Polycrystalline Er-Sc silicates (ErxSc2-xSiO5 and ErxSc2-xSi2O7) were fabricated using multilayer nanostructured films of Er2O3/SiO2/Sc2O3 deposited on SiO2/Si substrates by RF- sputtering and thermal annealing at high temperature. RBS, TEM, GIXD, and PL results show the presence of ErxSc2-xSiO5 with an emission peak at 1528 nm for annealing from 900 to 1100 °C, and ErxSc2-xSi2O7 with an emission peak at 1537 nm for higher annealing temperature. The PL intensity of the ErxSc2-xSi2O7 phase is five times stronger than that of the ErxSc2-xSiO5 phase at 1250 °C. From PLE and PL spectra of ErxSc2-xSi2O7 thin film, we schematically illustrate the Er3+ Stark energy levels of 4I13/2 to 4I15/2 manifolds due to the crystal field strength effect of Sc3+. Temperature-dependent PL of the ErxSc2-xSi2O7 phase exhibits a variation of the full-width at half-maximum (FWHM) from 1.1 to 2.3 nm. The narrow FWHM is due to the small ionic radii of Sc3+, which enhance the crystal field strength affecting the optical properties of Er3+ ions located at the well-defined lattice sites of Sc silicate. A large excitation cross-section (σex) is equal to 3.0x10−20 cm2 at λex = 1527.6 nm.
© 2015 Optical Society of America
The realization of photonic interconnects requires the development of new efficient and reliable on-chip optical device elements, including light sources, modulators, amplifiers, buffers, switches, and detectors. The transition from electronic to photonic interconnects will be cost effective only if it is performed with mature CMOS technology. This requires silicon compatibility of all the integrated optical components. A significant amount of work has already been done and different approaches to the realization of each of the mentioned components have been attempted: microdetectors based on III-V structures evanescently coupled to a silicon waveguide , Ge-on-Si [2, 3], and ion-implanted silicon  have been demonstrated; modulators based on the free carrier plasma dispersion effect  and electro-absorption effect [6–8] have been realized; and hybrid evanescent amplifiers , and switches  have been shown. The realization of a silicon-based light source is considered to be one of the most challenging tasks. Due to the indirect silicon band-gap, the emission efficiency of silicon is low and, even at liquid helium temperatures does not exceed ~10-4% . Consequently, alternative approaches are being considered, such as silicon Raman lasers , hybrid evanescent lasers , devices based on germanium-on-silicon , silicon-nanocrystal doped SiO2 [15, 16], silicon nanocrystal doped silicon nitride , erbium-doped Si-rich silicon nitride , erbium doped porous silicon [19–22], and erbium-doped silicon-rich SiO2 [23, 24].
One of the possible solutions for the realization of silicon based light source is to use the emission from Er-silicates. Er silicates (Er2SiO5 and Er2Si2O7) have been attracting considerable attention as Er based materials for small size and high optical gain light source in silicon photonics integration, because they contain a higher Er density of 1022 cm−3 than Er doped Si-based materials [25, 26]. H. Omi et al. have studied the mixture formation of ErxYb2-xSi2O7 and ErxYb2-xO3 on silicon for broadening the C-band in an optical amplifier . However, such a high concentration of Er results in up-conversion due to closely neighboring Er ions which limits the Er luminescence. Therefore, it is necessary to characterize and control the distance between Er ions in such Er silicates. An effective strategy for reducing this up-conversion is to incorporate yttrium (Y) cations into the structure, where they substitute Er ions in the silicate lattice and prevent neighboring Er ions from causing up-conversion due to the similar ionic radius between Y and Er [28, 29].
