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IZO sandwiched structure due to surface plasmon resonance of thin Ag film
Applied Surface Science 389 (2016) 906–910
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Emission enhancement in indium zinc oxide(IZO)/Ag/IZO sandwiched structure due to surface plasmon resonance of thin Ag film Takayuki Kiba a,∗ , Kazuki Yanome a , Midori Kawamura a , Yoshio Abe a , Kyung Ho Kim a , Junichi Takayama b , Akihiro Murayama b a b
Department of Materials Science and Engineering, Kitami Institute of Technology, Kitami 090-8507, Japan Graduate School of Information Science and Technology, Hokkaido University, Sapporo 060-0814, Japan
a r t i c l e
i n f o
Article history: Received 30 May 2016 Received in revised form 13 July 2016 Accepted 7 August 2016 Available online 9 August 2016 Keywords: Surface plasmon Plasmonic emission enhancement Photoluminescence Time-resolved spectroscopy Silver thin film
a b s t r a c t We report on a photoluminescence (PL) enhancement in IZO/Ag/IZO sandwiched structure via surface plasmonic effects of 14 nm-thick Ag film. In the presence of Ag thin film, the 2–8-fold enhancement was observed for the broad PL around 2.34 eV, which can be originated from defect states in amorphous IZO film. The results of time-resolved PL spectra suggested that the increase in radiative recombination rate, and the maximum Purcell factor of 19 was estimated from the analysis of the PL decay profiles. The comparison between the results of static- and dynamic-PL measurement suggests that the non-radiative process after the excitation of the surface plasmon of the silver film also affects the total efficiency of the emission enhancement. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Surface plasmon (SP) or localized plasmon (LP) have attracted increasing attention because their extensive applications in sensing [1,2], enhanced spectrum [3,4], enhancing transmission and emission [5–9], light harvesting antenna [10,11] and so on. The plasmon mediated enhancement of the intensity of photoluminescence (PL) in semiconductor or organic dye induced by metallic nanostructures are very attractive, because it can directly contribute to the efficiency of light emitting devices. The efficiency of SP or LP coupling on the PL enhancement is decided by plasmon resonance frequency, which can be controlled by the size, shape and the selection of metals for the nanostructures. Indium zinc oxide (IZO) has received much attention for its application to the light emitting devices as a transparent conducting electrode. Especially, IZO/Ag/IZO sandwiched film was successfully utilized for the anode of OLEDs [12], with high transmittance and low sheet-resistance, and reducing the indium consumption compared to the ITO electrode. Recently, this kind of dielectric/metal/dielectric sandwiched structures are investigated also in the view point of emission enhancement via SP or LP [13–15], because there are two SP modes (dielectric/metal interface) can
∗ Corresponding author. E-mail address: [email protected] (T. Kiba). http://dx.doi.org/10.1016/j.apsusc.2016.08.032 0169-4332/© 2016 Elsevier B.V. All rights reserved.
be involved, thus much larger emission enhancement is expected. Understanding the mechanism of the plasmonic PL enhancement in this sandwiched structure can be helpful for the emission device application, where the new functionality will add to this kind of dielectric/metal/dielectric transparent electrode. Here, we report on the PL enhancement in IZO/Ag/IZO via surface plasmonic effect of Ag thin film. The 2–8-fold enhancement was observed for the broad PL around 2.34 eV. The emission energy dependence of the time-resolved PL suggested that the increase in radiative recombination rate can be attributed to the SP resonance effects, whereas the non-radiative process at the silver thin film (=plasmonic loss) is not negligible for the total efficiency of the emission enhancement.
2. Material and methods IZO/Ag/IZO (IAI) sandwiched films and reference IZO films as shown in Fig. 1 were deposited on a synthetic quartz substrate using radio frequency magnetron sputtering of IZO target (90 wt% In2 O3 –10 wt% ZnO) and 99.99% pure Ag target accordingly at room temperature. The top and bottom sides of IAI films were a 30 nmthick IZO layer with using an rf-power of 30 W, and the middle 14 nm-thick Ag layer was deposited with the rf-power of 15 W. The base pressure of the reaction chamber was 1.8 × 10−7 Torr, and the working pressure of Ar gas was 1.0 × 10−2 Torr. The nominal
T. Kiba et al. / Applied Surface Science 389 (2016) 906–910
Fig. 1. Schematics of the sample structure. A thickness of each layer was shown in the figure.
