  Ferroelectric materials are characterized by degenerate ground states with
multiple polarization directions. In a ferroelectric capacitor this should
manifest as equally favourable up and down polarization states. However, this
ideal behavior is rarely observed in ferroelectric thin films and superlattice
devices, which generally exhibit a built-in bias which favors one polarization
state over the other. Often this polarization asymmetry can be attributed to
the electrodes. In this study we examine bias in PbTiO$_3$-based ferroelectric
superlattices that is not due to the electrodes, but rather to the nature of
the defects that form at the interfaces during growth. Using a combination of
experiments and first-principles simulations, we are able to explain the sign
of the observed built-in bias and its evolution with composition. Our insights
allow us to design devices with zero built-in bias by controlling the
composition and periodicity of the superlattices.
