本論文研究多鐵材料 BiFeO3 薄膜成長於不同基板 (LSAT、
DyScO3、LaAlO3) 的穿透光譜與拉曼散射光譜。實驗結果歸納為以 下三點結論：
第一，由於 BiFeO3 塊材在 ab 平陎的晶格常數 (a = 3.96 Å ) 皆 大於基板成長表陎的晶格常數 (aDSO = 3.940 Å ； aLSAT = 3.868 Å ；
a
LAO = 3.787 Å )，薄膜感受晶格壓縮應變，由 x 光繞射能譜，我們觀 察到成長於 DyScO3 及 LSAT 之 BiFeO3 薄膜，其光譜訊號與菱形 晶系 BiFeO3 薄膜相似 (BFO / STO)，為菱形晶系結構。成長於 LaAlO3 之 BiFeO3 薄膜，其光譜訊號與菱形晶系 BiFeO3 薄膜不相 符，較為接近類四方晶系的結果，確認 BiFeO3 / LaAlO3 為類四方晶 系。光譜響應可分為兩部分：(i) 菱形晶系 BiFeO3 薄膜樣品顯現 2 個吸收峰，隨晶格不匹配度增加，吸收峰能量往高能量移動，此為晶 格壓縮，使得電子與離子間交互作用力增強的結果。估算出能隙 BiFeO3 / LSAT 為 2.58 eV 與 BiFeO3 / LaAlO3 為 3.08 eV；(ii) 菱形 晶系 BiFeO3 薄膜樣品顯示兩個拉曼峰，其頻率位置為 143 cm-1 與 173 cm-1，類四方晶系 BiFeO3 薄膜樣品顯示 5 個拉曼峰其頻率位置
為 221 cm-1、269 cm-1、359 cm-1、586 cm-1 及 685 cm-1，我們與參考 文獻比較，發現 5 個拉曼峰皆來自於類四方晶系 BiFeO3 貢獻，且 利用二維拉曼散射掃描，顯現我們的薄膜樣品為純相的類四方晶系 BiFeO3 薄膜，同時觀察到類四方晶系 BiFeO3 薄膜具有週期性結構，
可能與 BiFeO3 因壓縮應力成長為類四方晶系結構，但是有些區域受 到之應力較小，結構改變不均勻導致。
第二，藉由低溫吸收光譜，估算能隙與溫度的變化關係，隨溫度 的降低，能隙值增大，這是因為晶格熱漲冷縮，電子與離子間交互作 用力增強，能隙因而增大，在低溫時 (TR = 100 K, TT = 25 K)，能隙 有偏離理論預測值的現象，推論與 BiFeO3 低溫自旋玻璃態的行為及 晶格不匹配度於低溫時增強有關。另外，我們利用總和定則，計算吸 收峰振盪子的權重，觀察到菱形晶系 BiFeO3 薄膜樣品在 200 K 附 近，振盪子權重有異常曲線變化，類四方晶系 BiFeO3 薄膜樣品在 150 K 附近，亦觀察到振盪子權重的異常變化，推論與已發表文獻於 低溫發現的自旋重新排列的行為有關。
第三，高溫拉曼散射光譜，觀察到菱形晶系 BiFeO3 薄膜樣品拉 曼峰參數 (頻率位置、半高寬與權重) 在接近尼爾溫度頻率位置 173 cm-1 拉曼峰隨溫度上升有明顯紅移變化，我們計算自旋聲子耦合係 數約為 12.9 mRy/Å2，此與文獻相符，顯示自旋間的關聯性與聲子有
強烈耦合行為。類四方晶系 BiFeO3 薄膜樣品在接近 360 K 附近，
低頻拉曼峰頻率位置與半高寬展現類似階梯狀的偏移，高頻拉曼峰參 數亦觀察到明顯的變化，此為結構從類四方晶系之 𝑀𝐶-Type 轉換成 𝑀𝐴-Type。
我們於類四方晶系 BiFeO3 薄膜樣品高溫拉曼散射光譜，在接近 600 K 附近，觀察到 586 cm-1 與 685 cm-1 拉曼峰，頻率位置明顯的 紅移，此可能為接近尼爾溫度，受到自旋聲子耦合交互作用影響，產 生的頻率偏移。
未來，我們希望對於類四方晶系 BiFeO3 薄膜樣品進行高溫的磁 化強度量測，進而了解類四方晶系 BiFeO3 薄膜於高溫時自旋有序性 隨溫度的變化關係，以釐清類四方晶系 BiFeO3 薄膜於高溫時晶格振 動與電子自旋間交互作用的變化關係。
參考文獻
[1] J. Wang, J. B. Neaton, H. Zheng, V. Nagarajan, S. B. Ogale, B. Liu, D. Viehland, V. Vaithyanathan, D. G. Schlom, U. V. Waghmare, N. A.
