Abstract
We investigate the surface states of topological insulator (TI) Bi2Se3 thin films grown on Si nanocrystals and Al2O3 substrates by using terahertz (THz) emission spectroscopy. Compared to bulk crystalline Bi2Te2Se, film TIs exhibit distinct behaviors in the phase and amplitude of emitted THz radiation. In particular, Bi2Se3 grown on Al2O3 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 Bi2Se3 films is inclined with respect to the plane of the Al2O3 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.
Highlights
Topological insulators (TIs) behave as a charge-gapped insulator in their interior but hosting a spinmomentum-locked Dirac state at the surface
As a quasi-two-dimensional system, Bi2Se3 can Conclusions We demonstrate that the terahertz (THz) emission spectroscopy can be a sensitive tool to investigate the fine details of the electrostatics and surface crystallography of topological insulator thin films
For the Bi2Se3 thin film grown on Si nanocrystals, the emitted THz electric field has an opposite phase to that from the bulk Bi2Te2Se crystal, and this can be attributed to different electrostatics related to the surface band bending of two samples
Summary
Topological insulators (TIs) behave as a charge-gapped insulator in their interior but hosting a spinmomentum-locked Dirac state at the surface. Surface-sensitive techniques can be utilized to characterize a Dirac dispersion of the surface state; Fermi surface information can be extracted from Shubnikov-de Haas oscillations, and in particular, the Terahertz (THz) spectroscopy can provide useful information about the surface state of the TIs. From conventional THz time-domain spectroscopy, Aguilar et al could retrieve optical response functions, such as optical conductivity, of the TI surface state, and determine electrodynamic parameters of Dirac fermions [18, 19]. Whereas the THz wave can be emitted from the acceleration of photocarriers generated by an illumination of a pulsed laser onto TIs, a change in THz intensity with a variation of the bulk carrier density could be satisfactorily explained by considering the contribution of Dirac fermions together with bulk charge carriers, which provided useful information about the mobility of surface carriers
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