Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 99354, USA.
J Phys Condens Matter. 2014 Apr 2;26(13):135005. doi: 10.1088/0953-8984/26/13/135005. Epub 2014 Mar 13.
The structural properties of phase-pure epitaxial (Fe1-xCrx)2O3 thin films deposited on α-Al2O3(0 0 0 1) substrates by oxygen-plasma-assisted molecular beam epitaxy are investigated across the composition range using x-ray photoelectron spectroscopy, high-resolution x-ray diffraction, scanning transmission electron microscopy and electron energy loss spectroscopy, and non-Rutherford resonant elastic scattering measurements. The films possess a columnar grain structure with uniform mixing of cations on the nanometer scale. Fe-rich films are relaxed and appear to be slightly oxygen-rich, while Cr-rich films remain partially strained to the Al2O3 substrate and are found to be oxygen deficient. A model is proposed to explain the oxygen stoichiometry results based on the energetics of oxygen defect formation and rate of oxygen diffusion in the corundum lattice, and the dependence on the cation composition. Deliberately introducing residual compressive biaxial strain into (Fe1-xCrx)2O3 thin films (x = 0, 0.41, 0.52) by employing a Cr2O3 buffer layer is shown to narrow the optical bandgap, from 1.80(1) eV for relaxed (Fe0.47Cr0.53)2O3 to 1.77(1) eV for partially strained (Fe0.48Cr0.52)2O3. The relationships which are elucidated between epitaxial film structure and optical properties can be applied to bandgap optimization in the (Fe,Cr)2O3 system.
通过氧等离子体辅助分子束外延在 α-Al2O3(0001)衬底上沉积的单相外延 (Fe1-xCrx)2O3 薄膜的结构特性在整个组成范围内使用 X 射线光电子能谱、高分辨率 X 射线衍射、扫描透射电子显微镜和电子能量损失光谱以及非卢瑟福共振弹性散射测量进行了研究。这些薄膜具有柱状晶粒结构，纳米级的阳离子均匀混合。富铁薄膜弛豫且似乎略微富氧，而富铬薄膜仍然部分应变到 Al2O3 衬底上，并发现贫氧。提出了一个模型来解释氧化学计量结果，该模型基于氧缺陷形成的能量学和氧在刚玉晶格中的扩散速率以及与阳离子组成的依赖性。通过在 (Fe1-xCrx)2O3 薄膜（x=0、0.41、0.52）中引入残余压缩双轴应变，Cr2O3 缓冲层可以将光学带隙从弛豫的 (Fe0.47Cr0.53)2O3 的 1.80(1) eV 缩小到部分应变的 (Fe0.48Cr0.52)2O3 的 1.77(1) eV。阐明的外延薄膜结构和光学性质之间的关系可应用于 (Fe,Cr)2O3 系统中的带隙优化。
