此實驗為金屬層-鐵電層-絕緣層-矽基底(MFIS)之電容器結構的研究，在鐵電層以鐵酸鉍(BiFeO3)摻雜鈮(Nb)來提高鐵電記憶體結構的性能並減少漏流產生，用(HfO2)作為絕緣層來抑制鐵電層與矽基底兩層之間的相互擴散、反應。
在此研究中，控制鈮的摻雜濃度，氬氧氣比例，RTA溫度，HfO2厚度，製備出鐵電電容器，再應用原子力顯微鏡(AFM)與X光繞射儀(XRD)來分析薄膜的表面形貌與晶相，另外應用C-V與I-V儀器來分析MFIS電容器結構的電器特性。從AFM與XRD圖片分析出得知，當退火溫度越高時薄膜的結晶訊號越明顯，當Nb和O2摻入BFO薄膜中經由C-V與I-V儀器的量測，發現Nb原子取代Fe位置和薄膜的氧空缺減少情形，使得鐵電記憶體的性能有顯著的改善。另外非晶態HfO2絕緣層的厚度增加其記憶視窗寬也增加但因為電荷注入現象產生使得記憶視窗寬隨之減少。但絕緣層的厚度太厚造成更大的壓降在絕緣層上使得記憶視窗寬減少。實驗結果顯示當掃描電壓為±8 V，最大的記憶視窗寬可達2.98 V，其製程參數是DC Power:Nb(15 W)，氬氧氣比例為10，絕緣層厚度為40 nm，退火溫度600 ℃。而此MFIS電容器結構之製程參數，擁有良好的鐵電記憶特性，因此可應用在非揮發性鐵電記憶體上。

關鍵字: BiFeO3，鐵電層，HfO2，金屬層-鐵電層-絕緣層-矽基底。

Metal-ferroelectric-insulator-semiconductor (MFIS) capacitors with Nb-doped BiFeO3 (BFO) ferroelectric layer and HfO2 insulator layer have been fabricated. The purpose of Nb doping is to improve the ferroelectric properties by reducing the leakage current; the insulator layer is to suppress the inter-diffusion and reaction between the ferroelectric layer and silicon substrate.
In this study, the process parameters include the Nb doping concentration, Ar/O2 ratio during sputtering, RTA temperature, and HfO2 thickness. The surface morphology and crystalline phase of annealed films were analyzed by AFM and XRD, respectively. Moreover, the electrical properties of the MFIS capacitors were characterized through capacitance-voltage (C-V) and current-voltage (I-V) measurements. AFM and XRD techniques indicate that the crystallization of annealed films increases as RTA temperature. C-V and I-V measurements reveal that the ferroelectric memory performance can be significantly improved through Nb doping and O2 incorporation in the BFO films due to Fe-site substitution by Nb atoms and reduction of oxygen vacancies. Moreover, for amorphous HfO2 insulator layers, the memory window increases with HfO2 thickness due to reduction of charge injection. However, on the other hand, a thicker HfO2 layer results in a larger voltage drop across the insulator layer, which may dominantly narrow down the memory window. The maximum ferroelectric memory window is 2.98 V obtained from a sweep voltage of ± 8 V for the samples with 40-nm-thick HfO2 insulator and RTA temperature of 600 ℃. This good ferroelectric memory performance of the MFIS structure shows its feasibility for nonvolatile memory applications.

Keywords: BiFeO3, Ferroelectric, HfO2, Metal-ferroelectric-insulator-
semiconductor

