Designer Topological Insulator with Enhanced Gap and Suppressed Bulk Conduction in Bi 2 Se 3 /Sb 2 Te 3 Ultrashort-Period Superlattices

A novel approach to suppress bulk conductance in three-dimensional (3D) topological insulators (TIs) using short-period superlattices (SLs) of two TIs is presented. Evidence for superlattice gap enhancement (SGE) was obtained from the reduction of bulk background doping from 1.2 × 10 cm to 8.5 × 10 cm as the period of Bi Se /Sb Te SLs is decreased from 12 nm to 5 nm. Tight binding calculations show that, in the ultrashort-period regime, a significant SGE can be achieved for the resulting SL. Ultrathin short-period SLs behave as new designer TIs with bulk bandgaps up to 60% larger than the bandgap of the constituent layer of largest bandgap, while retaining topological surface features. Evidence for gap formation was obtained from ellipsometric measurements. Analysis of the weak antilocalization cusp in low-temperature magneto-conductance confirms that the top and bottom surfaces of the SL structure behave as Dirac surfaces. This approach represents a promising platform for building truly insulating TIs.