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Optics Express

Optics Express

  • Editor: C. Martijin de Sterke
  • Vol. 19, Iss. 7 — Mar. 28, 2011
  • pp: 5895–5901
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Surface-plasmon-enhanced visible-light emission of ZnO/Ag grating structures

Minji Gwon, Eunsongyi Lee, Dong-Wook Kim, Ki-Ju Yee, Min Jung Lee, and Youn Sang Kim  »View Author Affiliations


Optics Express, Vol. 19, Issue 7, pp. 5895-5901 (2011)
http://dx.doi.org/10.1364/OE.19.005895


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Abstract

We investigated the optical properties of ZnO/Ag grating structures fabricated by sputtering and nanoimprint lithography. The grating structures exhibited multiple peak features in broad visible-range photoluminescence (PL) spectra. The PL intensity of the grating was larger than that of a planar thin film by up to two orders of magnitude. The surface plasmon (SP) dispersion relation suggested excitation of SPs with various energies of the grating, explaining the broad PL emission. The spectral dependence of the PL intensity was also well supported by the experimental reflectance spectra and the simulated electric field distribution at the ZnO/Ag interface.

© 2011 OSA

1. Introduction

Surface plasmons (SPs) are electromagnetic waves propagating along a metal interface. They generally arise from interactions between photons and electrons at the interface and exhibit various unique characteristics, including localization of electric fields at the interface and drastically increased density of modes near the resonance condition [1

1. E. Ozbay, “Plasmonics: merging photonics and electronics at nanoscale dimensions,” Science 311(5758), 189–193 (2006). [CrossRef] [PubMed]

4

4. S. Wedge and W. L. Barnes, “Surface plasmon-polariton mediated light emission through thin metal films,” Opt. Express 12(16), 3673–3685 (2004). [CrossRef] [PubMed]

]. Such properties provide the possibility of guiding light in subwavelength metallic structures and enhancing the light emission and absorption [1

1. E. Ozbay, “Plasmonics: merging photonics and electronics at nanoscale dimensions,” Science 311(5758), 189–193 (2006). [CrossRef] [PubMed]

8

8. B. J. Lawrie, R. F. Haglund Jr, and R. Mu, “Enhancement of ZnO photoluminescence by localized and propagating surface plasmons,” Opt. Express 17(4), 2565–2572 (2009). [CrossRef] [PubMed]

]. Thus, the SPs have attracted considerable research attention due to scientific interest and applications in optoelectronic devices.

ZnO, with a direct bandgap of 3.37 eV, has been widely used as a transparent conducting electrode and is a strong candidate for ultraviolet light-emitting devices and photodetectors [9

9. F.-J. Haug, T. Söderström, O. Cubero, V. Terrazzoni-Daudrix, and C. Ballif, “Plasmonic absorption in textured silver back reflectors of thin film solar cells,” J. Appl. Phys. 104(6), 064509 (2008). [CrossRef]

11

11. H. Kim, H. Kim, and D.-W. Kim, “Silver Schottky contacts to a-plane bulk ZnO,” J. Appl. Phys. 108(7), 074514 (2010). [CrossRef]

]. It has been reported that SPs at the ZnO/metal interface can enhance the spontaneous electron–hole recombination rate and subsequent photoluminescence (PL) intensity [5

5. D. Y. Lei, J. Li, and H. C. Ong, “Tunable surface plasmon mediated emission from semiconductors using metal alloys,” Appl. Phys. Lett. 91(2), 021112 (2007). [CrossRef]

8

8. B. J. Lawrie, R. F. Haglund Jr, and R. Mu, “Enhancement of ZnO photoluminescence by localized and propagating surface plasmons,” Opt. Express 17(4), 2565–2572 (2009). [CrossRef] [PubMed]

]. The SP dispersion relation at a metal–semiconductor interface is determined by the dielectric functions of both materials [1

