Structural characterization of GaAs and InAs nanowires by means of Raman spectroscopy

We report Raman studies of GaAs and InAs nanowires (NWs) grown on $SiO2$ and GaAs surfaces by means of catalyst-assisted molecular beam epitaxy. We have investigated several tens of NWs grown using either Mn or Au as a catalyst. The LO and TO phonon lines of the NWs showed an energy downshift and a broadening as compared to the lines usually observed in the corresponding bulk materials. A doublet is sometimes observed in the LO region due to the observation of a signal attributed to the surface optical (SO) phonon. The energy position of the SO phonon agrees with the values expected considering the section diameter of the NWs. LO and TO downshifts are due to the presence of structural defects within the NWs. The larger the energy downshift, the smaller the dimension of the defect-free regions. The results demonstrate that different catalysts provide wires with comparable crystal quality. The measurements also point out that differences in defect density can be found in wires coming from the same batch indicating that a substantial effort will be needed to obtain high homogeneities of the NW quality.