Preparation Of Hybrid Improper Ferroelectric Ca₃Ti₂O₇ Thin Films And Characterization Of Electrical Properties At Micro-nano Scale

Hybrid improper ferroelectrics have appealed lots of interest due to their great potential applications in multiferroicity with strong magnetoelectric coupling at room temperature.The layered hybrid improper ferroelectric Ca₃Ti₂O₇（CTO）with Rudlesden-Ropper structure exhibits novel properties such as negative piezoelectric behaviors and rich charged domain walls.As one of the important multi-functional materials,ferroelectric thin film has attracted great attention,but there is little research about CTO thin film at present.In this work,high-quality CTO thin films were prepared on different substrates by pulsed laser deposition（PLD）,and the ferroelectric and electrical transport characteristics were systematically investigated.（1）A series of CTO films were grown on conductive（001）oriented Nb-STO substrates,and the oxygen pressure and laser energy were adjusted to obtain flat films.Preferred conditions are 2 Pa and 350 m J.The lattice constants of the CTO pseudotetragonal structure and the STO cubic structure at room temperature were analyzed,and it was concluded that the long axis of the CTO film matched the five STO unit cells,and the mismatch was only 0.04%.The XRD pattern shows that the matching relationship is[010]CTO//[010]Nb-STO.?scans and RSM tests further verify the epitaxial relationship.CTO films with different oxygen pressures and different laser energies were grown on Pt-Si substrates when other growth conditions were unchanged.Oxygen pressure of 2 Pa and laser energy of 350 m J resulted in a smooth surface.The XRD pattern shows that CTO thin films on the Pt-Si substrates are polycrystalline films with the preferred orientation of（010）with a higher amount of b-axis-oriented grains compared to the bulk.CTO films were epitaxially grown on Dy Sc O₃ and Gd Sc O₃substrates,subjected to different degrees of tensile strain.Meanwhile,the CTO film on La Al O₃ substrate suffers from severe compressive strain.CTO on the Mg O substrate is the polycrystalline growth mode due to the large lattice mismatch.（2）The electrical properties of CTO films with different thicknesses on Nb-STO and Pt-Si substrates were analyzed.The CTO films on different substrates are relatively flat and uniform in the thickness range of 20-65 nm,which will not cause topographic errors in PFM images.Amplitude and phase images of the 5×5μm~2 area were acquired after the 3×3μm~2 area was poled by a negative voltage,and then,the central 1×1μm~2 area was poled by a positive voltage,respectively.By analyzing the contrast in PFM images,it was found that 180°ferroelectric polarization reversal occurred in the in-plane and out-of-plane of CTO films with different thicknesses.It is an unambiguous demonstration of the ferroelectricity in CTO/Nb-STO and CTO/Pt-Si at room temperature and also verifies the epitaxial matching relationship between the Nb-STO substrate and the CTO film.Applying positive and negative voltages to different regions of the sample through the AFM cantilever tip,the current mapping is obtained by c-AFM module with a reading sample bias of-10 V.A resistive switching behavior with at least two orders of magnitude is observed in CTO films driven by their ferroelectric polarization reversal.In addition,there are obvious self-polarization phenomena in the CTO films on Nb-STO and Pt-Si substrates,which may be due to the coupling between polarization and strain gradient.Amplitude-voltage curves were tested from-10 V to+10 V,and the coercive field of the films increased with increasing thickness.The polycrystalline CTO thin films on Pt-Si substrates,unlike their bulk polycrystalline ceramics,exhibit a positive piezoelectric effect.