Growth of Metamorphic InGaP for Wide-Bandgap Photovoltaic Junction by MBE

Metamorphic triple-junction solar cells can currently attain efficiencies as high as 41.1%. Using additional junctions could lead to efficiencies above 50%, but require the development of a wide bandgap (2.0-2.2eV) material to act as the top layer. In this work we demonstrate wide bandgap In_yGa _1-yP grown on GaAs_xP_1-x via solid source molecular beam epitaxy. Unoptimized tensile GaAs_xP_1-x buffers grown on GaAs exhibit asymmetric strain relaxation, along with formation of faceted trenches 100-300 nm deep in the [01-1] direction. Smaller grading step size and higher substrate temperatures minimizes the facet trench density and results in symmetric strain relaxation. In comparison, compressively- strained graded GaAs_xP_1-x buffers on GaP show nearly-complete strain relaxation of the top layers and no evidence of trenches. We subsequently grew In_yGa_1-yP layers on the GaAs _xP_1-x buffers. Photoluminescence and transmission electron microscopy measurements show no indication of phase separation or CuPt ordering. Taken in combination with the low threading dislocation densities obtained, MBE-grown In_yGa_1-yP layers are promising candidates for future use as the top junction of a multi-junction solar cell.