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

Nanoporous silicon tubes: the role of geometry in nanostructure formation and application to light emitting diodes

Obtaining light emission from silicon has been the holy grail of optoelectronics over the last few decades. One of the most common methods for obtaining light emission from silicon is to reduce it to a nanoscale structure, for example by producing porous silicon. Here, we present a method for the large-area fabrication of porous silicon microtubes by the stain etching of silicon micropillar arrays. We explain and model how the formation of the microtubes is influenced by the morphology of the substrate, especially the concave or convex character of the 3D features. Light emission is demonstrated at the micro- and nanoscale respectively by photo- and cathodoluminescence. Finally, we demonstrate a 0.55 cm(2) device that can work as a photodetector with 2.3% conversion efficiency under one sun illumination, and also as a broadband light emitting diode, illustrating the applicability of our results for optoelectronic applications

Polarization response of nanowires a la carte

Thanks to their special interaction with light, semiconductor nanowires have opened new avenues in photonics, quantum optics and solar energy harvesting. One of the major challenges for their full technological deployment has been their strong polarization dependence in light absorption and emission. In the past, metal nanostructures have been shown to have the ability to modify and enhance the light response of nanoscale objects. Here we demonstrate that a hybrid structure formed by GaAs nanowires with a highly dense array of bow-tie antennas is able to modify the polarization response of a nanowire. As a result, the increase in light absorption for transverse polarized light changes the nanowire polarization response, including the polarization response inversion. This work will open a new path towards the widespread implementation of nanowires applications such as in photodetection, solar energy harvesting and light emission

Morphology and composition of oxidized InAs nanowires studied by combined Raman spectroscopy and transmission electron microscopy

importance for semiconductor nanowires because of the high surface-to-volume ratio and only little is known about the consequences of oxidation for these systems. Here, we study the properties of indium arsenide nanowires which were locally oxidized using a focused laser beam. Polarization dependent micro-Raman measurements confirmed the presence of crystalline arsenic, and transmission electron microscopy diffraction showed the presence of indium oxide. The surface dependence of the oxidation was investigated in branched nanowires grown along the [0001] and [01 (1) over bar0] wurtzite crystal directions exhibiting different surface facets. The oxidation did not occur at the [ 011 (1) over bar 0] direction. The origin of this selectivity is discussed in terms transition state kinetics of the free surfaces of the different crystal families of the facets and numerical simulations of the laser induced heating

Chitosan-DNA complexes: Charge inversion and DNA condensation

The design of biocompatible polyelectrolyte complexes is a promising strategy for in vivo delivery of biologically active macromolecules. Particularly, the condensation of DNA by polycations received considerable attention for its potential in gene delivery applications, where the development of safe and effective non-viral vectors remains a central challenge. Among polymeric polycations, Chitosan has recently emerged as a very interesting material for these applications. In this study, we compare the observed aggregation behavior of Chitosan-DNA complexes with the predictions of existing models for the complexation of oppositely charged polyelectrolytes. By using different and complementary microscopy approaches (AFM, FESEM and TEM), light scattering and electrophoretic mobility techniques, we characterized the structures of the complexes formed at different charge ratios and Chitosan molecular weight. In good agreement with theoretical predictions, a reentrant condensation, accompanied by charge inversion, is clearly observed as the polycation/DNA charge ratio is increased. In fact, the aggregates reach their maximum size in correspondence of a value of the charge ratio where their measured net charge inverts its sign. This value does not correspond to the stoichiometric 1:1 charge ratio, but is inversely correlated with the polycation length. Distinctive "tadpole-like" aggregates are observed in excess polycation, while only globular aggregates are found in excess DNA. Close to the isoelectric point, elongated fiber-like structures appear. Within the framework of the models discussed, different apparently uncorrelated observations reported in the literature find a systematic interpretation. These results suggest that these models are useful tools to guide the design of new and more efficient polycation-based vectors for a more effective delivery of genetic material. (C) 2013 Elsevier B.V. All rights reserved.The design of biocompatible polyelectrolyte complexes is a promising strategy for in vivo delivery of biologically active macromolecules. Particularly, the condensation of DNA by polycations received considerable attention for its potential in gene delivery applications, where the development of safe and effective non-viral vectors remains a central challenge. Among polymeric polycations, Chitosan has recently emerged as a very interesting material for these applications. In this study, we compare the observed aggregation behavior of Chitosan–DNA complexes with the predictions of existing models for the complexation of oppositely charged polyelectrolytes. By using different and complementary microscopy approaches (AFM, FESEM and TEM), light scattering and electrophoretic mobility techniques, we characterized the structures of the complexes formed at different charge ratios and Chitosan molecular weight. In good agreement with theoretical predictions, a reentrant condensation, accompan

High Electron Mobility and Insights into Temperature-Dependent Scattering Mechanisms in InAsSb Nanowires

InAsSb nanowires are promising elements for thermoelectric devices, infrared photodetectors, high-speed transistors, as well as thermophotovoltaic cells. By changing the Sb alloy fraction the mid-infrared bandgap energy and thermal conductivity may be tuned for specific device applications. Using both terahertz and Raman noncontact probes, we show that Sb alloying increases the electron mobility in the nanowires by over a factor of 3 from InAs to InAs0.65Sb0.35. We also extract the temperature-dependent electron mobility via both terahertz and Raman spectroscopy, and we report the highest electron mobilities for InAs0.65Sb0.35 nanowires to date, exceeding 16,000 cm2 V–1 s–1 at 10 K

Unveiling Temperature-Dependent Scattering Mechanisms in Semiconductor Nanowires Using Optical-Pump Terahertz-Probe Spectroscopy

Optical-pump terahertz-prohe (OPTP) spectroscopy is a powerful, non-contact tool for extracting the electrical conductivity within a material. In this work, we show how OPTP spectroscopy can he used to extract the temperature-dependent electron mobility and photoconductivity lifetime within semiconductor nanowires (NWs), in order to reveal the underlying scattering mechanisms governing carrier transport in these materials

Tuning the response of non-allowed Raman modes in GaAs nanowires

We report on the use of photonic resonances in Raman spectroscopy on single nanowires for the enhancement of forbidden modes and the study of the interaction of phonons with free-carriers. This is achieved by suspending nanowire over a trench and detecting Raman scattered light with light polarized along the radial direction. Thanks to the photonic nature of the light-nanowire interaction, light polarization inside the nanowire is modified. This results in the excitation of LO modes, forbidden on {1 1 0} surfaces. We apply this new configuration to the measurement of carrier concentration on doped GaAs nanowires. These results open new perspectives for the study of the interaction of free-carriers or plasmons with optical phonons in nanostructures