Strain and lattice vibration mechanisms in GaN-AlxGa1-xN nanowire structures on Si substrate

In this work we use Raman scattering and X-ray diffraction (XRD) techniques to examine strain and lattice vibration mechanisms in self-assembled GaN-AlxGa1-xN nanowire (NW) structures grown by plasma-assisted molecular beam epitaxy on Si(1 1 1) substrate. The samples consisted of GaN NW parts overgrown by AlxGa1-xN segments with the nominal Al content x = 0.1, 0.25, 0.5, 0.75 and 1. The non-resonant Raman spectra revealed phonon modes originating from the Si substrate as well as from GaN and AlN. By analysing frequency position of the GaN-like and AlN-like E2high vibration modes as well as of GaN-like A1LO mode, we were able to study chemical composition and strain in NWs, as well as lattice vibrations mechanisms, such as: temperature effects, phonon confinement and phonon-plasmon interaction. All these factors were considered and analysed within this work to explain the observed phonon modes shifts. The biaxial in-plane strain in the NWs was calculated based on the estimated E2high Raman lines shifts with respect to that of bulk GaN and bulk AlN. XRD studies supplemented Raman measurements by the analysis of micro-strain in GaN-AlxGa1-xN NW structures. Based on the XRD reciprocal space maps the in-plane and out-of-plane strain values were estimated. The results obtained from XRD were in a good agreement with those derived from Raman measurements.