  Functional devices with ultrathin ferroelectric layers have been attracted as
a promising candidate for next-generation memory and logic device applications.
Using the ultrathin ferroelectric layers, particularly approaching the
two-dimensional limit, however, it is still challenging to control
ferroelectric switching and to observe ferroelectricity by spectroscopic tools.
In particular, conventional methods such as electrical measurements and
piezoelectric response force microscopy are very limited due to leakage
currents and the smallness of the ferroelectric signals. Here, we show that the
ferroelectricity of ultrathin SrRuO3/BaTiO3/SrRuO3 heterostructures grown on
SrTiO3(100) substrates can be measured using resonant x-ray reflectivity
(RXRR). This experimental technique can provide an element-specific electronic
depth profile as well as increased sensitivity to Ti off-center displacements
at the Ti K pre-edge. The depth-sensitivity of RXRR selectively detects the
strong polarization dependence of the Ti pre-edge features of ultrathin BaTiO3
layers while discriminating the contribution of the SrTiO3 substrate. This
technique verified that the BaTiO3 layer can be ferroelectric down to the
lowest experimental limit of a critical thickness of 2.5 unit cells. Our
results can open a novel way to explore ultrathin ferroelectric-based
nano-electronic devices.
