The effects of Sb concentration variation on the optical properties of GaAsSb/GaAs heterostructured nanowires

In this work we have investigated the variation of Sb concentration among and within zinc blende (ZB) GaAsSb inserts in wurtzite (WZ) GaAs bare-core and WZ GaAs/AlGaAs core–shell nanowires (NWs) grown by Au-assisted molecular beam epitaxy. The Sb concentration variation was related to the optical properties as determined by photoluminescence (PL). The NW structure and the Sb concentration were studied by transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDX) and quantitative high angle annular dark field scanning TEM (HAADF STEM). A clear trend relating the maximum Sb concentration with the insert length was observed: the longer the insert, the higher the Sb concentration. In addition, there are graded Sb concentration gradients both along and across the GaAsSb inserts. The influence of the Sb concentration variation on the PL emission from the GaAsSb inserts was investigated with correlated micro-PL and TEM-EDX on the same single NWs. Based on the PL results and the observed Sb concentration profiles, we propose a qualitative energy band diagram for a typical ZB GaAsSb insert in a WZ GaAs NW for the heterostructured NWs studied here. Type I transitions within the central region of the ZB GaAsSb inserts were found to dominate the insert-related PL emission. Weak type II transitions within the inserts due to the graded Sb concentration were observed as well. Using an existing empirical model, the Sb concentrations were additionally determined from the ground state PL energies (type I transition). For the average Sb concentration, the concentrations based on PL were in agreement with EDX and quantitative HAADF STEM results. (paper)