Spaldin, K. M. Rabe, M. Wuttig, and R. Ramesh, “Epitaxial BiFeO3
multiferroic thin film heterostructures”, Science 299, 1719 (2003).
[2] R. J. Zeches, M. D. Rossell, J. X. Zhang, A. J. Hatt, Q. He, C. H.
Yang, A. Kumar, C. H. Wang, A. Melville, C. Adamo, G. Sheng, Y.
H. Chu, J. F. Ihlefeld, R. Erni, C. Ederer, V. Gopalan, L. Q. Chen, D.
G. Schlom, N. A. Spaldin, L. W. Martin, and R. Ramesh, “A strain-driven morphotropic phase boundary in BiFeO3”, Science 326, 977 (2009).
[3] K. Takahashi, N. Kida, and M. Tonouchi, “Terahertz radiation by an ultrafast spontaneous polarization modulation of multiferroic BiFeO3 thin films”, Phys. Rev. Lett. 96, 117402 (2006).
[4] W. L. Chan, J. Deibel, and D. M Mittleman, “Imaging with terahertz radiation”, Rep. Prog. Phys. 70, 1325 (2007).
[5] F. Gao, Y. Yuan, K. F. Wang, X. Y. Chen, F. Chen, and J. M.
Liu, ”Preparation and photoabsorption characterization of BiFeO3
nanowires”, Appl. Phys. Lett. 89, 102506 (2006).
[6] P. Hermet, M. Goffinet, J. Kreisel, and P. Ghosez, “Raman and infrared spectra of multiferroic bismuth ferrite from first principles”, Phys. Rev. B 75, 220102 (2007).
[7] S. J. Clark and J. Robertson, “Band gap and Schottky barrier heights of multiferroic BiFeO3”, Appl. Phys. Lett. 90, 132903 (2007).
[8] S. R. Basu, L. W. Martin, Y. H. Chu, M. Gajek, R. Ramesh, R. C.
Tai, X. Xu, and J. L. Musfeldt, “Photoconductivity in BiFeO3 thin films”, Appl. Phys. Lett. 92, 091905 (2008).
[9] M. K. Singh, R. S. Katiyar, and J. F. Scott, “New magnetic phase transitions in BiFeO3”, J. Phys.: Condens. Matter 20, 252203 (2008).
[10] R. L. White, “Review of revent work on the magnetic and spectroscopic properties of the rare-earth orthoferrites”, J. Appl.
Phys. 40, 1061 (1969).
[11] M. K. Singh, W. Prellier, M. P. Singh, R. S. Katiyar, and J. F. Scott,
“Spin-glass transition in single-crystal BiFeO3”, Phys. Rev. B 77, 144403 (2008).
[12] J. Diouri, J. P. Lascaray, and M. El Amrani, “Effect of the magnetic
order on the optical-absorption edge in Cd1-xMnxTe”, Phys. Rev. B
31, 7995 (1985).
[13] M. K. Singh, S. Dussan, W. Prellier, and R. S. Katiyar, “One-magnon light scattering and spin-reorientation transition in epitaxial BiFeO3
thin films”, J. Magn. Magn. Mater. 321, 1706 (2009).
[14] X. S. Xu, T. V. Brinzari, S. Lee, Y. H. Chu, L. W. Martin, A. Kumar, S. McGill, R. C. Rai, R. Ramesh, V. Gopalan, S. W. Cheong, and J.
