本研究成功利用溶膠-凝膠法合成了Nd2(1-x)Ca2xTi2O7 (x = 0、0.05、0.09、0.13、0.17)薄膜，並探討不同薄膜厚度、熱處理和摻雜濃度對電阻轉換特性的影響。結果顯示，所有元件均表現出雙極性電阻轉換(BRS)行為。相較於未摻雜的NTO薄膜元件相比，摻雜濃度為x = 0.13的元件在性能方面顯著提升，開關循環次數從524次增加到1022次，Ron/Roff增至約102。此外，經過金屬後退火(PMA)處理後，開關循環次數可達1839次，−1.48 V/0.66 V之低的工作電壓，Ron/Roff則大於102，數據保持能力在25°C和85°C下達到104秒。這些電性的改善主要歸因於Ca2+離子少量取代Nd3+離子，有效增加了薄膜內的氧空位濃度，以及PMA處理後AlOx層的形成能幫助防止氧離子向外擴散。本研究展示了Al/Nd2(1-x)Ca2xTi2O7/ITO元件在低功耗RRAM應用中的潛力。

This study successfully synthesized Nd2(1-x)Ca2xTi2O7 (x = 0、0.05、0.09、0.13、0.17) thin films via sol-gel method, and systematically investigated the effects of different film thickness, thermal treatment, and doping concentration on the resistive switching characteristics. The results demonstrate that all devices exhibit bipolar resistive switching (BRS) behavior. Compared to undoped NTO thin film devices, doped devices exhibit significant improvements, particularly at a doping concentration of 13 mol%, where the switching cycles increase from 524 to 1022, and the Ron/Roff ratio increases to approximately 102. Furthermore, after post-metal annealing (PMA) treatment, the switching cycles further increase to 1839, with Vset/VReset of −1.48 V/0.66 V, Ron/Roff >102, and data retention capability reaching 104 seconds at both 25°C and 85°C. These improvements in electrical properties are attributed to the effective enhancement of oxygen vacancy concentration within the thin film due to Ca2+ doping, as well as the diffusion effects of Al and In ions after PMA treatment. In summary, this study demonstrates the potential application of Al/Nd2(1-x)Ca2xTi2O7/ITO devices in low-power non-volatile memory applications.

[1] M. Lanza, "A review on resistive switching in high-k dielectrics: a nanoscale point of view using conductive atomic force microscope," Materials, vol. 7, no. 3, pp. 2155-2182, 2014.
[2] H. Akinaga and H. Shima, "Resistive random access memory (ReRAM) based on metal oxides," Proceedings of the IEEE, vol. 98, no. 12, pp. 2237-2251, 2010.
[3] D.-W. Tao, Z.-J. Jiang, J.-B. Chen, B.-J. Qi, K. Zhang, and C.-W. Wang, "Stable resistive switching characteristics from highly ordered Cu/TiO2/Ti nanopore array membrane memristors," Applied Surface Science, vol. 539, p. 148161, 2021.
[4] J. Park and S. Kim, "Improving endurance and reliability by optimizing the alternating voltage in Pt/ZnO/TiN RRAM," Results in Physics, vol. 39, p. 105731, 2022.
[5] C. Wang, B. Song, and Z. Zeng, "Excellent selector performance in engineered Ag/ZrO2: Ag/Pt structure for high-density bipolar RRAM applications," AIP Advances, vol. 7, no. 12, 2017.
[6] G. Ma, X. Tang, H. Su, H. Zhang, J. Li, and Z. Zhong, "Effects of electrode materials on bipolar and unipolar switching in NiO resistive switching device," Microelectronic engineering, vol. 129, pp. 17-20, 2014.
[7] Y.-S. Hong et al., "Single-crystalline CuO nanowires for resistive random access memory applications," Applied Physics Letters, vol. 106, no. 17, 2015.
[8] P. K. Sarkar, M. Prajapat, A. Barman, S. Bhattacharjee, and A. Roy, "Multilevel resistance state of Cu/La2O3/Pt forming-free switching devices," Journal of Materials Science, vol. 51, pp. 4411-4418, 2016.
