Wide band gap topological insulator via band engineering

Molecular beam epitaxy (MBE) growth of Bi₂Se₃ and Sb₂Te₃ is hindered by a high bulk background doping due to selenium vacancies (n-type) and tellurium anti-sites defects (p-type). In order to explore the unique properties of these topological insulators (TIs), the bulk carriers must be suppressed. We previously presented the growth of TI/TI superlattices (SLs). By growing SLs of Bi₂Se₃ and Sb₂Te₃, which have a type III band alignment, a reduction of bulk background doping by more than one order of magnitude, from 1.2x10²⁰ cm^-3 to 8.5x10¹⁸ cm^-3 was observed as the period was decreased from 12 nm to 5 nm. We attributed this to the formation of SL bands that create an enhancement of the bulk bandgap. Here we present tight binding calculations, which predict a SL gap of up to 400% larger than the bandgaps of the constituent layers when the SL is grown with the appropriate thicknesses. The calculations also predict the preservation of the Dirac cone, along with these large bandgaps. Measurements of magnetoconductance confirm preservation of the surface states for a SL with period thickness of 5nm. Direct measurement of the bandgaps by FTIR spectroscopy will also be reported.