本實驗使用以氧化鉭作為主動層的電阻式記憶體元件作討論，元件結構分別為Ta/TaOx/Pt、Ta/TaOx/ITO。吾人藉由量測兩種元件的電流-電壓曲線、元件在不同阻態及在不同直流偏壓下的特徵阻抗並同時搭配材料分析(XPS縱深分析、 Cross-sectional TEM)來了解這二個元件的阻態轉換機制。
論文的第一部份為材料分析部分，從Cross-sectional TEM 得知元件的主動層TaOx為非晶(amorphous)；從XPS 縱深分析結果可知，在兩元件的Ta/TaOx在界面，同時存在Ta(0+)及Ta(+5)，故吾人可推測，在Ta//TaOx界面可能存一層富含氧空缺的過渡層(Transition Region)。
論文的第二部分為電性量測部分，在此部分，吾人除了利用了Agilent 4156C量測了元件的電阻轉換曲線之外，也利用了Agilent 4294A量測了元件在初始狀態(Initial state)、低阻態(Low resistance state, LRS)及高阻態(How resistance state, HRS)下的阻抗。從元件的電阻轉換曲線結果得知Ta/TaOx/Pt、Ta/TaOx/ITO元件均屬於正偏壓寫入，負偏壓抹除的元件。從阻抗量測分析結果知，Ta/TaOx/Pt、Ta/TaOx/ITO元件在三種阻態下等效電路皆可用Rs-R//C來表示，且兩種元件在高阻態時的R值，均會大於在低阻態時的R值。
從上述可知，兩種元件的R在不同阻態下，皆會產生顯著的差異，這即是因在電阻轉換的過程中，存在於過渡層及主動層內的氧空缺會隨著所施加偏壓大小及極性的不同，而在主動層內移動，進而生成或破壞導電路徑(Conductive filament)而產生電阻轉換的現象所致。
論文的第三部分，為探討Ta/TaOx/Pt、Ta/TaOx/ITO兩種元件在阻態轉換過程中的阻抗變化。在此部分，吾人在量測阻抗的同時，會施以不同極性及大小的直流正負偏壓，來觀察元件在寫入或抹除的過程中，所呈現出的特徵阻抗。此部分的實驗結果顯示，從高阻態轉換至低阻態的過程時，兩種元件在不同大小的直流正偏壓下所得到的阻抗相對應的等效電路皆會先從簡單的Rs-R//C轉變為一複雜且含有電感的等效電路，在移除直流正偏壓之後，再轉變回Rs-R//C，且其中的R值，已由原來高阻值，轉變為低阻值，而等效電路的轉變，即表示元件主動層內的導電路徑會在施加正偏壓的過程中形成，且其大小及形狀在施加偏壓的過程中會不斷改變。另一方面，當吾人在量測阻抗的同時，施加不同大小的直流負偏壓於低阻態的Ta/TaOx/Pt、Ta/TaOx/ITO兩種元件，在不同大小的直流負偏壓下所呈現的等效電路大致上皆還是維持著Rs-R//C，且R會隨著施加負偏壓的過程逐漸提升，而這表示兩種元件內部的導電路徑會在施加直流負偏壓的過程中，逐漸消失。
至於使用Pt下電極與ITO兩種下電極其差異在於，ITO下電極相較於Pt下電極，其會在阻態轉換過程中吸收及釋放氧空缺，因此會對元件的阻態轉換過程產生明顯的影響，故從實驗結果可發現明顯的差異。首先，由不同阻態的阻抗分析結果知，Ta/TaOx/Pt、Ta/TaOx/ITO兩種元件在高低阻態時，所呈現的等效電路皆為Rs-R//C，然而，在高阻態時，Ta/TaOx/Pt元件的R值明顯會高於Ta/ TaOx/ITO元件；相反地，在低阻態時，Ta/TaOx/Pt元件的R值會明顯低於Ta/ TaOx/ITO元件。另一方面，從元件的電阻轉換特性曲線所得到的Vset及on/off ratio的二種參數可以發現到，Ta/TaOx/Pt元件皆大於Ta/TaOx/ITO元件。