Scandium ions (Sc3+), on the other hand, are small (ionic radius = 0.75 Å) than erbium (Er3+) (ionic radius = 0.881 Å). Generally, this can result in enhancing the crystal field strength for Er doped silicates and oxides [30, 31]. In fact, Fornasiero et al. synthesized single crystal of Er doped Sc silicates using the Czochralski technique with the idea that Sc3+ ions would increase the Stark-splitting of the thermally populated erbium ground state as well as that of other electronic energy levels of the silicates and thereby reduce re-absorption losses . A. Najar et al. has fabricated ErxSc2-xSi2O7 compound after annealing at 1250 °C and estimated the erbium diffusion coefficient to be 1x10−15 cm2/s with a peak emission at 1537 nm . However, the effect of thermal annealing and Sc ions on the structural and optical properties of Er-Sc silicates with different compositions have not been sufficiently characterized yet, compared with those of Er-Y or Er-Yb silicates.
In this work, we synthesized a polycrystalline Er-Sc silicate and discilicate compounds in which Er and Sc are homogeneously distributed using RF-sputtering with multilayer Er2O3, Sc2O3, and SiO2 deposited on SiO2/Si (100) substrate and thermal annealing at high temperature. The concentration profiles of Er3+ and Sc3+ and the morphology structures were investigated by RBS, TEM-SAED and X-ray. The diagram energy of Er3+ ions in ErxSc2-xSi2O7 phase is presented based in the optical properties studies coupled with the morphology structure. The excitation cross-section will be determined and discussed.
2. Experimental conditions
Er-Sc multilayer thin films were grown by RF-sputtering by alternating 15 nm thick layers of Er2O3, and Sc2O3 separated by 15 nm thick SiO2 layer. These layers were deposited on 50 nm thick Er2O3 on SiO2 (1.3 µm)/Si (100) substrate at room temperature. After deposition, the samples were annealed in O2 at 900 °C, 1000 °C, 1100 °C, or 1250 °C for 1 h. The concentration of different chemical elements in the films was determined by Rutherford backscattering spectroscopy (RBS). He++ ions delivered by a 2.275 MeV Van de Graf accelerator with the normal detector angle of 160° was used. Spectra are fitted by applying a theoretical layer model and iteratively adjusted elemental concentrations and thickness until good agreement is found between the theoretical and the experimental spectra. The samples were analyzed from transmission electron microscopy (TEM) images obtained at 200 KeV, and by synchrotron grazing incidence X-ray diffraction (GIXD) experiments performed on the as-grown and annealed samples at the BL24 in SPring-8 using an X-ray wavelength of 1.24 Å and an incidence angle of 1.0◦. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were obtained by using two excitation sources at 980 and 1527.6 nm. The excitation laser was focused to a spot with a diameter of about 15 µm and an incident angle of 45 degrees through an objective lens. The luminescence from the sample was collected perpendicularly by a different objective lens with a numerical aperture of 0.40. The PL spectra were detected using a 0.5-m spectrometer and cooled InGaAs detector. Time-resolved PL was measured by using a pulsed laser formed by an acousto-optic modulator and a streak camera (Hamamatsu C11293S). The spectral and temporal resolutions of this system are 500 µeV and 20 ps (for a 1 ns time range), respectively.
3. Results and discussion
To determine the concentration distribution of chemical elements in the samples, we performed RBS measurements. RBS spectra in Fig. 1 show the profiles of Er, Sc, Si, and O atom distributions after deposition and annealing at 1100 and 1250 °C. As shown in Fig. 1, RBS signal intensities changed with increasing annealing temperature, suggesting that the concentration changes for Er3+ and Sc3+. Note that for the as-grown sample, the Er distribution is formed by peaks corresponding to Er2O3 layers. Whereas, the intensity decrease for sample annealed at 1100 °C and erbium diffuse in-depth. The concentration of Si and O has an atomic ratio of Si to O of about 1:4, indicating that the films are mainly formed by two phases ErxSc2-xSiO5 and ErxSc2-xSi2O7.