(a) IAI
8 6
40000
4 20000 2
Enhancement ratio
PL intensity (counts)
10 60000
IZO 0
2.2
2.4 2.6 2.8 3.0 Photon energy (eV)
3.2
3.4
0
9
(b)
80 60
Eg = 3.12 eV
2
2
-2
( h ) (eV cm )
100x10
40 20 0 1.5
2.0
2.5 3.0 Photon energy (eV)
3.5
Fig. 2. (a) PL spectra of bare IZO (blue solid line) and IAI sandwiched film (red solid line) excited by the femtosecond pulsed laser of 267 nm (=4.64 eV) at 6 K. Green line shows the PL enhancement ratio (IIAI /IIZO ) with- and without Ag thin film. (b) Tauc plot of the variation of (ah)2 with photon energy for the bare IZO, a was obtained from the spectroscopic ellipsometry. The direct optical gap is estimated by extrapolating to the energy axis as shown. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
thickness of Ag was determined from the deposition time with calibrated Ag deposition rate. The time-resolved photoluminescence spectra were measured by streak-camera (Hamamatsu, C4334) equipped with monochrometer. The samples are mounted on a cold-finger of closed-cycle He cryostat, and excited by third-harmonics of modelocked Ti:Sapphire laser (4.64 eV = 267 nm) whose time duration was 200 fs. The excitation power density was 3.8 W cm−2 . The timeresolution of this measurement was founded to be 50 ps after a deconvolution analysis. Extinction coefficient and refractive index of the films are determined by the spectroscopic ellipsometer (M500S, JASCO). 3. Results and discussion Fig. 2 shows the PL spectra of IZO and the IAI sandwiched films measured at 6 K. Both sample showed an intense broad green emission peaked around 2.34 eV, whereas a very weak emission band was also found at 3.10 eV. According to the literature, IZO thin film usually show a weak band-edge emission at 3.12 eV [13,14], and this assignment is also supported by the band gap value of 3.12 eV determined by the Tauc plot of our IZO thin film (shown in Fig. 2b) based on the ellipsometric data.
907
On the other hand, the emission band peaked at 2.76 eV is also reported by several paper [13,14], and it was attributed to the emission from defect states probably originated from the oxygen vacancy. In our case, the weak emission at 3.10 eV is originated from the band-edge emission, while the intensity is rather lower than the previous reports. The emission energy of the broad peak around 2.34 eV is slightly different from the literature value of defect related emission. The possible origin of this intense peak can be different type of defects, whose energy levels are much deeper. The lower intensity of the band edge emission is well corresponded to the reported PL spectra in the literature, where it usually appeared as shoulder on the blue side of the defect related emission [13,14]. This can be explained by the existence of large number of emissive defect formed in IZO, because it is amorphous film, and especially in our case, the IZO film was grown in Ar atmosphere thus the number of oxygen vacancy can be larger. Carriers, which are generated considerably higher than the band-edge upon excitation at 4.64 eV, are easily trapped by these emissive defect states much faster than the radiative recombination at the band-edge of IZO. Comparing the PL spectra of IZO and IAI sandwiched film, the PL intensity of IAI showed 2 fold increase in the integrated PL intensity from the reference IZO emission at the green emission band. The PL enhancement ratio depends on the emission energy, and increased up to 8 when it rises to the higher energy side of the green emission. The band edge emission located around 3.10 eV for IAI is also enhanced by 2–3 times compare to that of IZO. These emission enhancements can be attributed to the quasi-resonant or resonant coupling of the surface plasmon in IZO/Ag interface with the light emission. The plasmon resonant energy of bulk silver is 3.76 eV, but this SP energy of silver is lowered by the interface material of IZO up to 3.11 eV, according to the literature [13]. Therefore the observed emission energy dependence of the emission enhancement for green band shows a reasonable trend, where the enhancement ratio gradually increases as the energy moves toward the blue wing of the green emission band. For the band-edge emission, the enhancement ratio was limited up to 3 times. This is probably due to the balance of the competitive processes of surface plasmon excitation and the relaxation into the green emissive defect states. Concerning the Ag thickness, we have observed the PL enhancement by using 14 nm thick Ag thin film. This observation is consistent with the previous report of the same IZO/Ag/IZO materials [13], where the Ag thickness dependence of emission enhancement was examined and the increase in enhancement ratio with increasing the Ag thickness was observed from 12 nm, which was the onset for forming a continuous film, to 16 nm. There are several experimental studies showing the increase the emission enhancement ratio when the metal (Al, Ag etc.) thickness increasing up to 80–200 nm, especially in the ZnO/Metal system [16,17]. Neogi et al. showed the results of theoretical calculation of emission enhancement factor in the InGaN quantum well with Ag thin film [18], and the maximum enhancement factor increased as the Ag thickness increases below 20 nm, and it saturate with further increase of Ag thickness above 20 nm. The slight red shift of the resonance energy with peak broadening as decreasing the Ag thickness were also reported [18]. From these experimental results and theoretical simulation, the effect of surface plasmonic enhancement using 14 nm thick Ag film might be much weaker than that of thicker Ag film. However, the red shifted resonance energy and its broadened spectra may be also favorable for enhancing the broad emission of defect state observed at lower energy side. The localized plasmon due to the roughened surface of 14 nm-thick Ag film can be also excited. According to the AFM measurements of a similar thick Ag-film on glass substrate, and IZO bare film, the surface roughness of Ag film can be within a few nm.
T. Kiba et al. / Applied Surface Science 389 (2016) 906–910
Normalized PL inensty (arb. unit)
908
1 4 2
0.1 4 2
(a). IZO
0.01
Normalized PL intensity (arb. unit)
0
1
2 3 4 Time (ns)
5
6
1 4
Fig. 4. (a) PL time constants as a function of emission energy obtained from the fitting analysis for bare IZO (0 ) and IAI sandwiched sample (1 , 2 ) using single and double exponential functions, respectively. (b) The emission energy dependence of the Purcell factor which was derived by the experimentally obtained PL timeconstants.
2
0.1 4 2
(b). IAI
0.01 0
1
2 3 4 Time (ns)
5
6
Fig. 3. Typical PL decay profiles of (a) bare IZO film and (b) IAI sandwithed film with the energy window of 2.35–2.45 eV at 6 K. Solid lines are fitting results of the experimental data using single and double exponential decay functions for IZO and IAI, respectively.
