Atomic-scale Investigation of Structural evolution and Resistive Switching Behaviors in Lanthanum cobaltite -Based Resistive Random-Access Memory

Resistive random-access memory (RRAM) is considered for next-generation non-volatile memory (NVM) owing to its simple devices, low cost and high storage density. Resistive switching occurs in a wide range of materials among the transition metal oxides (TMO). In this work, we utilize epitaxial ternary metal oxides layer, LaCoOx (LCO), which grows on Nb-doped SrTiO3 (Nb-STO) substrate as RRAM device. We deposited Au/Ti metal as the top electrode, and measured the SET and RESET process with more than 900 cycles. To reveal the resistive switching behaviors, we use the high-resolution transmission electron microscope (TEM) and Atomic-scale scanning transmission electron microscopy (STEM) to observe the structural evolution and oxygen-ion migration in LaCoOx. From the TEM results and the corresponding Fast-Fourier-Transform Diffraction pattern (FFT-DP), the functionalities of LaCoOx films can be manipulated by distinct voltage. It is clearly demonstrated that the structure changes from monocrystalline to polycrystalline.<br/> Further demonstrate structural evolution, we change the top electrode to Pt/Ag. According to previous study, we know that the topotactic phase transformation between perovskite and brownmillerite phases can be performed for epitaxial LaCoOx films. In recent years, topotactic phase transformation has attracted much attention owing to their potential physical and electronical properties for RRAM devices. Under negative bias, the Pt/Ag/LCO /Nb-STO RRAM device can switch from the pristine high resistance state (HRS) to the LRS, which is known as the SET process. As we continuously applied voltages sweep cycles, the structure will transform from LaCoO2.67 to brownmillerite LaCoO2.5 with perovskite LaCoO3 regions. Additionally, we use X-ray photoelectron spectroscopy (XPS) to demonstrate the Co valence changed for further studying micro-structural evolution and the resistive switching behaviors. This study not only revealed the oxygen-ion migration of LaCoOx but also proved it to be the promising candidate for RRAM application.