近年來巨量的數據伴隨著物聯網、人工智慧及雲端運算等科技進步不斷產生，使得記憶體的需求正持續大幅增加。傳統的快閃記憶體隨著元件尺寸微縮，將面臨嚴苛考驗，過薄的穿隧氧化層，將產生漏電路徑，造成資料保存能力與元件可靠度劣化，因此發展新世代記憶體刻不容緩。在眾多新興記憶體技術中，電阻式記憶體具備簡單的金屬-絕緣體-金屬結構、低功耗與高操作性能等特點，因此具備潛力成為新世代的記憶體元件。
本論文首先以銦錫氧化物做為氧化鉿RRAM的電極材料，探討其元件特性與電阻切換機制，實驗結果顯示相較於一般的惰性金屬電極，運用ITO電極的元件，擁有較佳的電阻切換特性與可靠度，並具備低操作電流與較快的電阻切換速度等優點。藉由實驗的結果，我們提出一模型解釋，因氧離子能受電場效應進入ITO電極，形成一近似半導體區域，使得ITO電極的元件，具備自我限流、低操作電流等優點。此外因ITO電極具備體效應的儲氧效果，相較於一般金屬電極，以ITO為電極的元件擁有電阻切換速度較高的性能。
此外因過渡元素具有半滿的電子軌域，容易與氧離子形成鍵結，我們藉由在ITO電極中摻雜釓元素，形成ITO:Gd化合物藉以檢視電阻切換機制。實驗結果顯示，ITO:Gd為電極之RRAM具有高記憶窗口、低操作電流的特性，藉由傳導電流擬合與升溫實驗，確認了其低阻值狀態的電流機制由原先歐姆傳導改為蕭特基發射機制，我們認為因摻雜了過渡元素金屬，可以有效的提升ITO電極與氧離子鍵節能力，進而使得ITO:Gd擁有更好的電阻切換特性。
鑒於ITO材料擁有半導體的特性，我們亦嘗試運用氮氣摻雜的方式，將此材料運用於RRAM的絕緣層，實驗結果顯示，經摻雜後的ITON材料，亦具備電阻切換的效果。不僅如此，我們發現此元件亦具備較低的形成電壓，在正負3伏的範圍內即可完成Forming、寫入(SET)與抹除(RESET)的操作，具有優良的電阻切換特性。
除了運用氣體摻雜方式外，我們尚提出運用氧電漿處裡，來氧化ITO材料做為RRAM的中間層。實驗結果顯示，經氧電漿處理後的ITO薄膜具備絕緣體的特性，運用在RRAM之中亦具有良好電阻切換的特性，具備可操作的直流與交流讀寫能力，我們最後透過亦透過電流擬合與升溫實驗方式，確認其電阻傳導機制，並提出相關模型做為解釋。

With the advent of advanced technologies such as the Internet of Things (IoT), artificial intelligence (AI), and cloud computing, huge amounts of data are continuously produced. The demand for memory, therefore, is dramatically increasing. Traditional flash memory currently faces severe challenges when device size shrinks. A thin tunneling oxide layer may cause severe leakage path issues, resulting in degradation of data retention and component reliability. Therefore, it is imperative to develop a new generation of memory. Among next generation memories, the resistive random access memory (RRAM) has the advantages of being simple metal-insulator-metal structure, as well as having low power consumption and high operation performance, factors which are important in becoming a building block for the new generation of memory devices.
In this thesis, we applied an indium tin oxide (ITO) thin film as the electrode in a HfO2-based RRAM to investigate its resistance switching (RS) characteristics. The experimental results show that, compared to a traditional inert metal electrode, the ITO electrode has robust RS characteristics and better reliability. Both lower operating current and faster operating speed can be achieved simultaneously. On the basis of electrical measurements, we conclude that the oxygen ions can enter the ITO electrode by the given electric field, thus forming a semiconductor-like ITO region. This also causes the effects of a self-compliance current and low power consumption. In addition, due to the bulk oxygen-ion storage in the ITO electrode, the device exhibits better RS performance.
Since the transition element has a semi-full electron orbital region, it easily forms bonds with oxygen ions. We, therefore, doped gadolinium (Gd) into the ITO electrode as a ITO:Gd combination to examine the RS mechanism in the HO2-based RRAM. The experimental results show that the ITO:Gd device can produce a high memory window at a low operating current. By means of the current fitting method and a temperature effect experiment, we are able to confirm that a modification in the conducting mechanism has occurred, from the Ohmic conducting mechanism in the pure ITO electrode to Schottky emission in the ITO:Gd electrode. We conclude that the ITO:Gd electrode has better bonding ability with oxygen ions due to the Gd doping, which in turn allows the ITO:Gd device to have better resistance switching characteristics.