[9] X. Cao et al., "Forming-free colossal resistive switching effect in rare-earth-oxide Gd2O3 films for memristor applications," Journal of Applied Physics, vol. 106, no. 7, 2009.
[10] H.-I. Kim et al., "Improved environment stability of Y2O3 RRAM devices with Au passivated Ag top electrodes," Materials, vol. 15, no. 19, p. 6859, 2022.
[11] T.-M. Pan and C.-H. Lu, "Forming-free resistive switching behavior in Nd2O3, Dy2O3, and Er2O3 films fabricated in full room temperature," Applied Physics Letters, vol. 99, no. 11, 2011.
[12] J. Wang, B. Bedford, C. Chen, L. Miao, and B. Luo, "Optically Modulated Resistance Switching Polarities in BaTiO3 Thin Film," 2020.
[13] Y. Yang et al., "Moisture-modulated resistive switching behavior based on CaTiO3 prepared by the appropriate NaOH concentration," Chemical Physics, vol. 578, p. 112161, 2024.
[14] K. H. Chen, M. C. Kao, S. J. Huang, and J. Z. Li, "Bipolar Switching Properties of Neodymium Oxide RRAM Devices Using by a Low Temperature Improvement Method," Materials, Article vol. 10, no. 12, p. 8, Dec 2017, Art no. 1415, doi: 10.3390/ma10121415.
[15] V. Prusakova et al., "The development of sol-gel derived TiO2 thin films and corresponding memristor architectures," (in English), RSC Adv., Article vol. 7, no. 3, pp. 1654-1663, 2017, doi: 10.1039/c6ra25618j.
[16] G. Winfield, F. Azough, and R. Freer, "DiP224: Neodymium titanate (Nd2Ti2O7) ceramics," Ferroelectrics, vol. 133, no. 1, pp. 181-186, 1992.
[17] D. W. Hwang, J. S. Lee, W. Li, and S. H. Oh, "Electronic band structure and photocatalytic activity of Ln2Ti2O7 (Ln= La, Pr, Nd)," The Journal of Physical Chemistry B, vol. 107, no. 21, pp. 4963-4970, 2003.
[18] Y.-P. Jiang, X.-G. Tang, Q.-X. Liu, Z. Tang, and Y.-C. Zhou, "Resistive switching performance and synaptic behavior of La-doped HfO2 thin film," Thin Solid Films, vol. 774, p. 139842, 2023.
[19] T. Guo, H. Huang, X. Huang, Y. Wang, L. Duan, and Z. Xu, "Modulation of resistive switching and magnetism of HfOx film by Co doping," Journal of Alloys and Compounds, vol. 921, p. 166218, 2022.
[20] F. Lv, C. Gao, P. Zhang, C. Dong, C. Zhang, and D. Xue, "Bipolar resistive switching behavior of CaTiO3 films grown by hydrothermal epitaxy," RSC Adv., vol. 5, no. 51, pp. 40714-40718, 2015.
[21] W.-G. Kim and S.-W. Rhee, "Effect of the top electrode material on the resistive switching of TiO2 thin film," Microelectronic Engineering, vol. 87, no. 2, pp. 98-103, 2010.
[22] H. Y. Jeong, S. K. Kim, J. Y. Lee, and S.-Y. Choi, "Role of interface reaction on resistive switching of metal/amorphous TiO2/Al RRAM devices," Journal of The Electrochemical Society, vol. 158, no. 10, p. H979, 2011.
[23] S. Yu, Resistive random access memory (RRAM). Morgan & Claypool Publishers, 2016.
[24] Z. Shao et al., "An easy sol–gel route for deposition of oriented Ln2Ti2O7 (Ln=La, Nd) films on SrTiO3 substrates," Journal of Crystal Growth, vol. 311, no. 16, pp. 4134-4141, 2009/08/01/ 2009.
[25] A. G. Asadov et al., "The effects of high pressure on the crystal structure and vibration spectra of layered perovskite-like Nd2Ti2O7," Solid State Ionics, vol. 406, p. 116447, 2024.
[26] D. Panda and T.-Y. Tseng, "Growth, dielectric properties, and memory device applications of ZrO2 thin films," Thin Solid Films, vol. 531, pp. 1-20, 2013.