1. E. Ozbay, “Plasmonics: merging photonics and electronics at nanoscale dimensions,” Science 311(5758), 189–193 (2006). [CrossRef] [PubMed]

4

4. S. Wedge and W. L. Barnes, “Surface plasmon-polariton mediated light emission through thin metal films,” Opt. Express 12(16), 3673–3685 (2004). [CrossRef] [PubMed]

]. The SP density of modes (proportional to the inverse of dω/dk), and thus the emission rate, has peaks near the resonance energy [2

2. S. A. Maier and H. A. Atwater, “Plasmonics: localization and guiding of electromagnetic energy into metal/dielectric structures,” J. Appl. Phys. 98(1), 011101 (2005). [CrossRef]

]. In addition to the near band-edge UV emission, defect states in ZnO thin films usually result in broad visible-range luminescence [12

12. X. L. Wu, G. G. Siu, C. L. Fu, and H. C. Ong, “Photoluminescence and cathodoluminescence studies of stoichiometric and oxygen-deficient ZnO films,” Appl. Phys. Lett. 78(16), 2285–2287 (2001). [CrossRef]

]. In many studies of ZnO/metal structures without artificial periodicity, near-UV emission was enhanced while visible emission was suppressed, because the SP resonance was close to the bandgap energy of ZnO [5

5. D. Y. Lei, J. Li, and H. C. Ong, “Tunable surface plasmon mediated emission from semiconductors using metal alloys,” Appl. Phys. Lett. 91(2), 021112 (2007). [CrossRef]

8

8. B. J. Lawrie, R. F. Haglund Jr, and R. Mu, “Enhancement of ZnO photoluminescence by localized and propagating surface plasmons,” Opt. Express 17(4), 2565–2572 (2009). [CrossRef] [PubMed]

]. In contrast, periodic structures can allow multi-mode SP excitation in which the energy can be controlled by the periodicity and the properties of the constituent materials [4

4. S. Wedge and W. L. Barnes, “Surface plasmon-polariton mediated light emission through thin metal films,” Opt. Express 12(16), 3673–3685 (2004). [CrossRef] [PubMed]

,9

9. F.-J. Haug, T. Söderström, O. Cubero, V. Terrazzoni-Daudrix, and C. Ballif, “Plasmonic absorption in textured silver back reflectors of thin film solar cells,” J. Appl. Phys. 104(6), 064509 (2008). [CrossRef]

]. Influences of such multiple SP resonances on the optical properties of ZnO have not been explicitly investigated. Careful optical characterization of the grating structures will be very helpful for understanding the interplay between photons, excitons, and SPs in ZnO/metal structures.

In this article, we report the fabrication and optical characterization of ZnO/Ag nanograting structures. The experimental data clearly show that the optical properties of the grating structures were very different compared with the planar thin films. In particular, multiple PL peaks and notable enhancement of intensity were observed. The PL peaks were indicative of the resonant SP energies, which was supported by the reflectance spectra and the finite-difference time-domain simulation results.

2. Sample fabrication and characterization

We deposited ZnO and Ag thin films at room temperature using a radio-frequency (RF) magnetron sputtering technique. The base pressure of the sputtering system was 1 × 10−6 Torr, and the working pressure of pure Ar was 7.5 × 10−3 Torr during thin-film growth. The applied RF power was 50 W for both films. For the gratings, 100-nm-thick ZnO/100-nm-thick Ag layers were grown on polymer patterns fabricated by nanoimprint lithography [13

13. M. Lee, J. Kim, J. Lee, and Y. S. Kim, “Pressure-assisted printing with crack-free metal electrodes using an anti-adhesive rigiflex stamp,” J. Mater. Chem. 20(14), 2746–2748 (2010). [CrossRef]

]. Polydimethylsiloxane (PDMS) with nanoscale features was used as a stamp. Patterns were transferred to the UV-curable agent, trimethylolpropane propoxylate triacrylate (TPT), with a photoinitiator of 2-hydroxy-2-methylpropiophrnone. The patterns were cured under UV light (wavelength: 365 nm) for 30 minutes. The area of the pattern was 3 × 3 cm2, which was large enough for conventional optical measurements. For comparison, ZnO/Ag thin films were also prepared on planar Si substrates.

