Strain Analysis by Nano-Beam Electron Diffraction using convergent electron nanoprobes

半导体晶体中应变场的测量是理解新型GeSi/Si或（Al）GaN/蓝宝石异质结构中物理过程的先决条件。该建议首先处理GeSi/Si岛中的应变特性，以基本了解Ge在Si上的生长。其次，应变场和局部Ge组成的GeSi-MOSFET，是高性能计算机处理器的基本模块，是要测量的电子性能极大地依赖于应变分布。第三，GaN/蓝宝石伪衬底将被调查方面的应变，因为在这一层的应力降低的效率高度决定效率和后续的InGaN/GaN LED有源层的增长。为了访问这些应变场可靠，扫描透射电子显微镜（STEM）的应变测量方法是开发利用衍射光束的位置。特别地，STEM探针在样品上扫描，同时在每个扫描点处获取会聚束电子衍射（CBED）图案。通过专用算法，可以准确检测盘形CBED反射，精度可达7 x10 -4。为了大幅减少实验数据量，这些算法将在用于控制所使用的FEI Titan显微镜的软件中实现，以便在STEM探针扫描时进行原位CBED图案处理。这不仅可以实现2D应变映射，还可以在数据采集后立即提供应变结果。对于之前已经实现的0.5nm的空间分辨率，利用该方法用于2D映射的全部能力的一个主要先决条件是CBED图案获取的急剧加速，这是传统相机不可能实现的。因此，在项目开始时计划对延迟线探测器进行测试、修改和最后安装。为了研究微米范围内大延伸的应变场，将在实验和模拟中开发STEM进动方法。在旋进模式下，STEM探头的入射在圆锥上旋转，同时补偿所产生的衍射图案的偏移。以这种方式，可以消除CBED强度的强烈变化。此外，高阶反射的高应变灵敏度可以被利用，因为它们看起来更亮。此外，由于后放大透镜导致的衍射图案的失真将被测量并在应变测量中解释。这对于随后确定包括剪切在内的全应变张量非常重要。最后，这种技术将与所谓的Z对比信号相结合，对样品的化学成分和应变敏感。因此，应变和Z对比度的同时测量将提高组成测量的精度，因为应变和组成引起的Z对比度变化可以分离。

The measurement of strain fields in semiconductor crystals is a prerequisite for understanding physical processes in novel GeSi/Si or (Al)GaN/sapphire based heterostructures. This proposal firstly deals with the strain characterization in GeSi/Si islands for a basic understanding of Ge growth on Si. Secondly, strain fields and local Ge composition in GeSi-MOSFETs, being the basic module of high-performance computer processors, are to be measured as electronic properties drastically depend on the strain distribution. Thirdly, GaN/sapphire pseudosubstrates will be investigated with respect to strain since the efficiency of stress reduction in this layer highly determines efficiency and growth of subsequent InGaN/GaN LED active layers.In order to access these strain fields reliably, a scanning transmission electron microscopy (STEM) method for strain measurement is to be developed which exploits the positions of diffracted beams. In particular, a STEM probe is scanned over the specimen while a convergent beam electron diffraction (CBED) pattern is acquired at each scan point. With dedicated algorithms the disc-shaped CBED reflections can be detected accurately, leading to precisions of 7x10-4. To reduce the amount of experimental data drastically, these algorithms are to be implemented in a software for controlling the FEI Titan microscope used, so as to perform in-situ CBED pattern processing while the STEM probe is scanning. This not only enables 2D strain mapping but also provides the strain result immediately after data acquisition. One main prerequisite to exploit the full capability of the method for 2D mapping as to the spatial resolution of 0.5nm already achieved before is a drastic acceleration of CBED pattern acquisition which is not possible with conventional cameras. Therefore, test, modification and final installation of a delay-line detector are planned at the beginning of the project.To study strain fields of large extensions in the micron range, a STEM-precession method will be developed experimentally and in simulations. In precession mode, the incidence of the STEM probe is rotating on a cone while the resulting shift of the diffraction pattern is compensated for. In this way, strong variations of CBED intensities can be eliminated. In addition, the high strain sensitivity of high order reflections can be exploited as they appear brighter.Furthermore, distortions of the diffraction pattern due to post-magnifying lenses are to be measured and accounted for in strain measurements. This is important for the subsequent determination of the full strain tensor including shear.Finally, this technique is to be combined with the so-called Z-contrast signal, being sensitive to the chemical composition and to strain of the specimen. Consequently, the simultaneous measurement of both strain and Z-contrast will improve the precision of composition measurements since strain- and composition-induced Z-contrast variations can be separated.