Relaxed germanium on porous silicon substrates

In this paper we study the surfactant-mediated epitaxy (SME) of relaxed germanium films on (001)-orientated porous silicon wafers, which could be a first step towards a cost-efficient lift-off technique for lightweight high-efficiency multi-junction Ge/III-V solar cells. Transmission electron microscopy and high-resolution X-ray diffraction (XRD) investigations show full relaxation and high structural perfection of the epitaxial germanium. An abrupt interface showing no evidence for intermixing is formed between germanium and silicon. Similar to SME of germanium on standard Si(001) wafers the lattice mismatch is compensated by a periodic array of full edge dislocations parallel to the interface. XRD measurements indicate that part of the silicon is under a small tensile strain. We attribute this to partial strain accommodation in the top silicon layer of the porous substrate which acts as a compliant substrate.