Abstract
In this study, the resistance switching mechanisms of Ta/TaOx/Pt and Ta/TaOx/ITO devices are explored based on I-V sweeping curves, characteristic impedance and material characteristics.
First, TEM is used to investigate the thickness of the device and crystallinity of the TaOx films. Chemical bonding state in TaOx active layer is observed with XPS. Second, regarding to electrical properties of TaOx-based device, we not only use precision semiconductor parameter analyzer (Agilent 4156C) to get the resistive switching curve of TaOx-based device, but also use the precision impedance analyzer (Agilent 4294A)to get the characteristic impedance of TaOx-based device at different states and different dc bias.
The first part of this dissertation is about material characteristics of TaOx TEM, analysis reveals that TaOx is amorphous. Second, from the result of XPS, the chemical states of Ta(+0) and Ta(+5) are coexist near Ta/TaOx interface, so we can conclude that there is a oxygen vacancy-rich region, the Transition Region (TR), exist near Ta/TaOx interface.
The second part of this study is about electrical properties of TaOx-based devices. From resistive switching curves, both Ta/TaOx/Pt and Ta/TaOx/ITO devices can be operated in bipolar mode, with writing at positive bias and erasing at negative bias. The characteristic impedance at high and low resistance states of the two devices can be fitted with Rs-R//C model, and the value of R at high resistance state (HRS) is higher than at low resistance state (LRS) for both devices, which means that during the resistance-switching process, oxygen vacancies will be driven by different polarity bias to form or rupture the conductive filament.
In the third part of this study, we measure the impedance under positive and negative dc bias to observe the characteristic impedance during set and reset process. From the result of positive dc bias impedance measurement on the HRS devices, we found that the equilibrium circuit of the two devices will be first changed from Rs-R//C model into a complex model involving inductors, then turn back to Rs-R//C after removing positive bias. In the meaning time, the R changes from the initially high value to a low value, which means that the conductive filament will be formed and the shape of the filament will be changed during positive dc bias operation. From the result of negative dc bias impedance measurement on the LRS devices, the equilibrium circuit of the two devices will approximately maintain Rs-R//C, and the value of R for two devices is gradually become larger during negative biased impedance measurement, which means that the conductive filament in active layer will be ruptured during negative dc bias operation.
Regarding the differences of using Pt and ITO bottom electrodes is that, ITO bottom electrode will absorb or release oxygen vacancies during resistance switching process, which may affect the resistance-switching process, however, Pt bottom electrode will not.
So from the experiment, we can found that several difference between Ta/TaOx/Pt device and Ta/TaOx/ITO device. First, in high resistance state (HRS), the value of R fitted from Rs-R//C model in Ta/TaOx/Pt device is larger than Ta/TaOx/ITO device, however, in low resistance state (LRS) , the value of R fitted from Rs-R//C model in Ta/TaOx/Pt device is smaller than Ta/TaOx/ITO device. Second, the two electrical parameters of Vset and on/off ratio, which get from characteristic resistive switching curve, both in Ta/TaOx/Pt device are larger than in Ta/TaOx/ITO device.

Summary
The resistive switching process in oxide-based RRAM is generally originated from the formation-and-rupture of conductive filament via oxygen vacancies. However, to observe the filament evolution during resistive switching process is difficult. In our study, we measure impedance of RRAM at different resistance states to study the filament evolution during resistive switching.
Two TaOx-based resistive switching memories, Ta/TaOx/Pt and Ta/TaOx/ITO devices are investigated in this work. First, from the material analysis, we have found an oxygen vacancy rich region, the transition region, existing near Ta/TaOx interface. Secondly, from the analysis of impedance at different states for two devices, we can find that the impedance at different states all can be fitted with Rs-R//C circuit model, and the value of R for two devices in high resistance state is substantially larger than in low resistance state, which is the evidence to show that the filament rupture or formation during resistive switching process.

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