本研究利用鎳金屬靶進行反應性射頻磁控濺鍍製備氧化鎳薄膜，以未含金夾層氧化鎳薄膜(Al/NiO/Pt)、經氧氣氛熱處理未含金夾層氧化鎳薄膜(Al/anneal_NiO/Pt)及含金夾層氧化鎳薄膜(Al/NiO/Au/NiO/Pt)三個系統製作電阻式記憶體(RRAM)元件，由電流-電壓(I-V)及電容-電壓(C-V)特性曲線，搭配原子力顯微鏡及穿透式電子顯微鏡，嘗試了解氧化鎳電阻轉換的工作機制。
在氧化鎳的材料分析及RRAM元件電性量測方面，本研究利用穿隧式電子顯微鏡分析氧化鎳及金夾層的膜厚與微結構；利用拉塞福背向散射能譜儀及電子微探儀對氧化鎳薄膜中Ni、O原子成分進行分析；利用低掠角X光繞射儀進行氧化鎳薄膜結晶結構分析；利用四點探針及霍爾效應量測進行氧化鎳薄膜導電性質分析；利用精密半導體參數分析儀(Agilent 4156C)及電感電容電阻計量儀(Agilent 4284A)進行RRAM元件電性量測分析；最後，利用導電式原子力顯微鏡進行氧化鎳薄膜微區導電性分析。
實驗結果顯示，透過反應性射頻磁控濺鍍製備的氧化鎳薄膜，其初鍍薄膜及經熱處理後的薄膜具有多晶結構，O/Ni比值皆大於1，屬於P型半導體。但經熱處理的氧化鎳薄膜O/Ni比值會較趨近於1，且其電阻率和載子遷移率皆上升，但載子濃度大幅下降。
而三個系統的氧化鎳RRAM元件在I-V量測上，都可觀察到電阻轉換的現象。其中經熱處理後的元件，其電阻轉換成功率明顯下降。而含有金夾層的元件，藉由金夾層加入降低由高電阻態轉換至低電阻態的轉換電壓(switching voltage, Vset)， 且可降低轉換電壓值的標準差，增加元件轉換電壓的一致性。此外，三個系統在記憶時間穩定度方面，經過3600秒測試，其高電阻態與低電阻態皆能維持。三個系統RRAM元件，在高電阻態的導電機制皆屬於蕭特基發射效應，而在低電阻態的導電機制皆屬於歐姆行為。
本實驗亦量測未含金夾層氧化鎳薄膜與含金夾層氧化鎳薄膜RRAM元件的C-V特性曲線，二個系統在高電阻態的電容值皆小於低電阻態的電容值，代表於低電阻態時，薄膜內的電荷數目較高電阻態多。此外，含金夾層氧化鎳元件的高低電阻態電容值都較未含金夾層氧化鎳元件電容值高，故金夾層的加入，可能增加氧化鎳內薄膜的電荷數目。

In this research, nickel oxide(NiO) thin films were deposited by reactive sputtering from nickel(Ni) target. Resistance random access memory(RRAM) devices were fabricated using NiO film without embedded Au layer(Al/NiO/Pt), NiO film without embedded Au layer after thermal treatment in oxygen ambient(Al/anneal_NiO/Pt), and NiO film with embedded Au layer(Al/NiO/Au/NiO/Pt). The mechanism of resistive switching is explored based on the current-voltage(I-V) and capacitance-voltage(C-V) measurements of the devices, in combination with material characterization by atomic force microscopy(AFM) and transmission electron microscopy(TEM).
Thickness and micro-structure of NiO films and embedded Au layer were analyzed by using transmission electron microscopy. Rutherford Backscattering Spectrometry(RBS) and Electron Probe X-ray Microanalyzer (EPMA) were utilized to determine the composition of NiO films. The crystal structure of NiO films was identified by grazing incident angle X-ray diffraction (GIAXRD). Mobility, carrier concentration and resistivity of NiO films were determined by Hall measurement. Film resistivity of NiO films was also determined by four-point probe. The electrical properties of RRAM devices were measured by precision semiconductor parameter analyzer (Agilent 4156C) and LCR meter (Agilent 4284A). Finally, atomic force microscopy was utilized to analyze local conductivity of NiO films.
Experimental results reveal that all NiO films, before and after thermal treatment, are polycrystalline. The O/Ni atomic ratios are larger than 1, and the NiO is a P-type semiconductor. After thermal treatment at 500oC in oxygen ambient, the O/Ni atomic ratio approaches to 1. Carrier concentration is decreased, and the resistivity and mobility are increased.
Resistive switching phenomenon can be seen in all RRAM devices. Nevertheless, the device yield of resistive switching is greater for the devices without thermal treatment than the devices with thermal treatment. Switching voltage values as well as the standard deviation are both reduced for devices with embedded Au layer. In addition, current stress test indicates that the on-state and off-state maintain stable for at least 3600 seconds. The I-V conduction mechanism of all structures is ohmic at on-state, and that of all structures is Schottky emission at off-state.
RRAM devices of NiO films without and with embedded Au layer were also measured C-V curves. A distinct peak is seen in the C-V curves of all devices and the peak is larger for the device at on-state than at off-state, and it is larger for NiO with embedded Au layer than NiO without embedded Au layer. The C-V curves suggest that the number of charges in NiO is greater at on-state than at off-state. Moreover, the number of charges in NiO films increases when embedding an Au layer.

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