Characterizing the chemical potential disorder in the topological insulator (Bi${}_{1-x}$Sb${}_x$)${}_2$Te${}_3$ thin films

Abstract

We use scanning tunneling microscopy and spectroscopy under ultra-high vacuum and down to 1.7 K to study the local variations of the chemical potential on the surface of the topological insulator (Bi${}_{1-x}$Sb${}_x$)${}_2$Te${}_3$ thin films (thickness 7 - 30 nm) with varying Sb-concentration $x$, to gain insight into the charge puddles formed in thin films of a compensated topological insulator. We found that the amplitude of the potential fluctuations, $\Gamma$, is between 5 to 14 meV for quasi-bulk conducting films and about 30 - 40 meV for bulk-insulating films. The length scale of the fluctuations, $\lambda$, was found to span the range of 13 - 54 nm, with no clear correlation with $\Gamma$. Applying a magnetic field normal to the surface, we observe the condensation of the two-dimensional topological surface state into Landau levels and find a weak but positive correlation between $\Gamma$ and the spectral width of the Landau-level peaks, which suggests that quantum smearing from drift motion is the source of the Landau level broadening. Our systematic measurements give useful guidelines for realizing $({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x{)}_2{\mathrm{Te}}_3$ thin films with an acceptable level of potential fluctuations. In particular, we found that $x\approx 0.65$ realizes the situation where $\Gamma$ shows a comparatively small value of 14 meV and the Dirac point lies within $\sim$10 meV of the Fermi energy.

arXiv comment23 pages total; 13 pages of main text with 9 figures, 10 pages of supplement with 10 figures. The raw data are available at the online depository Zenodo with the identifier https://doi.org/10.5281/zenodo.13889543

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