Chemical Potential Fluctuations in Topological Insulator (Bi${}_{0.5}$Sb${}_{0.5}$)${}_2$Te${}_3$ Films Visualized by Photocurrent Spectroscopy

We investigate the photocurrent properties of the topological insulator (Bi${}_{0.5}$Sb${}_{0.5}$)${}_2$Te${}_3$ on SrTiO${}_3$-substrates. We find reproducible, submicron photocurrent patterns generated by long-range chemical potential fluctuations, occurring predominantly at the topological insulator/substrate interface. We fabricate nano-plowed constrictions which comprise single potential fluctuations. Hereby, we can quantify the magnitude of the disorder potential to be in the meV range. The results further suggest a dominating photo-thermoelectric current generated in the surface states in such nanoscale constrictions.