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

TL;DR

This study investigates the resonant Raman scattering in ZnCoO films, revealing inhomogeneous Co distribution, nano-region emissions, and a new interpretation of multi-phonon scattering involving localized excitons and charge transfer processes.

Contribution

It introduces a novel approach to interpret resonant enhancement in multi-phonon scattering in ZnCoO films under sub-band excitation, emphasizing the role of localized excitons and charge transfer.

Findings

Nanocrystalline structure with decreasing grain size at higher Co concentration

Inhomogeneous lateral distribution of Co atoms in the films

Resonant enhancement linked to localized excitons and charge transfer processes

Abstract

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 Fr\"{o}hlich 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.