Reformed Mesoporous Ge for Substrate Reuse in III-V Solar Cells

Mesoporous Ge was generated by bipolar electrochemical etching of (100) Ge. Through high-temperature annealing experiments, we found that the mesoporous Ge microstructure produces pore coalescence deep in the structure rather than at the surface. III-V epitaxy on annealed porous Ge with subsurface coalescence and coral-like structures on the surface produced shunted photovoltaic devices. The roughness of the annealed porous Ge film was reduced by a pre-annealing HBr treatment and a post-annealing ultrasonic DI water dip. A smoother reformed surface was obtained, on which a GaInAs upright device with an efficiency of 5.7% was grown, demonstrating a first step towards technical viability.