Defect-related trapping and recombination in metamorphic GaAs0.72P0.28 grown on GaAs

Since the bandgap of metamorphic GaAsP can be tuned through a range of energies in a high-radiance region of the solar spectrum, this alloy may be useful in multi-junction solar cells. For this application, achieving high conversion efficiency will require a good understanding of the defects that accompany lattice-mismatch in this system. We use photoluminescence and deep-level transient spectroscopy (DLTS) on GaAs0.72P0.28 to identify defect-related energy levels and assess their impact on photovoltaic device performance. Low temperature photoluminescence spectra reveal a broad defect-related band approximately 0.10 eV below the band-to-band emission. At 165 K and above, thermal activation out of the defect-related band appears to enhance the band-to-band radiative mechanism. DLTS measurements on p-type GaAs0.72P0.28 suggest the presence of a hole trap with a comparable depth (∼0.09 eV) and thermal escape temperature, but the collective results are difficult to interpret. An alternative approach assuming a reciprocal rather than logarithmic dependence on time is compelling, but the proposed hopping-transport model will require further testing.