Device and material characteristics of GalnP solar cells grown on Ge substrates by molecular beam epitaxy

摘要

We investigated the characteristics of GalnP solar cells grown directly on Ge substrates by molecular beam epitaxy (MBE). For comparison, GaAs solar cells were also grown on Ge substrates using similar initiation conditions. Compared to cells on GaAs, large degradation in open-circuit voltage (Voc) and short-circuit current density (Jsc) were observed in GaInP/Ge cells, while only small reductions were observed for GaAs/Ge. Transmission electron microscopy reveals an extremely high threading dislocation density (TDD) in the 109 cm-2 range in GaInP/Ge, four orders of magnitude larger than that of GaAs/Ge, despite the minimal lattice mismatch in both cells. GaInP/Ge initiation is far more challenging than GaAs/Ge initiation by MBE, and a homoepitaxial Ge layer will likely be necessary to obtain high-quality films.