Spectroscopic ellipsometry as an in-situ monitoring tool for Bi₂Se₃ films grown by molecular beam epitaxy

In-operando spectroscopic ellipsometry (SE) was used to measure the optical response of Bi₂Se₃ films grown on sapphire substrates in a molecular beam epitaxy (MBE) reactor during cool-down from a growth temperature of 225 °C. A temperature dependent dielectric model was refined for the topological insulator Bi₂Se₃ by taking SE spectra at different temperatures and fitting the amplitude ratio and phase difference of orthogonally polarized light using Bi₂Se₃ films with varying thickness. In-operando SE demonstrated here enabled determining the dielectric function of substrate and growing film unobscured by surface or interface reactions. Its sensitivity to sample temperature and film thickness variations allows determining growth temperature, absolute film thickness, and growth rate in real time, rendering it a reliable and universal approach for a direct comparison of growth conditions between different growth campaigns, thus offering the potential to improve reproducibility of the growth conditions for Bi₂Se₃ based films and heterostructures.