氧化物電阻式記憶體大多藉由氧空缺儲存槽提供或接收氧空缺，而帶正電氧空缺受電場或濃度梯度驅動而移動，以造成導電燈絲生成與斷裂，使得元件產生電阻轉換現象。本論文主要探討不同電極材料與氧化物主動層界面形成不同型式的氧空缺儲存槽，在阻態轉換過程中所扮演之角色的差異。同時，針對Ta/TaOx/Pt元件的多重阻態特性做進一步探討。
實驗第一部分以氧化鉭、氧化鋅做為主動層，試片結構(上電極/主動層/下電極)分別為Ta/TaOx/Ta、Ta/TaOx/Pt、Ta/TaOx/ITO、Ta/ZnOx/Ta、Ta/ZnOx/Pt、Ta/ZnOx/ITO。在此部分，主要是藉由分析元件各別的電流-電壓特徵曲線，包含操作電壓、操作極性，並搭配X光光電子能譜分析，進而推得元件的電阻轉換機制，並說明下電極Ta、Pt和ITO的差異、Ta/TaOx界面和ITO電極當作氧空缺儲存槽的差異，以及Ta/ZnOx界面和ZnOx/ITO界面當作氧空缺儲存槽的差異所造成導電燈絲生成與斷裂的不同機制。
在材料分析部分，由穿透式電子顯微鏡的影像，可以得知主動層氧化鉭為非晶相，而氧化鋅為結晶相；之後藉由Ta/TaOx/Pt、Ta/TaOx/ITO元件的X光光電子能譜儀的縱深分析，可以發現Ta/TaOx界面可能存在一富含氧空缺的過渡層，但是TaOx/ITO及TaOx/Pt界面應該沒有富含氧空缺的過渡層。此外藉由Ta/ZnOx/Pt、Ta/ZnOx/ITO兩種元件的X光光電子能譜儀的縱深分析，可以發現Ta/ZnOx界面和ZnOx/ITO界面可能存在一富含氧空缺的過渡層，但是ZnOx/Pt界面應該沒有富含氧空缺的過渡層。進一步，根據Ta/ZnOx/ITO元件中，不同界面位置O訊號的分峰處理計算缺氧次峰比例(ratio of oxygen deficiency subpeak)，可知Ta/ZnOx界面氧空缺儲存槽相較ZnOx/ITO界面氧空缺儲存槽可以提供和儲存較多數量的氧空缺。
在電性分析部分，藉由TaOx-based元件的電流-電壓量測結果可以發現，元件初始狀態為高阻態，須先施以正或負偏壓進行初始化(forming)達到低阻態，之後便可操作。以Pt當作下電極的元件，可以進行正偏壓寫入負偏壓抹除和負偏壓寫入負偏壓抹除；而以ITO當作下電極的元件，則是可以進行正偏壓寫入負偏壓抹除、負偏壓寫入負偏壓抹除，以及負偏壓寫入正偏壓抹除；若是以Ta當作下電極的元件，則是進行正偏壓寫入或負偏壓寫入之後，無法藉由施加正偏壓或負偏壓抹除低阻態。因而吾人根據元件操作極性組合的不同，確定TaOx/Pt界面及Pt本身皆不具有氧空缺儲存槽功能和ITO本身具有氧空缺儲存槽功能作區別。為區隔出TaOx/Ta界面和ITO電極兩種氧空缺儲存槽的角色差異，先比較Ta/TaOx/ITO元件之正、負偏壓初始化操作電壓(Vforming)絕對值大小，可以發現V-,forming,ITO(-15.16 V)>V+,forming,ITO(5.05 V)，推論負偏壓寫入時ITO下電極不主動提供氧空缺，正偏壓寫入時Ta/TaOx界面氧空缺儲存槽可以提供大量氧空缺，故可以較小偏壓進行初始化。進一步藉由比較Pt下電極元件和ITO下電極元件之負偏壓初始化(V-,forming)與負偏壓寫入(V-,set)的操作電壓大小，可以得知V-,forming,ITO(-15.16 V)>V-,forming,Pt(-4.71 V)，且V-,set (negative on negative off),ITO(-8.38 V)>V-,set (negative on negative off),Pt(-3.47 V)。由此結果推論，在施加負偏壓初始化或寫入的過程中，因氧空缺濃度梯度的影響，TaOx主動層部分氧空缺往下電極移動，若下電極為ITO，則會吸收來自主動層的氧空缺，而且ITO為一個不主動提供氧空缺的氧空缺儲存槽，使得ITO下電極元件需要較Pt下電極元件為大的電壓才能阻態轉換至低阻態。
由ZnOx-based元件的電流-電壓量測結果可以發現，Ta/ZnOx/Ta、Ta/ZnOx/Pt、Ta/ZnOx/ITO三元件初始狀態為低阻態。Pt或是Ta下電極元件，無法藉由施加正偏壓或負偏壓抹除初始低阻態，因此元件無法操作。而ITO下電極元件，則可先以負偏壓抹除初始低阻態，之後再進行正偏壓寫入負偏壓抹除、負偏壓寫入正偏壓抹除，以及負偏壓寫入負偏壓抹除。因而吾人根據元件是否能穩定操作，說明使用Pt或是ITO當作下電極對元件的電阻轉換機制具有影響力，並區隔出ZnOx/Ta界面氧空缺儲存槽和ZnOx/ITO界面氧空缺儲存槽在電阻轉換過程中的角色差異。藉由比較ITO下電極元件之正、負偏壓第一次寫入的操作電壓大小，可以發現V-,1st-set,ITO(-6.96 V)>V+,1st-set,ITO(3.28 V)。因而推論正偏壓寫入時Ta/ZnOx界面氧空缺儲存槽相較於負偏壓寫入時ZnOx/ITO界面氧空缺儲存槽提供較多數量的氧空缺，此推論與XPS分析氧空缺量之結果相符。
