Optical Properties of III-V Nanowires and Their Application for Charge Transport and Single-Photon Emission

This work covers the optical characterization of III-V semiconductor nanowires and their application for charge transport and for single-photon emission. InAs nanowires have been investigated by Raman scattering and PL spectroscopy. The possibility to grow nanowires with a crystal structure different from its bulk counterpart has aroused a lot of interest in their optical and electronic properties. Here, the optical phonon modes of wurtzite InAs nanowires have been studied by polarized Raman scattering. For the first time, Raman measurements on a single InAs nanowire have revealed the A1(TO) and E2h optical phonon modes of the wurtzite structure. Additional resonant Raman scattering experiments have shown that the electronic E1 transition in the wurtzite structure (close to the A point along the [0001] direction) is approximately 100 meV smaller than that for the zincblende phase (near the L point along the [111] direction). PL measurements of InAs ensembles have shown that the band gap of wurtzite InAs nanowires is substantially larger than the zincblende bulk band gap (0.415 eV) and a lower bound for the wurtzite gap has been estimated (0.458 eV). The measurements are in good agreement with available theoretical studies and close to experimental works. However, to the authors knowledge no band gap value on pure wurtzite nanowires has been published so far. In order to demonstrate the potential of nanowires in optoelectronic devices, GaAs/AlGaAs core/shell nanowires have been used for the acoustic transport of photoexcited carriers along the nanowire axis. In contrast to InAs, the advantages of this highly investigated GaAs/AlGaAs nanowires are the optical emission in the near infrared region, where optical detection is more efficient, and the high PL emission efficiency. The acoustic transport has been realized by surface acoustic waves (SAWs). These waves, also known as elastic surface waves, were first described theoretically by Lord Rayleigh in 1885. A particular type of these SAWs is the Rayleigh wave, named after his discoverer, which has been applied in this work. Rayleigh SAWs are localized near the surface and consist of the superposition of a longitudinal acoustic mode polarized parallel to the propagation direction and a transversal mode polarized perpendicular to the surface. This leads to alternating regions of tensile and compressive strain along the propagation direction. Due to the strain and its corresponding piezoelectric potential, a dynamic type-II modulation of the electronic band edges is imposed that can be used to transport carriers at the velocity of sound in quantum well structures. These SAWs have been applied to GaAs/AlGaAs core/shell nanowires transferred to a LiNbO3 substrate in order to transport photoexcited carriers along the nanowire axis to a second location, leading to remote emission of sub-nanosecond light pulses synchronized with the SAW phase. The high-frequency contactless manipulation of carriers by SAWs opens new perspectives for applications of nanowires in optoelectronic devices operating at GHz frequencies. The potential of this approach is demonstrated by the realization of a high-frequency source of antibunched photons based on the acoustic transport of electrons and holes in (In,Ga)As/GaAs axial nanowires with AlGaAs shell.