The morphology of the films and the fabricated patterns were examined using an atomic force microscopy (AFM) system (Digital Instruments, Dimension 3100) and a field-emission scanning electron microscope (SEM) (JOEL, JSM-6700F). For PL measurements, samples were excited by a HeCd laser (wavelength: 325 nm), and the luminescence spectra were obtained by a nitrogen-cooled charge-coupled device (CCD) detector that was attached to a monochromator with a spectral resolution of 1 nm. The optical reflectance of the films was studied using a UV-visible spectrophotometer (PerkinElmer, 750).

Figure 1(a)
Fig. 1 (a) Schematic diagram of a ZnO/Ag grating structure and (b) AFM morphology of a polymer pattern with a period of 1 μm and a line-to-space ratio of 1:1.
shows a schematic diagram of the ZnO/Ag grating with line-and-space structures such as one-dimensional square wave shapes. Figure 1(b) shows a typical AFM image of the polymer pattern. The period and amplitude of the grating were estimated to be 1000 and 500 nm, respectively. SEM measurements of the polymer patterns showed that the widths of the line and the space were nearly the same. After growth of the ZnO/Ag layers, the space-width was reduced, and the line-to-space ratio was 3:2, indicating that the sputtering method allowed very conformal growth of the layers on the polymer pattern. The root-mean-square (rms) roughnesses of ZnO and Ag thin films on flat Si substrates were estimated to be 3.2 and 1.9 nm, respectively, based on the AFM measurements. The surface roughness of both of the films was negligible compared with the peak-to-valley height of the grating.

3. Surface plasmon dispersion relation

SP resonance energies at the ZnO/Ag interface can be determined based on the dielectric functions of ZnO and Ag [2

2. S. A. Maier and H. A. Atwater, “Plasmonics: localization and guiding of electromagnetic energy into metal/dielectric structures,” J. Appl. Phys. 98(1), 011101 (2005). [CrossRef]

]. The dispersion relation of SPs at a metal/dielectric interface is given by the following expression:
kSP=ωcεm'εd'εm'+εd',
(1)
where kSP is a wave vector of SP, and εm' and εd' are real-part dielectric constants of the metal and the dielectric, respectively. Resonance occurs for flat interfaces when εm' + εd' = 0. Figure 2(a)
Fig. 2 (a) Real-part dielectric constants of Ag, ZnO, and air, and expected SPR and LSPR energies. (b) SP dispersion relation of a ZnO/Ag grating structure with a period of 1 μm. Blue dashed lines and dots indicate the SP (photon) energy that can couple photons (SPs).
shows plots of ε' for Ag (εAg') minusε' for ZnO (–εZnO') and –2ε' for air (–2εair') as functions of photon energy. The intersection of εAg' and –εZnO' indicates the SP resonance (SPR) energy, denoted as a circle in Fig. 2(a). On the other hand, the localized surface plasmon resonance (LSPR) within metal nanoparticles occurs when εm' + 2εd' = 0 [2

2. S. A. Maier and H. A. Atwater, “Plasmonics: localization and guiding of electromagnetic energy into metal/dielectric structures,” J. Appl. Phys. 98(1), 011101 (2005). [CrossRef]

]. The LSPR energy is also denoted in Fig. 2(a) by the intersection of εAg' and –2εair'. The SPR and the LSPR energies were 3.1 and 3.5 eV, respectively. These energies were close to the bandgap energy of ZnO. As discussed above, significant enhancement of the light emission was possible at the resonance energy. This explains why the near-UV bandgap emission was significantly enhanced using metal nanoparticles or textured metal layers on or under the ZnO layers [5

5. D. Y. Lei, J. Li, and H. C. Ong, “Tunable surface plasmon mediated emission from semiconductors using metal alloys,” Appl. Phys. Lett. 91(2), 021112 (2007). [CrossRef]

8

8. B. J. Lawrie, R. F. Haglund Jr, and R. Mu, “Enhancement of ZnO photoluminescence by localized and propagating surface plasmons,” Opt. Express 17(4), 2565–2572 (2009). [CrossRef] [PubMed]

].