Existence of this Ag roughness is necessary to scatter the excited surface plasmon coupled with the emission, and to convert it into the free space radiation which results in the emission enhancement. Therefore we considered that the localized plasmon of Ag roughened surface can be also affect the emission enhancement, and its resonance (LPR) energy can be slightly lower than that of the original surface plasmon resonance (SPR) energy of Ag thin film. Lowered energy of LPR from that of SPR (3.11 eV) is favorable for enhancing the emission of the defect state (2.34 eV). In order to gain more insight into the emission enhancement mechanism via surface plasmon effects, the time-resolved PL measurements of IZO and IAI were carried out. Fig. 3 shows the typical PL decay profiles of IZO and IAI films with the energy window of 2.35–2.45 eV, with the 267 nm excitation at 6 K. The obtained PL time-profiles are analyzed using single- or double exponential fittings. The PL decay curves of IZO samples can be fitted by single exponential function with the time constants (0 = 1.6–2.6 ns) for whole energy range of the green emission, whereas the faster decay component with the lifetime (1 = 110–210 ps) was necessary, in addition to the slower decay component (2 = 2.5–3.7 ns), for the PL decay curves of IAI. We attributed the nanosecondscale decaying components (0 and 2 ), found in both IZO and IAI, to the intrinsic recombination lifetime in the IZO defect states. The faster component (1 ) observed only in IAI can be attributed to the enhanced radiative recombination life affected by the Ag surface plasmonic resonance. The emission lifetime shortening is commonly observed in the case of SP or LP resonant emission enhancement. When the photogenerated excitons in the emissive material and the SP/LP at the metallic surface or nanostructured metal are spatially and energetically close each other, the efficient recombination via SP/LP emission occurs, as a results, the shorter lifetime in the PL decay profile is observed. Since this emissive process via SP/LP is faster than the other non-radiative recombination processes, the photogenerated excitons are effectively used for the radiation, which results in the plasmonic enhancement of emission. From the observed shortened PL decay time (1 ), the timescale of non-radiative recombination at the defect states of IZO can be much slower than about 200 ps. Based on the experimental results of 2–8 fold enhancement for defect state PL, the
slowest limit of non-radiative recombination rate can be comparable to the radiative lifetime of a few ns observed in bare IZO. The degree of enhancement of radiative recombination process should be affected by the matching of plasmonic resonance and the emission energy. Because the density of the plasmonic states become larger when the emission energy is close to the plasmon resonant energy, the spontaneous emission rate is accelerated by the coupling with the surface plasmon due to its dispersion relation. In the theoretical calculation, even it is 0.5 eV away from resonant energy, there are still plasmonic states which can interact with the emission dipole, the emission enhancement is expected for the quasi-resonant case [18]. According to the above mentioned situations, we expect that the PL decay constants can become shorter as the emission energy reaching to the resonant energy of SP, when the plasmonic effects is dominant for emission enhancement. The obtained time constants as the function of emission energy were plotted in Fig. 4(a). The intrinsic lifetimes of IZO in defect states (0 :IZO and 2 :IAI) are almost constant for whole energy range in the green emission. The small variation can be attributable to the different kind of defects or surface states. In contrast, the faster component (1 ) observed only in IAI become shorter as the emission energy increases. The plasmonic resonant energy of Ag/IZO interface was estimated to be 3.11 eV from the literature [13]. In order to evaluate the enhancement of radiative recombination rate without the contribution from the difference in intrinsic lifetime, we used the Purcell factor (Fsp ) which is analog of that used for emission enhancement in the resonant cavity [18]. In the case of emission enhancement of an emissive material with the presence of metal, the Purcell factor is defined as Fsp =
kr + knr + ksp with metal = , bare kr + knr
where bare the radiative recombination rate without metal, and with metal the enhanced radiative recombination rate affected by the plasmonic resonance of metal, and kr and knr are the radiative and non-radiative recombination rate, respectively. ksp is the excitation (coupling) rate of the plasmon. In our case, the non-radiative recombination process cannot be ignored, because knr might be similar or much larger to than kr in our estimation. By using the following relationship between the observed PL lifetimes and the rate constants; 1 (= k0 ) = kr + knr 0
,
1 (= k1 ) = kr + knr + ksp 1
,
T. Kiba et al. / Applied Surface Science 389 (2016) 906–910
(a)
Excitaon @4.64eV
between emissive material and the metal thin film is closer. Therefore, in this IZO/Ag/IZO sandwiched structure, it is suggested that a part, not all, of the excited Ag SP can contribute to the emission enhancement, and the plasmonic loss could not be ignored. The introduction of a spacer layer between the emissive materials and the plasmonic metal film can minimize the plasmon loss, and the proper design of the layered structure is necessary for optimizing the total efficiency for the plasmonic emission enhancement.
