Thickness-dependent bulk electronic properties in Bi2Se3 thin films revealed by infrared spectroscopy

We have investigated the electronic structure of Bi2Se3 epitaxial thin films with thicknesses between 15 and 99 quintuple layers (QL) on a Si substrate using a combination of variable angle spectroscopic ellipsometry and infrared transmission spectroscopy. The results we have obtained are consistent with a Fermi level that shifts relative to the conduction band as a function of sample thickness. We also present evidence that the bulk energy gap in these thin films is as much as 0.1 eV smaller than the value of 0.3 eV predicted by band structure calculations and confirmed by photoemission experiments. The thickness dependence of material properties in Bi2Se3 observed in this work reveals thickness to be a parameter that can be tuned to control and possibly optimize the bulk properties of Bi2Se3 thin films.