Ferroelectric resistive switching for non-volatile memory application

Abstract

Among the investigation of new emerging non-volatile memories, ferroelectric tunnel junction (FTJ) composed of an ultrathin ferroelectric layer sandwiched by two electrodes acting as a barrier for tunneling, is attracting great attention due to its non-destructive operation (write and read). FTJ also holds advantages of higher density and high scalability in design and improvement in resistive switching. Therefore, FTJ offers an opportunity to be one of the promising non-volatile memory alternatives for future memory industry. In this thesis, perovskite oxide based FTJs are studied for application in resistive switching memory, in particular, the role of ferroelectricity in ferroelectric tunneling and mechanism of polarization-dependent resistive switching are studied. FTJs using BaTiO₃ and (Bax,Sr1-x)TiO₃ as well as multiferroic tunnel junctions are fabricated by pulse-laser deposition and magnetron sputtering techniques. By comprehending the mechanism of switching and structure in ferroelectrics, significant enhancement of ON/OFF ratio (ratio of high resistance and low resistance that differentiate the logic state) has been achieved through strain engineering. Furthermore, by integrating tunnel electroresistance and tunnel magnetoresistance originated from ferroelectricity and ferromagnetism, respectively, multistate and multifunctional memory characteristics have been achieved. In the first part, by applying mechanical stress to the junction based on Pt/BaTiO₃/Nb:SrTiO₃ structure, dynamic strain modulation in FTJ with ultrathin ferroelectrics film has been successfully achieved. With an extra compressive strain induced by mechanical stress, which is dynamically applied beyond the lattice mismatch induced compressive strain, giant electroresistance changes at room temperature are obtained with the tunable potential barrier height and width. This result shows the enhancement in ON/OFF ratio from 10¹ to 10⁷ due to the variation of BaTiO3 polarization where in-plane lattice parameter is compressively stressed from ε = -0.18 % to -4.30 %. This result also confirms the role of ferroelectricity in the FTJ resistive switching. In the second part, investigation on ferroelectric-induced resistive switching in FTJ is also carried out in Pt/(Ba,Sr)TiO₃/Nb:SrTiO₃ heterostructure with strain engineering, where the role of ferroelectricity of BST on resistive switching in FTJ is revealed. The results not only enhance the ferroelectricity and ON/OFF ratio by strain engineering, but also offer another role for Ba₀.₈Sr₀.₂TiO₃ in memory application. In the third part, a multiferroic tunnel junction with the heterostructure of Ni₀.₈₁Fe₀.₁₉/BaTiO₃/ La₀.₇Sr₀.₃MnO₃/SrTiO₃ is studied. Electrical transport properties of the ferroelectric ultrathin BaTiO₃ film as well as magnetic properties of two dissimilar ferromagnetic layers Ni₀.₈₁Fe₀.₁₉ and La₀.₇Sr₀.₃MnO₃ are characterized. Ferroelectric switching-induced resistive change memory with ON/OFF ratio of 16 and 0.3% TMR effect have been successfully achieved in this tunneling structure at low temperatures. These results provide insight into the structural properties at ferroelectric perovskite oxide based FTJs and demonstrate the possibility for further enhancement in resistive switching for memory application which is related to the strength of ferroelectric polarization. By studying of mechanism in ferroelectric tunneling and polarization-dependent resistive switching, these results thus are promising for providing FTJs an alternative way to improve and fulfill the requirement in the field of non-volatile memory.