Multiple substrate reuse: a straightforward reconditioning of Ge wafers after porous separation

Epitaxial thin film detachment and substrate reuse is one of the promising approaches to reduce the weight and the cost of triple junction (3J) solar cells on Ge substrate for both terrestrial and space PV. This approach is based on epitaxial growth of high-quality solar cell materials on porosified Ge substrate. The mesoporous layer created by electrochemical etching undergoes thermal induced reconstruction leading to the formation of voided weak layer suitable for epilayers detachment. This approach is low-cost, scalable to large surfaces and allows the substrate reuse for several epitaxial cycles upon appropriate reconditioning. Accordingly, the success of the reconditioning step is conditional to both reliability and cost-effectiveness. In this context, we report the first successful proof-of-concept of Ge substrate reuse for epitaxy after the epilayer detachment. We demonstrate that chemical etching with HF-based mixture allows to recondition the detached substrate providing a low surface roughness of 1.3 nm without any CMP step. The reconditioned substrate was then porosified giving rise to homogenous porous layer suitable for epitaxial regrowth. A second growth cycle has been successively performed on the reconditioned and reporosified substrate. The epitaxial Ge layer from the second cycle is found to have high crystalline quality and low surface roughness as revealed by X-ray diffraction and atomic force microscopy investigations. Our results demonstrate a CMP-free reliable Ge substrate reconditioning process for epitaxy, which paves the way to the substrate multi-reuse for triple junction solar cell cost-reduction.