Optical properties of Bi₂Se₃: from bulk to ultrathin films

Abstract

We report on the determination of the dielectric functions of Bi 2 Se 3 thin films and bulk material. The Bi 2 Se 3 thin films with thicknesses ranging from 3–54 quintuple layers (QL) were grown by molecular beam epitaxy on GaAs(1 1 1)B substrates and the optical properties were determined from spectroscopic ellipsometry in the range of 0.5 eV–6 eV. We observed five absorption bands in the bulk sample, with a strong maximum near 2 eV, which were also present in the films down to 19 QL. Reducing the number below 19 QL in the Bi 2 Se 3 films caused dampening and broadening of the bulk absorption bands below 2 eV, and a shift to a higher energy of the band near 2 eV. Our experimental results thus provide evidence of marked changes in the joint density of states of Bi 2 Se 3 below 19 QL, indicating that the whole bulk band structure is affected for the ultrathin epilayers.