In contrast, at 1250 °C erbium starts to penetrate the silicate matrix and becomes more homogeneous. From the RBS analysis, the concentration of Si and O, has an atomic ratio of 2:7, indicating that the films are mainly composed of the ErxSc2-xSi2O7 phase. The RBS analysis indicates erbium and scandium doping concentrations close to [Er] = 12.7x1020, and [Sc] = 4.1 x1020 at/cm3 at 1100 °C and [Er] = 6.7x1021, and [Sc] = 1.4x1021 at/cm3 at 1250 °C.
To determine the microscopic structures after deposition, we performed TEM analysis of the cross-section coupled with selected area electron diffraction (SAED) images of the samples after deposition and annealing at 1100 °C and 1250 °C. Figure 2(a) shows a schematic cross-section of the different layers. The cross-sectional image in Fig. 2(b) obtained after deposition shows different layers of Er2O3, Sc2O3, and SiO2 with a total deposition thickness of around 109 nm. The inset SAED image from the Er2O3 layer at the bottom shows multicrystalline rings. The interplanar spacings d are about 1.29, 1.32 and 1.52 Å, corresponding respectively to the (203), (440), and (20-3) planes for Er2Si2O7, and with d of 1.32 and 1.52 Å corresponding to the (800) and (444) planes for Er2O3.
For the sample annealed at 1100 °C, the TEM image in Fig. 2(c) shows two layers separated by some voids. The appearance of these voids may be due to the diffusion of SiO2 layer. The interplanar spacings are about 1.7, 2.83 and 1.88 Å, corresponding respectively to the (233), (−402), and (−523) planes for Er2SiO5, and the (132), (130), and (22-2) planes for Er2Si2O7. These results show the mixture formation of ErxSc2-xSiO5, ErxSc2-xSi2O7 phases at 1100 °C.
After thermal annealing at 1250 °C, we formed a unique layer with an average thickness of 102 nm as shown in Fig. 2(d). Not that the size of the voids becomes very small at 1250 °C compared to 1100 °C. The SAED images show a single crystal compound. The interplanar spacings are 1.30, 1.54 and 2.61 Å, corresponding respectively to the (203), (33-2), and (220) planes for Er2Si2O7, and Er2SiO5 phase with (−442) plane for 1.54 Å spacing. TEM images show for both annealing temperatures, the existence of ErxSc2-xSiO5 and ErxSc2-xSi2O7 phases in the structures.
To confirm the dominant phase after annealing, we performed GIXD experiments on the as-grown and annealed samples. GIXD profiles of the crystalline structure after deposition and annealing are shown in Fig. 3. The GIXD profile of the sample after deposition shows the presence of Er2O3, Sc2O3, Er2SiO5, Sc2SiO5, and Er2Si2O7 in the films. After annealing at 1100 °C, the main intense peaks are close to Er2SiO5 and Sc2SiO5 with two peaks also close to Er2Si2O7. This result confirms the TEM and RBS measurements showing the presence of both ErxSc2-xSiO5 and ErxSc2-xSi2O7 phases. The size of the polycrystalline ErxSc2-xSiO5 is around 39 nm at 1100 °C according to the analysis of the diffraction peak width, whereas is equal to 50 nm for ErxSc2-xSi2O7 polycrystalline.
After annealing at 1200 °C (data not shown here) and 1250 °C, we have only Er2Si2O7 because of diffusion of Er and Sc into different layers and the formation of new polycrystalline mixed compounds assigned to ErxSc2-xSi2O7, which is the dominant phase. The size of the polycrystalline ErxSc2-xSi2O7 is around 35 nm at 1250 °C according to the analysis of the diffraction peak width.
To demonstrate a relation between composition stoichiometry and optical properties, we analyzed the annealed samples by PL. Figure 4 shows PL spectra of samples annealed between 900 and 1250 °C using λex = 980 nm at room temperature. For annealing temperature equal to 900 and 1100 °C, the spectra exhibit several emission peaks, corresponding to the 4I13/2 to 4I15/2 transition of Er3+ ions. The main intense peak is observed at 1528 nm and the second intense peak is at 1537 nm at 1100 °C. The PL spectrum with a single main intense peak at 1528 nm is similar to the PL spectrum of the Er silicates . The structure of the Er silicates has been confirmed to comprise Er2SiO5 crystalline films with a highly ordered layer structure [26, 33]. In our case, this peak corresponds to the ErxSc2-xSiO5 phase and confirms the previous observations at 1100 °C.
