Department of Physics & Astronomy, University of Glasgow, G12 8QQ, UK.
Ultramicroscopy. 2012 Mar;114:11-9. doi: 10.1016/j.ultramic.2011.10.015. Epub 2011 Nov 4.
Aberration-corrected high angle annular dark field (HAADF) imaging in scanning transmission electron microscopy (STEM) can now be performed at atomic-resolution. This is an important tool for the characterisation of the latest semiconductor devices that require individual layers to be grown to an accuracy of a few atomic layers. However, the actual quantification of interfacial sharpness at the atomic-scale can be a complicated matter. For instance, it is not clear how the use of the total, atomic column or background HAADF signals can affect the measured sharpness or individual layer widths. Moreover, a reliable and consistent method of measurement is necessary. To highlight these issues, two types of AlAs/GaAs interfaces were studied in-depth by atomic-resolution HAADF imaging. A method of analysis was developed in order to map the various HAADF signals across an image and to reliably determine interfacial sharpness. The results demonstrated that the level of perceived interfacial sharpness can vary significantly with specimen thickness and the choice of HAADF signal. Individual layer widths were also shown to have some dependence on the choice of HAADF signal. Hence, it is crucial to have an awareness of which part of the HAADF signal is chosen for analysis along with possible specimen thickness effects for future HAADF studies performed at the scale of a few atomic layers.
在扫描透射电子显微镜（STEM）中，现在可以进行像差校正的高角环形暗场（HAADF）成像，达到原子分辨率。这是最新半导体器件特性描述的重要工具，这些器件需要将各个层的生长精度控制在几个原子层的范围内。然而，在原子尺度上对界面清晰度进行实际量化可能是一个复杂的问题。例如，使用总原子列或背景 HAADF 信号如何影响测量的清晰度或单个层宽度尚不清楚。此外，还需要一种可靠且一致的测量方法。为了突出这些问题，通过原子分辨率 HAADF 成像对两种类型的 AlAs/GaAs 界面进行了深入研究。开发了一种分析方法，以便在图像上绘制各种 HAADF 信号，并可靠地确定界面清晰度。结果表明，界面清晰度的感知水平会随着样品厚度和 HAADF 信号的选择而显著变化。还表明，单个层宽度也与 HAADF 信号的选择有一定的关系。因此，对于在几个原子层的范围内进行的未来 HAADF 研究，必须意识到选择用于分析的 HAADF 信号的哪一部分以及可能的样品厚度效应至关重要。
