金屬電極對電阻式記憶體阻值切換機制以及碘化鉍結晶性之影響

Correlation of Bottom Metal Electrodes with the Properties and Resistive Switching Behaviors of BiI3

吳雨恬(Yu-Tien Wu)

指導教授 ： 吳志毅

國立臺灣大學/電機資訊學院/光電工程學研究所/碩士(2020年)

https://doi.org/10.6342/NTU202003344

摘要

本研究中，利用碘化鉍分別成長在活性金屬電極與惰性金屬電極上，探討其結晶性、型態與化學反應，進而研究其製成電阻式記憶體後的電阻切換效應。我們發現當碘化鉍成長於金電極上時，利用XPS和UPS皆可觀察到元件內部形成自建的金屬燈絲，故元件初始為低阻態，需透過Negative forming process方可切換至高阻態。但由於元件中缺少離子儲存層，故元件穩定性不佳。而當碘化鉍成長於銀電極上時，可明顯看到其結晶形態與在金電極上時不同，亦可由XPS觀察到銀與碘離子會反應使金屬鉍被還原出來，又藉由降低銀在元件中的含量，可降低Negative forming所需之電流與電壓。此種碘化鉍製成的電阻式記憶體元件，展現了良好的特性，可達9個級距的on/off ratio、10000秒以上的記憶儲存時間、1200次以上的開關，與≤500ns的高速操作在±1V內，藉由set process時的限流亦可控制元件的阻值，使該元件具有6個記憶儲存階段。這些特性顯示了碘化鉍是具有發展潛力的電阻切換材料，而本研究亦提供電阻式記憶體材料選擇的新觀點，有助於未來元件設計與電阻式記憶體結合光電元件的應用。

關鍵字

電阻式記憶體 ； 碘化鉍 ； 多階段儲存 ； 下電極

並列摘要

In this thesis, the layered material BiI3, which is one of the precursors of the lead-free perovskite, was proven to have good potential for use as a resistive switching layer in RRAMs, and demonstrated promising performance because of its anisotropic carrier transport, high absorption coefficient, and structural stability. The influence of the underlying metal substrate on the chemical and physical properties of the BiI3 top layer, and the device performance of the corresponding RRAM, were systematically studied. The surface roughness and morphology of the samples were studied by atomic force microscopy and scanning electron microscopy, respectively. The crystallinity and crystal phase of BiI3 were studied using X-ray diffraction. The chemical state and electronic structure of the BiI3/metal heterostructure were studied using X-ray and ultraviolet photoemission spectroscopy. The mechanism of resistive switching behavior was studied by analyzing the I–V characteristics and cross-sectional image of the device using tunneling electron microscopy.
The physical properties and chemical states of the BiI3 layer were found to be strongly dependent on the underlying metal substrates. The upward diffusion of Ag, and the self-formation of a conductive filament consisting of metallic Bi, were observed, and were found to affect the resistive switching behavior. In addition, the Ag plays a critical role in device stabilization and on the quantity of the conductive filament, which initially required a negative forming process.
By coating the Au bottom electrode with Ag, the forming voltage of the device was significantly reduced, and the device was much more stable than the Au/BiI3/Au device. The RRAM device with a structure of Au/10 nm Ag/BiI3/Au demonstrated an ultrahigh on/off ratio of 109, good retention of at least 104 s, 1200 switching cycles, at least 6 different storage state, and a short switch speed of ≤ 500 ns at a low operating pulse of ±1 V.
In summary, the BiI3 RRAM exhibits a high on/off ratio, good stability, multi-state data storage, and fast operation, and is hence an exciting new prospect for the development of RRAMs. In addition, this research on the effect of the underlying bottom electrode will assist the device design and material selection of the BiI3 RRAM and its optoelectronic devices.

並列關鍵字

resistive-random access memory ； bismuth iodide ； multi-state storage ； bottom electrode

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