Resistive random-access memory (RRAM) is considered a promising next generation nonvolatile memory for its simple metal-insulator-metal structure, lower operating voltage, faster switching speed, low power potential, and possible compatibility with semiconductor fabrication processes. Among many oxide switching materials, HfO₂ has attracted wide attention for its high dielectric constant, good thermal stability and CMOS compatibility. However, HfO₂-based RRAM devices still face important challenges, including forming voltage, leakage current, unstable memory window, poor cycle to cycle uniformity, and limited endurance. Therefore, development of forming-free, low-voltage, low-leakage, and stable HfO₂-based RRAM remains an important research topic.
In this thesis, Au/WTe₂/HfO₂/Ag RRAM devices were fabricated on Si/SiO₂ substrates to investigate the effects of WTe₂ interface insertion and defect control of RF-sputtered HfO₂ on forming-free switching behavior. Au was used as the bottom electrode, WTe₂ was introduced as an interfacial layer, HfO₂ served as the main switching layer and Ag was used as active top electrode. WTe₂ and HfO₂ were deposited by RF sputtering, while Au and Ag electrodes got deposited by electron-beam evaporation. Main purpose of this work was not only to obtain forming-free switching, but also to improve the quality of forming-free switching by lowering the switching voltage, suppressing leakage current, increasing the ON/OFF ratio, and improving switching stability and endurance.
Control devices were fabricated to clarify the role of each layer. The Au/WTe₂/Ag device showed ohmic or short-circuit-like behavior, indicating that WTe₂ alone cannot act as the main insulating switching layer. The Au/HfO₂/Ag control device showed forming-free resistive switching, but it exhibited higher SET/RESET voltage, lower ON/OFF ratio, poor cycle-to-cycle uniformity, and limited endurance. After inserting the WTe₂ interfacial layer, the Au/WTe₂/HfO₂/Ag device maintained forming-free switching while showing lower switching voltage, a larger memory window, better switching stability, and higher endurance.
The HfO₂ deposition condition strongly affected device performance. HfO₂ deposited at 100 W produced short-circuit-like or ohmic behavior for both tested deposition times. In contrast, reducing the sputtering power to 50 W and increasing the deposition time to 1200 s enabled clear forming-free resistive switching. In addition, oxygen flow during HfO₂ sputtering further improved device performance. The device prepared under the Ar:O₂ = 20:4 condition showed the best overall behavior, with an ON/OFF ratio of approximately 1.76 × 10⁵ and endurance of approximately 148 cycles, while still maintaining low-voltage operation. These results suggest that oxygen incorporation during HfO₂ sputtering reduces excessive defect-related leakage and stabilizes the high-resistance state.
Based on the electrical results, the switching mechanism is proposed to be mainly Ag-based electrochemical metallization (ECM) with oxygen-vacancy-assisted HfO₂ behavior. Under an applied electric field, Ag atoms may oxidize into Ag⁺ ions, migrate through the HfO₂ layer, and reduce to form a metallic conductive filament. During RESET, the filament may partially rupture or dissolve, and the device returns to high-resistance state. WTe₂ layer is proposed to modify the bottom interface and help stabilize filament formation and rupture. Overall, this work demonstrates that WTe₂ interface engineering combined with oxygen-flow-controlled HfO₂ sputtering is an effective approach for improving the quality of forming-free HfO₂-based RRAM devices.

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