實驗第二部份主要針對Ta/TaOx/Pt元件，以抹除停止電壓造成的多重阻態特性做進一步探討，由變溫電流-電壓特徵曲線，可以得知元件的電子傳輸行為和阻態間有密切關連，元件在低阻態1、低阻態2皆為歐姆傳導模型，中間阻態為跳躍傳導模型，高阻態為蕭特基發射模型。根據交流阻抗分析，元件於中間阻態及高阻態時可以用R1、R2、R3、C1，以及C2組成的等效電路描述，而於低阻態1、低阻態2時則用R1、R2、R3，及C1組成的等效電路描述。將實驗結果作比較可以發現R值和C值的改變和抹除停止電壓有關，而此現象可以藉由氧空缺模型和導電燈絲的形成與斷裂解釋，最後根據變溫電流-電壓量測和交流阻抗分析結果說明不同阻態時的電阻轉換機制。

The objective of research presented in this thesis is to understand the characters of oxygen vacancy reservoirs formed at the electrode-active layer interfaces using various electrodes and their influences on the resistive switching process of oxide-based resistance switching memories. Additionally, the multilevel resistive switching process and the characteristics of different resistance states of the Ta/TaOx/Pt device are investigated.
The first part of this study investigates the resistive switching mechanisms of Ta(TE：top electrode)/TaOx/Ta(BE：bottom electrode), Ta(TE)/TaOx/Pt(BE), Ta(TE)/TaOx/ITO(BE), Ta(TE)/ZnOx/Ta(BE), Ta(TE)/ZnOx/Pt(BE), Ta(TE)/ZnOx/ITO(BE) devices by analyzing individual current-voltage curves, including the operation voltage and operation polarity. The electrical characteristics are compared with X-ray photoelectron spectroscopy analysis at different depths of devices to apprehend the characters of oxygen vacancy reservoirs formed at the electrode-active layer interfaces, as well as their correlations to the formation and rupture of conducting filaments.
On the TEM images, it is observed that TaOx active layer is amorphous while the ZnOx active layer is crystalline. Based on results of X-ray photoelectron spectroscopy, it is found that there are oxygen vacancy-rich regions existing near Ta/ZnOx interface, Ta/TaOx interface and ZnOx/ITO interface. In addition, Ta/ZnOx interfacial oxygen reservoir could absorb and release more oxygen vacancies than ZnOx/ITO interfacial oxygen reservoir based on the ratio of oxygen deficiency subpeak deconvoluted from oxygen 1s signals at different interfaces.
