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

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


Bargar, J. - Presenter, SLAC National Accelerator Laboratory

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.