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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 23 — Nov. 18, 2013
  • pp: 28394–28402
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Efficient 1535 nm light emission from an all-Si-based optical micro-cavity containing Er3+ and Yb3+ ions

I. B. Gallo, A. Braud, and A. R. Zanatta  »View Author Affiliations


Optics Express, Vol. 21, Issue 23, pp. 28394-28402 (2013)
http://dx.doi.org/10.1364/OE.21.028394


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Abstract

This work reports on the construction and spectroscopic analyses of optical micro-cavities (OMCs) that efficiently emit at ~1535 nm. The emission wavelength matches the third transmission window of commercial optical fibers and the OMCs were entirely based on silicon. The sputtering deposition method was adopted in the preparation of the OMCs, which comprised two Bragg reflectors and one spacer layer made of either Er- or ErYb-doped amorphous silicon nitride. The luminescence signal extracted from the OMCs originated from the 4I13/24I15/2 transition (due to Er3+ ions) and its intensity showed to be highly dependent on the presence of Yb3+ ions. According to the results, the Er3+-related light emission was improved by a factor of 48 when combined with Yb3+ ions and inserted in the spacer layer of the OMC. The results also showed the effectiveness of the present experimental approach in producing Si-based light-emitting structures in which the main characteristics are: (a) compatibility with the actual micro-electronics industry, (b) the deposition of optical quality layers with accurate composition control, and (c) no need of uncommon elements-compounds nor extensive thermal treatments. Along with the fundamental characteristics of the OMCs, this work also discusses the impact of the Er3+−Yb3+ ion interaction on the emission intensity as well as the potential of the present findings.

© 2013 Optical Society of America

1. Introduction

Driven by their great technological potential, optical micro-cavities OMCs (or resonators) have been the subject of intensive research. Nowadays, OMCs can be found in various applications (lasers, LEDs, sensors, etc.) and there is room for much more by either improving their efficiencies or exploring new phenomena. The simplest OMC, a Fabry-Perot-like structure, involves one spacer layer and two reflectors [1

1. E. F. Schubert, Light-Emitting Diodes (Cambridge University, 2006).

]. Following this configuration, the OMC structure not only is simple but extremely flexible in the sense that it allows to tailor its resonance wavelength, spectral line-width, and optical gain. The effective control of these characteristics, however, depends on the preparation method and properties of the materials that constitute the OMC. The choice of the most appropriate method-material for the realization of OMCs should also consider their final cost, complexity, and compatibility to existing technologies. Within this context, the use of OMCs represents a crucial step towards the achievement of all-Si-based hybrid optical-electronic devices.

2. Experimental details

All samples investigated in this work were prepared by the radio frequency (13.56 MHz) sputtering method. The preparation involved the sequential deposition of a-Si and a-SiN layers − on different substrates − by using a 5 inches diameter Si target and a plasma of either argon or nitrogen. The planar OMCs consisted of one spacer layer (a-SiN film doped with Er or with Er + Yb) sandwiched in-between two identical Bragg reflectors [Fig. 1(a)
Fig. 1 (a) Schematic representation of the OMC formed by one spacer layer (Er- or ErYb-doped a-SiN film) between two Bragg reflectors (3 pairs of alternated a-Si/a-SiN layers). (b) Cross section view (SEM-FEG image) of the OMC-Er deposited on a crystalline Si substrate.
]. Each reflector, which comprised 3 pairs of alternated a-Si and a-SiN layers, was designed to be highly reflective in the ~1200−1600 nm range. Therefore, the thickness (t) and index of refraction (n at ~1535 nm) of the a-Si and a-SiN layers were: ta-Si = 95 nm, na-Si = 3.62, ta-SiN = 200 nm, and na-SiN = 1.94. For the a-SiN spacer layer, and in order to create a transmission window around 1535 nm, the thickness was 400 nm. Further information concerning the construction and main characteristics of the OMCs can be found elsewhere [10

10. I. B. Gallo and A. R. Zanatta, “A simple-versatile approach to achieve all-Si-based optical micro-cavities,” J. Appl. Phys. 113(8), 083106 (2013). [CrossRef]

].

