Structural Defects Improve the Memristive Characteristics of Epitaxial La_0.8Sr_0.2MnO₃‐Based Devices

Abstract

of Moore's law in mind, industry and academia moved the focus to new approaches and alternative materials. In this scenario, resistive random access memories (RRAM) are considered promising candidates for the next generation of nonvolatile memories, due to their fast writing speed, high storage density, high endurance, long retention times, and low energy consumption. [1] By applying an electric stimulus, these devices exhibit a reversible change of resistance retained under zero-field conditions, and hence high and low resistance states (HRS and LRS, respectively) can be defined. In recent years, a great effort has been carried out to explain the origin of the effective change in resistance, and a wide range of possible mechanisms have been reported in the literature. [1,2] Some of the switching mechanisms rely on purely electronic effects, such as charge-trapping at an interface, modification of a Schottky barrier, or changes induced by electronic charge in strongly correlated electron systems of oxides, which can lead to metal-insulator transitions (MIT). On the other hand, ionic transport (drift) has also been reported as a mechanism able to induce a change of resistance in memristive devices. In electrochemical metallization memories, cations from an active electrode migrate reversibly in an insulating matrix, reaching the inert electrode and leading to a change in resistance. In valence change memories (VCMs) with electrode/oxide/electrode configuration, where the oxide can be insulating, such as HfO 2 , [3] TaO x , [4,5] SrTiO 3 , [6] or semiconducting, [7] such as LaMnO 3 , [8] or La 2 NiO 4 , [9][10][11] the resistance is modified by the drift of oxygen ions (or oxygen vacancies) and the concomitant local valence change of the cationic sublattice. Nonetheless, in many cases electronic and ionic effects interplay to give raise to the memristive response. [12,13] The switching event in VCMs can take place at different scales and the memories can be classified as: i) filamentary, where the change of resistance and migration of oxygen takes place in a very localized region, ii) interface-type, where the mechanism occurs at the electrode/oxide interface, [2] or iii) volume-type, which can be considered an extension of the interface-type where eventually the whole volume of the oxide is involved in the change of resistance. [14] Although so far most of the technological effort has been devoted to filamentary-type memories, these devices suffer from strong cycle-to-cycle and cell-to-cell variability. The variability has been associated to the electroforming step, and Interface-type valence change memories (VCMs) are exciting candidates for multilevel storage in resistive random access memories (RRAM) and as artificial synapses for neuromorphic computing. Several materials have been proposed as VCM candidates and, depending on the materials and electrodes of choice, different switching mechanisms take place leading to the change in resistance. Here, the focus is on La 0.8 Sr 0.2 MnO 3-δ (LSM) perovskite and, par...

Epitaxial La0.5Sr0.5MnO3‐δ Bipolar Memristive Devices with Tunable and Stable Multilevel States

“…In our previous work, we showed that the LSM20/LAO-based devices show reproducible and multilevel resistive switching, that was largely improved by the presence of dislocations. [3] The LSM20 devices required larger operating voltage and the HRS/ LRS window was smaller, typically between 1.3 and 5 for V amp = 4 to 20 V. In the present work, we exploited the easy formation of oxygen vacancies in LSM50 to obtain larger operating windows at smaller voltages, i.e., 2.1 to 184 for V amp = 4 to 12 V. These results clearly show that it is possible to improve and optimize the memristive properties similar devices by modifying the Sr substitution of the manganite, leading to changes in ionic and electronic conductivity of the material.…”

“…[15,46] The metal-organic precursors, La(thd) 3 , Sr(thd) 2 -tetraglyme, and Mn(thd) 3 , [thd = 2,2,6,6,-tetramethyl-3,5-heptanedionato] were purchased from Strem Chemicals and were dissolved in m-xylene from Alfa Aesar. As an extension to the composition preliminary study, [3] the cationic concentration in the solution was optimized to grow films with 50% strontium substitution and [(La+Sr)/Mn] ratio close to 1. The total concentration of the solution was 22.5 mM, with [(La+Sr)/Mn] precursor ratio of 2.3 and [Sr/(La+Sr)] precursor ratio of 0.8.…”

“…[22] While the RS performance depends on many material properties and device parameters, for ease of comparison the geometry of the devices and the electrode material for LSM50 devices are the same as those previously reported for LSM20. [3] In LSM50, the La 3+ substitution by Sr 2+ is typically accommodated by the oxidation of Mn 3+ into Mn 4+ , which creates an electron-hole, and formation of oxygen vacancies (δ ). [22][23][24] According to Mizusaki et al [22] and supported by computational works, [25,26] the formation of oxygen vacancies in LSM50 is more favored than in any other Sr substituted LSM composition, in agreement with the differences in tracer bulk diffusion coefficient (10 -16 cm 2 s -1 and 2 × 10 -15 cm 2 s -1 for LSM20 and LSM50 at 700 °C, respectively).…”

“…The memristive properties of LSM50 are studied using titanium as an active electrode. As for other material combinations, [3,11,13,[33][34][35][36][37][38][39] the Ti/LSM50 stack can undergo a reversible redox reaction that originates two additional interlayers: oxidized metal electrode and oxygen-deficient oxide. The resistivity of both interlayers is higher than the parent compound.…”

The impact of Mn nonstoichiometry on the oxygen mass transport properties of La0.8Sr0.2Mn y O3±δ thin films

“…In this work, we focus on the impact of B-site deficiency on oxygen mass transport properties (i.e. oxygen diffusivity and surface exchange coefficient) of La 0.8 Sr 0.2 Mn y O 3±δ (LSM y ), a material relevant for diverse applications, such as solid oxide fuel cells and electrolyzers, supercapacitors [20] and resistive switching [21]. The defect chemistry of bulk LSM is strongly dominated by cation vacancies, which give rise to the oxygen hyper-stoichiometry observed in oxidizing conditions [22,23].…”