However, the samples annealed at 1200 °C and 1250 °C exhibit PL spectra with a different shape compared to 900 and 1100 °C, with the main intense peak at 1537 nm with narrower linewidth, indicating that a relatively homogeneous crystalline environment for Er3+ ions, and with sub-peaks at 1546.2 and 1551 nm. The PL peak intensity of annealed samples reaches the maximum at 1250 °C. The peak at 1537 nm appears as the main peak for the films at 1200 and 1250 °C, while the 1528 nm main PL peak typical of ErxSc2-xSiO5 is observed for the films with low intensity. It is considered that the 1537 nm peak come from another Er silicate structure formed by the reaction of the Er–O and Si–O precursors with the SiO2 inter-layers. We consider that high temperature annealing at 1200 and 1250 °C for 1 h enhances the reaction of the Er–O and Si–O precursors with the SiO2, converting the most of ErxSc2-xSiO5 to ErxSc2-xSi2O7. The surface morphology of the films has observed by AFM, which revealed that the surface is still flat, smooth, and noncracking after annealing up to 1250 °C .
To determine the Stark energy levels of the ErxSc2-xSi2O7 phase, PLE and PL tests were conducted at 4 K in Fig. 5. Figure 5(a) shows a PLE color plot under 4I13/2 manifold excitation. The dashed lines in this figure indicate the assigned energy levels of the 4I13/2 manifold measured for ErxSc2-xSi2O7 in the polycrystalline phase. The PLE spectrum of the sample annealed at 1250 °C in Fig. 5(b) shows peaks at 1512, 1519, and 1529 nm corresponding to the direct absorption mechanism with an FWHM of 2, 2.5, and 2.25 nm, respectively. The spectral linewidths of the PL emissions are 1.1, 1.76, and 1.37 nm, which correspond respectively to the emission peaks at 1537, 1546, and 1551 nm.
From the collected spectra, we obtained the energy level schematic in Fig. 6 corresponding to transition between the 4I15/2 and 4I13/2 manifolds of Er in ErxSc2-xSi2O7 for the sample annealed at 1250 °C. The excited state 4I13/2 level of Er3+ is composed of four levels instead of seven levels, and the ground state 4I15/2 level is composed of two levels instead of eight levels. The six component peaks corresponding to Y2-Z1, Y3-Z3, Y1-Z1, Y3-Z1, Y1-Z2, and Y1-Z3 transitions are presented in Fig. 6.
These level energies of ErxSc2-xSi2O7 are different from Er:Sc2O3 and Er in scandium silicate [34, 35]. J. H. Shin et al. have shown a peak emission at 1529 nm with an FWHM of 11 nm for ErxY2-xSiO5 annealed at 1200 °C using an excitation wavelength of 488 nm [36, 37]. In addition, M. Miritello et al. obtained a peak emission at 1535 nm for α-(Yb1-xErx)2Si2O7 with a 37 nm FWHM using 532 nm excitation wavelength after annealing at 1200 °C . Furthermore, the energy levels were different from Er2O3 with strong luminescence from Y2-Z1 and Y1-Z1 transitions in C3i site observed at wavelengths of 1544.7 and 1548.2 nm .
