\title{1.54~$\mu$m photoluminescence enhancement of Er$^{3+}$-doped ZnO films containing nc-Ge: joint effect from Er$^{3+}$ local environment changing and energy transfer of nc-Ge}

\author{Ranran Fan, Fei Lu, and Kaikai Li \\
School of Information Science and Engineering, Shandong University, Jinan 250100, China}

\begin{abstract}
ZnO films containing Er and Ge nanocrystals (nc-Ge) were synthesized and their photoluminescence (PL) properties were studied. Visible and near-infrared PL intensities are found to be greatly increased in nc-Ge-containing film. Er-related 1.54~$\mu$m emission has been investigated under several excitation conditions upon different kinds of Ge, Er codoped ZnO thin films. 1.54~$\mu$m PL enhancement accompanied by the appearance of nc-Ge implies a significant correlation between nc-Ge and PL emission of Er$^{3+}$. The increased intensity of 1.54~$\mu$m in Ge:Er:ZnO film is considered to come from the joint effect of the local potential distortion around Er$^{3+}$ and the possible energy transfer from nc-Ge to Er$^{3+}$.
\end{abstract}

\section{Introduction}
Doping erbium into semiconductors has attracted enormous interest in optical applications as Er-related 1.54~$\mu$m emission is within a wavelength range of minimum loss for silica optical fibers \cite{1}. It has been reported that oxygen codoping and the use of wide-bandgap materials as the host material are effective to enhance the Er-related 1.54~$\mu$m emission \cite{2,3}. ZnO is considered to be a promising candidate as a host material for Er doping because of being an oxide with a wide bandgap of about 3.37 eV \cite{4,5}. However, the efficiency of 1.54~$\mu$m emission in ZnO:Er thin films has not yet been satisfied. It has been confirmed that through the addition of Si nanocrystals (nc-Si) into SiO$_2$, strong Er$^{3+}$ photoluminescence (PL) at room temperature could be obtained \cite{6,7,8,9,10}. Ge nanocrystals (nc-Ge) have many properties (wide size-dependent emission tunability, larger Bohr radius, etc.), which is superior to nc-Si \cite{11}. The luminescence efficiency of Er$^{3+}$ may be enhanced by the introduction of nc-Ge through the change of local environment and recombination of photogenerated excitons in nanocrystals and subsequent energy transfer to Er$^{3+}$.

Energy transfer from nc-Si to RE$^{3+}$ in a silica host has been intensively investigated in recent years. In comparison, few studies have been found on nc-Ge and Er$^{3+}$ in a ZnO host. In this paper, we deposit ZnO film with codoped Ge, Er in it by magnetron sputtering. Visible and near-infrared (NIR) PL intensities are found to be greatly increased in 600$^{\circ}$C annealed Ge:Er:ZnO film compared to the as-deposited sample and the sample without Ge dopants. A 1.54~$\mu$m PL enhancement accompanied by the appearance of nc-Ge in an annealed sample implies a significant correlation between nc-Ge and Er$^{3+}$. The purpose of this work is to analyze how the nc-Ge participates in the PL enhancement. Our observed results suggest that the increased intensity of 1.54~$\mu$m in Ge:Er:ZnO film is the result of the joint effect of two factors: local distortion effect and the energy transfer from the nc-Ge participating in the efficient emission.

A ZnO:Er$_2$O$_3$ (Er $\sim$ 0.6 at.\%) target with small pieces of 5 mm $\times$ 5 mm Ge chips was cosputtered by radio frequency magnetron sputtering onto Al$_2$O$_3$ substrates. In the current experiment, Ge concentration was fixed to be about 4\% in volume fraction. The deposition was carried out in an oxygen and argon mixed gas atmosphere (Ar/O$_2$ = 2/5), and the chamber pressure was maintained at a constant value of 0.7 Pa. After deposition, the Ge:Er:ZnO film thickness was measured to be about 400 nm. Postannealing of the film was performed for 20 min at temperature of 600$^{\circ}$C in N$_2$ atmosphere. As a comparison, the Ge:Er:ZnO film (Er $\sim$ 0.3 at.\%) and the Er:ZnO film (Er $\sim$ 0.6 at.\%) were also deposited under the same