Mesoporous Germanium Layer Formation by Electrochemical Etching
Weight reduction of multi-junction III-V semiconductor solar cells is an important budget issue for space applications. Typically, space solar cells are epitaxially formed on a Ge or GaAs substrate wafer. The substrate material determines the lattice constant of the stack, provides mechanical stability during the cell process, and serves as bottom cell. The substrate wafer is typically more than 100 µm thick for reasons of mechanical stability during cell processing, whereas a few µm thickness are sufficient for the bottom cell to match the photogenerated currents in the top and middle cells and not to be current limiting. Unnecessarily heavy substrate wafers hence reduce the available payload for satellite missions. There are several techniques that permit the production of very-thin lightweight highly-efficient space solar cells. Ge or GaAs substrates are commonly removed by chemical wet etching, which reduces weight but has the disadvantage that the substrate wafer is lost for further use. Separating the electrically active solar cells from their substrates by a lift-off process, could save the substrate and reduce costs. The application of a layer transfer process for multi-junction III-V semiconductor space solar cells is hence of main interest for all space agencies. Lift-off processes based on epitaxial growth of the absorber layer onto a porous etched substrate already exist for the fabrication of monocrystalline silicon solar cells. Brendel demonstrated the so-called Porous Silicon (PSI) process for the production of monocrystalline thin-film Si solar cells. This method uses a double layer of mesoporous Si formed by means of electrochemical etching: A mesoporous layer with low porosity at the surface of the substrate is used as a seed layer for the Si epitaxy, while a buried high porosity layer is used as a pre-determined breaking-point. The formation of porous germanium (PGe) has been not intensively studied. This doctoral work focuses on the fabrication and characterization of porous germanium layers by means of electrochemical etching. This thesis evaluates the potential applications of porous Ge layers for the fabrication of very-thin space solar cells. Additionally, the formation of mesoporous GaAs and mesoporous Si layers with miscut orientations is investigated.