L. Musfeldt, “Optical properties and magnetochromism in multiferroic BiFeO3”, Phys. Rev. B 79, 134425 (2009).
[15] V. Železný, D. Chvostová, L. Pajasová, I. Vrejoiu, and M. Alexe,
“Optical properties of epitaxial BiFeO3 thin films”, Appl. Phys. A
100, 1217 (2010).
[16] I. C. Infante, S. Lisenkov, B. Dupe, M. Bibes, S. Fusil, E. Jacquet, G.
Geneste, S. Petit, A. Courtial, J. Juraszek, L. Bellaiche, A.
Barthelemy, and B. Dkhil, “Bridging multiferroic phase transitions by epitaxial strain in BiFeO3”, Phys. Rev. Lett. 105, 057601 (2010).
[17] M. N. Iliev, M. V. Abrashev, D. Mazumdar, V. Shelke, and A. Gupta,
“Polarized Raman spectroscopy of nearly tetragonal BiFeO3 thin films”, Phys. Rev. B 82, 014107 (2010).
[18]M. K. Singh and R. S. Katiyar, “Phonon anomalies near the magnetic phase transitions in BiFeO3 thin films with rhombohedral R3c symmetry”, J. Appl. Phys. 109, 07D916 (2011).
[19] K. Y. Choi, S. H. Do, P. Lemmens, D. Wulferding, C. S. Woo, J. H.
Lee, K. Chu, and C. H. Yang, “Anomalous low-energy phonons in nearly tetragonal BiFeO3 thin films”, Phys. Rev. B 84, 132408 (2011).
[20] I. C. Infante, J. Juraszek, S. Fusil, B. Dupe, P. Gemeiner, O. Dieguez, F. Pailloux, S. Jouen, E. Jacquet, G. Geneste, J. Pacaud, J. Iniguez, L.
Bellaiche, A. Barthelemy, B. Dkhil, and M. Bibes, “Multiferroic phase transition near room temperature in BiFeO3 films”, Phys. Rev.
Lett. 107, 237601 (2011).
[21] C. J. Cheng, C. Lu, Z. Chen, L. You, L. Chen, J. Wang, and T. Wu,
“Thickness-dependent magnetism and spin-glass behaviors in compressively strained BiFeO3 thin films”, Appl. Phys. Lett. 98, 242502 (2011).
[22] Y. Qi, Z. Chen, L. Wang, X. Han, J. Wang, T. Sritharan, and L. Chen,
“Temperature-driven evolution of hierarchical nanodomain structure in tetragonal-like BiFeO3 films”, Appl. Phys. Lett. 100, 022908
(2012).
[23] Z. Chen, S. Prosandeev, Z. L. Luo, Wei Ren, Y. Qi, C. W. Huang, Lu You, C. Gao, I. A. Kornev, T. Wu, J. Wang, P. Yang, T. Sritharan, L.
Bellaiche, and L. Chen, “Coexistence of ferroelectric triclinic phases in highly strained BiFeO3 films”, Phys. Rev. B 84, 094116 (2011).
[24] 鄧勃、寧永成、劉密新著，儀器分析，清華大學出版社出版，中 華民國八十年五月第一版。
[25] 王智弘 國立交通大學材料科學與工程學系碩士論文，民國 98 年。
[26] W. S. Choi, S. J. Moon, S. S. A. Seo, D. Lee, J. H. Lee, P.
Murugavel, and T. W. Noh, “Optical spectroscopic investigation on the coupling of electronic and magnetic structure in multiferroic hexagonal RMnO3 (R = Gd, Tb, Dy, and Ho) thin films”, Phys. Rev.
B 78, 054440 (2008).
[27]S. Gomez-Salces, F. Aguado, F. Rodriguez, R. Valiente, J. Gonzalez, R. Haumont, and J. Kreisel, “Effect of pressure on the band gap and the local FeO6 environment in BiFeO3”, Phys. Rev. B 85, 144109 (2012).
[28] P. Hannaford, “The oscillator strength in atomic absorption spectroscopy”, Microchemical journal 63, 43 (1999).
[29] W. W. Li, J. J. Zhu, J. D. Wu, J. Gan, Z. G. Hu, M. Zhu, and J. H.
Chu, “Temperature dependence of electronic transitions and optical properties in multiferroic BiFeO3 nanocrystalline film determined from transmittance spectra”, Appl. Phy. Lett. 97, 121102 (2010).