The pristine TaOx-based devices are at high resistance state (HRS) and can be turned to low resistance state (LRS) after a forming process with either positive or negative bias. The device using Pt BE could be set with positive bias and reset with negative bias, or set with negative bias and reset with negative bias. On the other hand, the device using ITO BE could be set with positive bias and reset with negative bias, set with negative bias and reset with negative bias, or set with negative bias and reset with positive bias. However, the device using Ta BE could not be reset after being set to low resistance state (i.e. staying at LRS after forming). According to the polarity-dependent I-V switching curves, neither TaOx/Pt interface nor Pt functions as an oxygen vacancy reservoir. To understand the roles of TaOx/Ta interfacial oxygen reservoir and ITO oxygen vacancy reservoir, it is found that the absolute value of forming voltage of Ta/TaOx/ITO is smaller when applying positive bias on TE (V-,forming,ITO(-15.16 V)>V+,forming,ITO(5.05 V)), suggesting that ITO BE will not provide oxygen vacancies, while the Ta/TaOx interface provide a significant amount. Therefore, the forming voltage is much lower under positive bias. Furthermore, by comparing the absolute values of forming and set voltages of Pt BE and ITO BE devices under negative bias, it shows that V-,forming,ITO(-15.16 V)>V-,forming,Pt(-4.71 V), and V-,set (negative on negative off),ITO(-8.38 V)>V-,set (negative on negative off),Pt(-3.47 V). On the above basis, we could speculate that some oxygen vacancies in the active layer moved toward BE due to concentration gradient and were absorbed by ITO BE during forming and set processes with negative bias. When BE is ITO, it will absorb oxygen vacancies from the active layer but doesn’t release oxygen vacancy actively. Accordingly, compared to Pt BE device, the ITO BE device needs a larger negative voltage to turn to LRS.
As for the ZnOx-based devices, the pristine devices (Ta/ZnOx/Ta, Ta/ZnOx/Pt, Ta/ZnOx/ITO) are at LRS. The devices using Pt BE or Ta BE could not be operated because they cannot be reset to HRS with any bias polarity. The device using ITO BE can be reset to HRS via applying negative bias on TE, and it could be subsequently set with positive bias and reset with negative bias, set with negative bias and reset with negative bias, or set with negative bias and reset with positive bias. Again, it points out the difference between ITO and Pt, as well as the difference between ZnOx/Ta interfacial oxygen vacancy reservoir and ZnOx/ITO interfacial oxygen vacancy reservoir. According to the operation voltages of ITO BE device in the first set process under positive and negative biases, V-,1st-set,ITO(-6.96 V)>V+,1st-set,ITO(3.28 V), we suggest that Ta/ZnOx interfacial oxygen vacancy reservoir can release more oxygen vacancies than ZnOx/ITO interfacial oxygen vacancy reservoir. This resultis in good agreement with the XPS analysis on the amount of oxygen vacancies.
Secondly, the multilevel characteristics of the Ta/TaOx/Pt device and the physics behind the resistive switching phenomena among these resistance states are further studied by analyzing current-voltage curves at various temperatures and characteristic impedance.
From the temperature-dependent I-V measurement, it is found that conduction mechanisms are different for three kinds of resistance states of the device. The LRS shows ohmic conduction. The HRS shows Schottky-emission dominant conduction. The intermediate resistance state (IRS) shows hopping conduction. Moreover, the impedance measurement suggests that HRS and IRS could be described with an equivalent circuit consisting of the major components R1, R2, R3, C1 and C2, and LRS could be characterized with an equivalent circuit consisting of the major components R1, R2, R3 and C1. The values of these components are strongly connected with the reset stop voltage, which could be explained with the oxygen vacancy model associated with the formation or rupture of filaments. In brief, different current conduction mechanisms and values of major resistance and capacitance components in the equivalent circuits for different resistance states could be attributed to changing of the filament structure related to the oxygen vacancy re-distribution during switching process.

[1] D. S. Jeong, R. Thomas, R. S. Katiyar, J. F. Scott, H. Kohlstedt, a Petraru, and C. S. Hwang, “Emerging memories: resistive switching mechanisms and current status,” Reports Prog. Phys., vol. 75, p. 076502, 2012.
[2] F. Pan, S. Gao, C. Chen, C. Song, and F. Zeng, “Recent progress in resistive random access memories: Materials, switching mechanisms, and performance,” Mater. Sci. Eng. R Reports, vol. 83, no. 1, pp. 1–59, 2014.
