菱形與類四方晶系結構鐵酸鉍薄膜之光譜性質研究

Abstract

本論文研究多鐵材料 BiFeO3 薄膜 (厚度約為 30 nm) 成長於不同基板 (LSAT、DyScO3、LaAlO3) 的光譜響應。x 光繞射能譜顯示 BiFeO3 成長於 DyScO3 及 LSAT 基板上呈現菱形晶系結構，成長於 LaAlO3 基板上呈現類四方晶系結構。我們量測薄膜的穿透光譜，計算吸收係數，估算 BiFeO3 薄膜成長於 LSAT 與 LaAlO3 基板上室溫能隙值約為 2.58 eV 與 3.08 eV。隨著溫度下降，菱形晶系 BiFeO3 薄膜 (LSAT 基板) 能隙值在低於 100 K 時，明顯偏離理論預測值，此可能與 BiFeO3 塊材於低溫具自旋玻璃態以及晶格不匹配度改變有關。此外，菱形晶系 BiFeO3 薄膜樣品於 2.8 ~ 3.2 eV 與 4.0 ~ 4.4 eV 能量區間，與類四方晶系 BiFeO3 薄膜樣品於 3.45 ~ 3.5 eV 與 4.27 ~ 4.3 eV 能量區間，其振盪子權重分別在 200 K 與 150 K 附近顯現極值，此亦與 BiFeO3 薄膜樣品於 200 K 與 150 K 的自旋重新排列行為有關。 其次，我們量測薄膜的高溫拉曼散射光譜，菱形晶系結構 BiFeO3 薄膜 (DyScO3 基板) 的 173 cm-1 拉曼峰於 600 K 附近展現不連續紅移現象，此與自旋聲子之交互作用有關，我們計算出自旋聲子耦合係數約為 12.9 mRy/Å2。另一方面，類四方晶系結構 BiFeO3 薄膜 (LaAlO3 基板) 在 360 K 附近，221 cm-1、269 cm-1、359 cm-1 及 586 cm-1 拉曼峰頻率位置與半高寬有階梯式的變化，造成此現象有兩種可能因素：(i) 類四方晶系結構從 MC-Type 轉換為 MA-Type；(ii) 反鐵磁有序相轉換。我們也觀察到 586 cm-1 與 685 cm-1 拉曼峰在接近 600 K 時有不連續紅移現象，此可能為接近尼爾溫度時，受到自旋聲子耦合交互作用影響所致。

We report the optical properties of 30 nm-thick multiferroic BiFeO3 (BFO) thin films. The x-ray diffraction data indicate that BiFeO3 thin film is rhombohedral phase grown on LSAT and DSO substrates while is tetragonal-like phase on a LAO substrate. The absorption coefficient of these thin films determined from the transmission spectra shows the value of bandgap to be about 2.58 eV and 3.08 eV for BFO/LAST and BFO/LAO. Furthermore, the bandgap of BFO/LSAT deviates from the theoretical predictions in terms of Bose-Einstein model below 100 K that is likely associated with spin-glass phase observed in bulk BFO or misfit strain change between thin film and substrate at low temperatures. Additionally, the spectral weight of two oscillators in the energy range of 2.8 ~ 4.4 eV shows an extreme behavior at 200 K in BFO/LSAT and at 150 K in BFO/LAO. These anomalies could be related to spin-reorientation transition. Raman-active phonon mode observed near 173 cm-1 in BFO/LSAT exhibits a discontinuous redshift at about 600 K, indicating a spin-phonon interaction with a coupling constant of 12.9 mRy/Å2. On the other hand, the peak position and linewidth of the 221 cm-1, 269 cm-1, 359 cm-1, 586 cm-1, and 685 cm-1 phonon modes observed in BFO/LAO show a step-like change at about 360 K, which could arise from either the structural transformation between tetragonal-like Mc- and MA-type or antiferromagnetic ordering. Finally, the 586 cm-1 and 685 cm-1 phonon modes observed in BFO/LAO also exhibits a discontinuous redshift at about 600 K driven by a spin-phonon coupling.