Relaxed Germanium on Porous Silicon Substrates

In this paper, the authors study the surfactant-mediated epitaxy (SME) of relaxed Ge films on (001)-orientated porous silicon wafers, which could be a first step towards a cost-efficient lift-off technique for lightweight high-efficiency multijunction Ge/III-V solar cells. Transmission electron microscopy (TEM) and high-resolution X-ray diffraction (XRD) investigations show full relaxation and high structural perfection of the epitaxial Ge. An abrupt interface showing no evidence for intermixing is formed between Ge and Si. Similar to SME of Ge 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. The authors attribute this to partial strain accommodation in the top silicon layer of the porous substrate which acts as a compliant substrate.