Lanthanum nickel oxide selector devices with composition-dependent forming-free threshold switching for cross-point arrays

Composition control of lanthanum nickel oxide (LNO) was investigated as a means of tuning the electrical characteristics of forming-free threshold-switching (TS) selectors. Pt/LNO/Pt devices were fabricated using LNO films with La concentrations ranging from 13% to 27%. Grazing-incidence X-ray diffraction showed that increasing the La content shifted the dominant NiO-like diffraction features toward lower 2θ angles while progressively weakening and broadening them, indicating changes in lattice spacing and a reduction in crystalline order. All compositions exhibited bidirectional TS from the first voltage sweep without a preceding electroforming process. As the La concentration increased from 13% to 27%, the threshold voltage decreased from approximately 2.54 to 1.41 V, whereas the off-state current increased by approximately 2.7-fold. The corresponding selectivity values were approximately 80, 37, and 30 for La concentrations of 13%, 18%, and 27%, respectively. The off-state current increased with electrode area, while the corresponding current density remained nearly area-independent, supporting spatially distributed, bulk-like subthreshold conduction rather than a highly localized filamentary pathway. On this basis, temperature-dependent transport was analyzed within a thermally assisted hopping (TAH) framework. With increasing La content, the activation energy decreased from 0.486 to 0.430 eV, the effective trap density increased from 3.38 × 10 19 to 9.55 × 10 19 cm −3 , and the effective inter-trap distance decreased from 3.093 to 2.189 nm. X-ray photoelectron spectroscopy independently revealed an increase in the non-lattice oxygen fraction from 2.4% to 7.9%, showing a composition dependence similar to that of the extracted trap density. Although the two quantities cannot be directly equated, their concurrent variation indicates that changes in subthreshold transport are accompanied by systematic changes in oxygen-related chemical states. Stable switching was further demonstrated over 10 10 pulse cycles in the La 0 . 18 Ni 0 . 82 O 1 . 44 device. Overall, the combined structural, electrical, and chemical-state results show that varying the La content systematically changes the off-state transport environment and thereby provides a material-design route for controlling the forming-free switching response of LNO selectors.