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Use of Cu – Ag bi-layer fi lms in oxide/metal/oxide transparent electrodes to widen their spectra of transmittance
Jean Chistian Bernède
a,n, Linda Cattin
b, Tahar Abachi
a,1, Yendoubé Lare
a,2, Mustapha Morsli
c, Mohammed Makha
aaL’UNAM, Université de Nantes, MOLTECH-Anjou, CNRS, UMR 6200, 2 rue de la Houssinière, BP 92208, 44322 Nantes Cedex3, France
bUniversité de Nantes, Institut des Matériaux Jean Rouxel (IMN), CNRS, UMR 6502, 2 rue de la Houssinière, BP 92208, 44322 Nantes cedex 3, France
cL’UNAM, Université de Nantes, Faculté des Sciences et des Techniques, 2 rue de la Houssinière, BP 92208, Nantes F-44000 France
a r t i c l e i n f o
Article history:
Received 16 July 2013 Accepted 7 September 2013 Available online 14 September 2013 Keywords:
Thinfilms Physical deposition ITO free electrode
Transparent conductive electrode Cu/Ag metal bi-layer
Transmittance Electrical properties
a b s t r a c t
Original ZnO/Cu/Ag/MoO3multilayer structures were deposited under vacuum. The optical transmittance spectrum of these structures is significantly broadened by using a double layer as metal interlayer. While the thickness of Ag was 6 nm, that of Cu was used as parameter. The highest averaged transmittance, 88% between 400 and 700 nm is obtained with the structure ZnO (20 nm)/ Cu (3 nm)/Ag (6 nm)/ MoO3. However, a better factor of merit is achieved,ΦM¼16103, when the Cu thickness is 4 nm, making that these innovative ZnO/Cu/Ag/MoO3structures are very promising for use as substitute to ITO electrodes.
&2013 Elsevier B.V. All rights reserved.
1. Introduction
The need for transparent conductive electrode (TCE) is increas- ing continuously due to applications in optoelectronic devices.
Indium tin oxide (ITO) is the current choice, however the scarcity of indium, the poorflexibility of ITO induce a need for alternative TCE [1,2]. Among all the possible solutions oxide/metal/oxide' (O/M/O') multilayers structures have been extensively studied[3].
If these structures exhibits small sheet resistance, their spectrum of transparency is narrower than that of ITO. The experience shows that the transmittance peak wavelength depends on the metal [4,5]. In afirst approach, Fresnel equations were used to predict the transmittance potential of ultra thin metalfilms[6]. It was shown that flat broad transmittance spectra are expected when two metals, Ag/Cu, are involved. In the present work we show that the use of a bilayer M¼Cu/Ag allows broadening significantly the value of the averaged (400–700 nm) transmit- tance of the O/M/O' structures.
2. Experimental procedures
We have shown that the structures MoO3 (20 nm)/M(9– 11 nm)/MoO3(35 nm) permit to achieve promising performances if Ag is used as metal, what is not the case if copper is the metal [4]. Indeed, Cu diffuses into MoO3, which limits the transmittance and the conductivity of the structures[5]. On the other hand, it is known that Cu does not diffuse when it is deposited onto ZnO[7].
According to all these results, we used ZnO for the bottom layer and MoO3for the top layer in ZnO (20 nm)/Cu/Ag/MoO3(35 nm) structures. If ZnO is used as bottom layer because it allows preventing the Cu diffusion, MoO3is used as top layer because it is well known that it induces efficient hole injecting (collecting) contact with organic materials[8]. Therefore, we studied ZnO/Cu/
Ag/MoO3structures, while ZnO/Ag/MoO3and ZnO/Cu/MoO3were used as references. The multilayer structures were deposited onto glass substrates, under vacuum.
The ZnOfilms were sputtered from a ZnO ceramic target. They were deposited by rf magnetron sputtering. The sputtering gas used was Ar/O2mixture with 2% O2gas sources, the pressure was 0.13 Pa.
The target-substrate distance was 60 mm, the power deposition 150 W, the flow gas 35 sccm and the sputtering time 20 min. The ZnOfilms obtained have high transmittance, above 90% in the visible range for a thickness around 0.5μm, their typical conductivity iss# 1105(Ωcm)1and they are quite smooth[9].
After deposition of the ZnO layer through the rf magnetron sputtering apparatus, a simple joule effect evaporation system was Contents lists available atScienceDirect
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Materials Letters
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http://dx.doi.org/10.1016/j.matlet.2013.09.039
nCorresponding author.
E-mail address:[email protected] (J.C. Bernède).
