ABSTRACT:

We have investigated wide-bandgap, metamorphic GaAs$_{1-x}$P$_x$ and In $_y$Ga $_{1-y}$P solar cells on GaAs as potential subcell materials for future 4–6 junction devices. We identified and characterized morphological defects in tensile GaAs $_{1-x}$P $_x$ graded buffers that lead to a local reduction in carrier collection and a global increase in threading dislocation density (TDD). Through adjustments to the graded buffer structure, we minimized the formation of morphological defects and, hence, obtained TDDs ≈ 10 $^6$ cm$^{-2}$ for films with lattice mismatch ≤1.2%. Metamorphic In $_y$Ga$_{1-y}$P solar cells were grown on these optimized GaAs$_{1-x}$P $_x$ graded buffers with bandgaps ($E_g$) as high as 2.07 eV and open-circuit voltages ($V_{oc}$) as large as 1.49 V. Such high bandgap materials will be necessary to serve as the top subcell in future 4–6 junction devices. We have also shown that the relaxed GaAs$_{1-x}$P $_x$ itself could act as an efficient lower subcell in a multijunction device. GaAs$_{0.66}$ P$_{0.34}$ single-junction solar cells with $E_g$ = 1.83 eV were fabricated with $V_{rm oc}$ = 1.28 V. Taken together, we have demonstrated that GaAs$_{1-x}$P $_x$ graded buffers are an appropriate platform for low-TDD, metamorphic GaAs$_{1-x}$P $_x$ and In $_y$Ga $_{1-y}$P solar cells, covering a wide bandgap range.