Space III-V Multijunction Solar Cells on Ge/Si virtual substrates

Virtual substrates based on thin Ge layers on Si substrates by direct deposition have recently achieved high quality. In this work, their application as low cost, removable substrates for the growth of high efficiency, lightweight and flexible multijunction solar cells for space applications is analyzed. Experimental Ge single-junction solar cells and GaInP/Ga(In)As/Ge triple-junction solar cells using the Ge/Si virtual substrate as an active bottom junction (being the Si inactive), are implemented using medium quality Ge/Si virtual substrates with a $5\ \boldsymbol{\mu} \mathbf{m}$ Ge layer thickness. A lower quality in the Ge material, as compared to standard substrates, but enough carrier collection efficiency for a standard triple-junction, are shown. The expected formation of cracks during growth, due to the large thermal expansion coefficient mismatch with the Si substrate, is confirmed, and is found to be a major limiting factor for the performance of the solar cells. Strategies such as thinning the Ge + III-V structure and minimizing the thermal cycling during growth are discussed. Using an embedded porous Si layer to serve as buffer for the strain is being investigated. This porous layer could also serve as sacrificial layer for high throughput mechanical epitaxial lift-off in the manufacturing of lightweight and flexible multijunction cells. These embedded porous Si layers need to be engineered for optimum performance and compatibility with the Ge and III-V deposition processes.