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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 16, Iss. 12 — Jun. 9, 2008
  • pp: 8896–8901
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Influence of substrates in ZnO devices on the surface plasmon enhanced light emission

Peihong Cheng, Dongsheng Li, and Deren Yang  »View Author Affiliations


Optics Express, Vol. 16, Issue 12, pp. 8896-8901 (2008)
http://dx.doi.org/10.1364/OE.16.008896


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Abstract

The substrates in emitting structure were found to have an influence on the surface plasmon mediated light emission of ZnO films. Ag film mediated photoluminescence was quenched for ZnO on silicon substrate but enhanced for ZnO on quartz or sapphire substrate. Through a theoretical simulation, the quenching for ZnO on silicon substrate is ascribed to the power lost to the substrate mode nonradiatively at the expense of the power coupled to the SP mode. The substrate with a high refractive index may capture and dissipate the emitting power which limits the efficiency of SP mediated light extraction. Therefore, a proper arrangement of the refractive index of the substrate and emitting layers in the device structure is decisive for the SP coupled light emission enhancement. Base on the theoretical analysis, a four-layered structure was advanced to recover SP mediated emission enhancement from ZnO film on silicon substrate.

© 2008 Optical Society of America

1. Introduction

It is well-known that an electric dipole placed in the proximity of metal surface may transfer energy to surface plasmon (SP), resulting in modification of its spontaneous emission decay rate [1

1. R. R. Chance, A. Prock, and R. Silbey, “Lifetime of an emitting molecule near a partially reflecting surface,” J.Chem. Phys. 60, 2744–2748 (1974). [CrossRef]

]. This effect has been used to enhance the photoluminescence (PL) of light emitters, further aiming to raise the emission efficiency of light emission diodes (LEDs) [2–6

2. K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, and Axel Scherer,“Surface-plasmon-enhanced light emitters based on InGaN quantum wells,” Nat. Mater. 3, 601–605 (2004). [CrossRef] [PubMed]

]. From the published reports [2–4

2. K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, and Axel Scherer,“Surface-plasmon-enhanced light emitters based on InGaN quantum wells,” Nat. Mater. 3, 601–605 (2004). [CrossRef] [PubMed]

], the morphology of metal films is found absolutely important for SP mediated emission enhancement, because after radiation energy is transferred to SP, only by scattering which bridges the momentum gap, the coupled SP energy can be transferred to free space radiation, otherwise it is only dissipated, inducing quenching of the emission. Thus, metal films with periodic grating or random corrugation was usually adopted to realize light emission enhancement.

In addition to the morphology of metal films, other factors such as the separation distance between metal films and light emitters and the energy matching between SP mode and emission band, are also found to be influential for SP coupled emission enhancement [5–7

5. J. S. Biteen, D. Pacifici, N. S. Lewis, and H. A. Atwater, “Enhance radiative emission rate and quantum efficiency in coupled silicon nanocrystal-nanostructured gold emitters,” Nano Lett. 5, 1768–1773 (2005). [CrossRef] [PubMed]

]. Whereas, there are few reports about the influence of emitting device structures on the SP coupled light emission. As we known, much of the power generated in light emitters is lost in LEDs before it is extracted to free space emission. Coupling with SP and then effective scattering radiatively is a promising way to recover part of the dissipated power, especially in top-emitting LED [8

8. S. Wedge and W. L. Barnes, “Surface plasmon-polariton mediated light emission through thin metal films,” Opt. Express. 12, 3673–3685 (2004). [CrossRef] [PubMed]

]. However, other power dissipating channels existing in device structures may compete with the beneficial SP coupling, resulting in the failure of SP mediated light extraction through the metal films. So the investigation on the dependence of the SP coupling on emitting structures is indispensable for designing high efficiency SP enhanced LEDs. In this paper, the emphasis was put on the influence of substrates in emitting devices on the SP coupled emission. It was found that the SP mediated emission from ZnO film based on silicon substrate was quenched, but that from ZnO based on quartz or sapphire substrate were enhanced. It is believe that the competition between the power transferred to the substrate mode or to the SP mode resulted in the emission enhancement or quenching.

2. Sample fabrication and measurement

The fabrication started from the ZnO film (40 nm thick) deposition on three types of substrates (quartz, sapphire and Si) by reactive direct current sputtering. Then the films were annealed in O2 at 800 °C for 2 hours. Subsequently, Ag films were sputtered onto the ZnO films at 200 °C with a deposition time of 60s. The three samples are referred as quartz/ZnO/Ag, sapphire/ZnO/Ag and Si/ZnO/Ag, respectively. Before Ag sputtering, half of each sample was covered to work as the reference samples for comparing the PL intensity. The schematics of the Si/ZnO/Ag sample structure is illustrated in Fig. 1(a). The PL measurements were performed with a He-Cd laser excitation at 325nm with an incidence angle of 45°, and detected by a spectrometer (Acton SP2500i) on the Ag film side. Cross-section scanning electron microscopy (SEM) images of the samples were obtained from a HITACHI S-4800 microscope.

