  Using interlayer interaction to control functional heterostructures with
atomic-scale designs has become one of the most effective interface-engineering
strategies nowadays. Here, we demonstrate the effect of a crystalline LaFeO3
buffer layer on amorphous and crystalline LaAlO3/SrTiO3 heterostructures. The
LaFeO3 buffer layer acts as an energetically favored electron acceptor in both
LaAlO3/SrTiO3 systems, resulting in modulation of interfacial carrier density
and hence metal-to-insulator transition. For amorphous and crystalline
LaAlO3/SrTiO3 heterostructures, the metal-to-insulator transition is found when
the LaFeO3 layer thickness crosses 3 and 6 unit cells, respectively. Such
different critical LaFeO3 thicknesses are explained in terms of distinct
characteristic lengths of the redox-reaction-mediated and
polar-catastrophe-dominated charge transfer, controlled by the interfacial
atomic contact and Thomas-Fermi screening effect, respectively. Our results not
only shed light on the complex interlayer charge transfer across oxide
heterostructures but also provides a new route to precisely tailor the
charge-transfer process at a functional interface.
