Raman investigation of small bandgap semiconductor nanowires

The study of fundamental properties of semiconductor nanowires is mandatory for their employment in future electronic devices. In this Thesis, we studied on one hand the modification of the wurtzite InAs E1 gap and of the wurtzite InAs and of the lattice dynamics response upon compression with respect to zincblende bulk InAs. High-pressure Raman spectroscopy experiment was performed on bundles of InAs nanowires with mainly wurtzite structure applying hydrostatic pressure up to 8.5 GPa. Combining high-pressure measurements and polarization dependent resonant Raman scattering varying excitation energy we found that E1 gap energy in wurtzite InAs is 2.4 eV and therefore lower than zincblende InAs. We also found that the phonon response due to a variation of the lattice volume is substantially different in wurtzite InAs with respect to the bulk counterpart.
On the other hand, we established a novel method for a complete thermoelectric characterization of individual semiconductor nanowires, applying the method to Se--doped InSb nanowires. Combined laser heating, Raman spectroscopy, and simple electrical measurements allow the determination of the nanowire thermoelectric figure of merit ZT, together with the estimate of the thermal contact resistances at the nanowires-source/drain contacts. The temperature and electrical data are well described by 1-D heating model which includes a Gaussian heat source. Comparison with field–effect transistor Seebeck measurements shows a good agreement between the obtained results, confirming the validity of the proposed method. Measured ZT values are between 0.014 and 0.025 and are, to the best of our knowledge, the best reported so far on single InSb nanowires.