Analysis of recombination processes in polytype gallium arsenide nanowires

Here we investigate recombination in polytype gallium arsenide (GaAs) nanowires (NWs) for photovoltaic applications, through photoluminescence studies coupled with rate equation analysis. Polytype NWs exhibit switching between zinc-blende (ZB) and wurtzite (WZ) crystal phases along the wire due to rotational twinning during self-catalyzed growth. When photons are absorbed in polytype NWs, electrons and holes separate: in the simplest case, electrons quickly thermalize to the band-edge of the ZB phase, while holes thermalize to the band-edge of the WZ phase, recombining indirectly in space across the type-II offset. The recombination mechanisms of this system are investigated experimentally through time-resolved photoluminescence (TRPL) at liquid helium temperature, and time-integrated photoluminescence (TIPL) at various temperatures, for the baseline case of AlGaAs capped GaAs NWs. The effects of the surface recombination on sub-bandgap transitions are also investigated using Al2O3 and no capping at the surface. We infer that carriers quickly thermalize to the spatially closest, lowest energy level, where they radiatively recombine across a sub-bandgap energy gap at a slower radiative rate than band-to-band. We use a rate equation model to investigate different configurations of polytype defects along the wire, including the effects of the surface and temperature, which compares well with experiment considering spatially indirect recombination between different polytypes, and defect-related recombination due to twin planes and other defects.