Metamorphic GaAsP buffers for growth of wide-bandgap InGaP solar cells

GaAs x P 1 − x graded buffers were grown via solid source molecular beam epitaxy (MBE) to enable the fabrication of wide-bandgap In y Ga 1 − y P solar cells. Tensile-strained GaAs x P 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 [ 0 1 ¯ 1 ] direction. We engineered a 6 μ m thick grading structure to minimize the faceted trench density and achieve symmetric strain relaxation while maintaining a threading dislocation density of ≤ 10 6 cm − 2 . In comparison, compressively-strained graded GaAs x P 1 − 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 In y Ga 1 − y P solar cells on our GaAs x P 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 0.39 Ga 0.61 P with a bandgap of 2.0 eV. Our results indicate MBE-grown In y Ga 1 − y P is a promising material for the top junction of a future multijunction solar cell.