In a periodic structure, SPs can be excited by incident photons, while the momentum mismatch in multiples of the reciprocal lattice vector are compensated according to the relationship
kSP=ωcsinθ0±2πDn,
(2)
where ω/c is a wave vector of the photon in vacuum, θ 0 is the incident angle of the photon with respect to the surface normal, D is the period of the grating, and n is an integer. The black solid line in Fig. 2(b) represents the SP dispersion given by Eq. (1), and green lines correspond to the right side of Eq. (2) with different values of n. The intercepts, denoted by blue filled circles on the vertical axis, indicate the photon energy that can couple SPs with the aid of the grating. θ 0 = 7° was chosen in Fig. 2, because the reflectance measurements were performed under such conditions. Figure 2(b) clearly shows that photons with various energies can excite SPs in ZnO/Ag grating structures, and vice versa.

4. Photoluminescence and reflectance spectra

Figure 3
Fig. 3 PL spectra of a ZnO/Ag planar thin film and a ZnO/Ag grating structure. Enhancement indicates the ratio of the PL intensity of the grating structure to that of the planar thin film.
shows the PL spectra of the ZnO/Ag films grown on a flat Si substrate (gray line) and on the nanograting structure (black line). For the planar sample, the PL spectra consisted of a peak at ~3.2 eV, originating from the band-to-band emission, and a broad peak in the visible range due to defect electronic states inside the bandgap region [12

12. X. L. Wu, G. G. Siu, C. L. Fu, and H. C. Ong, “Photoluminescence and cathodoluminescence studies of stoichiometric and oxygen-deficient ZnO films,” Appl. Phys. Lett. 78(16), 2285–2287 (2001). [CrossRef]

]. The PL spectra of the grating sample were drastically different from those of the flat sample: (1) the intensity increased significantly, and (2) wide-range (nearly white light) emission was observed.

Figure 3 shows the ratio of the PL intensity for the grating to that of the flat sample, which reached 102 at 2.8 eV. The PL enhancement ratio has been reported to be less than several tens for semiconductors with metal thin films and nanoparticles [3

3. K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, and A. Scherer, “Surface-plasmon-enhanced light emitters based on InGaN quantum wells,” Nat. Mater. 3(9), 601–605 (2004). [CrossRef] [PubMed]

8

8. B. J. Lawrie, R. F. Haglund Jr, and R. Mu, “Enhancement of ZnO photoluminescence by localized and propagating surface plasmons,” Opt. Express 17(4), 2565–2572 (2009). [CrossRef] [PubMed]

]. The electronic density of states in ZnO for the grating should have been similar to those for the flat samples. Thus, the grating structures likely played key roles in the modification of the spectral dependence and the enhancement of the PL intensity. First, the grating enabled SPs to exchange energy with photons efficiently, as mentioned previously. Diffraction can also occur in the grating; thus, the optical path can be increased [9

9. F.-J. Haug, T. Söderström, O. Cubero, V. Terrazzoni-Daudrix, and C. Ballif, “Plasmonic absorption in textured silver back reflectors of thin film solar cells,” J. Appl. Phys. 104(6), 064509 (2008). [CrossRef]

]. When the photons remain in the ZnO layers longer, more electron–hole pairs can be generated, resulting in emission enhancement. The electric field distributions in the grating should also be considered, and details will be discussed in Sec. 5.