[1]C. S. Hwang, “(Ba/Sr)TiO3 thin films for ultra large scale dynamic random access memory a review on the process integration”, Materials Science and Engineering B, Vol. 56, p. 178, 1998.
[2]B. H. Park, B. S. Kang, S. D. Bu, T. W. Noh, J. Lee, W. Jo, “Lanthanum-substituted bismuth titanate for use in non-volatile memories”, Nature, Vol. 401, p. 682, 1999.
[3]張勁燕，“工業電子學”，五南文化出版社，2001。
[4]鄭晃忠、史德智、黃全洲、鄭德宏、賴明駿、陳永宏、莊朝凱，“極大
型積體電路之鐵電材料”，電子月刊，1999。
[5]J. L. Moll, Y. Tarui, “A new solid state memory resistor”, IEEE Transactions on Electron Devices, Vol. 10, p. 338, 1963.
[6]P. M. Heyman, G. H. Hlilmeier, “A ferroelectric field effect device”, IEEE Transactions on Electron Devices, Vol. 54, p. 842, 1966.
[7]S. P. Stuart, H. L. Karl, “An adaptive thin-film transistor”, IEEE Transactions on Electron Devices, Vol. 14, p. 816, 1967.
[8]H. N. Lee, Y. T. Kim, S. H. Choh, “Comparison of memory effect between YMnO3 and SrBi2Ta2O9 ferroelectric thin films deposited on Si substrates”, Applied Physics Letters, Vol. 76, p. 1066, 1967.
[9]H. N. Lee, M. H. Lim, Y. T. Kim, “Characteristics of metal/ferroelectric/insulator/semiconductor field effect transistors using a Pt/SrBi2 Ta2O9 /Y2O3 /Si structure”, Japanese Journal of Applied Physics, Vol. 37, p. 1107, 1998.
[10]H. N. Lee, Y. T. Kim, Y. K. Park, “Memory window of highly c-axis oriented ferroelectric YMnO3 thin films”, Applied Physics Letters, Vol. 74, p. 3887, 1999.
[11]T. Hirai, Y. Fujisaki, K. Nagashima, “Preparation of SrBi2Ta2O9 film at low emperatures and fabrication of a metal/ferroelectric/insulator/semiconductor field effect transistor using Al/SrBi2Ta2O9/CeO2/Si (100) structures”, Japanese Journal of Applied Physics, Vol. 36, p. 5908, 1997.
[12]B. W. Shen, G. Chung, I. C. Chen, “Scalability of a trench capacitor cell for 64 Mbit RAM”, IEEE IEDM Technical Digest, p.2.2.1, 1989.
[13]M. S. Tsai, S. C. Sun, T. Y. Tseng, “Effect of bottom electrode materials on the electrical and reliability characteristics of (Ba/Sr)TiO capacitors”, IEEE Transactions on Electron Devices, Vol. 46, p. 1829, 1999.
[14]T. Eimori, H. Kimura, J. Matsufusa, “A newly designed planar stacked capacitor cell with high dielectric constant film for 256 Mbit DRAM”, IEEE IEDM Technical Digest, Vol. 93, p. 26.3, 1993.
[15]K. Igrashi, K. Koumoto, H. Yanagida, “Curie point of Perovskite - type oxides containing bivalent ions of the 4th period in the B - site”, Journal of Materials Science, Vol. 23, p. 2517, 2005.
[16]M. Houssa, “High-k Gate Dielectrics”, IMEC, Belgium, 2004.
[17]Y. P. Wang, L. Zhou, M. F. Zhang, X. Y. Chen, J. M. Liu, Z. G. Liu, “Room temperature saturated ferroelectric polarization in BiFeO3 ceramics synthesized by rapid liquid phase sintering”, Applied Physics Letters, Vol. 84, p. 1731, 2004.

[18]V. R. Palkar, K. G. Kumara, S. C. Malik, “Observation of room temperature magnetoelectric coupling in pulsed – laser – deposited Bi0.6Tb0.3La0.1FeO3 thin films”, Applied Physics Letters. Vol. 58, p. 1003, 2002.
[19]俞文海，“晶體結構的對稱群”，中國科技技術大學出版社，1991。
[20]J. F. Scott, C. A. P. D. Araujo, “Ferroelectric memories”, Science,Vol. 246, p. 1400, 2001.
[21]李雅明，“固態電子學”，全華科技圖書股份有限公司，1997。
[22]K. Igrashi, K. Koumoto, H. Yanagida, “Curie point of perovskite-type oxides containing bivalent ions of the 4th period in the B-site” , Journal of Materials Science, Vol. 23, p. 2517, 2005.
[23]吳朗，“電子陶瓷入門”，全欣出版社，1992。
[24]K. B. Uhlman, “Introduction to Ceramices”, Wilet - New York, 1976.
[25]呂正傑、詹世雄，“鐵電記憶體簡介”，NDL奈米通訊，1998。
[26]Y. L. Qin, C. L. Jia, K. Urban, “Structural and morphologic evolution of Pt/Ba0.7Sr0.3TiO3/Pt capacitors with annealing processes”, Applied Physics Letters, Vol. 80, p. 2728, 2002.
[27]H. F. Cheng, “Structural and optical properties of laser deposited ferroelectric (Ba0.6Sr0.4) TiO3 thin films”, Applied Physics Letters, Vol. 79 , p. 7965, 1996.
[28]D. Ricinschi, M. Okuyama, “Ab initio study of the giant spontaneous polarization of BiFeO3: Carrier doping effects”, Ferroelectrics, Vol.355, p. 101, 2007.

