Evidence of an Oxygen Recombination Center in p+-n GaInNAs Solar Cells

We have studied deep-level impurities in p⁺-n GaInNAs solar cells using deep-level transient spectroscopy (DLTS). These films were grown by atmospheric- and low-pressure metalorganic vapor-phase epitaxy. The base layer is doped with silicon, and the emitter layer is zinc doped. Two types of samples have been studied: samples were grown with and without the addition of oxygen impurity. Two electron traps were found in all samples. These are designated as: E1, at E_c -0.23-E_c-0.27 eV, E2 at E_c-0.45 eV, and E2* at 0.77 eV. With the addition of oxygen impurity, DLTS showed additional traps designated as E3 (electron) at E_c-0.59 eV and H3 (hole) at E_v+0.59 eV. Using secondary ion mass spectroscopy, the oxygen concentration was found to be about 2-3 × 10¹⁹ and 1 × 10¹⁷ cm^-3 in two sets of samples. However, only samples containing oxygen contained the two near-midgap levels (E3 and H3). We present evidence that these levels are associated with the oxygen defect. As we change the dc bias voltage, the E3 trap disappears in unison with the appearance of the H3 trap. Furthermore, E3 and H3 trap levels have comparable capture cross sections. This oxygen-related trap is an effective recombination center. The measured Shockley-Hall-Read lifetime for this center is about 0.6 μs.