  A novel approach to reduce bulk conductance by the use of short period
superlattices (SL) of two alternating topological insulator layers is
presented. Evidence for a superlattice gap enhancement (SGE) was obtained from
the observed reduction of bulk background doping by more than one order of
magnitude, from 1.2x1020 cm-3 to 8.5x1018 cm-3 as the period of Bi2Se3/Sb2Te3
SLs is decreased from 12 nm to 5 nm, respectively. Tight binding calculations
show that in the very thin period regime, a significant SGE can be achieved by
the appropriate choice of materials. The ultrathin SL of alternating Bi2Se3 and
Sb2Te3 layers behaves as a new designer material with a bulk bandgap as much as
60% larger than the bandgap of the constituent layer with the largest bandgap,
while retaining topological surface features. Analysis of the weak
antilocalization (WAL) cusp evident in the low temperature magneto-conductance
of a very thin period SL sample grown confirms that the top and bottom surfaces
of the SL structure behave as Dirac surface states. This approach represents a
promising and yet to be explored platform for building truly insulating bulk
TIs.
