Miniaturizing the erbium-doped optical fiber amplifier (~20 m in length) into a small, compact amplifier that can be integrated with other optical and electronic devices on a single chip (optoelectronics) offers great promise in optical communication as an alternative to the electronic technology.^1,2, The gain of these miniaturized devices is limited by the solubility, concentration, and distribution of optically-active Er³⁺ in a host material. ³ While incorporation of a high concentration of erbium is possible by ion implantation, the method does not allow for the control of spatial distribution or the activation of the ions in the host. This is critical at high Er concentrations, since non-radiative processes resulting from ion-ion interactions become dominant and significantly reduce the photoluminescence (PL) yield.

In this work, we demonstrated radical-enhanced atomic layer deposition (RE-ALD) as a viable technique to synthesize Er³⁺-doped dielectric thin films with a precise control of its concentration and spatial distribution, thus tailoring the PL property of Er³⁺ doped waveguides. Since the optically-active Er needs to be in the trivalent state, showing highest photoluminescence efficiency when coordinating with approximately six O atoms as in crystalline Er₂O₃, Y₂O₃ was chosen as the host material due to its identical crystal structure and very similar lattice constant to Er₂O₃.^4-5 In this case, typical problems such as lattice distortion and vacancy formation which are detrimental to the PL yield can essentially be eliminated. The thin film deposition was carried out in an ultra-high vacuum multi-beam reactor in which metal b-diketonate complexes and oxygen radicals were introduced independently and sequentially. Incorporation of Er in Y₂O₃ thin films at 350°C was accomplished by combining the self-limiting RE-ALD of Y₂O₃ and Er₂O₃ in an alternating fashion, with the Er doping level at a specific depth location controlled by varying the ratio of Y₂O₃:Er₂O₃ cycles during deposition.

Figure 1. (a) Proposed structures of Y2O3 films doped with low and high concentrations of Er3+, (b) EXAFS analysis of a 6 at.% Er3+ doped Y2O3 thin film, and (c) EXAFS analysis of a 14 at.% Er3+ doped Y2O3 thin film.

The nanostructure of Er-doped Y₂O₃ thin films was investigated by using a high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectrometry (EELS). Specifically, the distribution of Er separated by layers of Y₂O₃ was confirmed by elemental EELS mapping of Er M₄ and M₅, with the Er concentration controlled from 6 to 14 at.%, determined by X-ray photoelectron spectroscopy (XPS). This unique feature is characteristic of the alternating RE-ALD of Y₂O₃ and Er₂O₃. The photoluminescence yield was found to reduce by at least one order of magnitude when the Er doping level exceeded 8 at.%. This photoluminescence quenching, also commonly known as concentration quenching, is attributed to two main processes: Er immiscibility in the host matrix and/or Er ion-ion interaction. To delineate the origin of this photoluminescence reduction, we applied X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) analyses.

X-ray absorption near edge spectroscopy (XANES) study using the Er L_III edge at 8358 eV confirmed that Er was in the optically active trivalent state (Er³⁺), having an octahedral symmetry similar to Er in Er₂O₃. No other chemical state was found up to at least 14 at.%, indicating no formation of Er precipitate which is optically inactive. To obtain further insight into the Er local coordination, three different 4-Å local cluster models were constructed, corresponding to three different possible Er configurations in the Y₂O₃ thin film. In all three models, the first shell is O while the second shell can be all Er³⁺ (first model), all Y³⁺ (second model), or a mixture of Y³⁺ and Er³⁺ (third model). Because of the almost perfect crystal structure match between Y₂O₃ and Er₂O₃, the simulation of the Er-doped Y₂O₃ local structure was simply accomplished by replacing Y³⁺ with Er³⁺ in the Y₂O₃ lattice. Shown in Figure 1a (left) is a pictorial view of the Er-doped Y₂O₃ structure at low Er concentration. In this case, the center absorbing Er has a second coordination shell with a mixture of both Er and Y. At high Er concentration where the alternating growth of Y₂O₃ and Er₂O₃ resulting in an exsolution with Er₂O₃-rich domains (Figure 1a, right), the Er local environment is described by combining the first and third model.

