Compositional inhomogeneities in type-I and type-II superlattices for GaAsSbN-based solar cells: Effect of thermal annealing

GaAsSbN alloys is recognized as an effective candidate to incorporate in multijunction solar cell applications due to its advantages of being grown lattice-matched to GaAs and its capability to achieve bandgap around 1.0 eV. Recently, the use of superlattice (SL) structures that permit type-II (GaAsSb/GaAsN) and type-I (GaAsSbN/GaAs) alignments showed a strongly heightened luminescence and a significant EQE compared to their thicker quaternary bulk counterparts. In this work, the correlation between the compositional distribution and its optical properties in both SL structures before and after RTA treatment is analyzed. Firstly, while N is confined following the nominal design, Sb presents a strong segregation to the upper layers achieving similar compositional profiles along the growth direction even after the RTA. However, the SL type-II approach presents a higher interface quality and a much lower trend to cluster formation than SL type-I. Secondly, RTA gives place to an improvement of the interface roughness, together to a decrease of the area associated to clusters in both structures. The blueshift and the improvement of the PL intensity induced by annealing in both samples is more correlated to the decomposition of nitrogen pairs to isolated substitutional nitrogen than to the dissolution of Sb inhomogeneities. A different distribution of new substitutional N atoms around Sb clusters could explain the differences in the PL improvement of both SLs during the annealing. SL type-II approach is the best design to minimize cluster formation and to obtain more abrupt interfaces and mainly describe the optical enhancement compared to SL type-I structures.