Functional properties of III-V nanowires addressed by Raman spectroscopy

In recent years, semiconductor nanowires have attracted considerable attention as a result of their unique properties and potential applications in many fields. In particular, they can be very attractive materials for certain optoelectronic and electronic devices, such as lasers, detectors and solar cells, which benefit from the photonic properties of nanowires. In order for these future technologies to become a reality, a good understanding of the functional properties of nanowires is fundamental. In this thesis we have investigated the optical and the electrical properties of III-V semiconductors nanowires by the means of Raman spectroscopy. Thanks to its non-destructive and spatial resolution, Raman spectroscopy is a powerful contact-less tool for the characterization of semiconductor nanowires. Raman spectroscopy can provide information about crystallinity, orientation, size and chemical composition. In polar semiconductors it is also possible to characterize the free carriers, through the coupling of plasmons with longitudinal optical modes. Due to the small size and particular morphology of nanowires, the interaction of light can be more complex than in thin films. In particular, the existence of photonic modes alters significantly the corresponding light-matter interaction. In this thesis we exploit the use of photonic modes for the compositional mapping of nanowire core-shell heterostructures and also to circumvent the macroscopic selection rules. In the first part of this thesis, we have performed Raman scattering measurements on GaAs/AlGaAs core/shell nanowires. We have shown that it is possible to select and characterize regions of the structure with different Aluminum content, by performing the measurements at different laser wavelengths. Then, we have shown that the photonic modes can be modified by suspending the nanowires on a trench. We have shown that in this case it is possible to enhance the response of the longitudinal optical phonon mode. We have then applied this configuration for the characterization of the hole concentration on p-type GaAs nanowires in back-scattering geometry. The second part of the thesis focused on the assessment of free carriers by Raman spectroscopy in systems with an expected high electron mobility: GaAs nanowires with a modulation doped structure and InAs(Sb) nanowires. Raman measurements were performed as a function of the temperature on modulation doped GaAs/AlGaAs nanowire. By characterizing the coupling between free carrier and the LO phonons, we have extracted the concentration and the mobility of carriers. We have found that Si donors are almost ionized for a temperature above 50 K. We have shown that the mobility is limited by interface scattering, with values of 400 cm^2/Vs at room temperature and 2700 cm^2/Vs at low temperature. Finally, we investigated the electronic properties of InAs(Sb) nanowires as a function of the temperature. The effect of a dielectric coating on the electronic properties was also studied. We have found an increase of mobility and electron concentration with the antimony content , moving from 5100 cm^2/Vs for InAs nanowires, to 17500 cm^2/Vs for InAsSb with 35% of antimony at 14K. Moreover, we have shown that in the case of InAs electrons are located in the accumulation layer at the surface, while for InAsSb our measurements are consistent with the carriers located in the nanowire core.