[27] S. Dasgupta, "6T SRAM cell analysis for DRV and read stability," Journal of Semiconductors, vol. 38, no. 2, p. 025001, 2017.
[28] P. Asthana and S. Mangesh, "Design and implementation of 4T, 3T and 3T1D DRAM cell design on 32 NM technology," International Journal of VLSI Design & Communication Systems, vol. 5, no. 4, p. 47, 2014.
[29] R. Micheloni, G. Campardo, and P. Olivo, Memories in wireless systems. Springer Science & Business Media, 2008.
[30] M. A. Sanvido, F. R. Chu, A. Kulkarni, and R. Selinger, "NAND flash memory and its role in storage architectures," Proceedings of the IEEE, vol. 96, no. 11, pp. 1864-1874, 2008.
[31] H.-S. P. Wong et al., "Phase change memory," Proceedings of the IEEE, vol. 98, no. 12, pp. 2201-2227, 2010.
[32] X. Fong, S. K. Gupta, N. N. Mojumder, S. H. Choday, C. Augustine, and K. Roy, "KNACK: A hybrid spin-charge mixed-mode simulator for evaluating different genres of spin-transfer torque MRAM bit-cells," in 2011 International Conference on Simulation of Semiconductor Processes and Devices, 2011: IEEE, pp. 51-54.
[33] Y. Nishi and B. Magyari-Kope, Advances in non-volatile memory and storage technology. Woodhead Publishing, 2019.
[34] C.-F. Liu et al., "Resistive switching characteristics of HfO2 thin films on mica substrates prepared by sol-gel process," Nanomaterials, vol. 9, no. 8, p. 1124, 2019.
[35] S. Gao, C. Song, C. Chen, F. Zeng, and F. Pan, "Dynamic processes of resistive switching in metallic filament-based organic memory devices," The Journal of Physical Chemistry C, vol. 116, no. 33, pp. 17955-17959, 2012.
[36] M. Akbari, M.-K. Kim, D. Kim, and J.-S. Lee, "Reproducible and reliable resistive switching behaviors of AlOX/HfOX bilayer structures with Al electrode by atomic layer deposition," RSC Adv., vol. 7, no. 27, pp. 16704-16708, 2017.
[37] D. Berco and T.-Y. Tseng, "A numerical study of forming voltage and switching polarity dependence on Ti top electrode thickness in ZrO2 RRAM," Journal of Computational Electronics, vol. 15, no. 2, pp. 595-601, 2016.
[38] P. Sun et al., "Thermal crosstalk in 3-dimensional RRAM crossbar array," Scientific reports, vol. 5, no. 1, p. 13504, 2015.
[39] R. Liu, D. Mahalanabis, H. J. Barnaby, and S. Yu, "Investigation of single-bit and multiple-bit upsets in oxide RRAM-based 1T1R and crossbar memory arrays," IEEE Transactions on Nuclear Science, vol. 62, no. 5, pp. 2294-2301, 2015.
[40] S. Munjal and N. Khare, "Advances in resistive switching based memory devices," Journal of Physics D: Applied Physics, vol. 52, no. 43, p. 433002, 2019.
[41] B. Ku, Y. Abbas, A. S. Sokolov, and C. Choi, "Interface engineering of ALD HfO2-based RRAM with Ar plasma treatment for reliable and uniform switching behaviors," Journal of Alloys and Compounds, vol. 735, pp. 1181-1188, 2018.
[42] J.-Y. Lin, K.-Y. Wu, and K.-H. Chen, "Effects of Sm2O3 and V2O5 film stacking on switching behaviors of resistive random access memories," Crystals, vol. 9, no. 6, p. 318, 2019.
[43] H. Zheng et al., "Forming-free resistive switching characteristics of Ag/CeO2/Pt devices with a large memory window," Semicond. Sci. Technol., vol. 32, no. 5, p. 055006, 2017.
[44] T.-M. Pan, C.-H. Lu, S. Mondal, and F.-H. Ko, "Resistive Switching Characteristics of Tm2O3,Yb2O3, and Lu2O3-Based Metal–Insulator–Metal Memory Devices," IEEE transactions on nanotechnology, vol. 11, no. 5, pp. 1040-1046, 2012.
