Doping erbium into semiconductors has attracted enormous interest in optical applications as Er-related 1.54 μm emission is within a wavelength range of minimum loss for silica optical fibers ^[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 μm emission ^[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 ^[4,5]. However, the efficiency of 1.54 μm 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 SiO2, strong Er3+ photoluminescence (PL) at room temperature could be obtained ^[6–10]. Ge nanocrystals (nc-Ge) have many properties (wide size-dependent emission tunability, larger Bohr radius, etc.), which is superior to nc-Si ^[11]. The luminescence efficiency of Er3+ 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 Er3+.

Energy transfer from nc-Si to RE3+ in a silica host has been intensively investigated in recent years. In comparison, few studies have been found on nc-Ge and Er3+ 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°C annealed Ge:Er:ZnO film compared to the as-deposited sample and the sample without Ge dopants. A 1.54 μm PL enhancement accompanied by the appearance of nc-Ge in an annealed sample implies a significant correlation between nc-Ge and Er3+. 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 μm 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:Er2O3 (Er∼0.6 at. %) target with small pieces of 5mm×5mm Ge chips was cosputtered by radio frequency magnetron sputtering onto Al2O3 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/O2=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°C in N2 atmosphere. As a comparison, the Ge:Er:ZnO film (Er∼0.3 at. %) and the Er:ZnO film (Er∼0.6 at. %) were also deposited under the same experimental conditions. PL measurements were performed at room temperature using the 325 and 532 nm lasers, and a photomultiplier tube and a thermo-electrically cooled InGaAs detector were used to detect the visible and infrared PL signals, respectively. PL excitation (PLE) spectra measurements are carried out by excitation from the wide spectrum of a xenon lamp.

Crystalline Ge particles in nanometer size are formed by the annealing process. Figure 1(a) shows a typical cross-section high resolution transmission electron microscopy (HRTEM) image of the 600°C annealed Ge:Er:ZnO (Er∼0.6 at. %) film containing nc-Ge. The lattice fringes correspond to the (111) planes of Ge crystals, where the diamond structure can be seen. The fast Fourier transform process in Fig. 1(a) is carried out to distinguish nc-Ge and ZnO; it is shown that nc-Ge is formed in the square lattice while that of ZnO is the hexagonal structure. These nc-Ge distribute dispersedly in ZnO matrices, and the estimated average diameter of them is about 5 nm. The morphology and distribution of Ge analyzed by high angle annular dark-field imaging (HAADF), as shown in Fig. 1(b), match the results of HRTEM well.

Figure 2 shows the visible PL spectra from as-deposited, 600°C annealed Ge:Er:ZnO and 600°C annealed Er:ZnO films (all with Er∼0.6 at. %) under excitation at 325 nm. A broad luminescence with a peak at about 0.58 μm can only be found in the 600°C annealed Ge:Er:ZnO sample. The visible luminescence appears accompanied by the appearance of nc-Ge. It may come from different emission sources, such as new defect states in ZnO originated from the incorporation of nc-Ge ^[12] and the recombination of photogenerated excitons in nc-Ge due to their quantum confinement effect ^[13,14]. About the contribution from the latter, a zeroth-order approximation asymptotic wave function is used to calculate the possible PL emission of nc-Ge ^[14,15]. The energy E10=2.18eV (the emission wavelength peak equal to 0.569 μm) can be obtained, where the radius of nc-Ge, R=2.5nm, is employed according to our HRTEM result. This value is in good consistent with the visible PL result shown in Fig. 2.

To analyze the NIR PL of Er3+ ions in Ge:Er:ZnO, three possible excitation approaches are investigated: energy transfer from the 3.37 eV band-edge excitation of the ZnO host ^[16], direct excitation of the main absorption levels of Er3+ by the pump light, and indirect excitation through energy transfer from photogenerated excitons in nc-Ge to Er3+. Figure 3 shows the NIR PL spectra of the as-deposited, 600°C annealed Ge:Er:ZnO and 600°C annealed Er:ZnO films (all with Er∼0.6 at. %) under excitations at 325 and 532 nm. PL peaks at 0.998 and 1.54 μm correspond to the Er3+ intra-4f transition of I11/24 and I13/24 to the ground state I15/24, respectively ^[17]. Pumping light at 325 nm corresponds to the band-tail absorption of the ZnO host, and 1.54 μm emission can be achieved through an indirect excitation by energy transfer from recombination of electron–hole (e–h) pairs in the ZnO host to Er3+ ions. From Fig. 3(a), it can be seen that 1.54 μm emission of the samples without nc-Ge (as-deposited Ge:Er:ZnO and Er:ZnO films) are very low. It indicates that the energy transfer from the ZnO band-edge transition to Er3+ is extremely inefficient. However, this situation has been changed dramatically with the addition of nc-Ge: a strong Er3+ PL at 1.54 μm and an intra-4f transition at 0.998 μm are observed from the 600°C annealed Ge:Er:ZnO film. It provides evidence for the enhancement effect of nc-Ge on 1.54 μm PL. Beyond the band-tail absorption of ZnO, pumping with 532 nm excludes the possibility of the energy transfer mechanism through ZnO. Direct excitation of Er3+ exists in all the Er:ZnO film. 532 nm light directly excites Er3+ ions from ground state to higher-energy levels at H11/22 and consequently relax to I13/22 to give emission at 1.54 μm. However, it is shown in Fig. 3(b) that such direct excitation is demonstrated to be extremely low without nc-Ge. Compared to the weak NIR PL from Er:ZnO and as-deposited Ge:Er:ZnO films, 1.54 μm emission from nc-Ge-containing film is greatly enhanced. This indicates that nc-Ge plays a significant role in Er3+ excitation.

