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Experimental verification of polar structures in ultrathin BaTiO_{3} layers using resonant x-ray reflectivity

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.