X-ray Methods for Structural Characterization of III-V Nanowires: From an ex-situ Ensemble Average to Time-resolved Nano-diffraction

Understanding and controlling the highly dynamical self-catalyzed vapor–liquid–solid growth of GaAs nanowires remains being a challenge. In order to gain deeper insight into these processes, our approach is the analysis by in-situ synchrotron X-ray diffraction during MBE-growth. This allows recording time-resolved information about crystal structure and shape of the nanowires. Within this chapter, we give a detailed overview of how this goal can be achieved. Starting from crucial basics about the crystal structure of GaAs nanowires and the signal obtained by X-ray diffraction, we describe how an asymmetric three-dimensional mapping of the reciprocal space is possible using a fixed angle of incidence—a geometry that is highly advantageous for time-resolved in-situ characterization. Additionally, the experimental setup is introduced and accompanying challenges are discussed. Applying the described methods, we were able to directly observe and distinguish several different effects during the growth of GaAs nanowires as well as of (In,Ga)As core-shell heterostructures. Firstly, an increasing transition probability from wurtzite to zinc-blende is observed during the course of growth, which is converted into the corresponding energy difference of the nucleation barriers of both phases. Secondly, analyzing the shape of the nanowires during growth, the evolution of the Ga-droplet on top of the nanowire was derived and the contributions of facet growth and tapering to the total radial growth were distinguished. Additionally, the effect of interfacial strain during the epitaxial growth of an (In,Ga)As shell layer around GaAs nanowires is monitored by X-ray diffraction and described by applying basic model representations. Finally, most recent results on nanowire arrays grown on Si substrates patterned by electron beam lithography are discussed.