Post-growth tuning of detachable Ge membranes adhesion strength via porous Ge transformation

Single crystal germanium (Ge) membranes have recently gained increasing interest for lightweight and low-cost solar cells and flexible optoelectronic devices. These membranes achieved similar material quality as bulk Ge substrate. However, the control of the membrane detachment is still challenging. In this work, we explore post-growth engineering of the adhesion strength of a Ge membrane on a porous germanium (PGe) substrate by inducing morphological transformations in the separation layer through Thermal Budget (TB) control. Indeed, the pillars formed through PGe sintering during epitaxy are found to evolve with post-growth thermal annealing. Scanning electron microscopy (SEM) based analysis of the residue of the post-detachment broken pillars has been performed showing that the pillar's diameter and density can be tuned by thermal annealing. Depending on the post-growth annealing temperature, the membrane adhesion strength can be successively tailored from 0.5 to up to 3.5 MPa while ensuring 100 % detachment yield. The experimental results have been correlated with Finite Element Modeling (FEM) considering realistic pillar distribution revealing that pillar size and density are the dominant factors influencing the membrane adhesion strength.