True or False: Geometrical Phase Analysis (GPA) carried out in TEM measurements of the Ge QW with respect to a reference point in the SiGe barrier indicates an in-plane strain close to 0. This means that the QW is fully relaxed.
It actually indicates that the Ge QW has the same in-plane lattice constant as the SiGe barrier i.e. that the QW is fully strained. The strain in a GPA measurement is not defined in terms of the measured lattice parameter and the expected natural lattice parameters of the material but rather the measured lattice parameter as compared to a reference measured in another part of the image. In order to arrive at the true strain values, knowledge of the material is required (the composition can be found e.g. from electron energy loss spectroscopy if not from XRD or SIMS). In addition, the thin TEM section may elastically relax, further complicating the relationship between the measured strain values and the actual strain in the 2d QW.
See for example figures S5 and S6 in the Supplementary Information of Daniel Jirovec, Andrea Hofmann, Andrea Ballabio, Philipp M. Mutter, Giulio Tavani, Marc Botifoll, Alessandro Crippa, Josip Kukucka, Oliver Sagi, Frederico Martins, Jaime Saez-Mollejo, Ivan Prieto, Maksim Borovkov, Jordi Arbiol, Daniel Chrastina, Giovanni Isella, and Georgios Katsaros. A singlet triplet hole spin qubit in planar Ge. Nature Mater. 20 (8) 1106--1112 (2021). https://www.nature.com/articles/s41563-021-01022-2