Interplay of the strain and microstructure in ferroelectric epitaxial CaTiO3 Films

CaTiO3 (CTO) was predicted to become ferroelectric under lattice strain. However, other factors such as oxygen octahedral tilts or microstructural details can play a role. In this work, two 20 nm CTO films were grown on LSAT and NGO by PLD. They both show ferroelectricity, with Tc near 140 K on LSAT and near 70 K on NGO, and the remnant polarization at 10K of 5 and 2 $\mu $C/cm, respectively. This is surprising given that the strain of CTO on both substrates is similar. AC-STEM shows two major differences in microstructure between two CTO films: Firstly, the first few nm of CTO on NGO show perfect epitaxial growth, and after that grains start to develop, but the c-axis of CTO remains aligned with the c-axis of NGO, suggesting the presence of 180$^{\circ}$ grain boundaries only. However for CTO/LSAT, grains begin to develop at the interface and their c-axes have two possible orientations, resulting in both 180$^{\circ}$ and 90$^{\circ}$ grain boundaries. These grain boundaries are either dislocations or ferroelastic twins. Secondly, the octahedral tilt behavior at the film/substrate interface is different: CTO/LSAT has a 5-6 unit cell transition region from the untilted LSAT to the tilted CTO, which is not the case in CTO/NGO. The connection between the microstructure, substrate strain and connections to the ferroelectric properties will be discussed in detail.