[45] A. M. Rana et al., "Thickness effect on the bipolar switching mechanism for nonvolatile resistive memory devices based on CeO2 thin films," Materials Science in Semiconductor Processing, vol. 39, pp. 211-216, 2015.
[46] X. Pan et al., "Resistive switching behavior in single crystal SrTiO3 annealed by laser," Applied Surface Science, vol. 389, pp. 1104-1107, 2016.
[47] X. Jiang et al., "Effect of deposition temperature on ultra-low voltage resistive switching behavior of Fe-doped SrTiO3 films," Applied Physics Letters, vol. 116, no. 10, 2020.
[48] H. Tang, X.-G. Tang, Y.-P. Jiang, Q.-X. Liu, W.-H. Li, and L. Luo, "Bipolar resistive switching characteristics of amorphous SrTiO3 thin films prepared by the sol-gel process," J. Asian Ceram. Soc., vol. 7, no. 3, pp. 298-305, 2019.
[49] P. Xu, D. Chang, T. Lu, L. Li, M. Li, and W. Lu, "Search for ABO3 type ferroelectric perovskites with targeted multi-properties by machine learning strategies," Journal of chemical information and modeling, vol. 62, no. 21, pp. 5038-5049, 2021.
[50] X. Zhang et al., "Effect of Joule heating on resistive switching characteristic in AlOx cells made by thermal oxidation formation," Nanoscale Research Letters, vol. 15, pp. 1-8, 2020.
[51] Y. Huang et al., "Amorphous ZnO based resistive random access memory," RSC Adv., vol. 6, no. 22, pp. 17867-17872, 2016.
[52] L. Hu, W. Han, and H. Wang, "Resistive switching and synaptic learning performance of a TiO2 thin film based device prepared by sol–gel and spin coating techniques," Nanotechnology, vol. 31, no. 15, p. 155202, 2020.
[53] D.-W. Tao, J.-B. Chen, Z.-j. Jiang, B.-J. Qi, K. Zhang, and C.-W. Wang, "Making reversible transformation from electronic to ionic resistive switching possible by applied electric field in an asymmetrical Al/TiO2/FTO nanostructure," Applied Surface Science, vol. 502, p. 144124, 2020/02/01/ 2020.
[54] 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.
[55] F. C. Chiu, "A Review on Conduction Mechanisms in Dielectric Films," (in English), Adv. Mater. Sci. Eng., Review vol. 2014, p. 18, 2014.
[56] E. I. Morosanova, "Silica and silica-titania sol-gel materials: Synthesis and analytical application," Talanta, Article vol. 102, pp. 114-122, Dec 2012.
[57] K. Xie, Q. Fu, G. G. Qiao, and P. A. Webley, "Recent progress on fabrication methods of polymeric thin film gas separation membranes for CO2 capture," Journal of Membrane Science, vol. 572, pp. 38-60, 2019/02/15/ 2019.
[58] Z. Shao et al., "Structural and dielectric/ferroelectric properties of (La1−xNdx)2Ti2O7 synthesized by sol–gel route," Journal of Solid State Chemistry, vol. 183, no. 7, pp. 1652-1662, 2010.
[59] R. Dudric, R. Bortnic, G. Souca, R. Ciceo-Lucacel, R. Stiufiuc, and R. Tetean, "XPS on Nd0.6-xBixSr0.4MnO3 nano powders," Applied Surface Science, vol. 487, pp. 17-21, 2019.
[60] F. Gul, "Carrier transport mechanism and bipolar resistive switching behavior of a nano-scale thin film TiO2 memristor," Ceramics International, vol. 44, no. 10, pp. 11417-11423, 2018/07/01/ 2018.
[61] Z. Yu, X. Qu, W. Yang, J. Peng, and Z. Xu, "Hydrothermal synthesis and memristive switching behaviors of single-crystalline anatase TiO2 nanowire arrays," Journal of Alloys and Compounds, vol. 688, pp. 294-300, 2016.
[62] Y.-W. Zhang, R. Si, C.-S. Liao, C.-H. Yan, C.-X. Xiao, and Y. Kou, "Facile alcohothermal synthesis, size-dependent ultraviolet absorption, and enhanced CO conversion activity of ceria nanocrystals," The Journal of Physical Chemistry B, vol. 107, no. 37, pp. 10159-10167, 2003.
