  A modulation-doping approach to control the carrier density of the
high-density electron gas at a prototype polar/non-polar oxide interface is
presented. It is shown that the carrier density of the electron gas at a
GdTiO3/SrTiO3 interface can be reduced by up to 20% from its maximum value (~
3x10^14 cm^-2) by alloying the GdTiO3 layer with Sr. The Seebeck coefficient of
the two-dimensional electron gas increases concurrently with the decrease in
its carrier density. The experimental results provide insight into the origin
of charge carriers at oxide interfaces exhibiting a polar discontinuity.