[30] W. Baltensperger and J. S. Helman, “Influence of magnetic order in insulators on the optical phonon frequency”, Helv. Phys. Acta. 41, 668 (1968).
[31] J. Lu, A. Gunther, F. Schrettle, F. Mayr, S. Krohns, P. Lunkenheimer, A. Pimenov, V. D. Travkin, A. A. Mukhin, and A. Loidl, “On the room temperature multiferroic BiFeO3: magnetic, dielectric and thermal properties”, Eur. Phys. J. B 75, 451 (2010).
[32] S. Issing, A. Pimenov, V. Yu. Ivanov, A. A. Mukhin, and J. Geurts,
“Composition-dependent spin-phonon coupling in mixed crystals of the multiferroic maganite Eu1-xYxMnO3 (0 < x < 0.5) studied by
Raman spectroscopy”, Phys. Rev. B 81, 024304 (2010).
[33] O. Dieguez, O. E. Gonzalez-Vazquez, J. C. Wojdeł, and J. Iniguez,
“First-principles predictions of low-energy phases of multiferroic BiFeO3”, Phys. Rev. B 83, 094105 (2011).
“Temperature-driven structural phase transition in tetragonal-like BiFeO3”, Appl. Phys. Express 4, 095801 (2011).
[36] G. J. MacDougall, H. M. Christen, W. Siemons, M. D. Biegalski, J.
L. Zarestky, S. Liang, E. Dagotto, and S. E. Nagler,
“Antiferromagnetic transitions in tetragonal-like BiFeO3”, Phys. Rev.
B 85, 100406 (2012).
[37] O. Chaix-Pluchery and J. Kreisel, “Raman scattering of perovskite DyScO3 and GdScO3 single crystals”, J. Phys.: Condens. Matter 21, 175901 (2009).
[38] H. Fukumura, H. Harima, K. Kisoda, M. Tamada, Y. Noguchi, and M.
Miyayama, “Raman scattering study of multiferroic BiFeO3 single crystal”, J. Magn. Magn. Mater. 310, e367 (2007).
[39] M. V. Abrashev, A. P. Litvinchuk, M. N. Iliev, and R. L. Meng,
“Comparative study of optical phonons in the rhombohedrally distorted perovskites LaAlO3 and LaMnO3”, Phys. Rev. B 59, 4146 (1999).
[40] M. K. Singh, H. M. Jang, S. Ryu, and M. H. Jo, “Polarized Raman scattering of multiferroic BiFeO3 epitaxial films with rhombohedral
R3c symmetry”, Appl. Phys. Lett. 88, 042907 (2006).
[41] D. Mazumdar, V. Shelke, M. Iliev, S. Jesse, A. Kumar, S. V. Kalinin, A. P. Baddorf, and A. Gupta, “Nanoscale switching characteristics of nearly tetragonal BiFeO3 thin films”, Nano Lett. 10, 2555 (2010).
[42] S. A. Lourenc¸ I. F. L. Dias, J. L. Duarte, E. Laureto, E. A. Meneses, J. R. Leite, and I. Mazzaro, “Temperature dependence of optical transitions in AlGaAs”, J. Appl. Phys. 89, 6159 (2001).
[43] A. Palewicz, I. Sosnowska, R. Przeniosło, and A. W. Hewat,
“BiFeO3 crystal structure at low temperatures”, Acta. Phys. Pol. A
117, 296 (2010).
[44] S. Kamba, V. Goian, M. Orlita, D. Nuzhnyy, J. H. Lee, D. G.
Schlom, K. Z. Rushchanskii, M. Lezaic, T. Birol, C. J. Fennie, P.
Gemeiner, B. Dkhil, V. Bovtun, M. Kempa, J. Hlinka, and J. Petzelt,
“Magnetodielectric effect and phonon properties of compressively strained EuTiO3 thin films deposited on (001)(LaAlO3)0.29- (SrAl1/2Ta1/2O3)0.71”, Phys. Rev. B 59, 094435 (2012).
[45] S. A. Hayward, S. A. T. Redfern, and E. K. H. Salje, “Order parameter saturation in LaAlO3”, J. Phys.: Condens. Matter 14, 10131 (2002).