over a 1 × 1 μm2 area. Homoepitaxial thin film growth using molecular beam epitaxy on an atomically flat surface of CaWO4 results in high-quality thin films. Our study establishes guiding principles to realize a novel quantum optical system based on REI-doped CaWO4 thin films.