Probing the atmosphere-induced subtle changes of the optical properties of (Bi_xIn_1-x)₂Se₃ films

In this study, we have investigated several molecular beam epitaxy-grown (Bi_xIn_1-x)₂Se₃ films to evaluate the changes of their optical properties as samples are exposed to the atmosphere. Using a spectroscopic ellipsometer that is coupled to the growth chamber, we obtained in situ spectra during the entire growth cycle of the samples. Following their growth, we used X-ray diffraction/reflection, Raman spectroscopy, and atomic force microscopy experiments to characterize the samples. Finally, we also performed ex situ spectroscopic ellipsometry measurements so that comparisons could be made with the in situ data. Using the final thickness values obtained from X-ray reflectivity, the ellipsometry spectra were modeled to obtain the dielectric functions of (Bi_xIn_1-x)₂Se₃ films. This was accomplished by representing the electronic transitions with several Kramers-Kronig-consistent oscillators. In comparing the oscillator parameters of the dielectric functions obtained for the in situ and the ex situ spectra, we are able to deduce the subtle changes of the optical properties of these films as a result of their exposure to the atmosphere.