Strain analysis of Ge micro disk using precession electron diffraction

TL;DR

This paper uses precession electron diffraction in STEM to analyze local strain in Ge micro disks, revealing significant in-plane tensile strain that could enable direct bandgap transitions for photonic applications.

Contribution

It demonstrates the application of PED in STEM for detailed strain mapping in Ge micro disks and validates findings with FEM simulations, highlighting potential for Ge-based lasers.

Findings

Maximum in-plane strain of nearly 2% induced by SiN stressor

Strain distribution matches finite element model predictions

Tensile strain may enable direct bandgap transitions in Ge

Abstract

The recently developed precession electron diffraction (PED) technique in scanning transmission electron microscopy (STEM) has been used to elucidate the local strain distribution and crystalline misorientation in CMOS fabricated strained Ge micro disk structure grown on Si substrate. Such structures are considered to be a compact optical source for the future photonics due to the specific undercut for direct bandgap behaviour under strain. In this study, the strain maps are interpreted and compared with a finite element model (FEM) of the strain in the investigated structure. Results demonstrate that the SiN used as a stressor on top of the Ge disk induces an in-plane strain $\epsilon_{xx}$ of a maximum value of almost 2 % which is also confirmed by FEM simulations. This tensile strain can reduce the difference between the direct and indirect bandgaps leading to direct bandgap radiative transitions, with the potential for applications in strained Ge lasers.