在傳統的電阻式記憶體當中，元件的主動層通常是單層材料(single oxide layer)，但是單層材料的主動層無法滿足現今微縮化製程所需的多功能需求。因此在近年來有許多人投入了雙層主動層(bilayer oxide)的研究領域，希望能夠藉由兩種不同材料的特性結合來達到所需的功能並改善元件的性質，但對於雙層主動層的電阻轉換機制與電性表現的關係鮮有探討。
本實驗以磁控濺鍍法鍍製氧化鉭(TaOx)薄膜和以旋轉塗佈的方式製備溶液法合成的氧化鋅奈米顆粒(ZnO)薄膜，分別製作單層主動層元件: Ta/TaOx/Pt和Ta/ZnO/Pt；以及雙層主動層元件: Ta/TaOx/ZnO/Pt和Ta/ZnO/TaOx/Pt。藉由電性的表現與材料分析的結果嘗試探討電阻轉換機制。
在電性量測方面，利用精密半導體參數分析儀(Agilent 4156 C)量測電阻式記憶體元件的電流-電壓特性曲線及統計各元件電阻轉換時的寫入電壓(Vset)、寫入電流(Iset)、抹除電壓(Vreset)、抹除電流(Ireset)以及抹除功率(Preset)的結果並計算各電阻轉換參數的標準差值。在材料分析方面，利用穿透式電子顯微鏡鑑定各元件薄膜厚度和結晶情況與觀察界面平整度，利用X-ray繞射分析儀鑑定ZnO的晶體結構，利用X光光電子能譜儀(XPS)鑑定Ta/TaOx/ZnO/Pt和Ta/ZnO/TaOx/Pt元件在Ta/TaOx、Ta/ZnO、TaOx/ZnO、ZnO/TaOx界面處Ta的價數。最後利用同步輻射X光來分析Ta/TaOx/Pt與Ta/ZnO/Pt兩種不同元件的O K-edge吸收圖譜隨著入射光角度改變的變化。
由電性分析的結果顯示，Ta/ZnO/Pt元件不具電阻轉換行為，且元件的初始狀態為低電阻態。Ta/TaOx/Pt、Ta/TaOx/ZnO/Pt和Ta/ZnO/TaOx/Pt元件皆具有電阻轉換的行為，且元件初始的狀態皆為高電阻態，元件操作的特性皆有正偏壓寫入、負偏壓抹除以及負偏壓寫入、負偏壓抹除的行為。從元件I-V特性的比較，Ta/TaOx/Pt元件在負偏壓抹除之後於高阻態的電流最小，而Ta/TaOx/ZnO/Pt元件於高阻態的電流最大。在正偏壓寫入、負偏壓抹除的方式下掃描元件，統計的結果顯示，Ta/TaOx/Pt元件具有最大的平均寫入電壓(Vset)，其值為3.77 V，Ta/TaOx/ZnO/Pt元件有最小的平均寫入電壓，其值為3.03 V。由寫入電壓(Vset)、寫入電流(Iset)、抹除電壓(Vreset)、抹除電流(Ireset)以及抹除功率(Preset)所計算的標準差結果，Ta/TaOx/Pt元件所有電阻轉換參數的標準差皆為最小，Ta/TaOx/ZnO/Pt元件的標準差皆為最大。
在材料分析方面，由TEM橫剖面影像圖的結果觀察到Ta/TaOx/ZnO/Pt元件在Ta/TaOx與TaOx/ZnO界面都相當不平整，Ta/ZnO/TaOx/Pt元件在Ta/ZnO和ZnO/TaOx界面較為平整，TEM與X-ray繞射分析結果顯示ZnO薄膜為多晶結構。從XPS縱深分析及能譜分峰來看，Ta 4f的分峰結果顯示Ta/TaOx/ZnO/Pt元件與Ta/ZnO/TaOx/Pt元件在TaOx內部皆有Ta2+、Ta4+和Ta5+的化學鍵結能態，在Ta/TaOx/ZnO/Pt元件中，Ta5+的含量在TaOx/ZnO界面處為最高，其比例為47.5%，在Ta/TaOx界面處為最低，其比例為31.9%；在Ta/ZnO/TaOx/Pt元件中，Ta5+的含量在ZnO/TaOx界面為最高，其比例為51.5%，Ta/ZnO界面為最低，其比例為35.7%。從O 1s的分峰結果顯示，兩組元件的ZnO內部的氧空缺濃度皆比TaOx內部的氧空缺濃度大。最後XAS的分析結果觀察到Ta(20 nm)/TaOx/Pt結構的O K-edge的吸收圖譜在Ta/TaOx界面處有兩層不同氧含量的界面存在。
從電性分析的結果與材料分析的結果，我們建立了氧空缺移動的電阻轉換機制模型來解釋Ta/TaOx/Pt、Ta/TaOx/ZnO/Pt與Ta/ZnO/TaOx/Pt元件在高電阻態的電流大小與電阻轉換參數的差異。

In the traditional resistive random access memory device, the active layer is usually composed of a single active layer. However, the single active layer is not able to satisfy the specifications for a number of device requirements when a memory to be scalable down in recent years. Therefore, several studies on double active layer RRAMs have been reported in this research field. For this reason, many reports put in efforts on fulfilling the specific requirements and improve the device performance by combining two different materials characteristics. But the underlying switching mechanisms and its corresponding electrical characteristics are rarely studied.
