二價與四價摻雜鑭錳氧化物薄膜之電子結構與相分離現象研究

Abstract

我們成功地藉由脈衝雷射濺鍍法，在鈦酸鍶（100）基板上磊晶成長軸向（001）的電子摻雜La0.7Ce0.3MnO3薄膜。即時成長純相La0.7Ce0.3MnO3薄膜的條件落在一個狹小的範圍內：基板溫度∼720℃、雷射能量密度∼2 J/cm2以及氧壓0.35 Torr。鍍膜後續的退火熱處理，除了釋放基板施壓在平面方向上的張力使得c軸延長之外，也會造成些許的CeO2雜相的生成，並導致金屬-絕緣相變溫度的提升。此一結果顯示材料之電磁特性與晶格間作用力的強烈關連。雖然退火熱處理會造成薄膜中有些許的CeO2雜相產生，但並未如混相的塊材（La0.7Ce0.3-yMnO3 + yCeO2）會具有兩個金屬-絕緣相變的特徵。 我們亦以X光吸收光譜、反射率光譜以及霍爾效應實驗等技術來探討La0.7Ce0.3MnO3的電子結構及相關之物理特性。從X光吸收光譜的結果，我們證實La0.7Ce0.3MnO3是多數自旋載子且為電子摻雜的結論。而LDA+U的計算結果也顯示上述的結論，但也同時指出Ce4+的摻雜僅僅將費米面提高，並縮小hole pockets，未能將費米面提升到載子為電子的能帶，故傳輸特性仍表現出載子為電洞的行為。由低溫霍爾效應實驗的結果， 與磁場曲線之斜率在高磁場時為正值顯示其傳輸載子與LDA+U的計算結果相符（電洞）。而La0.7Ce0.3MnO3與La0.7MnO3的反射率光譜顯示La0.7Ce0.3MnO3載子的有效質量（effective mass）迥異於缺鑭錳氧化物系統的載子行為。 在龐磁阻（Colossal Magnetoresistance）錳氧化物中，溫度相關的相分離行為是解釋金屬-絕緣相變的一個重要機制。藉由掃瞄式穿隧電子顯微鏡（STM）可以清楚的觀察到奈米尺度下的相分離現象隨溫度的變化。雖然相分離現象在La0.7Ca0.3MnO3與La0.7Ce0.3MnO3中皆在居禮溫度（TC）附近發生與消散，但在金屬與絕緣相分佈的大小等等之上有著明顯的不同。我們推論這些相異之處主要是源於摻雜離子半徑的不同、基板的應力以及本身電子間的作用行為的差異所造成，並做進一步的討論。

Single-phase electron-doped manganite thin films with nominal composition of La0.7Ce0.3MnO3, substituting La3+ ions with Ce4+ ions to dope electrons, were prepared on SrTiO3 (100) substrates by UV pulsed-laser deposition. The conditions for obtaining purely single phase La0.7Ce0.3MnO3 films in-situ lie within a very narrow window, namely with the substrate temperature (TS □ 720℃) and laser energy density (ED □ 2 J/cm2), during deposition. In-situ post-deposition annealing, mainly to relax the possible epitaxial in-plane tensile strain between the film and the substrate, leads to an increasing c-axis lattice constant accompanied by the formation of secondary CeO2 phase and higher metal-insulator transition temperature. This is indicative of a strong coupling between the electron and lattice degree of freedom. Furthermore, the presence of CeO2 does not seem to cause previously observed double-peak behavior in metal-insulator transition and paramagnetic-ferromagnetic transitions. X-ray absorption spectroscopy (XAS), optical reflectance spectroscopy, and the Hall effect measurements were used to investigate the electronic structure of La0.7Ce0.3MnO3. The XAS results are consistent with those obtained from LDA+U calculations. In that the doping of Ce4+ has shifted up the Fermi level and resulted in marked shrinkage of hole pockets originally existing in La0.7Ca0.3MnO3. The Hall measurements indicate that in La0.7Ce0.3MnO3 thin films the carriers are still displaying the characteristics of holes as LDA+U calculations predict. Analyses of the optical reflectance spectra evidently disapprove the scenario that the present La0.7Ce0.3MnO3 thin films might have been dominated by the La-deficient phases. Finally, nano-scale phase separation in the divalent-doped La0.7Ca0.3MnO3 and tetravalent-doped La0.7Ce0.3MnO3 manganite thin films was investigated as a function of temperature using the scanning tunneling spectroscopic imaging techniques. The results show that, although phase separation starts to emerge at temperatures slightly above the paramagnetic-ferromagnetic transition temperature (TC) in both cases, the evolution and size distribution of the separated conductive and insulating phases exhibit noticeable differences in the respective films. Although the differences may have originated from the strain state induced by A-ion substitution and by substrate/film epitaxy, we argue that the intrinsic differences between the electronic structures of the two materials can be the predominant factor.