Since the ITO material also possesses semiconductor characteristics, we have also attempted co-sputtering the ITO thin film with nitrogen gas as the ITON thin film to act as the insulator layer of RRAM, thereby further simplifying the device fabrication process. Experimental results shown that the ITON thin film can also induce RS characteristics. In addition, a lower operating voltage can be achieved with only ±3 volts to complete the forming, set, and reset operations.
Apart from this gas co-sputtering method, we also introduced an oxygen plasma treatment to oxidize the ITO thin film as the insulator in the RRAM. The experimental results show that the oxidized ITO thin film exhibits an insulator-like property and exhibits good RS characteristics. Both DC and AC electrical measurements were conducted to verify the continuous RS characteristics. Finally, a conducting model is proposed to explain the possible RS mechanism on the basis of the current fitting method and a temperature experiment.

Acknowledgement iii
Chinese Abstract v
English Abstract vii
Contents xi
Figure Captions xv
Table Captions xxv
Chapter 1 Introduction 1
1.1 Overview of Memory Device 1
1.2 Current Emerging Memory 2
1.2.1 Ferroelectric Random Access Memory (FRAM) 2
1.2.2 Magnetoresistive Random Access Memory (MRAM) 2
1.2.3 Phase Change Random Access Memory (PCRAM) 3
1.2.4 Resistive Random Access Memory (RRAM) 4
1.3 Research Motivation 6
1.4 Origination of This Thesis 7
Chapter 2 Basic Principle of Resistive Random Access Memory 13
2.1 Introduction of RRAM 13
2.2 Materials Selection of RRAM 15
2.2.1 Transition Metal Oxides 15
2.2.2 Perovskite 16
2.2.3 Organic Materials 17
2.3 Resistive Switching Mechanism of RRAM 18
2.3.1 Schottky Barrier Model 18
2.3.2 Filament Conduction Model 18
2.3.3 Charge-Trap in Small Domain 21
2.4 Carrier Conduction Mechanism in RRAM 22
2.4.1 Ohmic Conduction 22
2.4.2 Schottky Emission 23
2.4.3 Hopping Conduction 23
2.4.4 Tunneling Conduction 24
2.4.5 Poole-Frenkel Emission 25
2.4.6 Space Charge Limited Current 25
Chapter 3 Bulk Oxygen-Ion Storage in Indium-Tin-Oxide Electrode for Improved Performance of HfO2-Based Resistive Random Access Memory 37
3.1 Introduction 37
3.2 Experiment 38
3.3 Results and Discussion 39
3.4 Conclusion 43
Chapter 4 Improving Performance by Doping Gadolinium into the Indium-Tin-Oxide Electrode in HfO2-based Resistive Random Access Memory 49
4.1 Introduction 49
4.2 Experiment 51
4.3 Results and Discussion 51
4.4 Conclusion 56
Chapter 5 Obtaining Lower Forming Voltage and Self-compliance Current by Using a Nitride Gas/Indium-Tin-Oxide Insulator in Resistive Random Access Memory 61
5.1 Introduction 61
5.2 Experiment 63
5.3 Results and Discussion 64
5.4 Conclusion 69
Chapter 6 Resistance Switching Characteristics Induced by O2 Plasma Treatment on Indium-Tin-Oxide Film for Use as Insulator in Resistive Random Access Memory 77
6.1 Introduction 77
6.2 Experiment 80
6.3 Results and Discussion 81
6.4 Conclusion 87
Chapter 7 Conclusion 93
References 97
Publication List 121
Vita 125

Chapter 1
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Chapter 2
[2.1] H-S. Philip Wong et al. “Metal-oxide RRAM,” Proc. IEEE, vol. 100, no. 6, pp. 1951–1970, Jun. 2012.
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Chapter 3
[3.1] K. C. Chang et al. “Physical and Chemical Mechanisms in Oxide-based Resistance Random Access Memory,” Nanoscale Res. Lett., vol. 10, pp.120, MAR 2015.
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[3.16] C. Ye et al. “Role of ITO electrode in the resistive switching behavior of TiN/HfO2/ITO memory devices at different annealing temperatures,” JPN. J. APPL. PHYS., vol 54, no. 5, pp. 054201, MAY 2015.
[3.17] R Zhang et al. “Characterization of Oxygen Accumulation in Indium-Tin-Oxide for Resistance Random Access Memory,” IEEE Electron Device Lett., vol. 35, no. 6, pp. 630-632, JUN. 2014.
[3.18] C. Ye et al. "Low-power bipolar resistive switching TiN/HfO2/ITO memory with self-compliance current phenomenon," Appl. Phys. Express, vol. 7, pp. 034101, 2014.
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Chapter 4
[4.1] R. Waser and M. Aono, “Nanoionics-based resistive switching memories,” Nat. Mater., vol. 6, no. 11, pp. 833–840, Nov. 2007.
