Development of germanium-on-germanium engineered substrates for III-V multijunction solar cells

Specific power is an important metric for solar arrays in the aerospace market. For High Altitude Pseudo Satellites (HAPS), solar cells specific powers above 1500 W/kg are targeted. The current standard technology for space applications, namely lattice matched triple junction solar cells grown on $145\ \mu\mathrm{m}$ thick germanium substrates, supplies the required efficiency but is not light enough. This paper will present the Ge-on-Ge engineered substrate concept as an approach that can be implemented to address the needed reduction in germanium substrate thickness. The lift-off of the top Ge foil from this Ge-on-Ge substrate will be demonstrated. Additionally, results of epitaxial growth of III-V layers onto Ge-on-Ge engineered substrates will be discussed.