Nickel oxide (NiO) is a binary metal oxide that is found to have resistive switching (RS) properties, which makes it a viable candidate for next generation resistive random access memories. The demand for high-density memories has concentrated on RS materials with scalable dimensions. In these nanoscaled devices, the presence of point and line defects, composition inhomogeneity, and atomic inter-diffusion interfaces have been shown to have a significant impact on properties.
The objective of this study is to investigate the role of interface morphology and chemistry in RS behaviour of NiO epitaxial nanocrystals fabricated on (001) SrTiO3 (STO) substrates [1]. NiO nanostructures are found to have {111} and {113} orientated facets. The epitaxial relationship between the nanostructure and the substrate can be confirmed. The crystallographic orientations of the nanostructures are [110]NiO//[110]STO inplane and [100]NiO//[100]STO out of plane, confirming that the nanostructure and substrate are aligned in a cubic-on-cubic fashion.
Aberration corrected high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) reveals the interface between the NiO nanocrystals and the underlying STO substrate to be rough, irregular, and have a lower average atomic number than the substrate or the nanocrystal [1]. Energy dispersive x-ray spectroscopy and electron energy loss spectroscopy confirm both chemical disorder and a shift of the energy of the Ti L2,3 peaks. Analysis of the O K-edge profiles in conjunction with this shift, implies the presence of oxygen vacancies at the interface. This sheds light into the origin of the previously postulated minority carriers’ model to explain resistive switching in NiO [2].
HRTEM and HAADF-STEM reveal that the substrate is deformed so as to back-fill around the base of the nanostructure. Remarkably, the substrate under the nanocrystal is physically pulled towards the nanostructure, thus forming a rim around the nanocrystal [4].