Quantitative scanning transmission electron microscopy studies on heterostructured GaAs nanowires

Abstract

This thesis presents a quantitative high-angle annular dark eld scanning transmission electron microscopy (HAADF STEM) study on heterostructured GaAs-based nanowires (NWs). Quantitative HAADF STEM was employed to investigate Sb concentration variations in axial GaAsSb inserts within GaAs NWs and in GaAsSb NWs, as well as Al concentration variations in the AlGaAs shell in GaAs/AlGaAs core-shell NWs. The NWs were grown with the vapor-liquid-solid (VLS) growth mechanism using Ga- and Au-assisted molecular beam epitaxy (MBE).

Compositional characterisation by quantitative HAADF STEM was realized by comparing experimental image intensities normalized to the incident beam intensity with simulated intensities. The HAADF STEM image simulations were performed using the frozen-phonon multislice approach. For correct compositional analysis of GaAs1-ySby, static atomic displacements (SAD) had to be included in the simulations, however this was not the case with AlxGa1-xAs.

GaAsSb inserts within GaAs NWs and GaAsSb NWs were studied with non-corrected STEM at a relatively low magnication using NWs in plane. In this case the known thickness prole of the hexagonal NWs could be exploited in the quantitative HAADF STEM analysis. In the GaAsSb inserts, concentration gradients axially along as well as radially across the insert were identied. The Sb concentration in the insert decreased axially towards the upper and lower interfaces with GaAs, and radially towards the outer surfaces. The axial concentration gradients were attributed to the reservoir eect related to the VLS growth process. The eects of the axial concentration gradients on the NWs optical properties were investigated.

The radial concentration gradients in the GaAsSb inserts result from a combined eect of radial GaAs overgrowth and out-diusion of Sb during the post-insert axial GaAs growth, causing an increased surface depletion of Sb with increasing post-insert GaAs growth time. In GaAsSb NWs, increased surface depletion of Sb towards the bottom of the NW was observed, and this was attributed to out-diusion of Sb during the NW growth.

GaAs/AlGaAs core-shell NWs were studied with aberration corrected STEM, using cross-sectional specimens prepared with ultramicrotomy. A method for mapping Al concentration in AlxGa1-x As at unit cell spatial resolution using the atomic resolution HAADF STEM images was developed. The method is independent of the eective source size and higher order lens aberrations. With the method, Al concentration variations in the AlGaAs shell could be quantied at unit cell spatial resolution.