Growth and Raman scattering studies of single GaN nanowires

Recently, GaN and related compounds semiconducting nanowires have attracted great attention as building blocks in electronics, optoelectronics and sensors. It has been reported that the optical properties of nanosized materials are strongly relevant to the crystalline quality, growth orientation, extended structural defects and surface defects. Among all characterization techniques, Raman scattering can be the best candidate because the above-mentioned information can be obtained in an easy as well as non-destructive way.In this work, GaN nanowires were grown on Si substrate via the chemical vapor deposition method with Ni as the catalyst in the vapor-liquid-solid mechanism. The resulting nanowires were found to be hexagonal wurtzite in structure and [100] in preferred growth orientation by using transmission electron microscopy. For single nanowires Raman measurement, GaN nanowires were further dispersed on Si substrate with pre-coated patterned Au film to prevent the background signal from Si.Raman scattering measurement was performed using confocal Raman microscope (HORIBA Jobin Yvon, LabRAM HR800) with 633 nm Laser. The polarization vector was rotated by a half-wave plate positioned at entrance of Laser.The GaN single nanowire shows conventional Raman characteristics of wurtzite structure, such as 529, 559, 567, 724 cm-1 for A1(TO), E1(TO), E2(high), A1(LO) phonon mode, respectively, and sharp line-width of E2(high) mode of 4 cm-1 indicating high crystalline quality. The polarized Raman spectra of GaN single nanowire exhibits unusual polarized response, which is out of expectation from a simple simulation based on a*-directed wire. The difference is assigned to the shape effect of the nano-size-wire, as understood by the distortion of linearly polarized scattering states of the electromagnetic field across the nanowire boundary. Additionally, the angle-dependent Raman spectra of individual nanowires reveal different polarized behaviors, as a result of various c-axis orientated directions. After combination with the simulation, the spatial orientation of GaN nanowire crystallographic axes, as well as crystal quality can be determined. Accordingly, this is a powerful analytical and in-situ technique in future application of nano-devices, such as single nanowire FET, single nanowire photonics, for probing structural and optical properties. urthermore, two broadband centered at 683 cm-1 and neighbored beside A1(LO) mode are supposed to the surface mode of nanostructure due to a large surface-to-volume ratio. Also, there is a good agreement with the prediction of surface mode but still essentially unexplored.In short, this report demonstrates a convenient method to obtain the profile of single GaN NW. For those who may concern the orientation, facets, defects of the NW, this technique is indispensable especially for realizing NW-integrated system.