Anomalous optical properties of bismuth ultrathin film using spectroscopic ellipsometry in the visible - Ultraviolet range

We report the study of optical properties of Bismuth (Bi) thin films with different thicknesses (tBi = 25 nm, 20 nm, 15 nm, and 5 nm) using spectroscopic ellipsometry in the visible – ultraviolet light range (1.4 – 5.3 eV). A broad absorption hump and three broad absorption peaks, as represented by the imaginary part of the dielectric constant and real part of optical conductivity, occur at 2.0 eV and 2.8 eV; 3.8 eV, and 5.0 eV, respectively, for tBi of 25 nm, 20 nm, and 15 nm. The absorption features might be related to electron transition involving px, py orbitals to orbital throughout the high symmetry line of Bi's first Brillouin zone. These absorption peaks decreased as tBi decreased from 25 nm to 15 nm. However, a significant increase in those absorption peaks when tBi = 5 nm accompanied by extra absorption peaks at 2.6 eV, 3.0 eV, and 4.1 eV. This anomalous is possibly due to the transition from three-dimension to two-dimension by the quantum confinement effect, leading to the increase of density of state as the Bi goes to ultrathin-film thickness. We did the first principle calculation (density functional theory) based on bulk Bi and Bi thin film to support this observation. By using more precise optical properties of this material, our study may provide insight into the future development of Bi-enabled technologies.