Optical spectra measurements are carried out to analyze how the nc-Ge participates in the PL enhancement. Figure 4(a) shows the PLE spectrum monitored at 1.54 μm as a function of the excitation wavelength in the 300–750 nm range for Er:ZnO without Ge and nc-Ge-containing Er:ZnO films. It can be found that compared to the sample without Ge dopants, both the energy transfer from the ZnO host and direct excitation of Er3+ are more efficient in nc-Ge-containing film, and the overall luminous efficiency is much higher in it. Meanwhile, the PLE peak of the nc-Ge-containing Er:ZnO film shifts to short wavelengths. This indicates that the local environment of 4f states of Er ions has been modified by the formation of nc-Ge. It has been reported that lowering of the symmetry of the crystal field around Er3+ is more suitable for Er intra-4f transitions ^[18,19]. In order to achieve such local potential distortion of the optically active centers, doping some impurities into the specimen proves to be very effective. The existence of nc-Ge may strongly affect the local environment of Er, which enhances the transition rates, resulting in a large enhancement of the 1.54 μm PL intensity. In addition, energy transfer from nc-Ge to Er3+ may be another possible reason for PL enhancement. Figure 4(b) shows the dependence of visible spectra for 600°C annealed nc-Ge-containing ZnO films with different Er concentrations (0.6 at. % and 0.3 at. %) under excitation at 325 nm. If the energy transfer from nc-Ge to Er3+ is assumed, the emission intensities between yellow band emission and Er-related 1.54 μm emission may correlate to each other depending on the number of Er3+ surrounding the nc-Ge. In Fig. 4(b), it can be found that the visible PL decreases and NIR PL at 1.54 μm increases as the Er concentration increases. According to semi-quantum theory, when the quantum dot size of nc-Ge is smaller than its effective Bohr radius (24 nm for Ge), the carrier (electrons, holes) movement will be limited, leading to the increase of kinetic energy and the discrete of energy levels in nc-Ge ^[20]. The quantum confinement effect of ∼5nm nc-Ge encourages the excitation at ∼2.3eV (0.52 μm in Fig. 2; 0.52 and 0.58 μm may be the most efficient energy transfer wavelength and the nc-Ge luminescence wavelength, respectively), and this energy is resonant with the H11/22−4I15/2 transition of Er3+ [inset of Fig. 3(a)]. The resonant energy transfer will make 1.54 μm emission more efficient. Although the PLE spectra do not reflect the absorption band of nc-Ge, probably due to the weak PL intensity from nc-Ge compared to that from Er3+ direct excitations or there may exist a fast energy exchange between nc-Ge and nearby Er3+, the possible energy transfer from nc-Ge to Er3+ cannot be ruled out. We have found that the 600°C annealed process resulted in the strongest PL intensity. If the annealing temperature is increased further, as the size of nc-Ge increases, 1.54 μm PL intensity decreases dramatically. This is probably due to the fact that with the decrease in the number of nc-Ge and the reduction of quantum effects, the energy exchange between nc-Ge and Er3+ becomes less efficient. The 1.54 μm PL enhancement may come from a joint effect of both the local environment change of Er3+ and the energy exchange between nc-Ge and Er3+. In order to find direct evidence of the energy transfer from nc-Ge to Er3+, a further investigation is being carried out.

In conclusion, visible and NIR luminescence properties of ZnO films containing Ge dopants and Er3+ are studied under different pumping wavelengths. Nc-Ge particles ∼5nm in size are formed in 600°C annealed Ge:Er:ZnO film. With the presence of nc-Ge, 1.54 μm emission has been greatly enhanced. The increased intensity of 1.54 μm in Ge:Er:ZnO film is considered to be the result of the joint effect of the local potential distortion around Er3+ and the energy transfer from nc-Ge to Er3+.