In Situ Composition and Thickness Monitoring during (Bi _x In _1-x ) ₂ Se ₃ Thin Film Growth: toward Automated Synthesis Control Using Spectroscopic Ellipsometry for Quantum and Spintronic Devices

In this work, we show that by coupling in situ spectroscopic ellipsometry with a molecular beam epitaxy growth chamber, the growth parameters of ternary compounds with potential for quantum and spintronic applications can be immediately ascertained during the entire growth cycle of a sample. Initially, several films of (BixIn1–x)2Se3 with stoichiometries ranging from x = 0 to x = 1 were grown and characterized by X-ray reflectivity, X-ray photoelectron spectroscopy, and Rutherford backscattering. Using this information, ellipsometry spectra were fitted by representing the dielectric functions with Kramers–Kronig-consistent oscillators. Consequently, composition-dependent dielectric functions of (BixIn1–x)2Se3 were parametrized to create a material file that determines the Bi content of an unknown (BixIn1–x)2Se3 film. By using this material file, therefore, both the Bi content and thickness of a (BixIn1–x)2Se3 film can be obtained immediately at any stage of the growth cycle. We tested the model for universality among MBE growth systems and found that the model was transferable between systems. Furthermore, the generalized model allowed us to monitor sticking and desorption coefficients for Bi2Se3 thin films in operando for the first time, with significant implications for quantum and spintronic applications by enabling more reproducible and controlled device fabrication, advancing the understanding of emergent physics, and helping address future societal bottlenecks in electronic performance and demand.