Defect landscape and electrical properties in solution-derived LaNiO3 and NdNiO3 epitaxial thin film

Details concerning the deposition of nanostructured ReNiO3 thin films by chemical solution deposition method grown on different single crystal substrates have not been investigated systematically.

Z-contrast images from LNO and NNO thin films grown onto LAO (a-b) and LSAT substrate (c-d). Misfit dislocations and RPFs are pointed out with orange and yellow arrows, respectively. Inset shows new spots appearing at the Fourier Transform (FFT) of the NNO layer due to the Nd rippling, which are marked with yellow circles. Scale bar: 5 nm.

We elucidate a fundamental link between strain and the most common defect observed in Nickelate films, the Ruddlesden-Popper fault (RPF)

Our results show that the presence of a large amount of RPFsis clearly detrimental to the conductivity, and that their generation and concentration depend on the used substrate, strain and growing conditions.

Rare-earth nickelates RNiO3 are known to display metal-to-insulator transitions which are associated with a variation of the unit cell volume as a function of temperature and the radius of the rare-earth material. The stabilization of a RENiO3 crystal demands a raise of Ni oxidation state from the most common oxidation state, +2, to +3, which in the bulk is possible only under very high oxygen pressure. An alternative route for obtaining large size single crystalline material is the growth of thin films. To the best of our knowledge, several works in literature report a significant understanding on the ReNiO3 thin films deposited with pulsed laser deposition, sputtering and all oxide molecular beam epitaxy. However, details concerning the deposition of nanostructured ReNiO3 thin films by chemical solution deposition method grown on different single crystal substrates have not been investigated systematically.

In this work we report the microstructure of chemical solution deposition epitaxial NdNiO3 and LaNiO3 thin films grown onto different single crystal substrates with thicknesses ranging between 6-25 nm. We have used aberration-corrected scanning transmission electron microscopy (STEM) to provide new structural insights on the occurrence of structural defects and distortions in the case of tensile and compressive strained NdNiO3 and LaNiO3 thin films. We have elucidated the link between strain and the most common defect observed in ReNiO3 films, the Ruddlesden-Popper fault, which ultimately affects the electrical properties of these films. In addition, we unveil an unforeseen polar-like distortion at either side of the Ruddlesden-Popper fault.

The combination of STEM with the geometrical phase analysis (GPA) software used in this work provides a way to obtain accurate depth-wise strain information, a key parameter in confined systems such as thin films.

Researchers

B. Mundet1, J. Jareño1, J. Gazquez1, M. Varela2, X. Obradors1 and T. Puig1

1 Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, Campus UAB, 08193 Bellaterra (Barcelona) Spain

2 GFMC, Dept. de Fisica de Materiales, Universidad Complutense de Madrid, 28040 Madrid, Spain.