FTIR Characterization of Bi₂Se₃/Sb₂Te₃ Ultrashort-Period Superlattices

Topological insulators (TIs) are novel materials that have a band structure comprising an insulating bulk and highly conductive surface states. These surface states exhibit exotic properties of interest in fundamental physics and novel applications, such as spintronics and quantum computing. In this context, much interest has developed surrounding the ability to alter and enhance the bandgap of the TI while reducing the bulk doping, which may compromise the effects of the unique surface properties. Previous studies of Bi₂Se₃/Sb₂Te₃ ultrashort-period superlattices predicted the effects that period thickness may have on the TI bandgap, with the intent to maximize topological phenomena [Nano Lett. 20, 3420 (2020)]. In this study, superlattices were grown by molecular beam epitaxy and characterized using Fourier Transform Infrared Spectroscopy (FTIR) to measure absorption and observe changes in bandgap as a function period thickness. Coupling variable temperature FTIR measurements and doping determination from Hall transport, we estimated the bandgap of each sample. The analysis shows shifts in the bandgap energy as the period is reduced. We present those results and discuss the influence of period thickness, confinement and doping on the optical transitions.