1Permanent address: ENS Kouba, Alger, Algérie, Laboratoire sur l'énergie solaire, BP 1515 Lomé, Togo.
2Permanent address: Université de Lomé, Faculté Des Sciences, Laboratoire sur l'énergie solaire, BP 1515 Lomé, Togo.
Materials Letters 112 (2013) 187–189
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used for the deposition of the following layers[4]. Deposition rate and film thickness were measured in situ by quartz monitor.
The deposition rate of the evaporated films was 0.5 nm/s and 3 nm/s for Cu and Ag respectively.
The surface topography was observed with a field emission scanning electron microscope (SEM, JEOL F-7600). The SEM opera- ting voltage was 5 kV.
The optical density was measured at wavelengths of 0.3μm to 1.2μm with a Carry spectrometer. The electrical resistivity was determined by measurements in a van der Pauw configuration.
The conductivity of Cu being slightly smaller than that of Ag, in order to broaden the transmittance spectrum of the multilayer structure without penalizing its conductivity, we choose using, for afirst study, an Ag/Cu ratio higher than unity. Since in the case of classical O/M/O structures with single Ag metal layer, the optimum metal layer thickness is 9–11 nm [4], we used a metal bilayer consisting in a Cu layer thick of 3–5 nm and an Ag layer thick of 6 nm.
3. Results and discussion
As shown in Table 1, the highest averaged transmittance achieved is 88%, between 400 and 700 nm, with the metal bi-layer Cu (3 nm)/Ag (6 nm). However, the value of its sheet resistanceRsh(Rsh¼79Ω/square) is fairly high. Therefore, in order to obtain high conductivity it is necessary to increase the Cufilm thickness. However, simultaneously, the transmittance decreases.
So, in order to determine the best structure we used the Figure of merit, which has been defined by Haacke[10]as followΦTC¼ T10/Rsh, withRshsheet resistance andTaveraged transmittance.
It can be seen in Table 1 that, when the Cu film thickness increases from 3 nm to 4 nm there is a strong improvement ofΦTC. It is remarkable that such small change in the thickness of the Cu interlayer, that was sandwiched between a pair of ZnO(20 nm) and Ag(6 nm)/MoO3(35 nm) makes a drastic change in the sheet resistance. As a matter of fact, the conductivity of the structures MoO3/Metal/MoO3depends strongly on the metalfilm thickness and morphology. It is known there is a commutation of the conductivity of the structures from an insulating state to a con- ductive state when there is percolation of the metalfilm, ie, when the metalfilm becomes continuous. The percolation threshold value is classically around 10 nm. In the case of MoO3/Ag/MoO3 we showed in theTable 1of a previous publication[11]that near the threshold value, for a variation of 1 nm, the resistance can change of 7 orders of magnitude.
In the present letter, the whole thickness of the metal (AgþCu) is 9 nm to 11 nm, therefore it is not unexpected to measure a drastic change in the sheet resistance, due to the fact that, we are situated on the verge of the switching. As a matter of fact for a sampling of 10 structures, which Cu thickness is situated between 4 and 5, ΦTC tends to be around 15103Ω1, which is a completely satisfactory value. Then, for thicker Cu layer, it decreases.
In Fig. 1 we compare the transmittance spectrum of the different structures. The structure ZnO/Cu(4 nm)/Ag(6 nm)/MoO3
exhibits a good factor of merit (16103) and an averaged transmittance of 83%. One can see that the maximum transmit- tance are, 77% atλ¼405 nm, in the case of ZnO/Ag/MoO3and 69%
at 590 nm, in the case of ZnO/Cu/MoO3. For the structure with Cu the maximum transparency is quite small. As a matter of fact we have shown in an earlier work [5], that Cu also diffuses in the upper layer of MoO3, which explains its poor performances. In the case of ZnO/Cu/Ag/MoO3, there is neither Cu diffusion into the dense ZnO bottom layer[7], nor in the MoO3upper layer, because the Ag layer separates the Cu from the upper oxide layer.
So, we note that, when a Cu/Ag bilayer is used as metal interlayer, there is a significant broadening of the transmittance spectrum of the multilayer structure. The transmittance maximum of the structure ZnO/Cu/Ag/MoO3is 86% at 586 nm.
These experimental results are in good agreement with thefirst theoretical predictions. They show that the combination of metals, which present transmittance peaks at different wavelengths in the visible region, allows achieving aflat wavelength response in the visible region. As a matter of fact, the ZnO/Cu/Ag/MoO3multilayer structure shows a quiteflat transmittance spectrum compared to the classical reference structures, ZnO/Ag/MoO3 and ZnO/Cu/
MoO3. The limits of the plateau of high transmittance correspond to the maximum transmittance of the ZnO/Ag/MoO3structure for the low wavelengths and ZnO/Cu/MoO3 structure for the high wavelengths. This broadening of the transmittance spectrum induces a red shift of its high wavelength side, which moves it towards the ZnO spectrum.
