(685c) Spatially Controlled Nano-Scale Doping by Atomic Layer Deposition

In this work, we demonstrated radical-enhanced atomic layer deposition (RE-ALD) as an ideal technique for incorporation of dopants in ultra-thin metal oxide films. Er-doped Y₂O₃ thin film is a potential waveguide core material for planar miniature optical amplifiers. Though SiO₂ has traditionally been used as the host material for Er ions in fiber amplifiers (~20 meters long), it is an unsuitable host in small, compact amplifiers due to its low solubility for erbium. In contrast, Y₂O₃ has a similar crystal structure and lattice constant as Er₂O₃, which in principle allows for a much higher concentration of Er to be incorporated, and hence higher gain values.

Extended X-ray Absorption Fine Structure (EXAFS) 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 6 to 14 atomic percent 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. Modeling of the EXAFS data revealed that the local structure of Er ion is similar to that of Er ion in Er₂O₃. Specifically, Er³⁺ is coordinated with 6 O at 2.24 and 2.32 Å. Excellent fits to the EXAFS for samples with Er³⁺ concentration less than 8 at.% 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 8 at.% is likely due to Er ion-ion interaction but not Er immiscibility in Y₂O₃. The critical inter-ionic distance between two Er³⁺ was determined to be 0.4 nm, thus setting an upper limit on the Er³⁺ concentration in Y₂O₃ at ~6×10²¹/cm³, at least three orders of magnitude higher than the Er³⁺ solubility limit in the conventional SiO₂ host (< 10¹⁸/cm³).

The nanostructure and photoluminescence of polycrystalline Er-doped Y₂O₃ thin films were also investigated. The controlled distribution of erbium separated by layers of Y₂O₃ was confirmed by elemental electron energy loss spectroscopy (EELS) mapping. This unique feature is characteristic of the alternating radical-enhanced ALD of Y₂O₃ and Er₂O₃. X-ray diffraction (XRD) and selected-area electron diffraction 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 1.54 micro-meter was observed for the 50 nm 8 at.% Er-doped Y₂O₃ thin film, showing various well-resolved Stark features. 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 (~50 nm) without a high temperature annealing.