The doping of the OMC spacer layers was achieved by partially covering the Si target with suitable chips of metallic Er and Yb. The process is known as cosputtering and allows the production of materials with chemical compositions determined by the relative areas and sputtering yields of the elements forming the target [11

11. B. Chapman, Glow Discharge Processes: Sputtering and Plasma Etching (Wiley, 1980).

]. Following this approach, the OMCs containing the spacer layer doped with Er (OMC-Er) and with Er + Yb (OMC-ErYb) were prepared by covering the Si target with 6 cm2 of Er, and with 6 cm2 of Er combined to 6 cm2 of Yb. For comparison purposes, Er- and ErYb-doped a-SiN films were also prepared with the same composition and thickness of the OMC spacer layers.

Consistent with the deposition method and conditions, all layers were amorphous as shown by Raman scattering spectroscopy [10

10. I. B. Gallo and A. R. Zanatta, “A simple-versatile approach to achieve all-Si-based optical micro-cavities,” J. Appl. Phys. 113(8), 083106 (2013). [CrossRef]

]. The morphological aspects of the OMCs were also probed by means of scanning electron microscopy imaging (SEM-FEG) and the results confirmed the existence of a regular array of a-Si/a-SiN layers separated by well-defined interfaces [Fig. 1(b)]. The atomic composition of the samples was investigated by energy dispersive x-ray (EDX) measurements. The results indicate that the a-SiN films are almost stoichiometric ([N] ~57 at.%), homogenous, and that both Er and Yb concentrations were at the nominal doping level (clearly above the detection limits of the technique). The optical properties were investigated through transmission and reflection measurements (Perkin-Elmer λ950). Photoluminescence (PL) and photoluminescence excitation (PLE) experiments were carried out by exciting the samples with either argon ion (488.0 nm) or titanium-sapphire (900−1000 nm) laser sources with the resulting radiation being detected by either Ge or InGaAs detectors. All measurements (transmission−reflection−photoluminescence) were conducted at room temperature.

3. Experimental results

Figure 2
Fig. 2 EDX survey spectra of the pure and ErYb-doped a-SiN films illustrating their main components (x-ray transitions). The copper contribution comes from the substrate. The inset is an expanded view of the EDX spectra centered at ~1.5 keV and denotes the Lβ x-ray transitions due to the presence of Er and Yb in the Er-, Yb-, and ErYb-doped a-SiN films.
shows the EDX spectra of the pure and ErYb-doped a-SiN films. Because of experimental constraints, mainly those involving charging effects, the spectra were recorded from films deposited on copper substrates. Despite the use of different substrates (copper instead of fused silica), both the doped films and the spacer layers within the OMCs were prepared during the same deposition runs and should be identical.

According to the EDX analyses, whereas the pure a-SiN film was constituted by silicon, nitrogen, and traces of oxygen (< 0.2 at.%, which can also be due to the substrate), the ErYb-doped a-SiN film also presented uniformly distributed erbium (0.4 ± 0.1 at.%) and ytterbium (0.4 ± 0.1 at.%) components. In fact, similar concentration values were obtained from a-SiN films doped with only Er or Yb (inset of Fig. 2). These numbers are consistent with the deposition method and chosen conditions and, most importantly, demonstrate the suitability of the present experimental approach in producing rather uniform RE-doped OMCs in a very controllable and reproducible manner.

The transmission windows λT's at 1510 nm [Fig. 3(a)] and 1540 nm [Fig. 3(b)] result from the structure adopted for the OMCs (λT = 2 tspacer nspacer [14

14. M. A. MacLeod, Thin-Film Optical Filters (Institute of Physics, 2001).

]) and were intentionally designed to match the 4I13/24I15/2 optical transition [15

15. G. Dieke, Spectra and Energy Levels of Rare-Earth Ions in Crystals (Wiley Interscience, 1968).

] of the Er3+ ions present in the a-SiN spacer layers. The coincidence between λT and the 4I13/24I15/2 transition is illustrated in Fig. 3 by means of the optical transmission profiles of the OMCs and PL spectra of the Er- and ErYb-doped a-SiN films. In order to be comparable, the doped films were identical (composition, thickness, thermal treatment) to the Er- and ErYb-doped spacer layers present in the OMCs.

