Anisotropic Terahertz Emission from Bi(2)Se(3) Thin Films with Inclined Crystal Planes

We investigate the surface states of topological insulator (TI) Bi(2)Se(3) thin films grown on Si nanocrystals and Al(2)O(3) substrates by using terahertz (THz) emission spectroscopy. Compared to bulk crystalline Bi(2)Te(2)Se, film TIs exhibit distinct behaviors in the phase and amplitude of emitted THz radiation. In particular, Bi(2)Se(3) grown on Al(2)O(3) shows an anisotropic response with a strong modulation of the THz signal in its phase. From x-ray diffraction, we find that the crystal plane of the Bi(2)Se(3) films is inclined with respect to the plane of the Al(2)O(3) substrate by about 0.27°. This structural anisotropy affects the dynamics of photocarriers and hence leads to the observed anisotropic response in the THz emission. Such relevance demonstrates that THz emission spectroscopy can be a sensitive tool to investigate the fine details of the surface crystallography and electrostatics of thin film TIs.