[29]A. M. Kadomtseva, Y. F. Popov, A. P. Pyatakov, G. P. Vorobev, A. K. Zvezdin, D. Viehland, “Phase transitions in multiferroic BiFeO3 crystals, thin-layers and ceramics enduring potential for a single phase, room-temperature magnetoelectric holy grail”, Phase Transitions, Vol. 79, p. 1019, 2006.
[30]R. Haumont, J. Kreisel, P. Bouvier, “Raman scattering of the model multiferroic oxide BiFeO3: effect of temperature, pressure and stress”, ferroelectrics, Vol. 79, p. 1043, 2006.
[31]P. Fischer, M. Polomska, I. Sosnowska, M. Szymanski, “Temperature dependence of the crystal and magnetic structures of BiFeO3”, Journal of. Physics C: Solid State Physics, Vol. 13, p. 1931, 1980.
[32]C. J. Fennie, “Ferroelectrically induced weak ferromagnetism by design”, Physical Review Letters, Vol. 100, p.167203, 2008.
[33]G. Catalan, J. F. Scott, “Physics and applications of bismuth ferrite”, Advanced Materials, Vol. 21, p. 2463, 2009.
[34]R. Teague, R. G. erson, W. J. James, “Dielectric hysteresis in single crystal BiFeO3”, Solid State Communications , Vol. 8, p. 1073, 1970.
[35]M. M. Kumar, V. R. Palker, K. Srinivas, S. V. Sueyanaryana, “Ferroelectricity in a pure BiFeO3 ceramic”, Applied Physics Letters, Vol. 76, p. 2764, 2000.
[36]Y. P. Wamg, L. Zhou, M. F. Zhang, X. Y. Chen, J. M. Liu, Z. G. Liu, “Room-temperature saturated ferroelectric polarization in BiFeO3 ceramics synthesized by rapid lique phase sintering”, Applied Physics Letters, Vol. 84, p. 1731, 2004.

[37]V. R. Palkar, K. g. Kumara, S. K. Malik, “Observation of room temperature magnetoelectric coupling in pulsed-laser-deposited Bi0.6Tb0.3La0.1FeO3 thin flims”, Applied Physics Letters, Vol. 84, p. 1003, 2004.
[38]K. Y. Yun, M. Noda, M. Okuyama, “Prominent ferroelectricity of BiFeO3 thin flims prepared bypulsed laser deposition”, Applied Physics Letters, Vol. 83, p. 3981, 2003.
[39]K. Y. Yun, M. Noda, M. Okuyama, H. Seaki, H. Tabata, K. Saito, “Structural and multiferroic properties of BiFeO3 thin flims at room temperature”, Journal of Applied Physics, Vol. 96, p. 3399, 2004.
[40]R. Ueno, S. Okaura, H. Funakubo, K. Saito, “Crystal structure and electric properties of epitaxial BiFeO3 thin flims grown by metal organic chemical vapor deposition”, Japanese Journal of Applied Physics, Vol. 44, p. 1231, 2005.
[41]Y. H. Lee, J. M. Wu, Y. C. Chen, Y. H. Lu, H. N. Lin, “Surface chemistry and nanoscale characterizations of multiferroic BiFeO3 thin flims”, Electrochemical and Solid-State Letters, Vol. 8, p. 43, 2005.
[42]H. S. Gu, J. M. Xue, X. Gao, J. Wang, “Doping effect of BiFeO3 in layered perovskite SrBi2Nb2O9”, Materials Chemistryand Physics, Vol. 75, p. 105, 2002.
[43]C. Cheon, J. Seog, K. Pyung, W. Jang, “Ferroelectric and magnetic properties of PrFeO3–PbTiO3 and PrFeO3–BiFeO3–PbTiO3 thin flims”, Japanese Journal of Applied Physics, Vol. 41, p. 6777, 2002.

