III-V Epitaxy on Detachable Porous Germanium 4” Substrates

Porous germanium multiple layer stacks were prepared using bipolar electrochemical etching. In dependance of the porosity within the individual porous layers they can be used after a high temperature process either for III-V epitaxy and/or detachment. Both characteristics are crucial to result in a thin detached multi-junction solar cell of which the germanium substrate occupies about 600 nm or less. We observe a strong correlation between the porosity of the detachment layer and the surface roughness after annealing. Finally an Al0.5Ga0.49In0.01As/Ga0.99In0.01As double heterostructure was grown in a MOVPE reactor on a reference CMP germanium sample and a comparable low roughness porous germanium substrate. An intensive material analysis was performed on both samples revealing a similar surface. Cathodoluminescence measurements expose a defect density of 5.2×104 cm-2 for the reference case and 2.4×105 cm-2 for the porous substrate. Time resolved photoluminescence measurements (TRPL) revealed a difference of approx. 2 ns in the lifetime of the minority carriers in the low injection regime for the two substrates (approx. 8 ns vs. 6 ns, respectively). In addition, XRD measurements confirmed the crystalline quality and in particular no change of the in-plane lattice parameter of the substrate due to the porosification or the annealing process. Finally, the results are very promising to achieve two goals at the same time. On the one hand, the weight reduction of a potential multi-junction solar cell especially for space applications. And on the other hand a cost reduction based on a lower consumption of the rare and valuable element germanium.