Strain analysis of a Ge micro disk using precession electron diffraction

The recently developed precession electron diffraction (PED) technique in scanning transmission electron microscopy has been used to elucidate the local strain distribution and crystalline misorientation in a CMOS fabricated strained Ge microdisk structure grown on a Si substrate. Tensile strained Ge and GeSn structures are considered to be potential CMOS compatible optical sources, as both Sn alloying and strain can lead to a direct band-structure and lasing. The ability to take nanometer resolution, experimental measurements of the cross-sectional strain distribution, is important to understand modal gain and, therefore, ultimate device performance. In this work, we demonstrate PED techniques to measure the cross-sectional strain field in tensile Ge microdisks strained by SiN stressors. The strain maps are interpreted and compared with a finite element model of the strain in the investigated structure, which shows good agreement, and, therefore, highlights the applicability of PED techniques for mapping strained photonic structures. The technique also allows for the observation of strain relaxation due to dislocation pileup, further demonstrating the benefit of such experimental techniques.