Differential phase contrast (DPC) imaging in scanning transmission electron microscopy (STEM) allows for measuring electric and magnetic fields in solids on scales ranging from picometres to micrometres. The DPC technique mainly uses the direct beam, which is deflected by the electric and magnetic fields of the specimen and measured with a beam position sensitive detector. The beam deflection and thus the DPC signal is strongly influenced by specimen thickness, specimen tilt and lens aberrations. Understanding these influences is critical for a solid interpretation and quantification of contrasts in DPC images. To this end, the present study employs DPC-STEM image simulations of SrTiO3 [001] at atomic resolution to analyse the influence of lens aberrations, specimen tilt and thickness and also to give a guideline for the detection of parameters affecting the contrast by performing an analysis of associated scattergrams. Simulations are obtained using the multislice algorithm implemented in the Dr. Probe software with conditions corresponding to a JEOL ARM200F microscope equipped with an octa-segmented annular detector, but results should be similar for other microscopes. Simulations show that due to a non-rigid shift of the detected intensity distribution correct values of projected potentials of specimens thicker than one unit-cell cannot be determined. Regarding the impact of residual lens aberrations, it is found that the shape of the lens aberration phase function determines the symmetry and features in the DPC image. Specimen tilt leads to an elongation of features perpendicular to the tilt axis. The results are confirmed by comparing simulated with experimental DPC images of Si [110] yielding good agreement. Overall, a high sensitivity of DPC-STEM imaging to lens aberrations, specimen tilt and diffraction effects is evidenced.

中文翻译：

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透镜像差、样品厚度和倾斜对微分相位对比 STEM 图像的影响

扫描透射电子显微镜 (STEM) 中的微分相衬 (DPC) 成像允许测量固体中从皮米到微米尺度的电场和磁场。DPC 技术主要使用直接光束，该光束被试样的电场和磁场偏转，并用光束位置敏感检测器进行测量。光束偏转以及 DPC 信号受到试样厚度、试样倾斜和透镜像差的强烈影响。了解这些影响对于 DPC 图像中对比度的可靠解释和量化至关重要。为此，本研究采用原子分辨率的 SrTiO3 [001] 的 DPC-STEM 图像模拟来分析透镜像差的影响，样品倾斜度和厚度，并通过对相关散点图进行分析，为检测影响对比度的参数提供指导。使用在 Dr. Probe 软件中实现的多层算法获得模拟，条件对应于配备八段环形探测器的 JEOL ARM200F 显微镜，但其他显微镜的结果应该相似。模拟表明，由于检测到的强度分布的非刚性偏移，无法确定比一个单元格厚的样品的投影电位的正确值。关于残余透镜像差的影响，发现透镜像差相位函数的形状决定了 DPC 图像中的对称性和特征。试样倾斜导致垂直于倾斜轴的特征伸长。通过比较模拟与 Si [110] 的实验 DPC 图像来确认结果，产生良好的一致性。总体而言，DPC-STEM 成像对透镜像差、样品倾斜和衍射效应的高灵敏度得到了证明。