FABRICATION AND CHARACTERIZATION OF METAL-OXIDE RESISTIVE RANDOM-ACCESS MEMORY DEVICES FOR DATA STORAGE AND ARTIFICIAL SYNAPSE

Abstract

Resistive random-access memory (ReRAM) employing metal-oxides can serve the main requirements of a non-volatile memory (NVM) technology, e.g., area scaling, low power operation, and multi-level/cell (MLC) storage as needed for both data storage and neuromorphic synapses. The thesis aims to address both valance change (VCM) and electrochemical metallization (ECM) ReRAM emphasizing the same objectives. To obtain MLC in a VCM candidate, a multi-layer dielectric stack containing ZrOx/AlOx/HfOx was optimized so that upto eight-level/cell low power MLC had been ensured based on the spatial location of AlOx layer within a fixed stack thickness. Furthermore, we did numerical modeling to demonstrate the amount of taper in conductive filament (CF) that primarily governs MLC characteristics in this ReRAM, which had been later validated by fabrication and characterization of the device. Moreover, it had been demonstrated that the same optimized ReRAM stack could behave as a synapse as its conductance variation was remarkably linearized with SET and RESET pulse optimization. We demonstrated pulse timing optimization and optimization of leakage characteristics of the device based on spatial location of AlOx barrier layer within the stack, which had been found crucial to ensure CF stability and also to having excellent synaptic behavior. As oxygen vacancy (VO) states determine CF stability in VCM, the importance of spatial and energy mapping of those states is of prime importance. For the same purpose, zinc oxide (ZnO) was selected as a reference storage medium for VO profiling through Al/ZnO/Al ReRAM, as bandgap and vacancy formation enthalpy of ZnO are lower than the conventional metal-oxide based dielectrics. Both shallow and deep level states were probed employing a novel pulsed low frequency noise (LFN) method which we have outlined here, and leakage that impacts the resistance contrast between high resistance state (HRS) and low resistance state (LRS) had been investigated through VO, photoluminescence (PL) and experimental current-voltage (I-V) characteristics correlation. Further, we have demonstrated grain boundary steered switching in an AlOx/SiOx VCM cell, and have demonstrated CF instability within a few quantum conductance scale (G0) due to formation of quantum point contact (QPC) at the conductance bottleneck within the dielectric medium, which was carefully tailored by selective O3 anneal provided to the middle of the stack. Finally, we have fabricated an ECM by embedding platinum Nanodisks (Pt-ND) within storage medium (IGZO) of an Ag/IGZO/Pt-ND/IGZO/ITO ReRAM, and demonstrated its light sensitive switching characteristics and degradation behavior due to the extraordinary optical activity of Pt-ND.