Figure 4
Fig. 4 PL spectra and reflectance of a ZnO/Ag grating structure. Dashed blue lines correspond to the SP mode energies expected from the dispersion relation.
shows the PL and total reflectance spectra of a ZnO/Ag grating structure. The reflectance data were obtained with an integrating sphere to minimize losses due to interference and diffraction. The incident angles of light were 0° and 7° from the surface normal for the PL and the reflectance experiments, respectively. Dashed blue lines indicate the excited SP modes expected from Fig. 2(b). According to Eq. (2), the term sin(7°) = 0.12 can cause a small difference in the SP energy involved in the PL and the reflectance data. The blue dashed lines agreed well with the PL peaks and the reflectance dips, suggesting that the coupling between the photons and SPs enhanced the PL intensity and modified the PL spectra, as discussed above. However, the reflectance dips and the corresponding PL peaks were located at somewhat higher energies than the blue lines (note the differences at 1.85 and 1.68 eV). Haug et al. reported similar reflectance results for their ZnO/Ag grating structures: the SP resonance at the Ag interface was not sensitive to the presence of the ZnO layer at low energies, and a transition between the relationships of air/Ag and ZnO/Ag was observed at photon energies above 2 eV [9

9. F.-J. Haug, T. Söderström, O. Cubero, V. Terrazzoni-Daudrix, and C. Ballif, “Plasmonic absorption in textured silver back reflectors of thin film solar cells,” J. Appl. Phys. 104(6), 064509 (2008). [CrossRef]

]. At low energies (i.e., long wavelengths), the propagation length in the ZnO layer can be less than one third of the effective wavelength, λeff=λ0/ε330 nm. For high-energy photons, the 100-nm-thick ZnO layer acted like a bulk material, resulting in the ZnO/Ag SP dispersion relation depicted in Fig. 2(b). As a result, the PL peaks and the reflectance dips were in good agreement with the dashed lines.

5. Finite-difference time-domain simulation

To describe the wave propagation within the nanograting structure, Maxwell’s equations had to be solved. Real-space electric field distributions can be obtained by finite-difference time-domain (FDTD) simulations, which numerically solve the differential equations. In this work, the nanograting structure was modeled in FDTD (Lumerical FDTD Solutions 7.0.1) as a unit cell (Fig. 5
Fig. 5 Electric field intensity distribution of a ZnO/Ag grating structure at various photon energies obtained by FDTD simulations. All simulations were carried out for p-polarized plane waves.
) embedded in air with a perfectly matched layer (PML) and periodic boundary conditions. Figure 5 shows the electric field intensity,|E|2, for four different photon energies () for a p-polarized plane wave at normal incidence. For = 1.0 eV, no SPP excitation was expected from Fig. 2(b), and no large field formed in the ZnO layer, as shown in Fig. 5(a). Figures 5(b), 5(c), and 5(d) show that the field intensity built up in the ZnO layers, especially at the ZnO/Ag interface, for = 2.1, 2.7, and 3.0 eV, respectively. For these energies, SPP excitation was possible according to the dispersion relation depicted in Fig. 2(b). At = 2.7 eV, a large field was observed at the top as well as the side of the ZnO/Ag interface. Notable field intensity enhancement and a large number of excited SP modes at = 2.7 eV accounted for the PL intensity enhancement, as shown in Fig. 3.

6. Conclusion

We demonstrated that the optical properties of ZnO/Ag grating structures were significantly different from those of ZnO/Ag planar thin films. PL spectra for the grating showed multiple peaks and nearly white light emission. The PL intensity of the grating was larger than that of the planar sample by two orders of magnitude. The PL peak positions and the reflectance dips were well matched, which was understood based on the SP dispersion relation. The spatial electric field distribution was simulated, revealing that the SPs influenced the optical characteristics significantly.

