Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronic Engineering, East China Normal University, Shanghai 200241, People's Republic of China.
ACS Appl Mater Interfaces. 2011 Dec;3(12):4844-52. doi: 10.1021/am201340d. Epub 2011 Dec 7.
Bismuth ferrite (BiFeO(3)) nanocrystalline films with the crystalline size of 27-40 nm have been grown on c-sapphire substrates under various oxygen pressures of 1 × 10(-4) to 1 Pa by pulsed laser deposition. The X-ray diffraction spectra show that the films are polycrystalline and present the pure rhombohedral phase. It was found that the Raman-active phonon mode E(TO1) shifts towards a higher energy side from 74 to 76 cm(-1) with increasing oxygen pressure, indicating a larger tensile stress in the films deposited at higher oxygen pressure. The X-ray photoelectron spectroscopy analysis suggests that the concentrations of both Fe(2+) ions and oxygen vacancies in the BiFeO(3) films increase with decreasing oxygen pressure. Moreover, the dielectric functions in the photon energy range of 0.47-6.5 eV have been extracted by fitting the transmittance spectra with the Tauc-Lorentz dispersion model. From the transmittance spectra, the fundamental absorption edge is observed to present a redshift trend with increasing the temperature from 8 to 300 K. Note that the optical band gap (E(g)) decreases with increasing the temperature due to the electron-phonon interactions associated with the interatomic distance in the BiFeO(3) films. However, the E(g) decreases from 2.88 to 2.78 eV with decreasing oxygen pressure at 8 K, which can be attributed to the increment of oxygen vacancies leading to the formation of some impurity states between the valence and conduction band. It can be concluded that the oxygen pressure during the film fabrication has the significant effects on microstructure, optical properties, and electronic band structure modification of the BiFeO(3) films.
在不同氧压（1×10(-4) 到 1 Pa）下，采用脉冲激光沉积法在 c-蓝宝石衬底上生长了晶粒尺寸为 27-40nm 的铋铁氧体（BiFeO(3)）纳米晶薄膜。X 射线衍射谱表明，薄膜为多晶态，呈现纯菱面体相。发现随着氧压的增加，拉曼活性声子模式 E(TO1)从 74 到 76cm(-1) 向高能侧移动，表明在较高氧压下沉积的薄膜中存在较大的拉伸应力。X 射线光电子能谱分析表明，BiFeO(3) 薄膜中 Fe(2+)离子和氧空位的浓度随氧压的降低而增加。此外，在光子能量范围为 0.47-6.5eV 时，通过用 Tauc-Lorentz 色散模型拟合透射谱提取了介电函数。从透射谱中可以观察到，随着温度从 8 到 300K 的升高，基本吸收边呈现出红移趋势。值得注意的是，由于 BiFeO(3) 薄膜中原子间距离的电子-声子相互作用，光学带隙（E(g)）随温度的升高而减小。然而，在 8K 时，随着氧压的降低，E(g)从 2.88 降低到 2.78eV，这可以归因于氧空位的增加导致价带和导带之间形成一些杂质态。可以得出结论，薄膜制备过程中的氧压对 BiFeO(3)薄膜的微观结构、光学性能和能带结构改性有显著影响。
