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 InyGa1-yP grown on GaAsxP1-x via solid source molecular beam epitaxy. Unoptimized tensile GaAsxP1-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 GaAsxP1-x buffers on GaP show nearly-complete strain relaxation of the top layers and no evidence of trenches. We subsequently grew InyGa1-yP layers on the GaAsxP1-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 InyGa1-yP layers are promising candidates for future use as the top junction of a multi-junction solar cell.