Reduction of Crosshatch Roughness and Threading Dislocation Density in Metamorphic GaInP Buffers and GaInAs Solar Cells

Surface crosshatch roughness typically develops during the growth of lattice-mismatched compositionally graded buffers and can limit misfit dislocation glide. In this study, the crosshatch roughness during growth of a compressive GaInP/GaAs graded buffer is reduced by increasing the phosphine partial pressure throughout the metamorphic growth. Changes in the average misfit dislocation length are qualitatively determined by characterizing the threading defect density and residual strain. The decrease of crosshatch roughness leads to an increase in the average misfit dislocation glide length, indicating that the surface roughness is limiting dislocation glide. Growth rate is also analyzed as a method to reduce surface crosshatch roughness and increase glide length, but has a more complicated relationship with glide kinetics. Using knowledge gained from these experiments, high quality inverted GaInAs 1 eV solar cells are grown on a GaInP compositionally graded buffer with reduced roughness and threading dislocation density. The open circuit voltage is only 0.38 V lower than the bandgap potential at a short circuit current density of 15 mA/cm ², suggesting that there is very little loss due to the lattice mismatch.