Grieb Tim, Tewes Moritz, Schowalter Marco, Müller-Caspary Knut, Krause Florian F, Mehrtens Thorsten, Hartmann Jean-Michel, Rosenauer Andreas
Institute of Solid State Physics, University of Bremen, 28359 Bremen, Germany.
Institute of Solid State Physics, University of Bremen, 28359 Bremen, Germany.
Ultramicroscopy. 2018 Jan;184(Pt B):29-36. doi: 10.1016/j.ultramic.2017.09.012. Epub 2017 Oct 13.
The chemical composition of four SiGe layers grown on silicon was determined from quantitative scanning transmission electron microscopy (STEM). The chemical analysis was performed by a comparison of the high-angle annular dark field (HAADF) intensity with multislice simulations. It could be shown that amorphous surface layers originating from the preparation process by focused-ion beam (FIB) at 30 kV have a strong influence on the quantification: the local specimen thickness is overestimated by approximately a factor of two, and the germanium concentration is substantially underestimated. By means of simulations, the effect of amorphous surface layers on the HAADF intensity of crystalline silicon and germanium is investigated. Based on these simulations, a method is developed to analyze the experimental HAADF-STEM images by taking the influence of the amorphous layers into account which is done by a reduction of the intensities by multiplication with a constant factor. This suggested modified HAADF analysis gives germanium concentrations which are in agreement with the nominal values. The same TEM lamella was treated with low-voltage ion milling which removed the amorphous surface layers completely. The results from subsequent quantitative HAADF analyses are in agreement with the nominal concentrations which validates the applicability of the used frozen-lattice based multislice simulations to describe the HAADF scattering of SiGe in STEM.
通过定量扫描透射电子显微镜（STEM）确定了在硅上生长的四个硅锗层的化学成分。化学分析是通过将高角度环形暗场（HAADF）强度与多切片模拟进行比较来进行的。结果表明，在30 kV下通过聚焦离子束（FIB）制备过程产生的非晶表面层对定量有很大影响：局部样品厚度被高估了约两倍，锗浓度被大幅低估。通过模拟，研究了非晶表面层对晶体硅和锗的HAADF强度的影响。基于这些模拟，开发了一种方法，通过考虑非晶层的影响来分析实验性HAADF-STEM图像，这是通过将强度乘以一个常数因子来降低强度来实现的。这种建议的改进后的HAADF分析给出的锗浓度与标称值一致。用低电压离子研磨处理相同的TEM薄片，完全去除了非晶表面层。随后定量HAADF分析的结果与标称浓度一致，这验证了所使用的基于冻结晶格的多切片模拟在描述STEM中硅锗的HAADF散射方面的适用性。
