  The epitaxial growth of multifunctional oxides on semiconductors has opened a
pathway to introduce new functionalities to semiconductor device technologies.
In particular, ferroelectric materials integrated on semiconductors could lead
to low-power field-effect devices that can be used for logic and memory.
Essential to realizing such field-effect devices is the development of
ferroelectric metal-oxide-semiconductor (MOS) capacitors, in which the
polarization of a ferroelectric gate is coupled to the surface potential of a
semiconducting channel. Here we demonstrate that ferroelectric MOS capacitors
can be realized using single crystalline SrZrxTi1-xO3 (x = 0.7) that has been
epitaxially grown on Ge. We find that the ferroelectric properties of
SrZrxTi1-xO3 are exceptionally robust, as gate layers as thin as 5 nm
corresponding to an equivalent-oxide-thickness of just 1.0 nm exhibit a ~ 2 V
hysteretic window in the capacitance-voltage characteristics. The development
of ferroelectric MOS capacitors with nanoscale gate thicknesses opens new
vistas for nanoelectronic devices.