The split of Stark levels are due to the smaller Sc3+ radius’s enhancing the crystal field strength, which affects the luminescence properties with a smaller FWHM compared to the effect of Y ions. However, Er can be substituted with Y ions in the silicate phase, but Sc has additional effect due to it is smaller radius compared to Er and Y.
where represents the non-zero crystal-filed parameters that contain the radially dependant parts (k and q) of the one-electron crystal-field. characterizes the interaction between ligands and the central ions and includes the radial integral of the wavefunction. The values of the crystal field strength NV have been calculated for Er3+ doped into different matrices Sc2O3 (5300 cm−1), Y2O3 (2700 cm−1), and Er2O3 (2200 cm−1) [40, 41], and they show clearly that the crystal-field strength increases regularly with decreasing ionic radius of the RE host cation, which generates the Stark-splitting energy levels. The presence of peaks at 1542, 1546, and 1551 nm is due to the presence of Sc ions, which increase the crystal field strength and thereby enhance the Stark-splitting of the thermally populated Er energy levels (4I15/2 and 4I13/2 levels) as well as that of the other electronic energy levels.
Figure 7 shows the variation of FWHM at 1537 nm from 4 K to room temperature of ErxSc2-xSi2O7. FWHM increases from 1.1 to 2.3 nm with increasing of temperature. The PL spectra are plotted as a function of temperature in the inset in Fig. 7. As can be seen, the Er3+ PL intensity becomes stronger at low temperature without shifting. The absence of shoulders peak emission for the main peak at 1537 nm like Er doped Si nanocluster that can be inducing larger inhomogeneous broadening of Er3+:4I13/2-4I15/2 transition .
The results of integrated PL intensity at 1537 nm as a function of excitation power are shown in Fig. 8. The PL intensity increases linearly and then exhibits a sublinear increase associated with the upconversion phenomena where two Er3+ ions both excited at the 4I13/2 level interact, with one being de-excited to the 4I15/2 ground state and the other being resonantly excited to the 4I9/2 level . At low pump powers, however, a nonradiative concentration quenching process may occur. In this case the excitation is resonantly transferred from one excited Er3+ ion to a nearby Er3+ ion in the ground state and hence it travels along the sample. In order to determine the excitation cross-section, it can be shown that the pump intensity dependence of the PL from erbium is given by the following Eq. (2) :
where IPLmax is the maximum PL intensity, h is Plank’s constant, c is the speed of light, pex is the pump power, and τ the lifetime at room temperature, which is equal to 12 μs fitted with single exponential for the excitation wavelength at λex = 1527.6 nm. Fitting the experimental data of this equation, an estimate of the excitation cross-section σex can be obtained. The value of σex is 3.0x10−20 cm2 at λex = 1527.6 nm. This is one order of magnitude higher than the σex of 1.4 10−21 cm2 at λex = 980 nm reported by us for the same structure . The difference between both values is due to the excitation wavelength (resonant excitation at 1527.6 nm) and the life time depending on excitation wavelength. In addition, σex at λex = 980 nm is on the same order as the excitation cross-section in α-(Y1-xErx)2Si2O7 of 2.0x10−21 cm2. This is means that Sc plays the same role as Y in the structure - an inactive element and acts as a disperser of Er ions – in addition, it presents a higher crystal field strength that enhances the Stark-splitting of Er energy levels. On the other hand, σex in ErxSc2-xSi2O7 (1.4 10−21 cm2) is smaller than that in α-(Yb1-xErx)2Si2O7, 2.0x10−20 cm2 at 980 nm, because of the energy transfer from Yb to Er ions .
The key advantage of ErxSc2-xSi2O7 is that present high excitation cross-section, small FWHM at 1537 nm, and high crystal field strength due to Sc ions that enhances the Stark-splitting of Er. ErxSc2-xSi2O7 compound is promising material by combining with photonic crystal structures to build light sources or on-chip optical amplifiers.
In summary, polycrystalline ErxSc2-xSiO5 and ErxSc2-xSi2O7 compounds were fabricated using RF-sputtering by alternating Er2O3, Sc2O3 layers separated by a SiO2 layer and annealing in O2 gas. We demonstrated that stoichiometric Er-Sc silicate films and crystalline structures can be controlled by changing the annealing temperature and we identified the stoichiometry. The Er-Sc silicate layer shows a sharp emission peak at room temperature centered at 1528 and 1537 nm corresponding respectively to ErxSc2-xSiO5 and ErxSc2-xSi2O7 phases. The PL efficiency in ErxSc2-xSi2O7 phase is due to the decrease of nonradiative transition rates.