[4.2] K. M. Kim et al. “Nanofilamentary resistive switching in binary oxide system; a review on the present status and outlook,” Nanotechnology, vol. 22, no. 25, pp. 254002-1–254002-17, Jun.2011.
[4.3] T. C. Chang et al. “Developments in nanocrystal memory,” Mater. Today, vol. 14, no. 12, pp. 608–615, Dec. 2011.
[4.4] R. Waser et al. “Redox-Based Resistive Switching Memories – Nanoionic Mechanisms, Prospects, and Challenges,” Adv. Mater., vol. 21, no. 25–26, pp. 2632–2663, Jul. 2009.
[4.5] H-S. Philip Wong et al. “Metal-oxide RRAM,” Proc. IEEE, vol. 100, no. 6, pp. 1951–1970, Jun. 2012.
[4.6] K. C. Chang et al. “Physical and Chemical Mechanisms in Oxide-based Resistance Random Access Memory,” Nanoscale Res. Lett., vol. 10, pp. 120-1–120-27, Mar. 2015.
[4.7] C. C. Lin and Y. Kuo, “Memory functions of nanocrystalline cadmium selenide embedded ZrHfO high-k dielectric stack,” J. Appl. Phys., vol. 115, no. 8, pp. 084113-1–084113-7, Feb. 2014.
[4.8] S. B. Long et al. “Cycle-to-cycle intrinsic RESET statistics in HfO2-based unipolar RRAM devices,” IEEE Electron Device Lett., vol. 34, no. 5, pp. 623–625, May 2013.
[4.9] K. C. Chang et al. “Dual Ion Effect of the Lithium Silicate Resistance Random Access Memory,” IEEE Electron Device Lett., vol. 35, no. 5, pp. 530–532, May 2014.
[4.10] Y. J. Chen et al. “Resistance Switching Induced by Hydrogen and Oxygen in Diamond-Like Carbon Memristor,” IEEE Electron Device Lett., vol. 35, no. 10, pp. 1016–1018, Oct. 2014.
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[4.18] R Zhang et al. “Characterization of Oxygen Accumulation in Indium-Tin-Oxide for Resistance Random Access Memory,” IEEE Electron Device Lett., vol. 35, no. 6, pp. 630–632, Jun. 2014.
[4.19] C. Ye et al. “Low-power bipolar resistive switching TiN/HfO2/ITO memory with self-compliance current phenomenon,” Appl. Phys. Express, vol. 7, no. 3, pp. 034101-1–034101-4, Mar. 2014.
[4.20] C. W. Zhong et al. “Effect of ITO electrode with different oxygen contents on the electrical characteristics of HfOx RRAM devices,” Surf. Coat. Technol., vol. 231, pp. 563–566, Sep. 2013.
[4.21] H. B. Zhao et al. “High mechanical endurance RRAM based on amorphous gadolinium oxide for flexible nonvolatile memory application,” J. Phys. D: Appl. Phys., vol. 48, no. 20, pp. 205104-1–205104-7, May. 2015.
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[4.25] S. M. Sze, and Kwok K. Ng, Physics of Semiconductor Devices (John Wiley & Sons, 2006) 3rd ed.
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Chapter 5
[5.1] D. S. Jeong et al. “Emerging memories: resistive switching mechanisms and current status,” Rep. Prog. Phys., vol. 75, no. 7, pp. 076502, Jul. 2012.
[5.2] R. Waser and M. Aono, “Nanoionics-based resistive switching memories,” Nat. Mater., vol. 6, no. 11, pp. 833–840, Nov. 2007.
[5.3] H. S. Philip Wong et al. “Metal-oxide RRAM,” Proc. IEEE, vol. 100, no. 6, pp. 1951–1970, Jun. 2012.
[5.4] T. C. Chang et al. “Resistance random access memory,” Mater. Today, vol. 19, no. 5, pp. 254–264, Jun. 2016.
[5.5] A. Sawa, “Resistive switching in transition metal oxides,” Mater. Today, vol. 11, no. 6, pp. 28–36, Jun. 2008.
[5.6] K. M. Kim et al. “Nanofilamentary resistive switching in binary oxide system; a review on the present status and outlook,” Nanotechnology, vol.22, no.25, pp. 254002-1–254002-17, Jun. 2011.
[5.7] W. Zhang et al. “Mechanism of triple ions effect in GeSO resistance random access memory,” IEEE Electron Device Lett., vol. 36, no. 6, pp. 552–554, Jun. 2015.
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[5.9] C. C. Kuo et al. “Galvanic effect of Au-Ag electrodes for conductive bridging resistive switching memory,” IEEE Electron Device Lett., vol. 36, no. 12, pp. 1321–1324, Dec. 2015.
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Chapter 6
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