Porous Germanium Layers by Electrochemical Etching for Layer Transfer Processes of High-Efficiency Multi-Junction Solar Cells

We demonstrate reproducible formation of mesoporous germanium layers suitable for solar energy applications by electrochemical etching in highly concentrated electrolytes. For long anodization times or thick layer formation a porosity gradient is observed leading eventually to high porosity regions and cavity formation at the bottom of the porous layer. A 30 min annealing step at a temperature of 575 degrees C in hydrogen atmosphere allows for reorganization and subsequent lift-off. The mean surface roughness increases from 0.31 nm for unprocessed Ge up to 7.85 nm for reorganized Ge as measured by atomic force microscopy. mu-Raman confocal spectroscopy analysis confirms that etching and annealing do not affect the crystalline structure.