[44]B. Ruette, S. Zvyagin, A. P. Pyatakov, A. Bush, J. F. Li, V. I. Belotelov, A. K. Zvezdin, D. Viehland, “Magnetic-field-induced phase transition in BiFeO3 observed by high-field electron spin order＂, Physical Review B, Vol. 69, p. 064114 , 2004.
[45]J. Allibe, I. C. Infante, S. Fusil, K. Bouzehouane, E. Jacquet, C. Deranlot, M. Bibes, A. Barthélémy, “Coengineering of ferroelectric and exchange bias properties in BiFeO3 based heterostructures”, Applied Physics Letters, Vol. 95, p. 182503, 2009.
[46]T. P. C. Juan, J. H. Lu, M. W. Lu, “Improvement on reliability properties of metal-ferroelectric(BiFeO3)-insulator(HfO2)-semiconductor structuresfabricated by oxygen-incorporated magnetron sputtering”, Journal of the Electrochemical Society, Vol. 155, p. 991, 2008.
[47]G. D. Hu, S. H. Fan, C. H. Yang, W. B. Wu, “Low leakage current and enhanced ferroelectric properties of Ti and Zn codoped BiFeO3 thin film”, Applied Physics Letters, Vol. 92, p. 192905, 2008.
[48]S. K. Singh, K. Maruyama, H. Ishiwara, “Reduced leakage current in La and Ni codoped BiFeO3 thin films”, Applied Physics Letters, Vol. 91, p. 112913, 2007.
[49]S. K. Singh, H. Ishiwara, K. Sato, K. Maruyama, “Microstructure and frequency dependent electrical properties of Mn-substituted BiFeO3 thin films”, Journal of Applied Physics, Vol. 102, p. 94109, 2007.
[50]Z. Q. Hu, M. Y. Li, B. F. Yu, L. Pei, J. Liu, J. Wang, X. Z. Zhao, “Enhanced multiferroic properties of BiFeO3 thin films by Nd and high-valence Mo co-doping”, Journal of Physics D-Applied Physics, Vol. 42, p. 185010, 2009.

[51]J. K. Kim, S. S. Kim, W. J. Kim, A. S. Bhalla, R. Guo, “Enhanced ferroelectric properties of Cr-doped BiFeO3 thin films grown by chemical solution deposition”, Applied Physics Letters, Vol. 88, p. 132901, 2006.
[52]S. T. Zhang, L. H. Pang, Y. Zhang, M. H. Lu, Y. F. Chen, “Preparation, structures and multiferroic properties of single phase Bi1-XLaXFeO3 (x=0.40) ceramics” , Journal of Applied Physics, Vol. 100, p. 114108, 2006.
[53]Z. C. Quan, W. Liu, H. Hu, S. Xu, B. Sebo, G. J. Fang, M. Y. Li, X. Z. Zhao, “Microstructure, electrical and magnetic properties of Ce-doped BiFeO3 thin films”, Journal of Applied Physics, Vol. 104, p. 084106, 2008.
[54]C. D. Zheng, J. Yu, D. M. Zhang, B. Yang, Y. Y. Wu, L. H. Wang, Y. B. Wang, W. L. Zhou, “Processing and ferroelectric properties of Ti-doped BiFeO3 ceramics”, Integrated Ferroelectrics, Vol. 94, p. 31, 2007.
[55]A. Z. Simoes, R. F. Pianno, E. C. Aguiar, E. Longo, J. A. Varela, “Effect of niobium dopant on fatigue characteristics of BiFeO3 thin films grown on Pt electrodes”, Journal of Alloys and Compounds Vol. 479 , p. 274, 2009.
[56]Q. Xiaoding, D. Joonghoe, T. Rumen, G. Mark, L. Judith, D. Macmanus, “Graetly reduced leakage current and onducvion Mechanismin aliovalent -ion- donped BiFeO3”, Applied Physics Letters, Vol. 86, p. 062903, 2005.
[57]H. N. Lee, Y. T. Kim, S. H. Choh, “Comparison of memory effect between YMnO3 and SrBi2Ta2O9 ferroelectric thin films deposited on Si substrates”, Applied Physics Letters, Vol. 76, p. 1066, 2000.
[58]B. Yu, H. Wang, C. Riccobene, Q. Xiang, M. R. Lin, “Limits of gate-oxide scaling in nano-transistors”, VLSI Technology Digest of Technical Papers, Vol. 36, p. 90, 2000.
[59]B. Cheng, M. C. Cao, R. Rao, A. Inani, P. V. Voorde, W. M. Greene, J. M. C. Stork, Z. Yu, M. Zeitzoff, J. C. S. Woo, “The impact of high-k gate dielectrics and metal gate electrodes on sub-100 nm MOSFETS”, IEEE Transactions on Electron Devices, Vol. 46, p. 1537, 1999.
[60]M. H. Cho, D. H. Ko, Y. G. Choi, K. Jeong, I. W. Lyo, D. Y. Noh, H. J. Kim, C. N. Whang, “Structural and electrical characteristics ofY2O3 films grown on oxidized Si (100) surface”, Journal of Vacuum Science and Technology A, Vol. 19, p. 192, 2001.
[61]B. H. Lee, Y. J. K. Zawadzki, W. J. Qi, J. Lee, “Effects of interfacial layer growth on the electrical characteristics of thin titanium oxide films on silicon”, Applied Physics Letters, Vol. 74, p. 3143, 1999.
[62]V. Mikhelashvili, G. Eisenstein, “Effects of annealing conditions on optical and electrical characteristics of titanium dioxide films deposited by electron beam evaporation”, Journal of Applied Physics, Vol.89, p. 3256, 2001.
[63]M. H. Cho, Y. S. Roh, C. N. Whang, K. Jeong, S. W. Nahm, D. H. Ko, J. H. Lee, N. I. Lee, K. Fujihara, “Thermal stability and structural characteristics of HfO2 films on Si (100) grown by atomic-layer deposition”, Applied Physics Letters, Vol. 81, p. 472, 2002.
[64]S. M. Hu, “Stress - related problems in silicon technology”, Journal Applied Physics, Vol. 70, p. 53, 1991.

