Germanium surface wet-etch-reconditioning for porous lift-off and substrate reuse

Reducing both the cost and weight of Germanium (Ge)-based devices is a key concern in extending these technologies to mainstream applications. In this framework, the porous Ge lift-off, based on a mesoporous Ge layer (PGe), shaped by bipolar electrochemical etching (BEE), constitutes an appealing strategy allowing the separation of lightweight, flexible, and low-cost devices and substrate reuse. However, after the device detachment, the broken pillar residues on the host substrate's surface prevent its reuse. Here, we report on the development and application of a reconditioning process based on an aqueous HF:H2O2:H2O (10:80:10, v-v-v) mixture without the need for Chemical Mechanical Polishing (CMP). We found that a mixed kinetic- and diffusion-controlled wet etching leads to surface polishing. Flat reconditioned substrates with low surface roughness (<2.5 nm RMS) are successfully obtained from detached surfaces with various pillar sizes up to 500 nm in diameter. The substrate reusability is demonstrated by achieving a new porous layer on a reconditioned substrate with an RMS roughness of 2.2 nm, ready for a second round of the membrane's epitaxial growth. These results demonstrate a CMP-free, reliable Ge substrate reconditioning process, paving the way towards substrate multi-reuse and consequent devices' weight and cost reduction.