  Polar discontinuities and redox reactions provide alternative paths to create
two-dimensional electron liquids (2DELs) at oxide interfaces. Herein, we report
high mobility 2DELs at interfaces involving SrTiO3 (STO) achieved using polar
La7/8Sr1/8MnO3 (LSMO) buffer layers to manipulate both polarities and redox
reactions from disordered overlayers grown at room temperature. Using resonant
x-ray reflectometry experiments, we quantify redox reactions from oxide
overlayers on STO as well as polarity induced electronic reconstruction at
epitaxial LSMO/STO interfaces. The analysis reveals how these effects can be
combined in a STO/LSMO/disordered film trilayer system to yield high mobility
modulation doped 2DELs, where the buffer layer undergoes a partial
transformation from perovskite to brownmillerite structure. This uncovered
interplay between polar discontinuities and redox reactions via buffer layers
provides a new approach for the design of functional oxide interfaces.