Whereas 488.0 nm photons can effectively (and quasi-resonantly [5

5. A. R. Zanatta and L. Nunes, “Green photoluminescence from Er-containing amorphous SiN thin films,” Appl. Phys. Lett. 72(24), 3127–3129 (1998). [CrossRef]

]) excite the Er3+ ions in the a-SiN films (Fig. 3), this is not the case for the OMCs. In fact, considering the distinctive optical transmission of the OMCs, the only possibility is to excite within the ~800−1100 nm range − ideally by using photons with wavelengths λexc that coincide with the energy levels of either Er3+ or Yb3+ ions. The most appropriate λexc's were found by means of PLE measurements by observing the 4I13/24I15/2 transition at ~1535 nm. These results are shown in Fig. 4
Fig. 4 Photoluminescence excitation spectra (with light detection at ~1535 nm) of OMCs containing a-SiN spacer layers doped with Er (OMC-Er) and Er + Yb (OMC-ErYb). The spectra show that the most intense emission occurs by exciting the OMC-Er and OMC-ErYb with 964 and 982 nm photons, respectively. The inset shows the 4I13/24I15/2 PL transition due to the Er3+ ions present in the OMC-Er, whose intensities have been indicated in the main PLE spectrum (colored dots).
, from which it is clear that the most appropriate λexc's take place at ~964 and ~982 nm for the OMCs containing Er and Er + Yb, respectively. Despite such small difference, which is associated with the energy levels due to Er3+ (4I11/2) and Yb3+ (2F5/2) ions when inserted in the a-SiN matrix, it is clear from the spectra of Fig. 4 that the use of non-resonant λexc's can reduce the light emission intensity by, at least, 25%.

4. Discussion

The optical transmission (or reflection) properties of any OMC is determined essentially by two factors: its design characteristics (Bragg reflectors and spacer layers), and by the differences in the optical path the photons experience as they cross the OMC. When a photon hits a single layer system (index of refraction n and thickness t), for example, the effective phase thickness δ varies with the angle of incidence such that δ = (2π nt cos θ)/λ [14

14. M. A. MacLeod, Thin-Film Optical Filters (Institute of Physics, 2001).

]. As a consequence, the increase of the angle of incidence induces a blue-shift of the resonance wavelength in order to keep δ constant. At this point, it is important to remark that a similar reasoning applies for the departing photons. In the specific case of an optical micro-cavity, the presence of various layers requires the use of an effective index of refraction, defined by neff = 2na-Sina-SiN/(na-Si + na-SiN), that properly describes the whole OMC structure. The “new” (tilted) resonant wavelengths λtilt can be expressed by [1

1. E. F. Schubert, Light-Emitting Diodes (Cambridge University, 2006).

]:
λtilt=λ0cos(θ/neff),
(1)
where λ0 stands for the original (non-tilted) resonance wavelength and neff = 2.44 [10

10. I. B. Gallo and A. R. Zanatta, “A simple-versatile approach to achieve all-Si-based optical micro-cavities,” J. Appl. Phys. 113(8), 083106 (2013). [CrossRef]

]. The theoretical resonance wavelengths, as provided by Eq. (1), are presented in Fig. 7. As can be seen, the agreement between the experimental and theoretical resonance wavelengths is almost perfect for the OMC-ErYb structure. In the case of the OMC-Er, on the contrary, the experimental results are ~12 nm smaller than the expected ones. Such a difference occurred because of the mismatch between the Er3+ transition and the center of the transmission window of the OMC-Er structure [Fig. 6(a)]. Actually, the experimental results indicate that only a small portion of the Er3+ PL signal (short-wavelength side) was enhanced by the influence of the micro-cavity, suggesting that slightly higher PL intensities could be achieved in the case of the OMC-Er. Therefore, along with the efficiency of the excitation process and wave-guiding effects, the mismatch between the light emission and transmission wavelengths can also explain the decrease of the PL intensity.

4. Conclusions

In summary, the present work reports on the construction and properties of optical micro-cavities OMCs entirely based on silicon thin films. The OMCs were formed by one spacer layer (either Er- or ErYb-doped a-SiN films) that was placed between two identical Bragg reflectors (three pairs of alternate a-Si/a-SiN films). The whole OMC structure was prepared by sputtering and was intended to present a transmission window at approx. 1535 nm. Er-, Yb-, and ErYb-doped a-SiN thin films were also prepared, following exactly the same deposition conditions, for comparison purposes. The details involving the construction, atomic composition, and optical characterization of the OMC were presented to some extent. Also, the basics of the OMCs and the mechanisms behind the Yb3+-to-Er3+ energy transfer were conveniently addressed.