(b) Ag SP
< 50 ps
Band edge
< 50 ps Band-edge emission @3.10 eV
Defect emission @2.34 eV
Defect States
909
Ag SP
200 ps Emission Enhancement
4. Conclusion Fig. 5. The photoexcited energy-flow diagram in (a) IZO and (b) IAI sandwiched structure. (a) In the case of IZO bulk sample without Ag surface, the photogenerated excitons can rapidly relaxed to the band-edge, subsequently, further relaxation process to the defect states occurs within the time-resolution of TRPL measurement system, it is much faster than the radiative-recombination takes place at band-edge. (b) In the presence of Ag surface plasmonic states, the excitons at defect states can coupled with Ag surface plasmon though the available states are less than the resonant case. As the results, emission enhancement was significantly observed at the defect states. In contrast, though the density of the plasmonic states is higher for the band-edge emission because it is resonant case, the plasmonic coupling can be less efficient due to the existence of fast competitive relaxation to the defect states.
we obtained the Purcell factor as Fsp =
kr + knr + ksp k1 0 = = 1 kr + knr k0
.
The calculated Fsp , which describes a degree of enhancement in radiative recombination rate, was plotted as a function of emission energy in Fig. 4(b). The Fsp clearly increased as the emission energy moves towards blue-side of green emission. This observed trend of Fsp for green emission band indicates that the emission enhancement in the IAI sandwiched sample is due to the surface plasmonic resonance effect of Ag thin film, though the emission energies are slightly detuned from its resonance energy. The maximum Fsp value of 19 was within a reasonable range, where it was same order compare with the literature value at 0.5 eV away from the resonant energy, obtained from the simulation of InGaN QW/Ag system [18]. While the reasonable increase of the Fsp towards the higher energy side was observed for the green emission band, there is no significant (enhancement) tendency for the band-edge emission though the emission energy is close to the resonance energy of SP. This is probably due to the fast relaxation to the emissive defect states, which result in a low-intensity of the band-edge PL, as schematically shown in Fig. 5. It is supported by the fact that the PL lifetime of band-edge emission was shorter than the instrumental response (<50 ps, data not shown) of the streak camera in the case of bare IZO. Therefore, the relaxation to the emissive defect states and the excitation of the SP occur within the same time scale at the band-edge, and these processes compete each other. This is because why the enhancement ratio stayed at lower level (∼2) for the bandedge emission even though its energy was almost resonant to the SP. Finally, we discuss the relationship between the observed enhancement ratio from the PL measurement and the estimated Fsp from the PL decay analysis. The similar tendencies for enhancement ratio and Fsp as the function of emission energy were observed for green emission band. However, its absolute values were different, where Fsp was much larger than the PL enhancement factor. This disagreement was originated from the assumption that there is no non-radiative relaxation path from the Ag SP states, that is, all energy used for the Ag SP excitation will spend for “re”-radiation via coupling with the emission light. Therefore, the obtained larger Fsp value can be slightly overestimated because there should be nonradiative (thermal) relaxation path, especially in the present case, Ag thin film and IZO are directly contacted without any spacer layer. This kind of “plasmonic loss” become significant when the distance
We report on the PL enhancement in IZO/Ag/IZO sandwiched structure due to the surface plasmonic effect of Ag film. The 2fold enhancement in integrated PL intensity was observed for the defect-related green emission peaked at 2.34 eV in the presence of Ag thin film. PL decay curves of defect-related emission in IAI had a faster lifetime component, which was absent in that in IZO without Ag film. By the relationship between the energy dependence of lifetime in IAI and Ag/IZO surface plasmon resonance energy, we conclude that the emission enhancement in IAI is due to surface plasmonic resonance effect of Ag thin film. The comparison between the results of static- and dynamic-PL measurement suggests that the non-radiative process after the energy transfer to the silver film also affects the total efficiency of the emission enhancement. Acknowledgement This work is supported in part by the Japan Society for the Promotion of Science (JSPS), Grant-in-Aid for Scientific Research (B) No. 16H04503. References [1] J.N. Anker, W.P. Hall, O. Lyandres, N.C. Shah, J. Zhao, R.P. Van Duyne, Biosensing with plasmonic nanosensors, Nat. Mater. 7 (2008) 442–453. [2] K.M. Mayer, J.H. Hafner, Localized surface plasmon resonance sensors, Chem. Rev. 111 (2011) 3828–3857. [3] K.A. Willets, R.P.V. Duyne, Localized surface plasmon resonance spectroscopy and sensing, Annu. Rev. Phys. Chem. 58 (2007) 267–297. [4] M. Takase, Y. Sawai, H. Nabika, K. Murakoshi, Detection of adsorption sites at the gap of a hetero-metal nano-dimer at the single molecule level, J. Photochem. Photobiol. A: Chem. 221 (2011) 169–174. [5] K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, A. Scherer, Surface-plasmon-enhanced light emitters based on InGaN quantum wells, Nat. Mater. 