[3] Y. Li, S. Long, Q. Liu, H. Lü, S. Liu, and M. Liu, “An overview of resistive random access memory devices,” Chinese Sci. Bull., vol. 56, no. 28–29, pp. 3072–3078, 2011.
[4] F. Pan, C. Chen, Z. Wang, Y. Yang, J. Yang, and F. Zeng, “Nonvolatile resistive switching memories-characteristics, mechanisms and challenges,” Prog. Nat. Sci. Mater. Int., vol. 20, pp. 1–15, 2010.
[5] J. S. Meena, S. M. Sze, U. Chand, and T.-Y. Tseng, “Overview of emerging nonvolatile memory technologies.,” Nanoscale Res. Lett., vol. 9, no. 1, p. 526, 2014.
[6] C. Ye, J. Wu, G. He, J. Zhang, T. Deng, P. He, and H. Wang, “Physical Mechanism and Performance Factors of Metal Oxide Based Resistive Switching Memory: A Review,” J. Mater. Sci. Technol., vol. 32, no. 1, pp. 1–11, 2016.
[7] A. C. Torrezan, J. P. Strachan, G. Medeiros-ribeiro, and R. S. Williams, “Sub-nanosecond switching of a tantalum oxide memristor,” vol. 485203.
[8] H. Y. Lee, Y. S. Chen, P. S. Chen, P. Y. Gu, Y. Y. Hsu, S. M. Wang, W. H. Liu, C. H. Tsai, S. S. Sheu, P. C. Chiang, W. P. Lin, C. H. Lin, W. S. Chen, F. T. Chen, C. H. Lien, and M. J. Tsai, “Evidence and solution of over-RESET problem for HfOX based resistive memory with sub-ns switching speed and high endurance,” Tech. Dig. - Int. Electron Devices Meet. IEDM, pp. 7–10, 2010.
[9] M.-J. Lee, C. B. Lee, D. Lee, S. R. Lee, M. Chang, J. H. Hur, Y.-B. Kim, C.-J. Kim, D. H. Seo, S. Seo, U.-I. Chung, I.-K. Yoo, and K. Kim, “A fast, high-endurance and scalable non-volatile memory device made from asymmetric Ta2O5−x/TaO2−x bilayer structures,” Nat. Mater., vol. 10, no. 8, pp. 625–630, 2011.
[10] J. Park, W. Lee, M. Choe, S. Jung, M. Son, S. Kim, S. Park, J. Shin, D. Lee, M. Siddik, J. Woo, G. Choi, E. Cha, T. Lee, and H. Hwang, “Quantized conductive filament formed by limited Cu source in sub-5nm era,” Tech. Dig. - Int. Electron Devices Meet. IEDM, pp. 63–66, 2011.
[11] B. Govoreanu, G. S. Kar, Y. Y. Chen, V. Paraschiv, S. Kubicek, A. Fantini, I. P. Radu, L. Goux, S. Clima, R. Degraeve, N. Jossart, O. Richard, T. Vandeweyer, K. Seo, P. Hendrickx, G. Pourtois, H. Bender, L. Altimime, D. J. Wouters, J. A. Kittl, and M. Jurczak, “10×10nm2 Hf/HfOx crossbar resistive RAM with excellent performance, reliability and low-energy operation,” Tech. Dig. - Int. Electron Devices Meet. IEDM, pp. 729–732, 2011.
[12] C. H. Cheng, C. Y. Tsai, A. Chin, and F. S. Yeh, “High performance ultra-low energy RRAM with good retention and endurance,” Tech. Dig. - Int. Electron Devices Meet. IEDM, pp. 448–451, 2010.
[13] M. Marinella, “Overview of Emerging Research Memory Devices,” 2015.
[14] J. J. Yang, M. X. Zhang, J. P. Strachan, F. Miao, M. D. Pickett, R. D. Kelley, G. Medeiros-Ribeiro, and R. S. Williams, “High switching endurance in TaOx memristive devices,” Appl. Phys. Lett., vol. 97, no. 23, 2010.
[15] J. P. Strachan, A. C. Torrezan, G. Medeiros-Ribeiro, and R. S. Williams, “Measuring the switching dynamics and energy efficiency of tantalum oxide memristors.,” Nanotechnology, vol. 22, no. 50, p. 505402, 2011.