Figure 1(b) shows the cross-section SEM image of the quartz/ZnO/Ag structure. The Ag film thickness was about 40nm. It can be seen that Ag island microstructures were formed, which ensures that the energy coupled to the SP can be effectively scattered and transferred into radiative emission [9

9. W. L. Barnes and P. T. Worthing, “Spontaneous emission and metal-clad microcavities,” Opt. Commun. 162, 16–20 (1999). [CrossRef]

, 10

10. C.-Y. Chen, D.-M. Yeh, Y.-C. Lu, and C. C. Yang, “Dependence of resonant coupling between surface plasmons and an InGaN quantum well on metallic structure,” Appl. Phys. Lett. 89, 203113-1-3 (2006). [CrossRef]

]. Figure 1(c) illustrates the schematics of the structure of the four-layered sample (Si/SiO2/ZnO/Ag). In this sample, before the ZnO film and Ag film sputtering, the SiO2 layer with a thickness of 200nm was first formed on silicon substrate by thermal oxidation. A reference sample without Ag sputtering was also prepared for PL comparison.

Fig. 1. (a) Schematics of the Si/ZnO/Ag sample structure; (b) Cross-section SEM image of the sample quartz/ZnO/Ag; (c) Schematics of the structure of the four-layer sample Si/SiO2/ZnO/Ag.

3. Results and discussion

Figure 2 shows the PL spectra of the Ag sputtered ZnO films on three substrates and their respective reference samples. It can be seen that the PL peak occurs at about 380nm for the three samples, which is the band edge emission of ZnO films. The PL intensity is about 3 times higher for the sample quartz/ZnO/Ag and sapphire/ZnO/Ag than for the respective reference samples without Ag sputtering. However, for the sample Si/ZnO/Ag, the PL intensity was decreased after Ag sputtering.

Fig. 2. Photoluminescence of (a) quartz/ZnO/Ag, (b)sapphire/ZnO/Ag, and (c) Si/ZnO/Ag; solid line: before Ag sputtering, broken line: after Ag sputtering.

As we known, the decay rates of a light emitter can be modified when it is situated near an interface or in a microcavity due to the modification of the photonic mode density (PMD) [11

11. R. R. Chance, A. Prock, and R. Silbey, “Comments on the classical theory of energy transfer,” J. Chem. Phys. 62, 2245–2253 (1975). [CrossRef]

]. In our experiment, the samples have a three-layered structure in which the ZnO film is situated between Ag layer and substrate. Therefore, the de-excited energy from the dipoles in ZnO may couple to the waveguide mode in the substrate and SPP mode at the ZnO/Ag interface. The coupled energy can be lost nonradiatively due to absorption in substrate waveguide and SPP absorption. In those cases, the radiative decay rate will be decreased and induces the quenching of PL. However, the SP coupled energy can also be scattered and reemitted into effective free space radiation. In this case, due to the high PMD associated with the SPP mode, the radiative decay rate of ZnO can be raised greatly, leading to the PL enhancement [11

11. R. R. Chance, A. Prock, and R. Silbey, “Comments on the classical theory of energy transfer,” J. Chem. Phys. 62, 2245–2253 (1975). [CrossRef]

]. So the PL intensity enhancement or quenching is mainly determined by three factors: power lost to the substrate waveguide mode, power coupled to the SP at ZnO/Ag interface, and the out-coupling efficiency from SP to free space emission. The third factor, i.e., out-coupling efficiency between SP and radiative emission, is mainly determined by the thickness and morphology of Ag layer [8

8. S. Wedge and W. L. Barnes, “Surface plasmon-polariton mediated light emission through thin metal films,” Opt. Express. 12, 3673–3685 (2004). [CrossRef] [PubMed]

]. In our experiment, the Ag layers for the three samples were same, due to the same preparation condition. Furthermore, the surface roughness of ZnO on each substrate is small, so the influence from the interface between ZnO and Ag layer can be neglected. Thus, the PL enhancement or quenching for the three samples is mainly determined by the first two factors: power dissipated to the substrate mode and SP mode.