[63] C. Chen et al., "Room-temperature electrically pumped near-infrared random lasing from high-quality m-plane ZnO-based metal-insulator-semiconductor devices," Nanoscale Research Letters, vol. 10, pp. 1-6, 2015.
[64] Z. F. Zhang, H. X. Gao, M. Yang, P. F. Jiang, X. H. Ma, and Y. T. Yang, "Effect of thickness of metal electrode on the performance of SiNx-based resistive switching devices," Applied Physics Letters, vol. 114, no. 4, 2019.
[65] S.-Y. Huang et al., "Resistive switching characteristics of Sm2O3 thin films for nonvolatile memory applications," Solid-State Electronics, vol. 63, no. 1, pp. 189-191, 2011.
[66] Z. Yong et al., "Tuning oxygen vacancies and resistive switching properties in ultra-thin HfO2 RRAM via TiN bottom electrode and interface engineering," Applied Surface Science, vol. 551, p. 149386, 2021.
[67] R. Dudric, A. Vladescu, V. Rednic, M. Neumann, I. Deac, and R. Tetean, "XPS study on La0.67Ca0.33Mn1− xCoxO3 compounds," Journal of Molecular Structure, vol. 1073, pp. 66-70, 2014.
[68] D.-W. Tao, Z.-J. Jiang, J.-B. Chen, X.-G. Wang, Y. Li, and C.-W. Wang, "The evolution of resistive switching behaviors dependent on interface transition layers in Cu/Al/FTO nanostructure," Journal of Alloys and Compounds, vol. 827, p. 154270, 2020.
[69] D. Kumar, R. Aluguri, U. Chand, and T.-Y. Tseng, "Conductive bridge random access memory characteristics of SiCN based transparent device due to indium diffusion," Nanotechnology, vol. 29, no. 12, p. 125202, 2018.
[70] S. W. Han, C. J. Park, and M. W. Shin, "The role of Al atoms in resistive switching for Al/ZnO/Pt resistive random access memory (RRAM) device," Surfaces and Interfaces, vol. 31, p. 102099, 2022.
[71] S. Ha et al., "Effect of annealing environment on the performance of sol–gel-processed ZrO2 RRAM," Electronics, vol. 8, no. 9, p. 947, 2019.
[72] T. T. Guo, T. T. Tan, Z. T. Liu, and B. J. Liu, "Oxygen vacancy modulation and enhanced switching behavior in HfOx film induced by Al doping effect," Journal of Alloys and Compounds, Article vol. 686, pp. 669-674, Nov 2016.
[73] T. T. Guo, T. T. Tan, and Z. T. Liu, "Resistive switching behavior of HfO2 film with different Ti doping concentrations," J. Phys. D-Appl. Phys., Article vol. 49, no. 4, p. 6, Feb 2016.
[74] J.-C. Wang, D.-Y. Jian, Y.-R. Ye, L.-C. Chang, and C.-S. Lai, "Characteristics of gadolinium oxide resistive switching memory with Pt–Al alloy top electrode and post-metallization annealing," Journal of Physics D: Applied Physics, vol. 46, no. 27, p. 275103, 2013.
[75] K. Shi et al., "Improved performance of Ta2O5−x resistive switching memory by Gd-doping: Ultralow power operation, good data retention, and multilevel storage," Applied Physics Letters, vol. 111, no. 22, 2017.
[76] K.-H. Chen, M.-C. Kao, S.-J. Huang, and J.-Z. Li, "Bipolar switching properties of neodymium oxide RRAM devices using by a low temperature improvement method," Materials, vol. 10, no. 12, p. 1415, 2017.
[77] V. Prusakova et al., "The development of sol–gel derived TiO2 thin films and corresponding memristor architectures," RSC Adv., vol. 7, no. 3, pp. 1654-1663, 2017.
[78] K. Kim et al., "Sol-gel-processed amorphous-phase ZrO2 based resistive random access memory," Mater. Res. Express, vol. 8, no. 11, p. 116301, 2021.