[65]T. M. Klein, D. Niu, W. S. Epling, W. Li, D. M. Maher, C. C. Hobbs, R. I. Hegde, I. J. R. Baumvol, G. N. Parsons, “Evidence of aluminum silicate formation during chemical vapor deposition on amorphous Al2O3 thin films on Si (100)”, Applied Physics Letters, Vol. 75, p. 4001, 1999.
[66]H. F. Luan, S. J. Lee, S. C. Song, Y. L. Mao, Y. Senzaki, D. Roverts, D. L. Kwong, “High quality Ta2O5 gate dielectrics with Tox.eq”, IDEM Techniccal Digest International, Vol. 19, p. 141, 1999.
[67]K. J. Hubbard, D. G. Schlom, “Thermodynamic stability of binaryoxides in contact with silicon”, Journal Material Reserch, Vol. 11, p. 2757, 1996.
[68]B. H. Lee, L. Kang, W. J. Qi, R. Nieh, Y. Jeon, K. Onishi, D. L. Kwong, “Deposited directly on Si ”, Technical Digest of the International Electron Devices Meet. Vol. 21, p. 133, 1999.
[69]I. Barin, G. Platzki, “Thermochemical data of pure substances”, VCH Weinheim-New York, 1995.
[70]J. C. Lee, “Ultra - thin gate dielectrics and high-k dielectrics”, IEEE EDS Vanguard Series of Independent Short Courses, Vol. 42, p. 71, 2001.
[71]R. Ueno, S. Okaura, H. Funakubo, “Crystal structure and electrical properties of epitaxial BiFeO3 thin films grown by metal organic chemical vapor deposition”, Japanese Journal of Applied Physics, Vol. 44, p. 1231, 2005.
[72]C. M. Foster, G. R. Bai, R. Csencsits, “Single-crystal Pb (ZrxTi1-x) O3 thin films prepared by metal-organic chemical vapor deposition: Systematic compositional variation of electronic and optical properties”, Journal of Applied Physics, Vol. 81, p. 2349, 1997.
[73]W. J. Lin, T. Y. Tseng, Y. Z. Wu, “Growth and fatigue properties of pulsed laser deposited Pb1-XLaX (ZryTiz)O3 thin films with [001] preferred orientation”, Journal Materials Science, Vol. 7, p. 409, 1996.
[74]C. C. Lee, J. M. Wu, C. P. Hsiung, “Highly (110) and (111) oriented BiFeO3 films on BaPbO3 electrode with Ru or Pt/Ru barrier layers”, Applied Physics Letters, Vol. 90, p. 182909, 2007.
[75]Y. W. Chiang, J. M. Wu, “Characterization of metal-ferroelectric (BiFeO3)-insulator(ZrO2)-silicon capacitors for nonvolatile memory applications”, Applied Physics Letters, Vol. 91, p. 142103, 2007.
[76]P. C. Juan, J. D. Jiang, W. C. Shih, J. Y. M. Lee, “The effect of annealing temperature on the electrical properties of metal ferroelectric (PbZr0.53Ti0.47O3)-insulator (ZrO2)-semiconductor (MFIS) thin-film capacitors”, Microelectronic Engineering, Vol. 84, p. 2014, 2007.
[77]T. P. C. Juan, J. H. Lu, M. W. Lu, “Influence of Ar/O2 ratio on the electrical properties of metal-ferroelectric (BiFeO3)-insulator (HfO2)-semiconductor capacitors fabricated by RF magnetron sputtering”, Journal of Vacuum Science and Technology B, Vol. 27, p. 313, 2009.