Acknowledgments

This work was supported by the Quantum Metamaterials Research Center (2009-0063324) and Pioneer Research Center Program (2010-0002231) through the National Research Foundation of Korea Grant funded by the Ministry of Education, Science and Technology (MEST). This work was also supported by MEST & DGIST (10-BD-0101, Convergence Technology with New Renewable Energy and Intelligent Robot).

References and links

1.

E. Ozbay, “Plasmonics: merging photonics and electronics at nanoscale dimensions,” Science 311(5758), 189–193 (2006). [CrossRef] [PubMed]

2.

S. A. Maier and H. A. Atwater, “Plasmonics: localization and guiding of electromagnetic energy into metal/dielectric structures,” J. Appl. Phys. 98(1), 011101 (2005). [CrossRef]

3.

K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, and A. Scherer, “Surface-plasmon-enhanced light emitters based on InGaN quantum wells,” Nat. Mater. 3(9), 601–605 (2004). [CrossRef] [PubMed]

4.

S. Wedge and W. L. Barnes, “Surface plasmon-polariton mediated light emission through thin metal films,” Opt. Express 12(16), 3673–3685 (2004). [CrossRef] [PubMed]

5.

D. Y. Lei, J. Li, and H. C. Ong, “Tunable surface plasmon mediated emission from semiconductors using metal alloys,” Appl. Phys. Lett. 91(2), 021112 (2007). [CrossRef]

6.

J. B. You, X. W. Zhang, Y. M. Fan, Z. G. Yin, P. F. Cai, and N. F. Chen, “Effects of the morphology of ZnO/Ag interface on the surface-plasmon-enhanced emission of ZnO films,” J. Phys. D Appl. Phys. 41(20), 205101 (2008). [CrossRef]

7.

K. W. Liu, Y. D. Tang, C. X. Cong, T. C. Sum, A. C. H. Huan, Z. X. Shen, L. Wang, F. Y. Jiang, X. W. Sun, and H. D. Sun, “Giant enhancement of top emission from ZnO thin film by nanopatterned Pt,” Appl. Phys. Lett. 94(15), 151102 (2009). [CrossRef]

8.

B. J. Lawrie, R. F. Haglund Jr, and R. Mu, “Enhancement of ZnO photoluminescence by localized and propagating surface plasmons,” Opt. Express 17(4), 2565–2572 (2009). [CrossRef] [PubMed]

9.

F.-J. Haug, T. Söderström, O. Cubero, V. Terrazzoni-Daudrix, and C. Ballif, “Plasmonic absorption in textured silver back reflectors of thin film solar cells,” J. Appl. Phys. 104(6), 064509 (2008). [CrossRef]

10.

H.-D. Um, S. A. Moiz, K.-T. Park, J.-Y. Jung, S.-W. Jee, C. H. Ahn, D. C. Kim, H. K. Cho, D.-W. Kim, and J.-H. Lee, “Highly selective spectral response with enhanced responsivity of n-ZnO/p-Si radial heterojunction nanowire photodiodes,” Appl. Phys. Lett. 98(3), 033102 (2011). [CrossRef]

11.

H. Kim, H. Kim, and D.-W. Kim, “Silver Schottky contacts to a-plane bulk ZnO,” J. Appl. Phys. 108(7), 074514 (2010). [CrossRef]

12.

X. L. Wu, G. G. Siu, C. L. Fu, and H. C. Ong, “Photoluminescence and cathodoluminescence studies of stoichiometric and oxygen-deficient ZnO films,” Appl. Phys. Lett. 78(16), 2285–2287 (2001). [CrossRef]

13.