[16] S. J. Choi, G. S. Park, K. H. Kim, S. Cho, W. Y. Yang, X. S. Li, J. H. Moon, K. J. Lee, and K. Kim, “In situ observation of voltage-induced multilevel resistive switching in solid electrolyte memory,” Adv. Mater., vol. 23, no. 29, pp. 3272–3277, 2011.
[17] M. Trapatseli, D. Carta, a. Regoutz, a. Khiat, a. Serb, I. Gupta, and T. Prodromakis, “Conductive Atomic Force Microscopy Investigation of Switching Thresholds in Titanium Dioxide Thin Films,” J. Phys. Chem. C, vol. 119, no. 21, pp. 11958–11964, 2015.
[18] Y. Du, A. Kumar, H. Pan, K. Zeng, S. Wang, P. Yang, and A. T. S. Wee, “The resistive switching in TiO2 films studied by conductive atomic force microscopy and Kelvin probe force microscopy,” AIP Adv., vol. 3, no. 8, pp. 0–7, 2013.
[19] U. Celano, L. Goux, A. Belmonte, K. Opsomer, A. Franquet, A. Schulze, C. Detavernier, O. Richard, H. Bender, M. Jurczak, and W. Vandervorst, “Three-dimensional observation of the conductive filament in nanoscaled resistive memory devices,” Nano Lett., vol. 14, no. 5, pp. 2401–2406, 2014.
[20] E. W. Lim and R. Ismail, “Conduction Mechanism of Valence Change Resistive Switching Memory: A Survey,” Electronics, vol. 4, no. 3, pp. 586–613, 2015.
[21] H. S. P. Wong, H. Y. Lee, S. Yu, Y. S. Chen, Y. Wu, P. S. Chen, B. Lee, F. T. Chen, and M. J. Tsai, “Metal-oxide RRAM,” Proc. IEEE, vol. 100, no. 6, pp. 1951–1970, 2012.
[22] H. Y. Lee, P. S. Chen, T. Y. Wu, Y. S. Chen, C. C. Wang, P. J. Tzeng, C. H. Lin, F. Chen, C. H. Lien, and M. J. Tsai, “Low power and high speed bipolar switching with a thin reactive ti buffer layer in robust HfO2 based RRAM,” Tech. Dig. - Int. Electron Devices Meet. IEDM, pp. 3–6, 2008.
[23] U. Russo, D. Kamalanathan, D. Ielmini, A. L. Lacaita, and M. N. Kozicki, “Study of multilevel programming in Programmable Metallization Cell (PMC) memory,” IEEE Trans. Electron Devices, vol. 56, no. 5, pp. 1040–1047, 2009.
[24] Y. Wang, Q. Liu, S. Long, W. Wang, Q. Wang, M. Zhang, S. Zhang, Y. Li, Q. Zuo, J. Yang, and M. Liu, “Investigation of resistive switching in Cu-doped HfO2 thin film for multilevel non-volatile memory applications.,” Nanotechnology, vol. 21, no. 4, p. 045202, 2010.
[25] C. He, Z. Shi, L. Zhang, W. Yang, R. Yang, D. Shi, and G. Zhang, “Multilevel resistive switching in planar graphene/SiO2 nanogap structures,” ACS Nano, vol. 6, no. 5, pp. 4214–4221, 2012.
[26] S.-Y. Wang, C.-W. Huang, D.-Y. Lee, T.-Y. Tseng, and T.-C. Chang, “Multilevel resistive switching in Ti/CuxO/Pt memory devices,” J. Appl. Phys. Appl. Phys, vol. 108, no. 102, pp. 114110–94101, 2010.
[27] S. Yu, Y. Wu, and H. S. P. Wong, “Investigating the switching dynamics and multilevel capability of bipolar metal oxide resistive switching memory,” Appl. Phys. Lett., vol. 98, no. 10, pp. 2009–2012, 2011.
[28] B. Hu, X. Zhu, X. Chen, L. Pan, S. Peng, Y. Wu, J. Shang, G. Liu, Q. Yan, and R. W. Li, “A multilevel memory based on proton-doped polyazomethine with an excellent uniformity in resistive switching,” J. Am. Chem. Soc., vol. 134, no. 42, pp. 17408–17411, 2012.
