Probing the growth quality of molecular beam epitaxy-grown Bi₂Se₃ films via in-situ spectroscopic ellipsometry

Multilayer heterostructures of the topological insulator Bi₂Se₃ and conventional semiconductor In₂Se₃, as well as solid solution layers of (Bi_1-xIn_x)₂Se₃ layers were grown by molecular beam epitaxy and analyzed in-operando using spectroscopic ellipsometry (SE). SE spectra were obtained after the deposition of each layer to determine the respective dielectric functions and thicknesses of each layer. In contrast to ex-situ SE, where uncertainty in the dielectric function and thicknesses of individual layer impose limitations to extract a correct model for the dielectric function of such multilayer heterostructures from a single set of SE data, the step-by-step in-situ SE data recover more precise dielectric functions for Bi₂Se₃, In₂Se₃, (Bi_0.7In _0.3)₂Se₃ and the cap-layer. The optical models developed for multilayer structures can decipher minute perturbations in layers as the growth progresses. Our models show that a ∼7 nm Bi₂Se₃ layer grown next to a sapphire substrate seems to disappear as the structure is annealed at 600 °C. Finally, when the dielectric functions were represented as a collection of Kramers-Kronig-consistent oscillators, in-situ SE predicted the quality of films; the weighted-average broadening parameter for oscillators used for Bi₂Se₃ films grown on (Bi_0.7In _0.3)₂Se₃ layer is ∼20% smaller compared to Bi₂Se₃ films directly grown on sapphire, confirming that the former film is of better quality, and providing a direct metric to quantify film quality and defect concentration. These conclusions were corroborated by transport data.