3 (2004) 601–605. ´ [6] A. Urbanczyk, G.J. Hamhuis, R. Nötzel, Coupling of single InGaAs quantum dots to the plasmon resonance of a metal nanocrystal, Appl. Phys. Lett. 97 (2010) 043105. [7] K. Nakaji, H. Li, T. Kiba, M. Igarashi, S. Samukawa, A. Murayama, Plasmonic enhancements of photoluminescence in hybrid Si nanostructures with Au fabricated by fully top-down lithography, Nanoscale Res. Lett. 7 (2012) 1–5. [8] Y. Zhong, K. Ueno, Y. Mori, X. Shi, T. Oshikiri, K. Murakoshi, H. Inoue, H. Misawa, Plasmon-assisted water splitting using two sides of the same srtio3 single-crystal substrate: conversion of visible light to chemical energy, Angew. Chem. Int. Ed. 53 (2014) 10350–10354. [9] K. Tateishi, M. Funato, Y. Kawakami, K. Okamoto, K. Tamada, Highly enhanced green emission from InGaN quantum wells due to surface plasmon resonance on aluminum films, Appl. Phys. Lett. 106 (2015) 121112. [10] Y. Nishijima, K. Ueno, Y. Yokota, K. Murakoshi, H. Misawa, Plasmon-assisted photocurrent generation from visible to near-infrared wavelength using a Au-nanorods/TiO2 electrode, J. Phys. Chem. Lett. 1 (2010) 2031–2036. [11] Y. Nishijima, L. Rosa, S. Juodkazis, Surface plasmon resonances in periodic and random patterns of gold nano-disks for broadband light harvesting, Opt. Express 20 (2012) 11466–11477. [12] T. Chiba, Y. Kudo, M. Kawamura, Y. Abe, K.H. Kim, Preparation of an indium zinc oxide—silver—indium zinc oxide multilayer film and its application in organic light-emitting diodes, Vacuum 121 (2015) 320–322. [13] D.H. Shin, H.T. Oh, S.H. Choi, J.W. Park, H.S. Lee, Y.S. Park, H.K. Kim, Surface-plasmon-mediated enhancement of photoluminescence from hybrid structures of indium zinc oxide/Ag/indium zinc oxide, J. Korean Phys. Soc. 56 (2010) 1164–1166. [14] J. Sun, J. Xu, X. Tang, Y. Huang, C. Tang, C. Han, Y. Gong, H. Gong, Unexpected violet and blue light emission from amorphous indium zinc oxide (IZO) with silver nanoparticle embedment, Opt. Mater. Express 5 (2015) 1331–1338.
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[15] X. Fang, C.L. Mak, J. Dai, K. Li, H. Ye, C.W. Leung, ITO/Au/ITO sandwich structure for near-infrared plasmonics, ACS Appl. Mater. Interfaces 6 (2014) 15743–15752. [16] C.W. Lai, J. An, H.C. Ong, Surface-plasmon-mediated emission from metal-capped ZnO thin films, Appl. Phys. Lett. 86 (2005) 251105.
[17] W.H. Ni, J. An, C.W. Lai, H.C. Ong, J.B. Xu, Emission enhancement from metallodielectric-capped ZnO films, J. Appl. Phys. 100 (2006) 026103. [18] A. Neogi, C.-W. Lee, H.O. Everitt, T. Kuroda, A. Tackeuchi, E. Yablonovitch, Enhancement of spontaneous recombination rate in a quantum well by resonant surface plasmon coupling, Phys. Rev. B 66 (2002) 153305.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Emission enhancement in indium zinc oxide(IZO)/Ag/IZO sandwiched structure due to surface plasmon resonance of thin Ag film Takayuki Kiba a,∗ , Kazuki Yanome a , Midori Kawamura a , Yoshio Abe a , Kyung Ho Kim a , Junichi Takayama b , Akihiro Murayama b a b
Department of Materials Science and Engineering, Kitami Institute of Technology, Kitami 090-8507, Japan Graduate School of Information Science and Technology, Hokkaido University, Sapporo 060-0814, Japan
a r t i c l e
i n f o
Article history: Received 30 May 2016 Received in revised form 13 July 2016 Accepted 7 August 2016 Available online 9 August 2016 Keywords: Surface plasmon Plasmonic emission enhancement Photoluminescence Time-resolved spectroscopy Silver thin film
a b s t r a c t We report on a photoluminescence (PL) enhancement in IZO/Ag/IZO sandwiched structure via surface plasmonic effects of 14 nm-thick Ag film. In the presence of Ag thin film, the 2–8-fold enhancement was observed for the broad PL around 2.34 eV, which can be originated from defect states in amorphous IZO film. The results of time-resolved PL spectra suggested that the increase in radiative recombination rate, and the maximum Purcell factor of 19 was estimated from the analysis of the PL decay profiles. The comparison between the results of static- and dynamic-PL measurement suggests that the non-radiative process after the excitation of the surface plasmon of the silver film also affects the total efficiency of the emission enhancement. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Surface plasmon (SP) or localized plasmon (LP) have attracted increasing attention because their extensive applications in sensing [1,2], enhanced spectrum [3,4], enhancing transmission and emission [5–9], light harvesting antenna [10,11] and so on. The plasmon mediated enhancement of the intensity of photoluminescence (PL) in semiconductor or organic dye induced by metallic nanostructures are very attractive, because it can directly contribute to the efficiency of light emitting devices. The efficiency of SP or LP coupling on the PL enhancement is decided by plasmon resonance frequency, which can be controlled by the size, shape and the selection of metals for the nanostructures. Indium zinc oxide (IZO) has received much attention for its application to the light emitting devices as a transparent conducting electrode. Especially, IZO/Ag/IZO sandwiched film was successfully utilized for the anode of OLEDs [12], with high transmittance and low sheet-resistance, and reducing the indium consumption compared to the ITO electrode. Recently, this kind of dielectric/metal/dielectric sandwiched structures are investigated also in the view point of emission enhancement via SP or LP [13–15], because there are two SP modes (dielectric/metal interface) can
∗ Corresponding author. E-mail address: [email protected] (T. Kiba). http://dx.doi.org/10.1016/j.apsusc.2016.08.032 0169-4332/© 2016 Elsevier B.V. All rights reserved.
