0000000000605861
AUTHOR
Karine Hestroffer
Resonant Raman scattering of core-shell GaN/AlN nanowires.
Abstract We have analyzed the electron–phonon coupling in GaN/AlN core–shell nanowires by means of Raman scattering excited at various wavelengths in the ultraviolet spectral range (335, 325 and 300 nm) and as a function of the AlN shell thickness. The detailed analysis of the multi-phonon spectra evidences important differences with excitation energy. Under 325 and 300 nm excitation the Raman process is mediated by the allowed A 1(LO) phonon mode, where the atoms vibrate along the NW axis. Considering its selection rules, this mode is easily accessible in backscattering along the wurtzite c axis. Interestingly, for 335 nm excitation the scattering process is instead mediated by the E 1(LO)…
The structural properties of GaN/AlN core-shell nanocolumn heterostructures.
International audience; The growth and structural properties of GaN/AlN core-shell nanowire heterostructures have been studied using a combination of resonant x-ray diffraction, Raman spectroscopy and high resolution transmission electron microscopy experiments. For a GaN core of 20 nm diameter on average surrounded by a homogeneous AlN shell, the built-in strain in GaN is found to agree with theoretical calculations performed using a valence force field model. It is then concluded that for an AlN thickness up to at least 12 nm both core and shell are in elastic equilibrium. However, in the case of an inhomogeneous growth of the AlN shell caused by the presence of steps on the sides of the …
Growth, structural and optical properties of GaN/AlN and GaN/GaInN nanowire heterostructures
Abstract After discussing the GaN NW nucleation issue, we will present the structural properties of axial and radial (i.e. core/shell) GaN/AlN NW heterostructures and adress the issue of critical thickness during the growth of such heterostructures. Next, we will present the growth of InGaN NWs on a GaN NW base. It will be shown that the morphology and structural properties of the InGaN NW sections depend on the In content: for high In content a flat top is observed and plastic relaxation is occuring, with mismatch dislocations formed at the InGaN/GaN interface. By contrast, for In content below 25% InGaN NWs exhibit a pencil-like shape assigned to a purely elastic strain relaxation process…
Nucleation of GaN nanowires grown by plasma-assisted molecular beam epitaxy: The effect of temperature
Abstract The growth of GaN nanowires by means of plasma assisted molecular beam epitaxy directly on Si(1 1 1) has been investigated as a function of temperature. Statistical analysis of scanning electron microscopy pictures taken for different growth temperatures has revealed that density, diameter, length and length dispersion of nanowires were strongly dependent on temperature. Length dispersion, in particular, was found to be significant at high temperature. These features have been assigned to the different duration of the nucleation process with temperature, namely to the dependence with temperature of the time necessary for the size increase of the three-dimensional precursors up to a…
Surface optical phonon modes in GaN nanowire arrays: Dependence on nanowire density and diameter
We present a systematic study, by means of Raman scattering, of the surface optical modes of GaN nanowires (NW) as a function of the fill factor, defined as the relative concentration of GaN NWs and the surrounding air. We show that changes in the fill factor from 0.8 to 0.3 result in a shift of the surface optical mode of more than 60 cm${}^{\ensuremath{-}1}$, which is explained by theoretical calculations of surface vibrations for a cylindrical nanowire based on the dielectric continuum model. Two cases are considered: the effective dielectric function approximation (Maxwell-Garnett approximation) and a model for isolated NWs with various diameters. We conclude, in agreement with the Maxw…