Shown in Figure 1b are the k³-weighted EXAFS of the Y₂O₃ thin films doped with 6 at.% Er, representing samples with high photoluminescence yield. The best fit to the EXAFS using the 1st and 2nd model agreed fairly well with the EXAFS up to k ~ 6 Å^-1, mainly from the first O shell, but failed to describe the oscillations at higher k (Figure 1b top). This indicates that the second nearest neighbors are neither all Er³⁺ in which case the local environment of Er³⁺ would be similar to that of Er³⁺ in Er₂O₃, nor all Y³⁺ which would otherwise indicate an infinite dilution of Er³⁺ in Y₂O₃. A best fit to the EXAFS was achieved with the third model (Figure 1b bottom) when the Y:Er cation ratio is specified to be 3:1, as determined by XPS compositional analysis In this case, a coordination number of 6 for the first O shell and 8-9 for the second shell were obtained. Furthermore, Er³⁺ was found to be completely miscible in the Y₂O₃ matrix up to at least 8 at.%, showing no evidence of Y₂O₃ and Er₂O₃ phase segregation. There is also no indication of Er-Er coordination within 4-Å proximity.

For the 14 at.% Er-doped Y₂O₃ thin film, representing samples with low photoluminescence yield, a combination of the first (~60%) and third model (~40%) best fitted the EXAFS spectrum (Figure 1c), with the Y:Er cation ratio specified at 1:3, as determined by XPS. This is consistent with the alternating RE-ALD process, resulting in a layer-like structure under these deposition conditions. Since there is no indication of Er-Er coordination in all samples doped with 6 to 14 at.% Er³⁺, it is concluded that the photoluminescence quenching observed in samples with Er concentration exceeding 8 at.% is not due to Er immiscibility in Y₂O₃ but likely due to Er ion-ion interaction. When the Er³⁺ concentration is sufficiently small, the ions are evenly distributed in the Y₂O₃ matrix with relatively large inter-ionic distances, impeding ion-ion interaction. Consequently, the photoluminescence yield is relatively high in the absence of these competing processes. As the Er³⁺ concentration increases, there is more Er³⁺ within a 4-Å proximity of each other that they can interact, resulting in cooperative energy upconversion or energy migration, leading to reduced photoluminescence yield.

Using EXAFS, the origin of the observed concentration quenching of photoluminescence for the Er³⁺:Y₂O₃ system was delineated. The study also suggests that, in order to prevent ion-ion interaction, no Er³⁺ should have another Er³⁺ as a second nearest neighbor. This criteria sets an upper limit on the Er³⁺ concentration in the Y₂O₃ host at ~6x10²¹/cm³, or ~10 at.%, estimated by systematically replacing Y³⁺ in the Y₂O₃ unit cell by Er³⁺ while ensuring there is no direct Er-O-Er bonding. These results are essential to the understanding of the Er³⁺ optical properties in correlation to its local structure, allowing for the optimization of the photoluminescence yield by controlling its distribution in the host lattice.

Primary Citation

T.T. Van, Bargar, J.R., and Chang, J.P. (2006) Structural investigation of Er coordination in Y₂O₃. J. Applied Physics 100, 023115

References

E. Desurvire, Erbium-doped Fiber Amplifiers: Principles and Applications (John Wiley & Sons, Inc., New York, 1994). A. Polman and F. C. J. M van Veggel, J. Opt. Soc. Am. B 21, 871 (2004). F. Auzel, in Spectroscopic Properties of Rare Earths in Optical Materials, 1st edition, edited by G. Liu & B. Jacquier (Springer, New York, 2005), Vol. 83, Chap. 5, p.266. D. J. Eaglesham, J. Michel, E. A. Fitzgerald, D. C. Jacobson, and J. M. Poate, Appl. Phys. Lett. 58, 2797 (1991). E. C. M. Pennings, G. H. Manhoudt, and M. K. Smit, Electron. Lett. 24, 998 (1988).

| PDF Version |     | Lay Summary |     | Highlights Archive |   SSRL is supported by the Department of Energy, Office of Basic Energy Sciences. The SSRL Structural Molecular Biology Program is supported by the Department of Energy, Office of Biological and Environmental Research, and by the National Institutes of Health, National Center for Research Resources, Biomedical Technology Program, and the National Institute of General Medical Sciences.

 

Last Updated:	25 SEP 2006
Content Owner:	Jane Chang, UCLA
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