Evolution of the surface state in Bi₂Se₂Te thin films during phase transition

Abstract

Topological insulators, a new quantum state of matter, have created exciting opportunities for studies in topological quantum physics and for exploring spintronics applications due to their gapless helical metallic surface states. In this study, thin films composed of alternate layers of Bi and Se (Te) ({Bi(3 Å)Te(9 Å)}n/{Bi(3 Å)Se(9 Å)}n) were fabricated by controlling the layer thickness within the atomic scale using thermal evaporation techniques. The high-purity growth of uniform Bi2Se2Te1 thin films has not yet been achieved using a thermal evaporation method. However, as a result of a self-ordering process during annealing, an as-grown amorphous film with p-type polarity could transform into single crystalline Bi2Se2Te1 with n-type polarity. Using THz-time domain spectroscopy (THz-TDS) and ultraviolet photoemission spectroscopy (UPS), we concluded that the conductivity is dominated by the Drude contribution, suggesting the presence of a quantum well state and surface states. Moreover we demonstrated that the emission of terahertz waves from the (001) surface of the single crystalline Bi2Se2Te1 thin film would be possible under the excitation of 790 nm femtosecond optical pulses, indicating the presence of a Dirac-fermion, a photo-Dember effect at the surface state and the transient current within the surface depletion region. The results reported herein provide useful information regarding a valuable deposition method that can be useful in studies of the evolution of surface state electrons in topological insulators.

Enhanced Photoinduced Carrier Generation Efficiency through Surface Band Bending in Topological Insulator Bi2Se3 Thin Films by the Oxidized Layer

“…Moreover, electron− hole separation is affected by the built-in potential at the interface between the oxide layer and Bi 2 Se because the difference in the work function induces a built-in potential. 32,33 As the results for the maximum band bending and the maximum photocurrent occurred at an exposure time of 24 h, the cause of the increment in the photocurrent is related to surface band bending. In the valence band region shown in Figure 3b, the point at which the kinetic energy is 21.22 eV (binding energy is zero) is the Fermi level, and a peak is observed up to 6 h. As this peak disappears on increasing the exposure time, it could be caused by the Dirac surface state of the TIs.…”

“…In the XPS results, as Bi–oxide was formed up to 24 h, the change in the work function of Bi 2 Se 3 may be caused by the surface band bending from the Bi–oxide. Moreover, electron–hole separation is affected by the built-in potential at the interface between the oxide layer and Bi 2 Se because the difference in the work function induces a built-in potential. , As the results for the maximum band bending and the maximum photocurrent occurred at an exposure time of 24 h, the cause of the increment in the photocurrent is related to surface band bending. In the valence band region shown in Figure b, the point at which the kinetic energy is 21.22 eV (binding energy is zero) is the Fermi level, and a peak is observed up to 6 h. As this peak disappears on increasing the exposure time, it could be caused by the Dirac surface state of the TIs. , In the Bi 2 Se 3 capped by the selenium, to prevent oxidation, the peak was maintained until the selenium was removed just before the UPS measurement (Figure S2).…”

MAX-Phase Films Overcome Scaling Limitations to the Resistivity of Metal Thin Films

“…To further investigate carrier transport, the TDS signals were Fourier-transformed to generate frequency-dependent spectral functions. Subsequently, the TDS transmittance was converted into complex conductance spectra based on Tinkham’s formula that is valid for the ultrathin-film limit . This formula is given by t fs / t s = 1/[1 + Z 0 G /(η s + 1)], where t fs and t s are the complex transmission coefficients of the sample (film and substrate) and bare substrate, respectively.…”

“…Subsequently, the TDS transmittance was converted into complex conductance spectra based on Tinkham's formula that is valid for the ultrathin-film limit. 32 This formula is given by t fs /t s = 1/ [1 + Z 0 G/(η s + 1)], where t fs and t s are the complex transmission coefficients of the sample (film and substrate) and bare substrate, respectively. In addition, Z 0 is the vacuum impedance, G is the conductance of the film, and η s is the refractive index of the bare substrate.…”

Quasicrystalline phase-change memory

“…Internal reflections in substrate were excluded and time-domain signals were then Fourier-transformed to obtain energy (frequency)dependent spectral functions. Complex transmittance was normalized and converted to complex conductance spectra based on Tinkham's formula 38 , which is valid for the ultrathin-film limit studied here. This formula is given by t fs /t s = 1/[1 + Z 0 G/(η s + 1)], where t fs and t s are the complex transmission coefficients of the sample (film plus substrate) and the bare substrate, respectively.…”