Investigation of Oxide-Based Materials Applied to Nonvolatile Memory Devices

Abstract

In this dissertation some kinds of resistive random access memory (RRAM) devices with various kinds of oxide-based materials as the resistive switching layers are fabricated and its characteristics are investigated First the fabrication and characterization of a RRAM with a Ti/TaO2/Pt structure at room temperature are reported According to the X-ray photoelectron spectroscopy (XPS) analysis of the deposited TaOx film and the data fitting of the XPS spectrum data the deposited TaOx film was then confirmed to be in the TaO2 phase The fabricated device exhibits bipolar resistance switching behavior over one hundred DC switching cycles and shows stable retention characteristics for over 104 s under a 100 mV stress It is also found that the electrical conduction mechanism in set process is firstly dominated by Ohmic conduction and then transforms to the space-charge-limited process in the high resistance state Finally the conduction mechanism turns to Ohmic conduction again in low resistance state The other part is described the fabrication and analysis of the RRAM cell with Ta dopant into silicon oxide layer by co-sputtering at room temperature From the I-V measurements it is found that the current conduction in high resistive state is first dominated by Ohmic conduction caused by the intrinsic carriers in the Ta:SiOx thin film and then turned to space-charge-limited-current mechanism In low resistance state the current conduction mechanism from higher voltage to lower voltage are dominated by Fowler–Nordheim tunneling and then SCLC mechanism and finally dominated by Ohmic conduction mechanism Furthermore it is found that the fabricated Ti/Ta:SiOx/Pt RRAM cell is durable and reliable Additionally the fabrication and characterization of resistance switching for a RRAM with a Ti/In2O3:SiO2/Pt structure in room temperature are reported It is found that the device exhibited bipolar resistance switching behavior over one hundred switching cycles and shows stable retention characteristics for over 104 sec under 100mV stress condition The asymmetric phenomenon of the carrier conduction mechanism at high resistance state is also explored by fitting the I-V curves and explained by the schematic energy band diagram It is also found that the switching behavior is due to the migration of oxygen ions Furthermore the fabrication and characterization of RRAM with Ni/ZnO/HfO2/Ni structure at room temperature are reported It is found that the proposed device exhibited bipolar switching behavior with multilevel characteristics in reset process and it exhibited two-step reset stage under high reset bias By applying a 2nd set process after the transformation of the 2nd reset stage it is found that the RRAM can return to the initial state From I-V curves measured in these two reset stages it is found that the current conduction is dominated by Schottky emission due to the migration of oxygen ions and recombination with oxygen vacancies This reaction can break the conducting filament so the carrier transport mechanism transforms to Schottky emission This also results in the simultaneous transformation from low resistance state to high resistance state Finally the fabrication and characterization of RRAM with Ti/MgZnO/Pt structure at room temperature were reported Four different resistive states are obtained by applying different stop voltages for the reset process These four resistance states show good retention characteristics without any degradation and can be clearly distinguished from one another by more than 10 000 second under 100mV stress The current transport mechanism is dictated by a Schottky emission as the stop voltage increases from 1 to 1 5 V The mechanism of multilevel RS is investigated and band diagrams are used to explain the multilevel RS phenomenon associated with Ti/MgZnO/Pt RRAM devices

Date of Award	2016 Jul 5
Original language	English
Supervisor	Shoou-Jinn Chang (Supervisor)