Layer-dependent optical properties of PtSe₂ determined from a single film

In this work, we have used in situ spectroscopic ellipsometry to investigate the thickness-dependent dielectric functions of PtSe₂, a transition metal dichalcogenide material that exhibits interesting optical and electrical properties. Continuous ellipsometry measurements were obtained while a ∼25 nm-thick PtSe₂ film was grown using molecular beam epitaxy. By using Kramers-Kronig-consistent oscillators to represent the dielectric functions, we fit the ellipsometry spectra to obtain the dielectric function and the thickness of PtSe₂ during its entire growth cycle. The dielectric function changes significantly with thickness, where the centroid of three oscillators used to represent the band-to-band transitions of PtSe₂ redshifts as the thickness increases, reaching a saturation value as the film approaches ∼20 nm. Furthermore, the Drude contribution also changes with thickness. More significantly, we can decipher the sample composition as a function of time by using an effective medium approximation-model to represent the film.