Complex optical conductivity of Bi2Se3 thin film: Approaching two-dimensional limit

Two-dimensional Bi2Se3 thin films have attracted widespread attention as an ideal platform of high-performance optoelectronic applications. Understanding the intrinsic optical/electronic properties of Bi2Se3 thin films is vital for Bi2Se3-based optoelectronic applications. Here, the complex optical conductivities of a series of Bi2Se3 thin films with a varying number of quintuple layers are investigated by combining spectroscopic ellipsometry with the classical slab model over a broad spectral range of 0.73–6.43 eV. Results show that the zero-cross point of the imaginary complex optical conductivity exhibits a blueshift trend due to the enhanced coupling between the surface states as the thickness of Bi2Se3 thin film approaches the two-dimensional limit. Five feature peaks (A–E) are identified in the complex optical conductivity spectra, and their center energies exhibit interesting thickness dependencies, which are mainly attributed to the increased surface state gap due to the finite-size effects when the Bi2Se3 thin film gradually approaches the two-dimensional limit. Our work not only gives insights into the tunable optical properties of Bi2Se3 thin films but also reveals its intrinsic physical origin, which are essential and imperative for accurate modeling and design of Bi2Se3-based optoelectronic devices.