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

Using the methods of scanning electron (SEM) and atom force microscopy (AFM) as well as photoluminescence (PL) and Raman micro-spectroscopy, we investigated $Zn_{1-x}Co_xO$ 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 $\sim150$ down to $28$ nm at changing the $Co$ concentration from $5$ to $15\%$. High-resolution SEM images of $Zn_{0.85}Co_{0.15}O$ 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 $Zn_{1-x}Co_xO$ nano-regions enriched and depleted with $Co$. In low-temperature PL spectra under sub-band excitation of $Zn_{1-x}Co_xO$ films, there observed are intra-center optical transitions due to $Co^{2+}$ center: $^2E(G)$, $^2A_1(G)$, $^2T_1(G)$, $^4T_1(P)$, $^2T_2(G)$ $\longrightarrow$ $^4A_2(F)$. Offered is a new approach to interpretation of resonant enhancement observed in multi-phonon scattering by $LO$ phonons in $Zn_{1-x}Co_xO$ 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 $Co^{2+}$ 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.