Using a well-established theory of the dipole radiation in multi-layered media [11

11. R. R. Chance, A. Prock, and R. Silbey, “Comments on the classical theory of energy transfer,” J. Chem. Phys. 62, 2245–2253 (1975). [CrossRef]

, 12

12. K. G. Sullivan and D. G. Hall, “Enhancement and inhibition of electromagnetic radiation in plane-layered media,” J. Opt. Soc. Am. B 14, 1149–1159 (1997). [CrossRef]

], we simulated how much power radiated from ZnO is coupled to substrate and SP modes in the three-layered structure. A convenient approach adopted in this theory was treating light emitters as source radiating dipoles. The reflections by the interfaces between the different materials that make up the multilayer structure act back on the source dipoles, and the effect of these reflections has on the power dissipated by the dipole was calculated. The obtained dissipated power as a function of the in-plane wave vectors is a measure of power lost to each mode in the layered structure [11

11. R. R. Chance, A. Prock, and R. Silbey, “Comments on the classical theory of energy transfer,” J. Chem. Phys. 62, 2245–2253 (1975). [CrossRef]

, 12

12. K. G. Sullivan and D. G. Hall, “Enhancement and inhibition of electromagnetic radiation in plane-layered media,” J. Opt. Soc. Am. B 14, 1149–1159 (1997). [CrossRef]

]. In our computation, the ZnO film is assumed as isotropic oriented dipoles situated in the median plane of the ZnO layer. The obtained power dissipation spectrum S (u) as a function of in-plane wavevector u is showed in Fig.3 (a). The refractive indexes of the layers at 380nm used in the calculation are 1.47, 1.76, 6.06+0.63i and 0.3+3.1i for quartz, sapphire, silicon and Ag island films respectively [13

13. S. Kawabata, K. Ishihara, Y. Hoshi, and T. Fukazawa, “Observation of silver film growth using an in situ ultra-high vacuum spectroscopic ellipsometer,” Thin Solid Film 313–314, 516–521 (1998). [CrossRef]

, 14

14. Edward D. Palik, Handbook of optical constant of solid (Academic, 1985)

].

Fig. 3. (a) Calculated power spectra versus the normalized transverse wavenumber of the quartz/ZnO/Ag, sapphire/ZnO/Ag and Si/ZnO/Ag; (b) Calculated power spectra versus the normalized transverse wavenumber for substrate/ZnO/Ag, The refractive index of the substrate varies from 2 to 6.

Due to the lower refractive index for quartz and sapphire comparing to that of ZnO, no substrate waveguide mode was expected in the power spectra in Fig. 3(a). However, it was also not observed in the power spectrum of the Si/ZnO/Ag. We consider that it is probably due to the influence from the relative large imaginary part of the complex refractive index of silicon substrate which will dissipate most of the substrate mode power by absorption. Then the substrate waveguide mode may be broadened and screened in the spectrum [18

18. J. Kalkman, H. Gersen, L. Kuipers, and A. Polman, “Excitation of surface plasmons at SiO2/Ag interface by silicon quantum dots:experiment and theory,” Phys. Rev. B 73, 075317-1-8 (2006). [CrossRef]

].

Fig. 4. (a) Calculated power spectrum versus the normalized transverse wavenumber of quartz/ZnO/Ag, sapphire/ZnO/Ag, and Si/SiO2/ZnO/Ag; (b) Photoluminescence of Si/SiO2/ZnO/Ag and reference sample.

To overcome the quenching problem of the SP coupled emission in the ZnO on silicon substrate, a four-layered structure is proposed. As Fig. 1(c) shown, a layer of SiO2 was inserted between silicon substrate and ZnO film. This layer introduces two interfaces and has a low refractive index. The calculated power dissipation spectrum of this structure is shown in Fig. 4(a). The power spectrum of the three layered sample (Si/ZnO/Ag) is also included for comparison. It is found that the power amplitude of the SP mode for the four-layered structure was increased significantly comparing with that for the Si/ZnO/Ag. The power coupled to SP mode is greatly raised, and PL enhancement can be expected from this structure. To confirm the theoretical prediction, the PL of the structure was measured, and the results are illustrated in Fig. 4(b). The PL intensity of the Si/SiO2/ZnO/Ag is really enhanced more than two fold comparing to the control sample. Despite the fact that the enhancement ratio for this four-layer silicon based sample is less than that for the three-layer quartz or sapphire based sample, probably due to losses of absorption and scattering at the SiO2/Si interface, this structure conquers the problem of SP coupled emission quenching of ZnO on silicon substrate, which is promising in design SP enhanced silicon based emission devices.

