Polarized raman scattering from single GaN nanowires

Raman spectra of single (hexagonal wurtzite) GaN nanowires grown by vapor-liquid-solid synthesis were recorded. By investigating the polarization dependence of the Raman intensities of the observed bands as a function of the angle between the long axis of the nanowire and the electric vector of the incident (and scattered) light, one is able to distinguish between nanowires whose crystallographic c axis corresponds to (c(*) directed), or is substantially perpendicular to (a(*) directed) the long axis of the nanowire-the two preferred directions of nanowire growth. For a(*)-directed nanowires, polarization-dependent Raman intensity analysis can also determine, within tolerable limits, the orientation of the crystalline c axis with respect to laboratory-fixed coordinates. Simultaneous transmission electron microscopy (TEM) analyses confirm the conclusions derived from the Raman measurements. The polarization-dependent Raman intensities could only be understood if one assumed a complex-valued Raman tensor. The detection of resonance enhancement together with the tendency of the nanowires to grow with a high concentration of defects and the observation of yellow luminescence in the photoluminescence spectrum may account for the complex-valued Raman tensor we observe for GaN even at sub-band-gap photon energies.