Finally, the present experimental approach and results suggest the feasibility of producing efficient light-emitting materials (or structures) entirely based on Si and rare-earth ions. Furthermore, a proper combination of different Si compounds and (groups of) rare-earth ions is expected to generate customized-tunable light sources at wavelengths ranging from the ultraviolet to the infrared regions.

Acknowledgments

The authors acknowledge the financial support received from CAPES-COFECUB, CNPq and FAPESP (Brazil).

References and links

1.

E. F. Schubert, Light-Emitting Diodes (Cambridge University, 2006).

2.

S. M. Sze, Semiconductor Devices - Physics and Technology (John Wiley, 1985).

3.

B. Jalali and S. Fathpour, “Silicon photonics,” J. Lightwave Technol. 24(12), 4600–4615 (2006). [CrossRef]

4.

H. Ennen, G. Pomrenke, A. Axmann, K. Eisele, W. Haydl, and J. Schneider, “1.54-μm electroluminescence of erbium-doped silicon grown by molecular beam epitaxy,” Appl. Phys. Lett. 46(4), 381–383 (1985). [CrossRef]

5.

A. R. Zanatta and L. Nunes, “Green photoluminescence from Er-containing amorphous SiN thin films,” Appl. Phys. Lett. 72(24), 3127–3129 (1998). [CrossRef]

6.

A. J. Kenyon, “Recent developments in rare-earth doped materials for optoelectronics,” Prog. Quantum Electron. 26(4–5), 225–284 (2002). [CrossRef]

7.

B. S. Richards, “Luminescent layers for enhanced silicon solar cell performance: Down-conversion,” Sol. Energy Mater. Sol. Cells 90(9), 1189–1207 (2006). [CrossRef]

8.

B. S. Richards, “Enhancing the performance of silicon solar cells via the application of passive luminescence conversion layers,” Sol. Energy Mater. Sol. Cells 90(15), 2329–2337 (2006). [CrossRef]

9.

A. R. Zanatta, “Photoluminescence quenching in Er-doped compounds,” Appl. Phys. Lett. 82(9), 1395–1397 (2003). [CrossRef]

10.

I. B. Gallo and A. R. Zanatta, “A simple-versatile approach to achieve all-Si-based optical micro-cavities,” J. Appl. Phys. 113(8), 083106 (2013). [CrossRef]

11.

B. Chapman, Glow Discharge Processes: Sputtering and Plasma Etching (Wiley, 1980).

12.

A. R. Zanatta and F. L. Freire Jr., “Optical study of thermally annealed Er-doped hydrogenated a-Si films,” Phys. Rev. B 62(3), 2016–2020 (2000). [CrossRef]

13.

A. R. Zanatta, “Visible light emission and energy transfer process in Sm-doped nitride films,” J. Appl. Phys. 111(12), 123105 (2012). [CrossRef]

14.

M. A. MacLeod, Thin-Film Optical Filters (Institute of Physics, 2001).

15.

G. Dieke, Spectra and Energy Levels of Rare-Earth Ions in Crystals (Wiley Interscience, 1968).

16.

C. Strohhöfer and A. Polman, “Relationship between gain and Yb3+ concentration in Er3+–Yb3+ doped waveguide amplifiers,” J. Appl. Phys. 90(9), 4314–4320 (2001). [CrossRef]

17.

C. T. M. Ribeiro, A. R. Zanatta, L. Nunes, Y. Messaddeq, and M. Aegerter, “Optical spectroscopy of Er3+ and Yb3+ co-doped fluoroindate glasses,” J. Appl. Phys. 83(4), 2256–2260 (1998). [CrossRef]

18.

K. S. Repasky, L. E. Watson, and J. L. Carlsten, “High-finesse interferometers,” Appl. Opt. 34(15), 2615–2618 (1995). [CrossRef] [PubMed]

19.

M. Grün, P. Miska, X. Devaux, H. Rinnert, and M. Vergnat, “Optical properties of a silicon-nanocrystal-based-microcavity prepared by evaporation,” Opt. Mater. 33(8), 1248–1251 (2011). [CrossRef]

20.