4. Conclusion

In conclusion, the substrate in the emitting device structure was found to be influential on the SP mediated light emission through metal films. Theoretical simulation shows that competition between the power dissipated to the SPP and substrate mode was the cause of PL enhancement or quenching. Proper arrangement of the refractive index of the substrates and emitting layers in the devices is decisive for the SP coupled light emission enhancement.

Acknowledgments

The authors express their appreciations to the Natural Science Foundation of China (No. 60606001, National Basic Research Program of China (973 Program, No. 2007CB613403) and PCSIRT(IRT0651) project for the financial support.

References and links

1.

R. R. Chance, A. Prock, and R. Silbey, “Lifetime of an emitting molecule near a partially reflecting surface,” J.Chem. Phys. 60, 2744–2748 (1974). [CrossRef]

2.

K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, and Axel Scherer,“Surface-plasmon-enhanced light emitters based on InGaN quantum wells,” Nat. Mater. 3, 601–605 (2004). [CrossRef] [PubMed]

3.

C. J. Yates, I. D. W. Samuel, P. L. Burn, S. Wedge, and W. L. Barnes, “Surface plasmon-polariton mediated emission from phosphorescent dendrimer light-emitting diodes,” Appl. Phys. Lett. 88, 161105-1-3 (2006). [CrossRef]

4.

P. Cheng, D. Li, Z. Yuan, P. Chen, and D. Yang, “Enhancement of ZnO light emission via coupling with localized surface plasmon of Ag island film,” Appl. Phys. Lett. 92, 041119-1-3 (2008). [CrossRef]

5.

J. S. Biteen, D. Pacifici, N. S. Lewis, and H. A. Atwater, “Enhance radiative emission rate and quantum efficiency in coupled silicon nanocrystal-nanostructured gold emitters,” Nano Lett. 5, 1768–1773 (2005). [CrossRef] [PubMed]

6.

D. Y. Lei, J. Li, and H. C. Ong, “Tunable surface plasmon mediated emission from semiconductors by using metal alloys,” Appl. Phys. Lett. 91, 021112-1-3 (2007). [CrossRef]

7.

T. D. Neal, K. Okamoto, and A. Scherer, “Surface plasmon enhanced emission from dye doped polymer layers,” Opt. Express. 13, 5522–5528 (2005). [CrossRef] [PubMed]

8.

S. Wedge and W. L. Barnes, “Surface plasmon-polariton mediated light emission through thin metal films,” Opt. Express. 12, 3673–3685 (2004). [CrossRef] [PubMed]

9.

W. L. Barnes and P. T. Worthing, “Spontaneous emission and metal-clad microcavities,” Opt. Commun. 162, 16–20 (1999). [CrossRef]

10.

C.-Y. Chen, D.-M. Yeh, Y.-C. Lu, and C. C. Yang, “Dependence of resonant coupling between surface plasmons and an InGaN quantum well on metallic structure,” Appl. Phys. Lett. 89, 203113-1-3 (2006). [CrossRef]

11.

R. R. Chance, A. Prock, and R. Silbey, “Comments on the classical theory of energy transfer,” J. Chem. Phys. 62, 2245–2253 (1975). [CrossRef]

12.

K. G. Sullivan and D. G. Hall, “Enhancement and inhibition of electromagnetic radiation in plane-layered media,” J. Opt. Soc. Am. B 14, 1149–1159 (1997). [CrossRef]

13.

S. Kawabata, K. Ishihara, Y. Hoshi, and T. Fukazawa, “Observation of silver film growth using an in situ ultra-high vacuum spectroscopic ellipsometer,” Thin Solid Film 313–314, 516–521 (1998). [CrossRef]

14.

Edward D. Palik, Handbook of optical constant of solid (Academic, 1985)

15.

B. J. Soller and D. G. Hall, “Energy transfer at optical frequencies to silicon-based waveguiding structures,” J. Opt. Soc. Am. A 18, 2577–2584 (2001). [CrossRef]

16.

T. Nakamura, M. Fujii, K. Imakita, and S. Hayashi, “Modification of energy transfer from Si nanocrystals to Er3+ near a Au thin film,” Phys. Rev. B 72, 235412-1-6 (2005) [CrossRef]

17.

R. M. Amos and W. L. Barnes, “Modification of the spontaneous emission rate of Eu3+ ions close to a thin metal mirror,” Phys. Rev. B 55, 7249–7254 (1997). [CrossRef]

18.

