Specific features of optical phonons in raman spectra of an array of vertical ZnO microrods on silicon

The polarized Raman spectra of an array of vertical zinc oxide microrods prepared by the carbothermal synthesis on a Si(001) substrate have been investigated. The length and diameter of the rods are 9 $\pm$ 0.5 $\mu$m and 210 $\pm$ 50 nm, respectively. X-ray diffraction studies have confirmed the mutual crystallographic orientation [001]$_{\mathrm{ZnO}}$ $\parallel$ [001]$_{\mathrm{Si}}$ in the direction normal to the substrate plane. In the Raman spectra of the rods, there are modes forbidden by selection rules for the single crystal: the $A_1(TO)$ $E^{\mathrm{high}}_2$ and modes in the scattering geometry $x(yz)\bar{x}$ and the $E_1(LO)$ mode in the geometry $x(zz)\bar{x}$. It has been found that the spectra contain quasi-modes $Q(TO)$ and $Q(LO)$, the spectral position of which shifts with a variation in the angle of incidence of the exciting radiation. The angle range of propagation of the exciting radiation in the array of rods is estimated as $\sim$ 25$^\circ$ for a fixed angle of incidence by comparing the spectral shift of the quasi-modes with experimental data for the single crystal.