  Ferroelectric tunnel junctions (FTJs) have attracted increasing research
interest as a promising candidate for non-volatile memories. Recently,
significant enhancements of tunneling electroresistance (TER) have been
realized through modifications of electrode materials. However, direct control
of the FTJ performance through modifying the tunneling barrier has not been
adequately explored. Here, adding a new direction to FTJ research, we
fabricated FTJs with BaTiO3 single barriers (SB-FTJs) and BaTiO3/SrTiO3
composite barriers (CB-FTJs), and reported a systematic study of FTJ
performances by varying the barrier thicknesses and compositions. For the
SB-FTJs, the TER is limited by pronounced leakage current for ultrathin
barriers and extremely small tunneling current for thick barriers. For the
CB-FTJs, the extra SrTiO3 barrier provides an additional degree of freedom to
modulate the barrier potential and tunneling behavior. The resultant high
tunability can be utilized to overcome the barrier thickness limits and enhance
the overall CB-FTJ performances beyond those of SB-FTJ. Our results reveal a
new paradigm to manipulate the FTJs through designing multilayer tunneling
barriers with hybrid functionalities.
