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

TL;DR

This study uses scanning tunneling microscopy to analyze local chemical potential variations in (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ thin films, revealing fluctuation amplitudes, length scales, and effects of magnetic fields, aiding in optimizing film quality.

Contribution

It provides detailed characterization of chemical potential disorder in topological insulator thin films, offering guidelines for reducing potential fluctuations in material synthesis.

Findings

Potential fluctuation amplitude ranges from 5 to 40 meV.

Fluctuation length scales are between 13 and 54 nm.

Magnetic fields induce Landau levels with broadening related to quantum drift.

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.