Metamorphic GaAsP Buffers for Growth of Wide-Bandgap InGaP Solar Cells

GaAs_xP1_-x graded buffers were grown via solid source molecular beam epitaxy (MBE) to enable the fabrication of wide-bandgap InyGa1-yP solar cells. Tensile-strained GaAsxP_1-x buffers grown on GaAs using unoptimized conditions exhibited asymmetric strain relaxation along with formation of faceted trenches, 100-300 nm deep, running parallel to the [01̄1] direction. We engineered a 6 μm thick grading structure to minimize the faceted trench density and achieve symmetric strain relaxation while maintaining a threading dislocation density of ≤10⁶ cm ^-2. In comparison, compressively-strained graded GaAs _xP1_-x buffers on GaP showed nearly-complete strain relaxation of the top layers and no evidence of trenches but possessed threading dislocation densities that were one order of magnitude higher. We subsequently grew and fabricated wide-bandgap InyGa_1-yP solar cells on our GaAs_xP_1-x buffers. Transmission electron microscopy measurements gave no indication of CuPt ordering. We obtained open circuit voltage as high as 1.42 V for In_0.39Ga_0.61P with a bandgap of 2.0 eV. Our results indicate MBE-grown InyGa_1-yP is a promising material for the top junction of a future multijunction solar cell.