[29] W. C. Chien, Y. C. Chen, K. P. Chang, E. K. Lai, Y. D. Yao, P. Lin, J. Gong, S. C. Tsai, S. H. Hsieh, C. F. Chen, K. Y. Hsieh, R. Liu, and C. Y. Lu, “Multi-level operation of fully CMOS compatible WOX resistive random access memory (RRAM),” 2009 IEEE Int. Mem. Work. IMW ’09, vol. 91, pp. 1168–1169, 2009.
[30] S. C. Na, J. J. Kim, M. Chul Chun, D. Hee Jin, S. E. Ahn, and B. Soo Kang, “Mechanism of the reset process in bipolar-resistance-switching Ta/TaOx/Pt capacitors based on observation of the capacitance and resistance,” Appl. Phys. Lett., vol. 104, no. 12, pp. 0–4, 2014.
[31] H. K. Li, T. P. Chen, S. G. Hu, P. Liu, Y. Liu, P. S. Lee, X. P. Wang, H. Y. Li, and G. Q. Lo, “Study of Multilevel High-Resistance States in HfOx-Based Resistive Switching Random Access Memory by Impedance Spectroscopy,” IEEE Trans. Electron Devices, vol. 62, no. 8, pp. 2684–2688, 2015.
[32] Y. Zhang, H. Wu, Y. Bai, A. Chen, Z. Yu, J. Zhang, and H. Qian, “Study of conduction and switching mechanisms in Al/AlOx/WOx/W resistive switching memory for multilevel applications,” Appl. Phys. Lett., vol. 102, no. 23, pp. 1–5, 2013.
[33] J. Wu, C. Ye, J. Zhang, T. Deng, P. He, and H. Wang, “Multilevel characteristics for bipolar resistive random access memory based on hafnium doped SiO2 switching layer,” Mater. Sci. Semicond. Process., vol. 43, pp. 144–148, 2016.
[34] L. Liu, D. Yu, W. Ma, B. Chen, F. Zhang, B. Gao, and J. Kang, “Multilevel resistive switching in Ag/SiO2/Pt resistive switching memory device,” Jpn. J. Appl. Phys., vol. 54, no. 2, p. 021802, 2015.
[35] B. Díaz, J. Światowska, V. Maurice, A. Seyeux, E. Härkönen, M. Ritala, S. Tervakangas, J. Kolehmainen, and P. Marcus, “Tantalum oxide nanocoatings prepared by atomic layer and filtered cathodic arc deposition for corrosion protection of steel: Comparative surface and electrochemical analysis,” Electrochim. Acta, vol. 90, pp. 232–245, 2013.
[36] Y. C. Chen, Y. L. Chung, B. T. Chen, W. C. Chen, and J. S. Chen, “Revelation on the interrelated mechanism of polarity-dependent and multilevel resistive switching in TaOx-based memory devices,” J. Phys. Chem. C, vol. 117, no. 11, pp. 5758–5764, 2013.
[37] K. Chennakesavulu, M. M. Reddy, G. R. Reddy, A. M. Rabel, J. Brijitta, V. Vinita, T. Sasipraba, and J. Sreeramulu, “Synthesis, characterization and photo catalytic studies of the composites by tantalum oxide and zinc oxide nanorods,” J. Mol. Struct., vol. 1091, pp. 49–56, 2015.
[38] H. Xue, Y. Chen, X. L. Xu, G. H. Zhang, H. Zhang, and S. Y. Ma, “X-ray diffraction spectroscopy and X-ray photoelectron spectroscopy studies of Cu-doped ZnO films,” Phys. E Low-dimensional Syst. Nanostructures, vol. 41, no. 5, pp. 788–791, 2009.
[39] C. L. Lin, C. C. Tang, S. C. Wu, P. C. Juan, and T. K. Kang, “Impact of oxygen composition of ZnO metal-oxide on unipolar resistive switching characteristics of Al/ZnO/Al resistive RAM (RRAM),” Microelectron. Eng., vol. 136, pp. 15–21, 2015.
[40] S. Bae, D.-S. Kim, S. Jung, W. Jeong, J. Lee, S. Cho, J. Park, and D. Byun, “Bipolar Switching Behavior of ZnOx Thin Films Deposited by Metalorganic Chemical Vapor Deposition at Various Growth Temperatures,” J. Electron. Mater., vol. 44, no. 11, pp. 4175–4181, 2015.
[41] Z. M. Detweiler, S. M. Wulfsberg, M. G. Frith, A. B. Bocarsly, and S. L. Bernasek, “The oxidation and surface speciation of indium and indium oxides exposed to atmospheric oxidants,” Surf. Sci., vol. 648, pp. 188–195, 2016.
