In situ composition and thickness probing of (Bi_xIn_1-x)₂Se₃ films by spectroscopic ellipsometry

Developing in-situ tools that will produce instant feedback on the growth dynamics of thin films is an important feature to ascertain high-quality films with greater reliability. While laser interferometry and RHEED techniques have been used to monitor the growth characteristics, they do not provide reliable data on alloy concentration and thickness of a film during the growth. In this work, we show that spectroscopic ellipsometry is an excellent in situ tool that can be used to monitor growth characteristics and with sufficient calibration, it can detect angstrom-scale alloy and thickness fluctuations during the entire growth cycle of a thin film. Using a spectroscopic ellipsometer that was coupled to a molecular-beam epitaxy growth chamber, we obtained time-dependent spectra during the growth of several (Bi_xIn_1-x)₂Se₃ films, with compositions ranging from x=0 to x=1. After their growth, we performed X-ray reflectivity, X-ray photoemission spectroscopy and Rutherford backscattering to obtain their final alloy concentration and the thicknesses. Using this information, ellipsometry spectra were fitted by representing the dielectric functions with Kramers-Kronig-consistent oscillators. Consequently, composition-dependent dielectric functions of (Bi_xIn_1-x)₂Se₃ were parameterized to create a material file that determined the composition of an unknown (Bi_xIn_1-x)₂Se₃ film. By using this material file, therefore, both the composition and thickness of a (Bi_xIn_1-x)₂Se₃ film grown subsequently can be obtained immediately at any stage of the growth cycle.