In this work, we have prepared tantalum oxide (TaOx) thin film by reactive sputtering from tantalum target (Ta), as well as zinc oxide nanoparticles (ZnO NPs) thin films by spin-coating with the ZnO NPs synthesized via solution process. Next, we deposited tantalum by reactive sputtering as top electrode to fabricate single active layer: Ta/TaOx/Pt and Ta/ZnO/Pt, as well as double active layer: Ta/TaOx/ZnO/Pt and Ta/ZnO/TaOx/Pt devices on Pt/Ti/SiO2 substrates respectively. Then, we compare the electrical characteristics of the Ta/TaOx/Pt, Ta/ZnO/Pt, Ta/TaOx/ZnO/Pt and Ta/ZnO/TaOx/Pt devices. On the basis of material analysis results, we try to develop the resistive switching mechanism of double layer devices.
For the aspects of electrical characteristics, the I-V curves of the devices are measured by precision semiconductor parameter analyzer (Agilent 4156C). Thereafter, we show the statistical distribution of the set voltage (Vset), the set current (Iset), the reset voltage (Vreset), the reset current (Ireset) and the reset power (Preset) on the Ta/TaOx/Pt, Ta/TaOx/ZnO/Pt and Ta/ZnO/TaOx/Pt devices when operating in bipolar and unipolar switching modes and derive standard deviation of the resistive switching parameters. Regarding to the material characteristics, TEM cross-section image is carried out to examine the thin film thickness, crystallinity and interface roughness in these devices. The characteristics of the ZnO crystalline structure is determined from X-ray diffraction technique (XRD). X-ray photoelectron spectroscopy (XPS) analysis was applied to investigate the chemical state of TaOx at the Ta/TaOx, Ta/ZnO, TaOx/ZnO, ZnO/TaOx interface in Ta/TaOx/ZnO/Pt and Ta/ZnO/TaOx/Pt devices. Finally, the X-ray absorption spectroscopic measurements were carried out to investigate the O K-edge absorption spectra depending on the incident light angle in Ta/TaOx/Pt and Ta/ZnO/Pt devices.
The I-V curve measurement shows that the Ta/ZnO/Pt device has no switching phenomenon and its initial is in the low resistance state (LRS). On the contrary, the Ta/TaOx/Pt, Ta/TaOx/ZnO/Pt and Ta/ZnO/TaOx/Pt devices perform the resistive switching behavior and the initial state are kept in high resistance state (HRS). On the other hand, the Ta/TaOx/Pt, Ta/TaOx/ZnO/Pt and Ta/ZnO/TaOx/Pt devices can be wrote with positive bias, erased with negative bias and wrote with negative bias, erased with negative bias respectively. Comparing from the I-V characteristics, the current flow in the Ta/TaOx/ZnO/Pt device is higher than the Ta/TaOx/Pt device in HRS. The average set voltage in Ta/TaOx/Pt device is 3.77 V, which is the highest and the average set voltage is 3.03 V in Ta/TaOx/ZnO/Pt device which is the lowest among the Ta/TaOx/Pt, Ta/TaOx/ZnO/Pt and Ta/ZnO/TaOx/Pt devices from the statistical measurements and the standard deviation obtained from resistive switching parameters in Ta/TaOx/Pt is the lowest and the highest in Ta/TaOx/ZnO/Pt device.
The TEM cross-section image show that the Ta/TaOx and TaOx/ZnO interface in Ta/TaOx/ZnO/Pt device are quite uneven, however, the Ta/ZnO and ZnO/TaOx interface are smooth in Ta/ZnO/TaOx/Pt device. The ZnO thin film is polycrystalline from the observation of the TEM cross-section image and X-ray diffraction pattern. The XPS depth profile spectra and deconvoluted spectra show that Ta2+, Ta4+ and Ta5+ chemical bonding states obtained from TaOx films in Ta/TaOx/ZnO/Pt and Ta/ZnO/TaOx/Pt devices. The amount of Ta5+ at TaOx/ZnO is 47.5%, which is the highest, and the amount of Ta5+ at Ta/TaOx is 31.9% which is the lowest in Ta/TaOx/ZnO/Pt device. The amount of Ta5+ at ZnO/TaOx is 51.5%, which is the highest, and the amount of Ta5+ at Ta/ZnO is 35.7% which is the lowest in Ta/ZnO/TaOx/Pt device. The concentration of oxygen vacancy in ZnO are higher than in TaOx from the O 1s fitted binding energy peaks no matter in Ta/TaOx/ZnO/Pt and Ta/ZnO/TaOx/Pt devices. Finally, we observe two distinct interface near Ta/TaOx which contain different amount of oxygen in Ta(20 nm)/TaOx/Pt structure from the O K-edge XAS spectra. According to the I-V characteristics and material characteristics, we establish the oxygen vacancy based resistive switching model to illustrate the current difference in HRS and resistive parameters difference among the Ta/TaOx/Pt, Ta/TaOx/ZnO/Pt and Ta/ZnO/TaOx/Pt devices.

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