TEM characterization of GaAs/GaAsSb heterostructured nanowires for laser applications
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The structureof self-catalyzed GaAs NWs (length ~10 µm and thickness ~400 nm) with six GaAsSb/GaAs superlattices(SL), have been analyzed. Four samples with varying Sb flux and GaAsSb well growthtime, which all depicted lasing when optically pumped, were studied by conventional transmissionelectron microscopy. Crystal phase analysis has been done by dark-field imaging, and comparedto high-resolution lattice images. The aim of this work was to quantify the crystal phasesin the different NWs for all the SLs and link the findings to the growth conditions and observedlasing. The sample with the lowest Sb flux has a very high stacking fault density along the entireNW, and phase distribution is not further analyzed. The spacers between SLs are wurtzite(WZ) GaAs, and the SLs consist of defect-free zinc-blende (ZB) phase segments separated byregions of high stacking fault density and mixed phases (ie. twinned ZB, stacking faults and incases WZ). When considering crystal phase, SL-3, -4 and -5 are quite similar, while SL-6 is ingeneral shorter than other SLs. The ZB segments become larger and more stable, mixed phaseregions reduce in size, over all stacking fault density goes down and the amount of defect-freeWZ phase increases with Sb concentration. In addition, some specific structural features havebeen observed; SL-1 has quantum dot-like features, the NW tips have distinct facet planes, andone sample has surface damage from oxidation of the AlGaAs shell due to incomplete, or absenceof, the GaAs protective cap. The structural characterization carried out in this work helpsto understand the observed lasing behavior and compositional analysis from other studies onthe same NW batches. Together these findings can contribute to optimize the growth of NWswith GaAs/GaAsSb SLs and the understanding as well as performance of NW-based lasers basedon these heterostructures.