be involved, thus much larger emission enhancement is expected. Understanding the mechanism of the plasmonic PL enhancement in this sandwiched structure can be helpful for the emission device application, where the new functionality will add to this kind of dielectric/metal/dielectric transparent electrode. Here, we report on the PL enhancement in IZO/Ag/IZO via surface plasmonic effect of Ag thin film. The 2–8-fold enhancement was observed for the broad PL around 2.34 eV. The emission energy dependence of the time-resolved PL suggested that the increase in radiative recombination rate can be attributed to the SP resonance effects, whereas the non-radiative process at the silver thin film (=plasmonic loss) is not negligible for the total efficiency of the emission enhancement.
2. Material and methods IZO/Ag/IZO (IAI) sandwiched films and reference IZO films as shown in Fig. 1 were deposited on a synthetic quartz substrate using radio frequency magnetron sputtering of IZO target (90 wt% In2 O3 –10 wt% ZnO) and 99.99% pure Ag target accordingly at room temperature. The top and bottom sides of IAI films were a 30 nmthick IZO layer with using an rf-power of 30 W, and the middle 14 nm-thick Ag layer was deposited with the rf-power of 15 W. The base pressure of the reaction chamber was 1.8 × 10−7 Torr, and the working pressure of Ar gas was 1.0 × 10−2 Torr. The nominal
T. Kiba et al. / Applied Surface Science 389 (2016) 906–910
Fig. 1. Schematics of the sample structure. A thickness of each layer was shown in the figure.
(a) IAI
8 6
40000
4 20000 2
Enhancement ratio
PL intensity (counts)
10 60000
IZO 0
2.2
2.4 2.6 2.8 3.0 Photon energy (eV)
3.2
3.4
0
9
(b)
80 60
Eg = 3.12 eV
2
2
-2
( h ) (eV cm )
100x10
40 20 0 1.5
2.0
2.5 3.0 Photon energy (eV)
3.5
Fig. 2. (a) PL spectra of bare IZO (blue solid line) and IAI sandwiched film (red solid line) excited by the femtosecond pulsed laser of 267 nm (=4.64 eV) at 6 K. Green line shows the PL enhancement ratio (IIAI /IIZO ) with- and without Ag thin film. (b) Tauc plot of the variation of (ah)2 with photon energy for the bare IZO, a was obtained from the spectroscopic ellipsometry. The direct optical gap is estimated by extrapolating to the energy axis as shown. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
thickness of Ag was determined from the deposition time with calibrated Ag deposition rate. The time-resolved photoluminescence spectra were measured by streak-camera (Hamamatsu, C4334) equipped with monochrometer. The samples are mounted on a cold-finger of closed-cycle He cryostat, and excited by third-harmonics of modelocked Ti:Sapphire laser (4.64 eV = 267 nm) whose time duration was 200 fs. The excitation power density was 3.8 W cm−2 . The timeresolution of this measurement was founded to be 50 ps after a deconvolution analysis. Extinction coefficient and refractive index of the films are determined by the spectroscopic ellipsometer (M500S, JASCO). 3. Results and discussion Fig. 2 shows the PL spectra of IZO and the IAI sandwiched films measured at 6 K. Both sample showed an intense broad green emission peaked around 2.34 eV, whereas a very weak emission band was also found at 3.10 eV. According to the literature, IZO thin film usually show a weak band-edge emission at 3.12 eV [13,14], and this assignment is also supported by the band gap value of 3.12 eV determined by the Tauc plot of our IZO thin film (shown in Fig. 2b) based on the ellipsometric data.
907
On the other hand, the emission band peaked at 2.76 eV is also reported by several paper [13,14], and it was attributed to the emission from defect states probably originated from the oxygen vacancy. In our case, the weak emission at 3.10 eV is originated from the band-edge emission, while the intensity is rather lower than the previous reports. The emission energy of the broad peak around 2.34 eV is slightly different from the literature value of defect related emission. The possible origin of this intense peak can be different type of defects, whose energy levels are much deeper. The lower intensity of the band edge emission is well corresponded to the reported PL spectra in the literature, where it usually appeared as shoulder on the blue side of the defect related emission [13,14]. This can be explained by the existence of large number of emissive defect formed in IZO, because it is amorphous film, and especially in our case, the IZO film was grown in Ar atmosphere thus the number of oxygen vacancy can be larger. Carriers, which are generated considerably higher than the band-edge upon excitation at 4.64 eV, are easily trapped by these emissive defect states much faster than the radiative recombination at the band-edge of IZO. Comparing the PL spectra of IZO and IAI sandwiched film, the PL intensity of IAI showed 2 fold increase in the integrated PL intensity from the reference IZO emission at the green emission band. The PL enhancement ratio depends on the emission energy, and increased up to 8 when it rises to the higher energy side of the green emission. The band edge emission located around 3.10 eV for IAI is also enhanced by 2–3 times compare to that of IZO. These emission enhancements can be attributed to the quasi-resonant or resonant coupling of the surface plasmon in IZO/Ag interface with the light emission. The plasmon resonant energy of bulk silver is 3.76 eV, but this SP energy of silver is lowered by the interface material of IZO up to 3.11 eV, according to the literature [13]. Therefore the observed emission energy dependence of the emission enhancement for green band shows a reasonable trend, where the enhancement ratio gradually increases as the energy moves toward the blue wing of the green emission band. For the band-edge emission, the enhancement ratio was limited up to 3 times. This is probably due to the balance of the competitive processes of surface plasmon excitation and the relaxation into the green emissive