Resistance Switching Effect and Mechanism in Transition Metal Oxide Based RRAM Devices
|School||Wuhan University of Technology|
|Keywords||RRAM Resistive swiching ZnO CuO_x/Si WO3|
The resistive random access memory (RRAM) designed based on Electric field-induced resistive switching (RS) in transition metal oxide films has attracted considerable attention due to its potential applications in nonvolatile memory devices (NMDs). Transition-metal-oxides-based RRAM is a promising candidate due to the low price, high speed, CMOS competibility and plenty physical properties. The device fabrication and meterials exploring had a significant progress in the last ten years. However, lots of issures remains to be solved, and exactly resistance switching mechanism is still unclear. Moreover, scattering swiching parameters and uncontrollable switching hidered the industrialization progress. This paper focus on the promising RS materials:ZnO, CuOx/Si and WO3, and try to solve the key issues through designing new structures, developing new mechanism and using new research methods.By introducing AgOx buffer layer, Ag/AgOx/Zn0.8Mg0.20/Pt device with stable bipolar resistive switching (BRS) properties and minimal threshold voltages is obtained. The Ag/AgOx/Zn0.8Mg0.20/Pt device exhibits a narrow set/reset voltages and LRS/HRS distribution. The σ/μ values are6.7%,11.8%,9%and29.5%for Vset, Vreset, LRS and HRS, respectively. Both the set and reset voltages (Vset:0.11V～0.19V; Vreset:-0.14V--0.18V) are obviously lower than those in most reported oxide based rs device, which is benefical to the practical application of RRAM.The controllable transform between BRS and unipolar resistive switching (URS) is realized in Ag/AgOx/Zn0.8Mg0.2O/Pt device by pulse treatment and DC sweeps. A conductive filament model based on the competition of electrical bias caused ions migration and Joule heating induced thermal dissolution is introduced to explain the switching mechanisms and origin of switching mode transition. The field induced Ag+ions migration and the Joule heating effect are responsible for the BRS mode and URS mode, respectively.The RS became URS when electric pulses are applied, and the URS to BRS state can be occasionally triggered by simply performing I-V cycling in the negative branch. The device could be transiformed to either URS or BRS when they are required, which will expand the device’s applications. The uniform-like RS machenism is obtained in Pt/CuOx/Si/Pt device relied on the SiOx insulator layer which is formed through the redox reactions in CuOx/Si interface. The device exhibits a gradual electroforming process, marked by a gradually increasing of the device resistance and gradually decreasing of the device capacitance. The presence of SiOx layer is confirmed through the comparative test, the complex impedance spectra analysis, the symmetric Ⅰ-Ⅴ curves, the redox reactions in CuOx/Si interface, the change of the effective thickness and the AES depth profiling analysis. The SiOx layer played the key role in the uniform-like RS property in the Pt/CuOx/Si/Pt device. Acturaly, the SiOx layer grows gradually in DC sweeps, leading to the GE process.The transport properties, relaxation of LRS and the swiching machenism are studied in detail by analying Ⅰ-Ⅴ curves and AC conduction processes. The electronic transport machenism of the device belongs to space-charge-limited-current mechanism (SCLC) with the initial state without shallow trap SCLC (S-SCLC) process, indicating the fine structure of SiOx layer with little defects. However, after DC sweeps, Cu+/Cu2+ions migrate into the SiOx layer, and act as traps for electrons, leading to the S-SCLC process. Cu+/Cu2+ions migrating out from the SiOx layer result the relaxation of LRS, in which both the relaxtion time and diffusion coefficient increased. The f type (β～1) AC conductance behavior indicate the uniform contribution of relaxation center or jump barriers, which we could considered the Cu+/Cu2+ions distrubuted uniformly in SiOx layer. The present work would be meaningful for the preparation of homogeneous memristive devices.The conductive properties of WO3-x films were detected using Conducted Atomic Force Microscope (C-AFM). The conductive tunnels in the planar Au/WO3-x/Au devices could be divided into two parts, the one close to the white area (the high resistance region) exhibit arc shape and lowest resistance. The local current distributions demonstrated the lower conductivity at the grains than at grain boundaries in WO3-x/glass films. However, WO3-x/Pt films showed the higher conductivity at grains than at grain boundaries. Most of the grains have homogeneous conductivity, but in some grains, the edges have lower resistance due to the high oxide concentration.