A Steep-slope Threshold Switching Selector Using Silver-doped Polycrystalline Zinc Oxide: Fabrication, Characterization, & Application for 3D X-point Memory & Neuromorphic Devices

An assortment of emerging non-volatile memory (NVM) devices has displayed a surge of interest in being investigated for their implementation in energy-efficient bio-inspired neuromorphic computing. The intrinsic device physics of NVMs give them the capability to be employed for emulating the dynamics of a biological neuron and synapse. NVM devices are connected in a dense cross (X)-point circuit architecture thus enabling massive system-level parallelism necessary for a neural network. However, the leakage/sneak current that typically arises from neighboring unselected memory cells is considered as a stumbling block in enlarging X-point arrays. Metalfilament threshold switch has been suggested as a selector device, demonstrated on low leakage characteristics, that holds potentiality due to its straightforward metal-insulator-metal structure, superior performance, and excellent CMOS process compatibility. This dissertation demonstrates research study on the electrical and surface characterization of nano-polycrystalline silver-doped zinc oxide (ZnO) thin films for threshold switching selector device, to propose a way for amending the prevalent selector drawbacks: threshold voltage (Vth) variabilities i.e., intercell and cycle-to-cycle shifts and lousy DC cycling endurance. The current work demonstrates a novel approach to subside system variabilities by uniformly doping a crystalline selector medium i.e., ZnO with Ag metal atoms, rather than incorporating an Ag active metal layer/electrode. First, electrochemical deposition (ECD) process has been employed to slightly dope ZnO with Ag, because of its admirable dopant concentration controllability having atomic percent precision. ECD process helps in demonstrating the proof-of-concept experiment and provides an understanding of volatile switching behavior when ZnO is lightly doped with Ag. Next, “supercycle ALD” technique has been evaluated, where alternating ZnO ALD and Ag metal ALD was employed for lightly doping/delta doping ZnO with Ag. To fend off the shortcomings/drawbacks associated with both the ECD and ALD processes, RF magnetron co-sputtering process is the last fabrication method put to evaluation. Co-sputtering technique provides the wherewithal to control Ag doping levels when lightly doped composite targets (ZnO/Ag 100-x/x at. %, x=1,3,10) are employed. The switching parameters were observed to significantly improve and the trends have been explained based on surface characterizations with XPS, GIXRD, AFM, SEM, EDAX, ICP-MS, HR-TEM, and semiconductor parameter analyzer.