J. Kalkman, H. Gersen, L. Kuipers, and A. Polman, “Excitation of surface plasmons at SiO2/Ag interface by silicon quantum dots:experiment and theory,” Phys. Rev. B 73, 075317-1-8 (2006). [CrossRef]

OCIS Codes
(160.6000) Materials : Semiconductor materials
(240.6680) Optics at surfaces : Surface plasmons

ToC Category:
Optics at Surfaces

History
Original Manuscript: April 23, 2008
Revised Manuscript: May 23, 2008
Manuscript Accepted: May 26, 2008
Published: June 2, 2008

Citation
Peihong Cheng, Dongsheng Li, and Deren Yang, "Influence of substrates in ZnO devices on the surface plasmon enhanced light emission," Opt. Express 16, 8896-8901 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-12-8896


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References

  1. R. R. Chance, A. Prock, and R. Silbey, "Lifetime of an emitting molecule near a partially reflecting surface," J.Chem. Phys. 60, 2744-2748 (1974). [CrossRef]
  2. K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, and Axel Scherer,"Surface-plasmon-enhanced light emitters based on InGaN quantum wells," Nat. Mater. 3, 601-605 (2004). [CrossRef] [PubMed]
  3. C. J. Yates, I. D. W. Samuel, P. L. Burn, S. Wedge, and W. L. Barnes, "Surface plasmon-polariton mediated emission from phosphorescent dendrimer light-emitting diodes," Appl. Phys. Lett. 88,161105-1-3 (2006). [CrossRef]
  4. P. Cheng, D. Li, Z. Yuan, P. Chen, and D. Yang, "Enhancement of ZnO light emission via coupling with localized surface plasmon of Ag island film," Appl. Phys. Lett. 92,041119-1-3 (2008). [CrossRef]
  5. J. S. Biteen, D. Pacifici, N. S. Lewis, and H. A. Atwater, "Enhance radiative emission rate and quantum efficiency in coupled silicon nanocrystal-nanostructured gold emitters," Nano Lett. 5, 1768-1773 (2005). [CrossRef] [PubMed]
  6. D. Y. Lei, J. Li, and H. C. Ong, "Tunable surface plasmon mediated emission from semiconductors by using metal alloys," Appl. Phys. Lett. 91, 021112-1-3 (2007). [CrossRef]
  7. T. D. Neal, K. Okamoto, and A. Scherer, "Surface plasmon enhanced emission from dye doped polymer layers," Opt. Express. 13, 5522-5528 (2005). [CrossRef] [PubMed]
  8. S. Wedge and W. L. Barnes, "Surface plasmon-polariton mediated light emission through thin metal films," Opt. Express. 12, 3673-3685 (2004). [CrossRef] [PubMed]
  9. W. L. Barnes and P. T. Worthing, "Spontaneous emission and metal-clad microcavities," Opt. Commun. 162, 16-20 (1999). [CrossRef]
  10. C.-Y. Chen, D.-M. Yeh, Y.-C. Lu, and C. C. Yang, "Dependence of resonant coupling between surface plasmons and an InGaN quantum well on metallic structure," Appl. Phys. Lett. 89, 203113-1-3 (2006). [CrossRef]
  11. R. R. Chance, A. Prock, and R. Silbey, "Comments on the classical theory of energy transfer," J. Chem. Phys. 62, 2245-2253 (1975). [CrossRef]
  12. K. G. Sullivan and D. G. Hall, "Enhancement and inhibition of electromagnetic radiation in plane-layered media," J. Opt. Soc. Am. B 14, 1149-1159 (1997). [CrossRef]
  13. S. Kawabata, K. Ishihara, Y. Hoshi, and T. Fukazawa, "Observation of silver film growth using an in situ ultra-high vacuum spectroscopic ellipsometer," Thin Solid Film 313-314, 516-521 (1998). [CrossRef]
  14. EdwardD.  Palik, Handbook of optical constant of solid (Academic, 1985)
  15. B. J. Soller and D. G. Hall, "Energy transfer at optical frequencies to silicon-based waveguiding structures," J. Opt. Soc. Am. A 18, 2577-2584 (2001). [CrossRef]
  16. T. Nakamura, M. Fujii, K. Imakita, and S. Hayashi, "Modification of energy transfer from Si nanocrystals to Er3+ near a Au thin film," Phys. Rev. B 72, 235412-1-6 (2005) [CrossRef]
  17. R. M. Amos and W. L. Barnes, "Modification of the spontaneous emission rate of Eu3+ ions close to a thin metal mirror," Phys. Rev. B 55, 7249-7254 (1997). [CrossRef]
  18. <jrn>. J. Kalkman, H. Gersen, L. Kuipers, and A. Polman, "Excitation of surface plasmons at SiO2/Ag interface by silicon quantum dots:experiment and theory, " Phys. Rev. B 73, 075317-1-8 (2006).</jrn> [CrossRef]

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