  In ferroelectric materials, spontaneous symmetry breaking leads to a
switchable electric polarization, which offers significant promise for
nonvolatile memories. In particular, ferroelectric tunnel junctions (FTJs) have
emerged as a new resistive switching memory which exploit
polarization-dependent tunnel current across a thin ferroelectric barrier. Here
we demonstrate FTJs with CMOS-compatible Zr-doped HfO$_2$ (Zr:HfO$_2$)
ferroelectric barriers of just 1 nm thickness, grown by atomic layer deposition
on silicon. These 1 nm Zr:HfO$_2$ tunnel junctions exhibit large
polarization-driven electroresistance (19000$\%$), the largest value reported
for HfO$_2$-based FTJs. In addition, due to just a 1 nm ferroelectric barrier,
these junctions provide large tunnel current (> 1 A/cm$^2$) at low read
voltage, orders of magnitude larger than reported thicker HfO$_2$-based FTJs.
Therefore, our proof-of-principle demonstration provides an approach to
simultaneously overcome three major drawbacks of prototypical FTJs: a
Si-compatible ultrathin ferroelectric, large electroresistance, and large read
current for high-speed operation.
