Fundamental Properties of Semiconductor Nanowires

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.

Raman scattering is a powerful inelastic light scattering technique able to probe the vibrational properties of materials. This technique has been successfully employed in semiconductor nanowires to provide information on their fundamental properties, such as the phononic properties, the crystal composition, and the electronic band structure. When performed in a polarization-resolved manner on a single nanowire, Raman spectroscopy can even allow addressing the nanowire’s crystal structure. This is a fact of pivotal importance, as crystal phase is emerging as a novel degree of freedom in the bandgap engineering and phonon engineering of materials, and the control of the crystal phase is a possibility uniquely offered by nanowires. Indeed, recent advances in the synthetic growth of nanowires have given access to crystal phases (e.g., hexagonal phase in Si and Ge) that in the bulk can only be obtained under extreme pressure conditions, and it is possible to controllably switch between different crystal phases during the growth of nanowires. The realization and, even more, the interpretation of polarized Raman experiments on nanowires can be non-trivial, as several issues have to be considered. Therefore, in this chapter, we provide the basic theoretical background necessary to calculate Raman selection rules and interpret polarization-resolved Raman spectra of semiconductor nanowires. We also discuss the main ingredients of a Raman setup, with a focus on the scattering geometries typically used for nanowires. We highlight the main differences in the Raman spectra of nanowires with cubic and hexagonal crystal symmetries, and we treat also the case of the most challenging type of heterostructure: a nanoscale crystal-phase homostructure. Finally, we discuss resonant Raman experiments that allow the determination of the energy of some electronic transitions in nanowires. We focus mostly on a very new material system, namely Ge nanowires with controlled crystal phase, but the general procedure that we establish can be applied to several types of nanostructures.