Correlated transmission electron microscopy and micro-photoluminescence studies of GaAs-based heterostructured semiconductor nanowires

Abstract

The focus of this work was to study the nanowire (NW) optical and structural properties on a single NW level. The GaAs-based heterostructured semiconductor NWs, grown by Au-assisted molecular beam epitaxy (MBE), were studied throughout this work. Due to diversity of the NWs within each growth batch, it is essential to study the same single NWs both by di erent micro-photoluminescence (PL) and transmission electron microscopy (TEM) techniques. An efficient method for such a correlated study was developed (paper I), and applied in all following studies. The key feature is that the single NWs of desired morphology are preselected by low voltage scanning TEM (STEM) for the micro-PL study. Subsequently, TEM characterization is performed, and in such a way a detailed set of information is obtained for each single NW.

Wurtzite (WZ) GaAs/AlGaAs core-shell NWs were studied to determine the fundamental optical properties of WZ GaAs, a crystal phase found only in NW form for GaAs (paper III). To this cause, low temperature powerdependent, temperature-dependent, polariztion-resolved and time-resolved micro-PL were employed in correlation with conventional TEM characterization. The room temperature (294 K) WZ GaAs bandgap was determined to be 1:444 eV±1 meV which is ~ 20 meV higher than the well known bulk zinc blende (ZB) GaAs value. On the other hand, the low temperature free exciton emission energy was found to be approximately the same as in ZB GaAs, 1:516 eV at 15 K. The conduction band symmetry of WZ GaAs is proposed to be Ƭ8. In addition, the results demonstrate that the Au-assisted MBE grown NWs can have high PL brightness.

WZ GaAs/ZB GaAsSb axially heterostructured NWs were studied in order to understand the cause of variations of the ZB GaAsSb insert optical properties among the NWs (paper I, II, IV). The Sb concentration variation among and within ZB GaAsSb inserts was studied by energy dispersive X-ray spectroscopy (EDX), quantitative high angle annular dark eld (HAADF) STEM and correlated PL-TEM (paper IV). A clear trend of increasing Sb concentration with increasing insert length was observed. It was found that the Sb concentration increases gradually from the lower insert interface along the NW axis, reaches a maximum value and decreases towards the upper insert interface. The Sb concentration variation within the GaAsSb inserts induces both type I and type II optical transitions within the insert. To verify the EDX and quantitative HAADF STEM results independently, the Sb concentration was calculated from the ground state energies of the GaAsSb PL emission using an empirical model for low temperature bandgap of unstrained bulk GaAsSb. The calculated Sb concentration values were in agreement with quantitative HAADF STEM results, and systematically higher than the values obtained by the EDX quantification.