defect states. Concerning the Ag thickness, we have observed the PL enhancement by using 14 nm thick Ag thin film. This observation is consistent with the previous report of the same IZO/Ag/IZO materials [13], where the Ag thickness dependence of emission enhancement was examined and the increase in enhancement ratio with increasing the Ag thickness was observed from 12 nm, which was the onset for forming a continuous film, to 16 nm. There are several experimental studies showing the increase the emission enhancement ratio when the metal (Al, Ag etc.) thickness increasing up to 80–200 nm, especially in the ZnO/Metal system [16,17]. Neogi et al. showed the results of theoretical calculation of emission enhancement factor in the InGaN quantum well with Ag thin film [18], and the maximum enhancement factor increased as the Ag thickness increases below 20 nm, and it saturate with further increase of Ag thickness above 20 nm. The slight red shift of the resonance energy with peak broadening as decreasing the Ag thickness were also reported [18]. From these experimental results and theoretical simulation, the effect of surface plasmonic enhancement using 14 nm thick Ag film might be much weaker than that of thicker Ag film. However, the red shifted resonance energy and its broadened spectra may be also favorable for enhancing the broad emission of defect state observed at lower energy side. The localized plasmon due to the roughened surface of 14 nm-thick Ag film can be also excited. According to the AFM measurements of a similar thick Ag-film on glass substrate, and IZO bare film, the surface roughness of Ag film can be within a few nm.
T. Kiba et al. / Applied Surface Science 389 (2016) 906–910
Normalized PL inensty (arb. unit)
908
1 4 2
0.1 4 2
(a). IZO
0.01
Normalized PL intensity (arb. unit)
0
1
2 3 4 Time (ns)
5
6
1 4
Fig. 4. (a) PL time constants as a function of emission energy obtained from the fitting analysis for bare IZO (0 ) and IAI sandwiched sample (1 , 2 ) using single and double exponential functions, respectively. (b) The emission energy dependence of the Purcell factor which was derived by the experimentally obtained PL timeconstants.
2
0.1 4 2
(b). IAI
0.01 0
1
2 3 4 Time (ns)
5
6
Fig. 3. Typical PL decay profiles of (a) bare IZO film and (b) IAI sandwithed film with the energy window of 2.35–2.45 eV at 6 K. Solid lines are fitting results of the experimental data using single and double exponential decay functions for IZO and IAI, respectively.
Existence of this Ag roughness is necessary to scatter the excited surface plasmon coupled with the emission, and to convert it into the free space radiation which results in the emission enhancement. Therefore we considered that the localized plasmon of Ag roughened surface can be also affect the emission enhancement, and its resonance (LPR) energy can be slightly lower than that of the original surface plasmon resonance (SPR) energy of Ag thin film. Lowered energy of LPR from that of SPR (3.11 eV) is favorable for enhancing the emission of the defect state (2.34 eV). In order to gain more insight into the emission enhancement mechanism via surface plasmon effects, the time-resolved PL measurements of IZO and IAI were carried out. Fig. 3 shows the typical PL decay profiles of IZO and IAI films with the energy window of 2.35–2.45 eV, with the 267 nm excitation at 6 K. The obtained PL time-profiles are analyzed using single- or double exponential fittings. The PL decay curves of IZO samples can be fitted by single exponential function with the time constants (0 = 1.6–2.6 ns) for whole energy range of the green emission, whereas the faster decay component with the lifetime (1 = 110–210 ps) was necessary, in addition to the slower decay component (2 = 2.5–3.7 ns), for the PL decay curves of IAI. We attributed the nanosecondscale decaying components (0 and 2 ), found in both IZO and IAI, to the intrinsic recombination lifetime in the IZO defect states. The faster component (1 ) observed only in IAI can be attributed to the enhanced radiative recombination life affected by the Ag surface plasmonic resonance. The emission lifetime shortening is commonly observed in the case of SP or LP resonant emission enhancement. When the photogenerated excitons in the emissive material and the SP/LP at the metallic surface or nanostructured metal are spatially and energetically close each other, the efficient recombination via SP/LP emission occurs, as a results, the shorter lifetime in the PL decay profile is observed. Since this emissive process via SP/LP is faster than the other non-radiative recombination processes, the photogenerated excitons are effectively used for the radiation, which results in the plasmonic enhancement of emission. From the observed shortened PL decay time (1 ), the timescale of non-radiative recombination at the defect states of IZO can be much slower than about 200 ps. Based on the experimental results of 2–8 fold enhancement for defect state PL, the
slowest limit of non-radiative recombination rate can be comparable to the radiative lifetime of a few ns observed in bare IZO. The degree of enhancement of radiative recombination process should be affected by the matching of plasmonic resonance and the emission energy. Because the density of the plasmonic states become larger when the emission energy is close to the plasmon resonant energy, the spontaneous emission rate is accelerated by the coupling with the surface plasmon due to its dispersion relation. In the theoretical calculation, even it is 0.5 eV away from resonant energy, there are still plasmonic states which can interact with the emission dipole, the emission enhancement is expected for the quasi-resonant case [18]. According to the above mentioned situations, we expect that the PL decay constants can become shorter as the emission energy reaching to the resonant energy of SP, when the plasmonic effects is dominant for