Abstract
Background: Based on the resistive switching mechanism such as electrochemical migration and redox, chemical bonding character and structural distortion from elemental substitution may affect the resistive switching properties. Therefore, in this study, the resistive switching properties of a reactive metal Ti electrode and Ln0.7Sr0.3MnO3 films, which were synthesized with differently sized trivalent lanthanide cations (Pr and Sm), were investigated in terms of their structural, chemical, and electrical properties in order to elucidate the chemical and structural effects of the lanthanide cation on the resistive switching properties and the interface characteristics.
Methods: Ln0.7Sr0.3MnO3 (Ln=Pr and Sm) films were prepared on Pt(111)/Ti/SiO2/Si(100) substrates by RF magnetron sputtering from powder targets. The crystal structures of the films were analyzed by glancing angle X-ray diffraction and the surface morphologies were investigated by atomic force microscopy analysis in non-contact mode. The chemical states of the films were analyzed by X-ray photoelectron spectroscopy and the current-voltage characteristics were measured using a semiconductor device analyzer.
Results: An internal lattice strain in Ln0.7Sr0.3MnO3 (Ln=Pr and Sm) films was increased with decreasing trivalent cation size, orthorhombic structure for Pr3+ and monoclinic structure for Sm3+ as confirmed with XRD results. An increase in the covalent Mn-O bond character was confirmed by shifts in the binding energies of Mn 2p and Mn 3s. The Mn 3s splitting magnitude remained unchanged as the trivalent cation size was decreased and the formal oxidation state of Mn ions was calculated to be +3.33. The PSMO film shows a resistive switching curve with low resistance and this was related to the larger covalent character of the Mn-O bond, which obstructs a formation of interfacial oxide with Ti top electrode.
Conclusion: Structural analysis with X-ray diffraction showed that larger internal lattice strain was developed with Sm0.7Sr0.3MnO3 film, crystallized with monoclinic structure. The Mn-O bond character became more covalent with increasing Ln3+ cation size in case of Pr3+ and this covalent bond character inhibited the formation of TiOx at the interface between Ti top electrode and Ln0.7Sr0.3MnO3 film, which affects the redox-based resistive switching characteristics and IV behavior. The smaller lattice strain with higher crystal symmetry and larger covalent character in Mn-O bond were found to induce enlarged resistive switching ratio and operating current level, respectively. The larger internal lattice strain with lower crystal symmetry by smaller size of Sm3+ trivalent cation in Ln0.7Sr0.3MnO3 film induced a reduced resistive switching ratio due to obstacled electrochemical migration.
Keywords: Cation size, crystallinity, Ln0.7Sr0.3MnO3, ReRAM, resistive switching, XPS.
Graphical Abstract
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