Multiphonon resonant Raman scattering inZnOcrystals and nanostructured layers

Abstract

Multiphonon resonant Raman scattering (RRS) was studied in unintentionally doped bulk $\mathrm{ZnO}$ crystals and layers, including nanostructured and highly conductive films, excited by 351.1 and $363.8\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ laser lines in the temperature interval from 10 to $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The variation of resonant conditions with sample temperature and wavelength of the excitation laser line allowed us to discriminate between the incoming and outgoing exciton mediated RRS by LO phonons. The quenching of luminescence and enhancement of Raman scattering in nanostructured $\mathrm{ZnO}$ layers grown on single-layer opals enabled us to observe the Raman signal in resonant configuration thus making possible to study multiphonon RRS by Fr\ohlich-type vibrational modes related to nanocrystallites with sizes less than $50\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The emission spectra of highly conductive $\mathrm{ZnO}$ films grown on porous $\mathrm{InP}$ substrates were found to consist of multiphonon RRS lines superimposed on a broad asymmetric near band gap photoluminescence (PL) band. The occurrence of PL and RRS in highly conductive layers is attributed to tailing of the density of states caused by potential fluctuations and to the breakdown of the wave-vector conservation due to randomly distributed impurities (intrinsic defects).