Investigation of the interlayer surface of $p$-Bi$_{2-x}$Sb$_x$Te$_3$ films of topological thermoelectrics by scanning tunneling spectroscopy and microscopy

The morphology of the interlayer van der Waals surface (0001) has been investigated by scanning tunneling microscopy in layered films of topological insulators $p$-Bi$_{0.5}$Sb$_{1.5}$Te$_3$ and $p$-Bi$_2$Te$_3$ prepared by discrete evaporation. A systematization of the impurity and intrinsic defects arising in the film formation process were fulfilled. It is found that in the film of $p$-Bi$_{0.5}$Sb$_{1.5}$Te$_3$ solid solution with low thermal conductivity, the density of tellurium vacancies $V_\mathrm{Te}$ and the height distortions in the distribution of Te (1) atoms on the (0001) surface increase compared to $p$-Bi$_2$Te$_3$. Local characteristics of the surface electronic states of the Dirac fermions were determined by scanning tunneling spectroscopy. The Dirac point $E_D$ shifts to the top of the valence band in the $p$-Bi$_{0.5}$Sb$_{1.5}$Te$_3$ film with high thermoelectric figure of merit. Despite the fact that the bulk films under investigation exhibit $p$-type conductivity, electrons are found on the surface of the films, as the Fermi level $V_{\mathrm{Te}}$ is located above the Dirac point $E_D$. Fluctuations of the Dirac point energy $\Delta E_D/\langle E_D\rangle$, the valence band edge $\Delta E_V/\langle E_V\rangle$, and the energy of the surface defect levels $E_p$ in $p$-Bi$_{0.5}$Sb$_{1.5}$Te$_3$ films are reduced compared to $p$-Bi$_2$Te$_3$ films due to the variation of the density of states on the (0001) surface. The obtained values of the energy gap $E_g$ in the studied films is higher than estimated by optical data due to the inversion the edges of the valence and conduction bands in topological insulators.