Investigation of Switching mechanism, Thermal, Electrochemical and Structural properties of Solid Electrolytic, Superionic α-AgI based Silver Molybdate glass for Resistive Memory (RRAM) Applications

Abstract

Developing efficient, fast performing and thermally stable AgI-Ag2O-MoO3glasses are of great interest for Resistive Random Access Memory (RRAM) applications; however there many challenges such as metallization in bulk, behavior of Vth profile over composition and corrosion reactions. In this thesis work, fast ion conducting (FIC) AgI-Ag2O-MoO3 glasses have been investigated with an idea to solve some technical challenges such as thermal stability, corrosion etc. with the help of deep understanding of the material. Employing various experimental and characterization techniques, this research work aims to identify the links between various material and technical aspects and how to tune these aspects to solve the challenges envisaged. Bulk AgI-Ag2O-MoO3 (50:25:25) glasses have been prepared by melt quenching method (Microwave heating and quenched between two heavy steel plates). The electrical switching experiments have been carried out using a Keithley Source Meter (model 2410) controlled by Lab VIEW 6i, on samples of thicknesses (d) 0.1, 0.2 and 0.3 mm at different ON state currents (Imax) (3 mA, 2 mA, 1 mA, 0.6 mA, 0.4 mA and 0.25 mA); It has been found that these samples exhibit fast near ideal memory switching. The power dissipation (P) increases with both d and Imax. It is also found that the threshold voltage (Vth) increases with d; and for a given thickness, the Vth decreases with increasing Imax. A sample of d = 0.1 mm exhibits near ideal memory switching with the least P for Imax = 0.25 mA. These samples can be used for fast switching applications with minimum power dissipation. Further, the electrical switching behavior of bulk, FIC (AgI)50+x-(Ag2O)25-(MoO3)25-x, for 10 ≤ x ≤ -10 glasses has been investigated, in order to understand the switching mechanism of bulk samples with the inert electrodes. It is found that by using inert electrodes, the switching becomes irreversible, memory type. In these samples, the switching mechanism is an electrochemical metallization process. The inert electrodes restrain ionic mass transfer; however exhibit a low barrier to electron transfer allowing the cathodic metallization reaction to reach Nernst equilibrium faster. The cations involved in this process transport thorough the free volume within the glass structure and follows Mott-Gurney (MG) model for electric field driven thermally activated ion hopping conductivity. This model along with the thermal stability profile provide a narrow region within composition with better switching performance based on swiftness to reach Vth and less power loss. It is found that traces of anionic contribution to metallization are absent. Moreover, anodic oxidation involves reactions that cause bubble formation and corrosion which becomes evident from SEM (Scanning electron Microscope) micrographs of the switched and un-switched parts of the sample. Rigidity percolation phenomena in (AgI)50+x-(Ag2O)25-(MoO3)25-x, for 5 ≤ x ≤ -12.5 has been observed by performing calorimetry (ADSC) and photoelectron spectroscopy experiments (XPS). The temperature dependence of heat capacity (normalized Cp) at glass transition temperature (Tg), exhibits fluctuations for samples with higher AgI concentration indicating the fragile nature of the glass. The composition range chosen in the present study, accommodates both the fragile and strong glasses, and the fragility threshold. Cp (absolute) values, at Tg, exhibits abrupt sign shift at this threshold. The negative Cp is identified as a thermodynamic behavior of nanoclusters. The XPS study shows the formation of covalent structural units, [‒Mo‒O‒Ag‒O‒] and complex molybdenum oxides in the positive Cp region. Finally, the non-reversing enthalpy profile, exhibits square well minima, sandwiched between floppy and stress rigid region, which has been identified to be the intermediate phase, within the range 32.25 ≤ MoO3 concentration ≤ 35. Electrochemical Impedance Spectroscopy (EIS) and Raman studies have been performed on this glass, over a wide range of composition ((AgI)50+x-(Ag2O)25-(MoO3)25-x, for 3.75 ≤ x ≤ -10.5) to understand the features of structure, ion migration and their correlation. These features essentially involve diffusion and relaxation. The coefficients associated with diffusion process, especially, the diffusion coefficient, diffusion length and relaxation time has been determined by applying Nguyen-Breitkopf method. Besides, by tuning the concentration of the constituents, it is possible to obtain samples which exhibit two important structural characteristics, namely fragility and polymeric phase formation. The present study essentially addresses these issues and endeavors to figure out the corroboration among them. The relaxation behavior, when scrutinized in the light of Diffusion Controlled Relaxation (DCR) model, ascertains the fragility threshold which is also identified as the margin between the two types of polymeric phases. Simultaneously, it fathoms into the equivalent circuitry, its elements and their behavioral changes with above mentioned features. The power law behavior of A.C. conductivity exhibits three different non-Jonscher type dispersive regimes along with a high frequency plateau. The sub-linearity and super-linearity remain significantly below and above the Jonscher’s carrier transport limit, 0.5 ≤ n ≤ 0.9. Finally, by observing the behavior of the crossover between these sun-linear and super-linear (SLPL) regime, an intuitive suggestion has been proposed for the appearance of SLPL: oxygen vacancy formation.