In situ growth monitoring of strain-balanced quantum-well solar cells by metal-organic vapor phase epitaxy

In situ monitoring of surface reflectivity and reflectance anisotropy (RA) was examined as a monitor of strain during the metal-organic vapor phase epitaxy (MOVPE) of strain-balanced InGaAs/GaAsP quantum-well stacks, aiming at a middle cell that improves current matching in tandem solar cells. The RA, which is in principle sensitive only to the topmost surface of the crystal, changed linearly with time during the growth of InGaAs and GaAsP with opposite slopes according to the layers. This behavior seems to reflect the accumulation of strain with the growth of InGaAs layer and its release during the growth of GaAsP layer, allowing us to manage strain balance in the stacking of quantum wells based on in situ observation. Such strain balance enabled 100 stacks of InGaAs/GaAsP quantum wells without threading dislocations. The GaAs pin junction with the strain-balanced quantum wells in the i layer exhibited the absorption edge at 1.2 eV.