Transmission Electron Microscopy on GaAs Nanowires – Spontaneous Polarization and MnAs Nanocrystals

The research on semiconductor nanowires – tiny semiconductor crystals of sub-micron diameter and some microns length – has, although around only for about one and a half decade, already significantly diversified and developed. However, it has relied on characterization methods provided by electron microscopy since the earliest days, on the one hand as their sheer size keeps them out of reach of light-based imaging techniques and on the other hand due to their interesting structural properties down to the atomic level, which is one of the core capabilities of transmission electron microscopy when it comes to characterization. While the first years of research were governed mainly by exploration of their most fundamental properties and finding out the mechanisms of their synthesis, meanwhile already a lot of progress has been made towards the controlled growth and manipulation of their properties. Also, people quite early extended the activities beyond practising nanowire research for its own sake and started to seek ways to make use of their unique features for applications – both in the literal sense, i.e. the development of nanowire based devices and functionalization, as well as in the less applied sense, where the nanowires are used as a model system to investigate fundamental material properties. Applying several transmission electron microscopical techniques, this thesis contains contributions to both of the fields mentioned above: Concerning the research on fundamental material properties spontaneous polarization is a property unique to the wurtzite crystal structure, which was within the III–V semiconductors up to now only known to be present in the nitride based ones since they are the only ones that grow by default in the wurtzite crystal structure. Only since the discovery of nanowire growth this crystal structure has gotten available also for the other III–V semiconductor materials and provides a model system to access its properties experimentally and verify the theoretical calculations that have been done on such (up to then hypothetical) crystal structures. The first part of this work therefore contains the first direct experimental observation of the spontaneous polarization in the III–V semiconductor materials gallium arsenide (GaAs) and gallium phosphide (GaP). More towards the application oriented side is the field of spintronics, i.e. the development of electronic devices that make use of the electron spin instead of the charge as the fundamental property to transfer, process and store information. This field requires the fabrication of ferromagnetic device structures that allow to generate and detect spin polarized electrons within the semiconductor materials it should integrate with. One candidate for such a ferromagnetic material is manganese arsenide (MnAs) which includes semiconducting and ferromagnetic properties within one material and is hence considered to be an important building block with respect to the development of spin-based electronic devices. The second part of this work is the characterization of such MnAs nanocrystals that have been grown on top of GaAs nanowires. The document at hand is organized in five main chapters of which the first three describe the theoretical and experimental framework in which this work took place, while the remaining two present the results that have been achieved. Chapter 2 gives an introduction into the topic of semiconductor nanowires, about their synthesis, and their properties of interest, with a special focus on III–V semiconductor nanowires and especially gallium arsenide (GaAs) and gallium phosphide (GaP) nanowires. Since one of the features of semiconductor nanowires is their polytypism, i.e. the possibility to adopt different crystal structures, in Chapter 3 the occurring crystal structures and some of their properties and implications are discussed. Chapter 4 explains the transmission electron microscope – the experimental instrument used – and the analytical methods available in this device which were employed in this work. The first experimental chapter, Chapter 5, presents the experimental evidence of spontaneous polarization in gallium arsenide and gallium phosphide. In addition, based on the issues that arose with the quantitative evaluation of the measurements, the properties and experimental limitations of the differential phase contrast (DPC) microscopy technique are discussed. Finally in Chapter 6, the second experimental chapter, the characterization of samples from a growth study on the route towards the inclusion of manganese arsenide (MnAs) is described.