Y. G. Li and R. M. Almeida, “Simultaneous broadening and enhancement of the 1.5 μm photoluminescence peak of Er3+ ions embedded in a 1-D photonic crystal microcavity,” Appl. Phys. B 98(4), 809–814 (2010). [CrossRef]

OCIS Codes
(160.5690) Materials : Rare-earth-doped materials
(140.3945) Lasers and laser optics : Microcavities

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: September 27, 2013
Revised Manuscript: October 31, 2013
Manuscript Accepted: October 31, 2013
Published: November 11, 2013

Citation
I. B. Gallo, A. Braud, and A. R. Zanatta, "Efficient 1535 nm light emission from an all-Si-based optical micro-cavity containing Er3+ and Yb3+ ions," Opt. Express 21, 28394-28402 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-23-28394


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References

  1. E. F. Schubert, Light-Emitting Diodes (Cambridge University, 2006).
  2. S. M. Sze, Semiconductor Devices - Physics and Technology (John Wiley, 1985).
  3. B. Jalali and S. Fathpour, “Silicon photonics,” J. Lightwave Technol.24(12), 4600–4615 (2006). [CrossRef]
  4. H. Ennen, G. Pomrenke, A. Axmann, K. Eisele, W. Haydl, and J. Schneider, “1.54-μm electroluminescence of erbium-doped silicon grown by molecular beam epitaxy,” Appl. Phys. Lett.46(4), 381–383 (1985). [CrossRef]
  5. A. R. Zanatta and L. Nunes, “Green photoluminescence from Er-containing amorphous SiN thin films,” Appl. Phys. Lett.72(24), 3127–3129 (1998). [CrossRef]
  6. A. J. Kenyon, “Recent developments in rare-earth doped materials for optoelectronics,” Prog. Quantum Electron.26(4–5), 225–284 (2002). [CrossRef]
  7. B. S. Richards, “Luminescent layers for enhanced silicon solar cell performance: Down-conversion,” Sol. Energy Mater. Sol. Cells90(9), 1189–1207 (2006). [CrossRef]
  8. B. S. Richards, “Enhancing the performance of silicon solar cells via the application of passive luminescence conversion layers,” Sol. Energy Mater. Sol. Cells90(15), 2329–2337 (2006). [CrossRef]
  9. A. R. Zanatta, “Photoluminescence quenching in Er-doped compounds,” Appl. Phys. Lett.82(9), 1395–1397 (2003). [CrossRef]
  10. I. B. Gallo and A. R. Zanatta, “A simple-versatile approach to achieve all-Si-based optical micro-cavities,” J. Appl. Phys.113(8), 083106 (2013). [CrossRef]
  11. B. Chapman, Glow Discharge Processes: Sputtering and Plasma Etching (Wiley, 1980).
  12. A. R. Zanatta and F. L. Freire., “Optical study of thermally annealed Er-doped hydrogenated a-Si films,” Phys. Rev. B62(3), 2016–2020 (2000). [CrossRef]
  13. A. R. Zanatta, “Visible light emission and energy transfer process in Sm-doped nitride films,” J. Appl. Phys.111(12), 123105 (2012). [CrossRef]
  14. M. A. MacLeod, Thin-Film Optical Filters (Institute of Physics, 2001).
  15. G. Dieke, Spectra and Energy Levels of Rare-Earth Ions in Crystals (Wiley Interscience, 1968).
  16. C. Strohhöfer and A. Polman, “Relationship between gain and Yb3+ concentration in Er3+–Yb3+ doped waveguide amplifiers,” J. Appl. Phys.90(9), 4314–4320 (2001). [CrossRef]
  17. C. T. M. Ribeiro, A. R. Zanatta, L. Nunes, Y. Messaddeq, and M. Aegerter, “Optical spectroscopy of Er3+ and Yb3+ co-doped fluoroindate glasses,” J. Appl. Phys.83(4), 2256–2260 (1998). [CrossRef]
  18. K. S. Repasky, L. E. Watson, and J. L. Carlsten, “High-finesse interferometers,” Appl. Opt.34(15), 2615–2618 (1995). [CrossRef] [PubMed]
  19. M. Grün, P. Miska, X. Devaux, H. Rinnert, and M. Vergnat, “Optical properties of a silicon-nanocrystal-based-microcavity prepared by evaporation,” Opt. Mater.33(8), 1248–1251 (2011). [CrossRef]
  20. Y. G. Li and R. M. Almeida, “Simultaneous broadening and enhancement of the 1.5 μm photoluminescence peak of Er3+ ions embedded in a 1-D photonic crystal microcavity,” Appl. Phys. B98(4), 809–814 (2010). [CrossRef]

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