Advanced Transmission Electron Microscopy of III-V Nanowires

Abstract

Semiconducting nanowires have many interesting applications and a promising future as building blocks for optoelectronic devices. In this work, the aim is to improve the characterization of the nanowire structure and composition. By this, their optical and electronic properties can be better understood and the synthesis of nanowires and nanowire-based devices further developed.

Several different material systems, mostly based on ternary III-V nanowires, have been studied. This include GaAs/AlGaAs core-shell nanowires, GaAsSb superlattice structures, InGaN quantum wells in GaN nanowires and Pd/Ge/Au contacts to n-GaAs nanowires. The focus has been to more accurately determine the composition of the different components in the material systems using transmission electron microscopy. To achieve this, non-standard analysis methods for X-ray energy dispersive spectroscopy have been applied and new methods developed. In addition, structural characterization, including scanning precession electron diffraction, is used to complement and confirm the composition analysis. In relation to the compositional and structural analysis, it has been investigated how the different viewing directions affect the characterization, as well as the possibility of large-scale, semi-automated analysis.

Findings include that complicated absorption effects can influence the measurements in different ways and thus impede linking the composition to the optical and electronic properties. A more reliable result can be obtained by combining several characterization techniques or using refined analysis techniques. Specifically, determining the composition internally within the specimen has shown to be promising. It is concluded that in order to explain the observed behavior of the nanowires, a more robust characterization is needed compared to what is default today.