Microstructure and local electrical behavior in [(Nd₂Ti₂O₇)₄/(SrTiO₃) ]₁₀ (n = 4-8) superlattices

Artificial [(Nd₂Ti₂O₇)₄/(SrTiO₃) ]₁₀ superlattices (n = 4 and 8) were successfully epitaxially grown on SrTiO₃ substrates by pulsed laser deposition using the in situ high energy electron diffraction reflection diagnostic. The crystallographic relationships between Nd₂Ti₂O₇ (NTO) and SrTiO₃ (STO) (layers and substrate) were: [100]_NTO//[001]_STO, [010]_NTO//[1̄10]_STO, and (00l)_NTO//(110)_STO. Nanoscale current variation was detected on both superlattices, with the (NTO₄/STO₄)₁₀ heterostructure showing a higher density. The (NTO₄/STO₄)₁₀ sample did not show a piezoelectric response when measured by piezo-force microscopy (PFM), while ambiguous piezoactivity was observed on the (NTO₄/STO₈)₁₀ superlattice. Scanning transmission electron microscopy energy dispersive spectroscopy analysis showed the diffusion of Nd³⁺ cations on Sr²⁺ sites in SrTiO₃ structure into the multilayers, which was more pronounced when the value of n was lower. These particular nanoscale electrical behaviors, evidenced by electrical conducting channels and misleading PFM signals, were mainly attributed to the presence of oxygen vacancies in the SrTiO₃ layers at higher concentrations near the interface and to the mixed valence state of the titanium (Ti³⁺/Ti⁴⁺). This work showed the strong influence of interface structure on nanoscale electrical phenomena in complex oxide superlattices.