emission enhancement. The obtained time constants as the function of emission energy were plotted in Fig. 4(a). The intrinsic lifetimes of IZO in defect states (0 :IZO and 2 :IAI) are almost constant for whole energy range in the green emission. The small variation can be attributable to the different kind of defects or surface states. In contrast, the faster component (1 ) observed only in IAI become shorter as the emission energy increases. The plasmonic resonant energy of Ag/IZO interface was estimated to be 3.11 eV from the literature [13]. In order to evaluate the enhancement of radiative recombination rate without the contribution from the difference in intrinsic lifetime, we used the Purcell factor (Fsp ) which is analog of that used for emission enhancement in the resonant cavity [18]. In the case of emission enhancement of an emissive material with the presence of metal, the Purcell factor is defined as Fsp =
kr + knr + ksp with metal = , bare kr + knr
where bare the radiative recombination rate without metal, and with metal the enhanced radiative recombination rate affected by the plasmonic resonance of metal, and kr and knr are the radiative and non-radiative recombination rate, respectively. ksp is the excitation (coupling) rate of the plasmon. In our case, the non-radiative recombination process cannot be ignored, because knr might be similar or much larger to than kr in our estimation. By using the following relationship between the observed PL lifetimes and the rate constants; 1 (= k0 ) = kr + knr 0
,
1 (= k1 ) = kr + knr + ksp 1
,
T. Kiba et al. / Applied Surface Science 389 (2016) 906–910
(a)
Excitaon @4.64eV
between emissive material and the metal thin film is closer. Therefore, in this IZO/Ag/IZO sandwiched structure, it is suggested that a part, not all, of the excited Ag SP can contribute to the emission enhancement, and the plasmonic loss could not be ignored. The introduction of a spacer layer between the emissive materials and the plasmonic metal film can minimize the plasmon loss, and the proper design of the layered structure is necessary for optimizing the total efficiency for the plasmonic emission enhancement.
(b) Ag SP
< 50 ps
Band edge
< 50 ps Band-edge emission @3.10 eV
Defect emission @2.34 eV
Defect States
909
Ag SP
200 ps Emission Enhancement
4. Conclusion Fig. 5. The photoexcited energy-flow diagram in (a) IZO and (b) IAI sandwiched structure. (a) In the case of IZO bulk sample without Ag surface, the photogenerated excitons can rapidly relaxed to the band-edge, subsequently, further relaxation process to the defect states occurs within the time-resolution of TRPL measurement system, it is much faster than the radiative-recombination takes place at band-edge. (b) In the presence of Ag surface plasmonic states, the excitons at defect states can coupled with Ag surface plasmon though the available states are less than the resonant case. As the results, emission enhancement was significantly observed at the defect states. In contrast, though the density of the plasmonic states is higher for the band-edge emission because it is resonant case, the plasmonic coupling can be less efficient due to the existence of fast competitive relaxation to the defect states.
we obtained the Purcell factor as Fsp =
kr + knr + ksp k1 0 = = 1 kr + knr k0
.
The calculated Fsp , which describes a degree of enhancement in radiative recombination rate, was plotted as a function of emission energy in Fig. 4(b). The Fsp clearly increased as the emission energy moves towards blue-side of green emission. This observed trend of Fsp for green emission band indicates that the emission enhancement in the IAI sandwiched sample is due to the surface plasmonic resonance effect of Ag thin film, though the emission energies are slightly detuned from its resonance energy. The maximum Fsp value of 19 was within a reasonable range, where it was same order compare with the literature value at 0.5 eV away from the resonant energy, obtained from the simulation of InGaN QW/Ag system [18]. While the reasonable increase of the Fsp towards the higher energy side was observed for the green emission band, there is no significant (enhancement) tendency for the band-edge emission though the emission energy is close to the resonance energy of SP. This is probably due to the fast relaxation to the emissive defect states, which result in a low-intensity of the band-edge PL, as schematically shown in Fig. 5. It is supported by the fact that the PL lifetime of band-edge emission was shorter than the instrumental response (<50 ps, data not shown) of the streak camera in the case of bare IZO. Therefore, the relaxation to the emissive defect states and the excitation of the SP occur within the same time scale at the band-edge, and these processes compete each other. This is because why the enhancement ratio stayed at lower level (∼2) for the bandedge emission even though its energy was almost resonant to the SP. Finally, we discuss the relationship between the observed enhancement ratio from the PL measurement and the estimated Fsp from the PL decay analysis. The similar tendencies for enhancement ratio and Fsp as the function of emission energy were observed for green emission band. However, its absolute values were different, where Fsp was much larger than the PL enhancement factor. This disagreement was originated from the assumption that there is no non-radiative relaxation path from the Ag SP states, that is, all energy used for the Ag SP excitation will spend for “re”-radiation via coupling with the emission light. Therefore, the obtained larger Fsp value can be slightly overestimated because there should be nonradiative (thermal) relaxation path, especially in the present case, Ag thin film and IZO are directly contacted without any spacer layer. This kind of “plasmonic loss” become significant when the distance
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