The narrow FWHM of ErxSc2-xSi2O7 is due to the small ionic radii of Sc3+ which enhance the crystal field strength. The excitation cross-section is 3.0x10−20 cm2 at λex = 1527.6 nm. The narrow emission is very promising for photonic crystal light emitting devices because the extraction efficiency can be increased with a pronounced narrowing of the emission.
The synchrotron radiation experiments were performed at the BL24XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2013A3102 and 2013B3102). We thank Dr. Shingo Takeda for his help in the synchrotron radiation experiments at beam line BL24XU in SPring-8. This work was partially supported by JSPS KAKENHI Grant Number 24360033.
References and links
1. G. Roelkens, L. Liu, D. Liang, R. Jones, A. Fang, B. Koch, and J. Bowers, “III-V/silicon photonics for on-chip and inter-chip optical interconnects,” Laser Photonics Rev. 4(6), 751–779 (2010). [CrossRef]
2. Y. Kang, H. Liu, M. Morse, M. J. Paniccia, M. Zadka, S. Litski, G. Sarid, A. Pauchard, Y. H. Kuo, H. W. Chen, W. S. Zaoui, J. E. Bowers, A. Beling, D. C. McIntosh, X. Zheng, and J. C. Campbell, “Monolithic germanium/silicon avalanche photodiodes with 340 GHz gain–bandwidth product,” Nat. Photonics 3(1), 59–63 (2009). [CrossRef]
3. T. Yin, R. Cohen, M. M. Morse, G. Sarid, Y. Chetrit, D. Rubin, and M. J. Paniccia, “31 GHz Ge n-i-p waveguide photodetectors on Silicon-on-Insulator substrate,” Opt. Express 15(21), 13965–13971 (2007). [CrossRef] [PubMed]
4. A. P. Knights, J. D. B. Bradley, S. H. Gou, and P. E. Jessop, “Silicon-on-Insulator Waveguide Photo-detector with Self-ion Implantation Engineered Enhanced Infrared Response,” J. Vac. Sci. Technol. A 24(3), 783 (2006). [CrossRef]
6. A. Emboras, R. M. Briggs, A. Najar, S. Nambiar, C. Delacour, P. Grosse, E. Augendre, J. M. Fedeli, B. Salvo, H. A. Atwater, and R. Espiau de Lamaestre, “Efficient coupler between silicon photonic and metal-insulator-silicon-metal plasmonic waveguides,” Appl. Phys. Lett. 101(25), 251117 (2012). [CrossRef]
7. A. Emboras, A. Najar, S. Nambiar, P. Grosse, E. Augendre, C. Leroux, B. de Salvo, and R. E. de Lamaestre, “MNOS stack for reliable, low optical loss, Cu based CMOS plasmonic devices,” Opt. Express 20(13), 13612–13621 (2012). [CrossRef] [PubMed]
8. J. Liu, M. Beals, A. Pomerene, S. Bernardis, R. Sun, J. Cheng, L. C. Kimerling, and J. Michel, “Waveguide-integrated, ultralow-energy GeSi electro-absorption modulators,” Nat. Photonics 2(7), 433–437 (2008). [CrossRef]
9. H. Park, Y. H. Kuo, A. W. Fang, R. Jones, O. Cohen, M. J. Paniccia, and J. E. Bowers, “A hybrid AlGaInAs-silicon evanescent preamplifier and photodetector,” Opt. Express 15(21), 13539–13546 (2007). [CrossRef] [PubMed]
10. Y. Vlasov, W. M. J. Green, and F. Xia, “High-throughput silicon nanophotonic wavelengthinsensitive switch for on-chip optical networks,” Nat. Photonics 2(4), 242–246 (2008). [CrossRef]
