Influence of post-deposition annealing on the transport properties of sputtered Bi2Se3 thin films

Abstract

The fabrication of chalcogenide-based topological insulator (TI) thin films with low defects and vacancies is an important research problem. In the past decade, Bi2Se3 is one of the most investigated TI materials. The physical properties of Bi2Se3 are dictated by selenium vacancies irrespective of the growth method. Controlling the number of vacancies is very challenging, but it can be influenced by substrate and growth temperature. In this work, we demonstrate that post-deposition annealing temperature is also effective in controlling the carrier concentration of Bi2Se3 thin films. Ultra-thin Bi2Se3 films were fabricated on quartz substrates using magnetron sputtering at room temperature, and their resistivity, bulk carrier concentration, and bulk mobility are measured as a function of the post-deposition annealing temperature under vacuum conditions ranging from 180–350 °C. We find that the carrier concentration can vary by an order of magnitude, and we obtained values as low as ~1 × 1019 cm−3 between 200-250 °C which compares very well with some of the best literature reports on films made by molecular beam epitaxy. Room temperature bulk mobility values in excess of 100 cm2/Vs were recorded for the optimally annealed samples. Energy dispersive x-ray spectroscopy measurements showed that selenium-rich Bi2Se3 films have the highest carrier concentration values. X-ray diffraction measurements showed that the best crystallographic properties are obtained for stoichiometric Bi2Se3. Our results indicate that higher mobility values may be possible if better crystal structure and lower resistivity are obtained in selenium-rich films.