M. Lee, J. Kim, J. Lee, and Y. S. Kim, “Pressure-assisted printing with crack-free metal electrodes using an anti-adhesive rigiflex stamp,” J. Mater. Chem. 20(14), 2746–2748 (2010). [CrossRef]

OCIS Codes
(240.6680) Optics at surfaces : Surface plasmons
(160.4236) Materials : Nanomaterials

ToC Category:
Optics at Surfaces

History
Original Manuscript: January 19, 2011
Manuscript Accepted: March 1, 2011
Published: March 15, 2011

Citation
Minji Gwon, Eunsongyi Lee, Dong-Wook Kim, Ki-Ju Yee, Min Jung Lee, and Youn Sang Kim, "Surface-plasmon-enhanced visible-light emission of ZnO/Ag grating structures," Opt. Express 19, 5895-5901 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-7-5895


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References

  1. E. Ozbay, “Plasmonics: merging photonics and electronics at nanoscale dimensions,” Science 311(5758), 189–193 (2006). [CrossRef] [PubMed]
  2. S. A. Maier and H. A. Atwater, “Plasmonics: localization and guiding of electromagnetic energy into metal/dielectric structures,” J. Appl. Phys. 98(1), 011101 (2005). [CrossRef]
  3. K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, and A. Scherer, “Surface-plasmon-enhanced light emitters based on InGaN quantum wells,” Nat. Mater. 3(9), 601–605 (2004). [CrossRef] [PubMed]
  4. S. Wedge and W. L. Barnes, “Surface plasmon-polariton mediated light emission through thin metal films,” Opt. Express 12(16), 3673–3685 (2004). [CrossRef] [PubMed]
  5. D. Y. Lei, J. Li, and H. C. Ong, “Tunable surface plasmon mediated emission from semiconductors using metal alloys,” Appl. Phys. Lett. 91(2), 021112 (2007). [CrossRef]
  6. J. B. You, X. W. Zhang, Y. M. Fan, Z. G. Yin, P. F. Cai, and N. F. Chen, “Effects of the morphology of ZnO/Ag interface on the surface-plasmon-enhanced emission of ZnO films,” J. Phys. D Appl. Phys. 41(20), 205101 (2008). [CrossRef]
  7. K. W. Liu, Y. D. Tang, C. X. Cong, T. C. Sum, A. C. H. Huan, Z. X. Shen, L. Wang, F. Y. Jiang, X. W. Sun, and H. D. Sun, “Giant enhancement of top emission from ZnO thin film by nanopatterned Pt,” Appl. Phys. Lett. 94(15), 151102 (2009). [CrossRef]
  8. B. J. Lawrie, R. F. Haglund, and R. Mu, “Enhancement of ZnO photoluminescence by localized and propagating surface plasmons,” Opt. Express 17(4), 2565–2572 (2009). [CrossRef] [PubMed]
  9. F.-J. Haug, T. Söderström, O. Cubero, V. Terrazzoni-Daudrix, and C. Ballif, “Plasmonic absorption in textured silver back reflectors of thin film solar cells,” J. Appl. Phys. 104(6), 064509 (2008). [CrossRef]
  10. H.-D. Um, S. A. Moiz, K.-T. Park, J.-Y. Jung, S.-W. Jee, C. H. Ahn, D. C. Kim, H. K. Cho, D.-W. Kim, and J.-H. Lee, “Highly selective spectral response with enhanced responsivity of n-ZnO/p-Si radial heterojunction nanowire photodiodes,” Appl. Phys. Lett. 98(3), 033102 (2011). [CrossRef]
  11. H. Kim, H. Kim, and D.-W. Kim, “Silver Schottky contacts to a-plane bulk ZnO,” J. Appl. Phys. 108(7), 074514 (2010). [CrossRef]
  12. X. L. Wu, G. G. Siu, C. L. Fu, and H. C. Ong, “Photoluminescence and cathodoluminescence studies of stoichiometric and oxygen-deficient ZnO films,” Appl. Phys. Lett. 78(16), 2285–2287 (2001). [CrossRef]
  13. M. Lee, J. Kim, J. Lee, and Y. S. Kim, “Pressure-assisted printing with crack-free metal electrodes using an anti-adhesive rigiflex stamp,” J. Mater. Chem. 20(14), 2746–2748 (2010). [CrossRef]

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