11. D. Kovalev, H. Heckler, G. Polisski, and F. Koch, “Optical Properties of Si Nanocrystals,” Phys. Status Solidi 215(2), 871–932 (1999). [CrossRef]
13. A. W. Fang, H. Park, O. Cohen, R. Jones, M. J. Paniccia, and J. E. Bowers, “Electrically pumped hybrid AlGaInAs-silicon evanescent laser,” Opt. Express 14(20), 9203–9210 (2006). [CrossRef] [PubMed]
16. M. L. Brongersma, P. G. Kik, A. Polman, K. S. Min, and H. A. Atwater, “Size-dependent electron-hole exchange interaction in Si nanocrystals,” Appl. Phys. Lett. 76(3), 351–353 (2000). [CrossRef]
17. K. S. Cho, N.-M. Park, T.-Y. Kim, K.-H. Kim, G. Y. Sung, and J. H. Shin, “High efficiency visible electroluminescence from silicon nanocrystals embedded in silicon nitride using a transparent doping layer,” Appl. Phys. Lett. 86(7), 071909 (2005). [CrossRef]
18. S. Yerci, R. Li, S. O. Kucheyev, T. V. Buuren, S. N. Basu, and L. Dal Negro, “Energy transfer and 1.54 μm emission in amorphous silicon nitride films,” Appl. Phys. Lett. 95(3), 031107 (2009). [CrossRef]
19. A. Najar, J. Charrier, H. Ajlani, N. Lorrain, H. Elhouichet, M. Oueslati, and L. Hajim, “Optical properties of erbium-doped porous silicon waveguides,” J. Lumin. 121(2), 245–248 (2006). [CrossRef]
20. A. Najar, H. Ajlani, J. Charrier, N. Lorrain, S. Haesaert, M. Oueslati, and L. Haji, “Optical study of erbium-doped-porous silicon based planar waveguides,” Physica B 396(1), 145–149 (2007). [CrossRef]
21. A. Najar, N. Lorrain, H. Ajlani, J. Charrier, M. Oueslati, and L. Haji, “Er3+Doping conditions of planar porous silicon waveguides,” Appl. Surf. Sci. 256(3), 581–586 (2009). [CrossRef]
22. A. Najar, H. Elhouichet, N. Lorrain, and M. Oueslati, “Excitation mechanisms and localization sites of erbium-doped porous silicon,” Appl. Surf. Sci. 252(16), 5808–5813 (2006). [CrossRef]
23. A. J. Kenyon, M. Wojdak, I. Ahmad, W. H. Loh, and C. J. Oton, “Generalized rate-equation analysis of excitation exchange between silicon nanoclusters and erbium ions,” Phys. Rev. B 77(3), 035318 (2008). [CrossRef]
24. D. Pacifici, G. Franzò, F. Priolo, F. Iacona, and L. D. Negro, “Modeling and perspectives of the Si nanocrystals-Er interaction for optical amplification,” Phys. Rev. B 67(24), 245301 (2003). [CrossRef]
25. Y. Yin, K. Sun, W. J. Xu, G. Z. Ran, G. G. Qin, S. M. Wang, and C. Q. Wang, “1.53 µm photo- and electroluminescence from Er(3+) in erbium silicate,” J. Phys. Condens. Matter 21(1), 012204 (2009). [CrossRef] [PubMed]
26. X. J. Wang, T. Nakajima, H. Isshiki, and T. Kimura, “Fabrication and characterization of Er silicates on SiO2/Si substrates,” Appl. Phys. Lett. 95(4), 041906 (2009). [CrossRef]
27. H. Omi, Y. Abe, M. Anagnosti, and T. Tawara, “Mixture formation of ErxYb2-xSi2O7 and ErxYb2-xO3 on Si for broadening the C-band in an optical amplifier,” AIP Advances 3(4), 042107 (2013). [CrossRef]
28. X. J. Wang, G. Yuan, H. Isshiki, T. Kimura, and Z. Zhou, “Photoluminescence enhancement and high gain amplification of ErxY2−xSiO5 waveguide,” J. Appl. Phys. 108(1), 013506 (2010). [CrossRef]