[42] B. Zhou, A. V. Rogachev, Z. Liu, D. G. Piliptsou, H. Ji, and X. Jiang, “Effects of oxygen/argon ratio and annealing on structural and optical properties of ZnO thin films,” Appl. Surf. Sci., vol. 258, no. 15, pp. 5759–5764, 2012.
[43] F. A. Akgul, C. Gumus, A. O. Er, A. H. Farha, G. Akgul, Y. Ufuktepe, and Z. Liu, “Structural and electronic properties of SnO2,” J. Alloys Compd., vol. 579, pp. 50–56, 2013.
[44] F. G. Aga, J. Woo, S. Lee, J. Song, J. Park, J. Park, S. Lim, C. Sung, and H. Hwang, “Retention modeling for ultra-thin density of Cu-based conductive bridge random access memory (CBRAM),” AIP Adv., vol. 6, no. 2, 2016.
[45] S. Choi, J. Lee, S. Kim, and W. D. Lu, “Retention failure analysis of metal-oxide based resistive memory,” Appl. Phys. Lett., vol. 105, no. 11, pp. 2012–2017, 2014.
[46] K. Chiang, J. Chen, and J. Wu, “Aluminum Electrode Modulated Bipolar Resistive Switching of Al/Fuel-Assisted NiOx/ITO Memory Devices Modeled with a Dual-Oxygen-Reservoir Structure,” ACS Appl. Mater. Interfaces, vol. 4, pp. 4237–4245, 2012.
[47] Z. L. Liao, Z. Z. Wang, Y. Meng, Z. Y. Liu, P. Gao, J. L. Gang, H. W. Zhao, X. J. Liang, X. D. Bai, and D. M. Chen, “Categorization of resistive switching of metal-Pr0.7Ca0.3MnO3-metal devices,” Appl. Phys. Lett., vol. 94, no. 25, p. 253503, 2009.
[48] Q. Zhou and J. Zhai, “Resistive switching characteristics of Pt/TaOx/HfNx structure and its performance improvement,” AIP Adv., vol. 3, no. 3, 2013.
[49] A. Prakash, J. Park, J. Song, J. Woo, E. J. Cha, and H. Hwang, “Demonstration of low power 3-bit multilevel cell characteristics in a TaOx-Based RRAM by stack engineering,” IEEE Electron Device Lett., vol. 36, no. 1, pp. 32–34, 2015.
[50] C. Chen, S. Gao, F. Zeng, G. S. Tang, S. Z. Li, C. Song, H. D. Fu, and F. Pan, “Migration of interfacial oxygen ions modulated resistive switching in oxide-based memory devices,” J. Appl. Phys., vol. 114, no. 1, 2013.
[51] W. Kim, S. Menzel, D. J. Wouters, R. Waser, and V. Rana, “3-Bit Multilevel Switching by Deep Reset Phenomenon in Pt/W/TaOX/Pt-ReRAM Devices,” vol. 37, no. 5, pp. 564–567, 2016.
[52] V. Y. Q. Zhuo, Y. Jiang, R. Zhao, L. P. Shi, Y. Yang, T. C. Chong, and J. Robertson, “Improved switching uniformity and low-voltage operation in TaOx-based RRAM using Ge reactive layer,” IEEE Electron Device Lett., vol. 34, no. 9, pp. 1130–1132, 2013.
[53] P. S. Chen, H. Y. Lee, Y. S. Chen, F. Chen, and M. J. Tsai, “Improved bipolar resistive switching of HfOx/TiN stack with a reactive metal layer and post metal annealing,” Jpn. J. Appl. Phys., vol. 49, no. 4 PART 2, 2010.
[54] J. Lee, Y. C. Choi, and B. S. Lee, “Effects of O2/Ar ratio and annealing on the properties of (Ba,Sr)TiO3 films prepared by RF magnetron sputtering,” Japanese J. Appl. Physics, Part 1 Regul. Pap. Short Notes Rev. Pap., vol. 36, no. 6 A, pp. 3644–3648, 1997.
[55] S. Kim, S. Choi, and W. Lu, “Comprehensive physical model of dynamic resistive switching in an oxide memristor,” ACS Nano, vol. 8, no. 3, pp. 2369–2376, 2014.