A high concentration of erbium doping was achieved in Y2O3 thin films on Si (100) by depositing Y2O3 alternatively with Er2O3 using radical-enhanced atomic layer deposition (ALD). Specifically, the erbium doping level was controlled by varying the ratio of Y2O3:Er2O3 cycles during deposition, and a 10:5 ratio yielded ∼9at.% erbium incorporation in Y2O3, confirmed by the compositional analysis using x-ray photoelectron spectroscopy. Room-temperature photoluminescence was observed in a 320-Å Er-doped (9 at. %) Y2O3 film deposited at 350 °C. This result is very promising, since the film was fairly thin and no annealing at high temperature was needed to activate the erbium ions. This suggests that radical-enhanced ALD was able to preserve the optically active trivalent state of the erbium ion from its precursor state. The effective absorption cross section for Er3+ ions incorporated in Y2O3 was estimated to be on the order of 10−18cm2, about three orders of magnitude larger than the direct optical absorption cross section reported for Er3+ ions in a stoichiometric SiO2 host. These results validate Y2O3 as a promising Er3+ host material and demonstrate that radical-enhanced ALD is a viable technique for synthesizing these materials.

The nanostructure and photoluminescence of polycrystalline Er-doped Y2O3 thin films, deposited by radical-enhanced atomic layer deposition (ALD), were investigated in this study. The controlled distribution of erbium separated by layers of Y2O3, with erbium concentrations varied from 6to14at.%, was confirmed by elemental electron energy loss spectroscopy (EELS) mapping of Er M4 and M5. This unique feature is characteristic of the alternating radical-enhanced ALD of Y2O3 and Er2O3. The results are also consistent with the extended x-ray absorption fine structure (EXAFS) modeling of the Er distribution in the Y2O3 thin films, where the EXAFS data were best fitted to a layer-like structure. X-ray diffraction (XRD) and selected-area electron diffraction (SAED) patterns revealed a preferential film growth in the [111] direction, showing a lattice contraction with increasing Er doping concentration, likely due to Er3+ of a smaller ionic radius replacing the slightly larger Y3+. Room-temperature photoluminescence characteristic of the Er3+ intra-4f transition at 1.54μm was observed for the 500Å, 8at.% Er-doped Y2O3 thin film, showing various well-resolved Stark features due to different spectroscopic transitions from the I13∕24→I15∕24 energy manifold. The result indicates the proper substitution of Y3+ by Er3+ in the Y2O3 lattice, consistent with the EXAFS and XRD analyses. Thus, by using radical-enhanced ALD, a high concentration of optically active Er3+ ions can be incorporated in Y2O3 with controlled distribution at a low temperature, 350°C, making it possible to observe room-temperature photoluminescence for fairly thin films (∼500–900Å) without a high temperature annealing.

Extended x-ray absorption fine structure (EXAFS) spectroscopy was employed to study the Er coordination in polycrystalline Y2O3 thin films, which was found to dictate their photoluminescence (PL) properties. Incorporation of Er with concentrations varying from 6to14at.% was achieved by radical-enhanced atomic layer deposition at 350°C. In all samples, Er was found to be in the optically active trivalent state, confirmed by their x-ray absorption near-edge spectroscopy spectra. Modeling of the EXAFS data revealed that the local structure of Er3+ is similar to that of Er3+ in Er2O3. Specifically, Er3+ is coordinated with six O at 2.24 and 2.32Å. Excellent fits to the EXAFS for samples with Er3+ concentration less than 8at.% were achieved when the second coordination shell was modeled as a mixture of Y3+ and Er3+, indicating a complete miscibility of Er3+ in the Y2O3 matrix under these experimental conditions. This behavior is attributed to the almost perfect ionic size match between Y3+ and Er3+, having identical valence state and coordination characteristics. For thin films with higher Er concentrations, the EXAFS analysis revealed an exsolution with Er2O3 domain. Since there is no indication of Er clustering, it is concluded that the PL quenching observed in samples with the Er doping level exceeding 8at.% is likely due to Er ion-ion interaction but not Er immiscibility in Y2O3. Specifically, an increase in the Er3+ concentration implied an increase in the average number of Er3+ in the second coordination shell, thus making ion-ion interaction possible. The critical interionic distance between two Er3+ was determined to be ∼4Å, thus setting an upper limit on the Er3+ concentration in Y2O3 at ∼6×1021cm−3, at least three orders of magnitude higher than the Er3+ solubility limit in the conventional SiO2 host (<1018cm−3).

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