29. R. Lo Savio, M. Miritello, A. Shakoor, P. Cardile, K. Welna, L. C. Andreani, D. Gerace, T. F. Krauss, L. O’Faolain, F. Priolo, and M. Galli, “Enhanced 1.54 μm emission in Y-Er disilicate thin films on silicon photonic crystal cavities,” Opt. Express 21(8), 10278–10288 (2013). [CrossRef] [PubMed]
30. L. Fornasiero, K. Petermann, E. Heumann, and G. Huber, “Spectroscopic properties and laser emission of Er3+ in scandium silicates near 1.5 µm,” Opt. Mater. 10(1), 9–17 (1998). [CrossRef]
31. C. R. Stanek, K. J. McClennan, B. P. Uberuaga, and K. E. Sickafus, “Determining the site preference of trivalent dopants in bixbyite sesquioxides by atomic-scale simulations,” Phys. Rev. B 75(13), 134101 (2007). [CrossRef]
33. K. Masaki, H. Isshiki, T. Kawaguchi, and T. Kimura, “The effect of annealing conditions on the crystallization of Er–Si–O formed by solid phase reaction,” Opt. Mater. 28(6-7), 831–835 (2006). [CrossRef]
34. M. Miritello, P. Cardile, R. Lo Savio, and F. Priolo, “Energy transfer and enhanced 1.54 μm emission in Erbium-Ytterbium disilicate thin films,” Opt. Express 19(21), 20761–20772 (2011). [CrossRef] [PubMed]
35. N. Ter-Gabrielyan, V. Fromzel, and M. Dubinskii, “Performance analysis of the ultra-low quantum defect Er3+:Sc2O3 laser,” Opt. Mater. Express 1(3), 503–513 (2011). [CrossRef]
36. J. H. Shin and M. Lee, “Reducing optical losses and energy-transfer upconversion in ErxY2-xSiO5 waveguides,” IEEE Photon. Technol. Lett. 25(18), 1801–1804 (2013). [CrossRef]
37. K. Suh, M. Lee, J. S. Chang, H. Lee, N. Park, G. Y. Sung, and J. H. Shin, “Cooperative upconversion and optical gain in ion-beam sputter-deposited ErxY2-xSiO5 waveguides,” Opt. Express 18(8), 7724–7731 (2010). [CrossRef] [PubMed]
38. T. Tawara, H. Omi, T. Hozumi, R. Kaji, S. Adachi, H. Gotoh, and T. Sogawa, “Population dynamics in epitaxial Er2O3 thin films grown on Si(111),” Appl. Phys. Lett. 102(24), 241918 (2013). [CrossRef]
39. F. Auzel and O. Malta, “A scalar crystal field strength parameter for rare-earth ions: meaning and usefulness,” J. Phys. 44(2), 201–206 (1983). [CrossRef]
40. E. Antic-Fidancev, J. Holsa, and M. Lastusaari, “Crystal field strength in C-type cubic rare earth oxides,” J. Alloy. Comp. 341(1-2), 82–86 (2002). [CrossRef]
41. I. Trabelsi, R. Maâlej, M. Dammak, A. Lupei, and M. Kamoun, “Crystal field analysis of Er3+ in Sc2O3 transparent ceramics,” J. Lumin. 130(6), 927–931 (2010). [CrossRef]
42. N. Daldosso, D. N. Urrios, M. Melchiorri, C. Garcia, P. Pellegrino, B. Garrido, C. Sada, G. Battaglin, F. Gourbilleau, R. Rizk, and L. Pavesi, “Er-coupled Si nanocluster waveguide,” IEEE J. Sel. Top. Quantum Electron. 12(6), 1607–1617 (2006). [CrossRef]
44. A. Najar, T. Tawara, and H. Omi, “Structural characterization and luminescence properties of ErxSc2-xSi2O7 prepared by RF sputtering,” in CLEO: Application and technology, paper JW2A. 61 (2014). [CrossRef]