Investigation of Raman depolarization ratio in topological insulator Bi2Se3 epitaxial films

Bismuth selenide (Bi2Se3) epitaxial films were grown at different substrate temperatures (T (s)) by molecular beam epitaxy. In the optimization of T (s), a Bi2Se3 film with the smallest root mean square roughness (Rq) and the lowest electron density was grown at T (s) = 120 degrees C. In the optimized growth condition, Bi2Se3 epitaxial films with 3-85 quintuple layers (QL: 1 QL approximate to 1 nm) were grown. In unpolarized Raman spectra, the positions of A (1) (1g) and E (2) (g)-modes shifted to the low wavenumbers at film thickness below 10 QL. The polarized Raman spectra showed that the depolarization ratio rho also changed with film thickness. The change in rho was more significant than the change in the position shift. This indicates that the measurement of rho is an effective method to evaluate the thickness of Bi2Se3 ultrathin films.