Nature of resonant Raman scattering in ZnCoO films under sub-band excitation

Abstract

Using the methods of scanning electron (SEM) and atom force microscopy (AFM) as well as photoluminescence (PL) and Raman micro-spectroscopy, we investigated $Z{n}_{1-x}C{o}_{x}O$ films ($x=5$ and $15\%$) grown by molecular beam epitaxy on sapphire substrates. It is found that the films have a nanocrystalline structure with the grain size decreased from $\sim 150$ down to $28$ nm at changing the $Co$ concentration from $5$ to $15\%$. High-resolution SEM images of $Z{n}_{0.85}C{o}_{0.15}O$ film have been interpreted as inhomogeneous lateral distribution of $Co$ atoms. Two broad emission bands observed in spectra of band-to-band PL are ascribed to emission from $Z{n}_{1-x}C{o}_{x}O$ nano-regions enriched and depleted with $Co$. In low-temperature PL spectra under sub-band excitation of $Z{n}_{1-x}C{o}_{x}O$ films, there observed are intra-center optical transitions due to $C{o}^{2+}$ center: ${}^{2}E(G)$, ${}^2{A}_1(G)$, ${}^2{T}_1(G)$, ${}^4{T}_1(P)$, ${}^2{T}_2(G)$ $⟶$ ${}^4{A}_2(F)$. Offered is a new approach to interpretation of resonant enhancement observed in multi-phonon scattering by $LO$ phonons in $Z{n}_{1-x}C{o}_{x}O$ under sub-bandgap excitation combined with the extrinsic Fröhlich interaction mediated via a localized exciton bound to the isoelectronic impurity in which the electron is strongly localized at the magnetic $C{o}^{2+}$ ion. This conclusion is confirmed by the dependence of Raman spectra on the quantum energy of exciting radiation. It assumes formation of intermediated sub-band electron excited states of isoelectron $Co$ dopant, they are also referred to as charge transfer processes.

16 pages, 5 Figures