The broadening of the transmittance spectrum of the ZnO/Cu/
Ag/MoO3structure through the use of the Cu/Ag metal bilayer can be explained as follow. It is known that the transmittance of ultra thin metalfilms is not uniform in the visible. As a matter of fact, they exhibit a transmittance peak due to the plasma formation at a characteristic frequency (ωp¼(4πne2/mnε0)1/2), where n is the density of electrons,eis the electric charge, mn is the effective mass of the electron andε0is the permittivity of free space.
This behavior induces limited averaged transmittance and non- uniform wavelength response. Afirst possibility to counteract the decrease of the transmittance associated to the reflection above the plasma frequency consists in covering the metal of a layer of oxide, such as MoO3, ZnO,…However the width of the transmit- tance range remains limited. Here, in addition to this effect, we broaden this range by combining two different ultra thin metal films which present transmittance peak at different wavelengths in the visible. As shown above, this original combination presents a transmittance spectrum flatter than that of individual metal layer.
InFig. 2we present the surface visualization of structures glass/
ZnO/Cu and glass/ZnO/Cu/Ag. We can see that the surface morphology Table 1
Variation of the optical and electrical properties of the ZnO/Cu/Ag/MoO3with the Cu layer thickness, that of Ag beingfixed at 6 nm.
Thickness Cu (nm)
Tmax(%)
(λ¼580 nm) Taveraged (%)
s (Ωcm)1
Rsh(Ω/
square) ΩTC
3 92 88 1.95103 79 3.6103
3.5 86.5 83 1.40104 11 14103
4 83 81 2.05104 7.55 16103
400 600 800 1000 1200
0 20 40 60 80 100
Transmittance (%)
λ (nm) a- ZnO/Ag/MoO3
b- ZnO/Cu/MoO3 c- ZnO/Cu/Ag/MoO3
d- ZnO
Fig. 1.Transmittance spectra of spectra of different ZnO/M1,2/MoO3 multilayer structures. (a) ZnO(20 nm)/Ag (10 nm)/MoO3(35 nm), (b) ZnO (20 nm)/Cu (10 nm)/
MoO3(35 nm), (c) ZnO (20 nm)/Cu (4 nm)/Ag (6 nm)/MoO3(35 nm) and (d) ZnO.
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of the structures are different. While the Cu film is homogeneous (Fig. 1a), some black features, corresponding to freeAg chanel, are visible in the case of Ag (Fig. 1b). That means that the interaction between Cu and Ag is not very strong which is not unexpected, the energy of metal atoms being small since they are deposited by evaporation.
These optical, electrical and morphological properties make that these ZnO/Cu/Ag/MoO3structure, which originality is based on the utilization of a double metal interlayer, are suitable for application to optoelectronic devices.
About the transversal conductivity of the MoO3top layer thick of 35 nm, different authors have shown that such oxide layers are efficient in many devices[12–14]. Different explanations for this property were proposed. For instance Hancox et al. have proposed that it is due to the presence of traps in the MoO3band gap, traps due to the fact that it is well known that MoO3films are slightly oxygen deficient [14]. Another possibility is that the known presence of Ag (1–4 at%) in oxide introduces conducting levels in its band gap[15].
4. Conclusion
Original ZnO/Cu/Ag/MoO3 multilayer structures were deposited under vacuum. The optical transmittance spectrum of these structures
is significantly broadened by using a double layer as metal interlayer.
When the Cu layer is thick of 3 nm and that of Ag of 6 nm, the averaged transmittance of the structure ZnO (20 nm)/Cu(3 nm)/Ag (6 nm)/MoO3 is 88%. If these structures have very high averaged transmittance, their conductivity is not as high as necessary. Due a total metal thickness of only 9 nm, the sheet resistance is not as small as expected. However, when the Cu thickness is 4 nm, that of Ag staying equal to 6 nm, the sheet resistance decreases, which result in a factor of meritΦM¼16103, making that these innovative ZnO/Cu/
Ag/MoO3multilayer structures are very promising for use as substitute to ITO electrodes.
Acknowledgments
This work has been financially supported by the OTC-2012- 2013 project (Nanorgasol network of Mission Ressources et Compétences Technologiques du CNRS FRANCE).
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Fig. 2. Scanning electron microphotography showing the surface morphology of (a) glass/ITO/Cu and (b) glass/ITO/Cu/Ag.
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