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Multilayer transparent electrodes consisting of a tri-layer structure of Al-doped zinc oxide and silver (AZO/Ag/AZO) prepared by inline DC magnetron sputtering at room temperature were investigated. The AZO working gas pressure during deposition was identified as a crucial parameter to influence both the transmittance and sheet resistance of the transparent electrode. By a reduction of the pressure to an optimal value of 0.15 Pa, highest Figure-of-Merit values reported so far for suchlike prepared AZO/Ag/AZO systems could be achieved. In the course of layer characterization, a clear correlation between the coating microstructure and measured electrical and optical properties could be established. Furthermore, we present a model that describes the transmittance spectra of real-structure AZO/Ag/AZO tri-layer systems in a quantitative manner while explicitly considering the specific optical response of the AZO-silver interfaces. Using a generalized Maxwell-Garnett approach with a Gaussian distribution of the Depolarization factors, the interface roughness was described as an effective interfacial layer leading to an improved agreement between measured and simulated spectra.
© 2016 Optical Society of America
1. Introduction
For applications where both a high transmittance as well as a low sheet resistance is required, Al-doped zinc oxide (ZnO:Al, AZO) is a promising candidate that does not require the use of the most studied, but also cost-intense, material indium tin oxide (ITO). These materials are used to realize transparent electrodes in optoelectronic devices such as LED/OLEDs, flat panel displays and touch screens, and solar cells [1–4]. However, despite the intense research on AZO thin films, the achieved performance is often still poorer than that of ITO [5,6]. Moreover, high temperatures (>300°C) are necessary to achieve increased conductivity and transparency [7]. Both these circumstances result in high AZO film thicknesses and inefficient deposition conditions since, for example, a sheet resistance of only a few Ω/sq is required for large displays and large area lighting applications [3].
In order to increase the conductivity while also maintaining a high transmittance, a thin metal film with a thickness of only a few nanometers can be embedded in between two AZO layers. Originally, the structure Oxide/Metal/Oxide was invented for low-emissivity coatings [8]. The combination of a conductive AZO/metal/AZO layer structure allows the deposition at low temperatures as the electrical properties of the layer system are mainly governed by the silver film, which is why the AZO layer can exhibit a lower conductivity.
Such structures have been studied using different metals such as Ag, Au, and Cu [9–11] as well as different sandwich layers e.g. ITO, SnO2, and AZO [12–15]. However, the combination of AZO and Ag is the first choice as AZO provides a low-cost substitute for ITO and Ag has the lowest resistivity of all metals [16] and enables color-neutral coatings in the visible spectral range [17]. Different techniques have been applied to produce AZO/Ag/AZO tri-layers such as electron beam evaporation [15], ion beam sputtering [18], RF magnetron sputtering [19–21], and DC magnetron sputtering [22–25]. Most studies focus on the optimization of the AZO and Ag layer thicknesses in order to obtain a low sheet resistance, high visible transmittance, and resulting high Figure-of-Merit [15,18–20]. Also the silver growth parameters and their influence on the resulting sheet resistance and transmittance have been investigated [26,27]. We have recently studied the influence of the AZO dispersion on the optical properties of the tri-layer system AZO/Ag/AZO [28].
However, no study has investigated the influence of the AZO deposition conditions on the performance of the transparent electrode. The goal of this work is to understand the correlation between the AZO working gas pressure and the resulting properties of the AZO/Ag/AZO structure and to develop low-resistive and highly transparent multilayers. In addition to this, an optical modeling approach is developed to describe real-structure AZO/Ag/AZO thin films.
2. Experimental details
The AZO/Ag/AZO samples were prepared in an inline DC magnetron sputtering system MRC 903 (Kenotec SRL) with a carrier size of 30x30 cm2. The multilayers were deposited at room temperature on Borofloat glass substrates (25 mm diameter) using a ceramic ZnO:Al2O3 (98:2 wt%) target and a metallic Ag target (4N), respectively. The target-to-substrate distance amounted to 60 mm for both materials AZO and Ag. The power applied to the target was 2000 W in case of AZO and 400 W for Ag. Prior to the deposition, the chamber was evacuated to a base pressure of 5·10−5 Pa. Argon was used as working gas and the pressure was set to 0.3 Pa during the silver deposition. For the AZO-deposition also pure Argon was used as working gas. Therefore, potential oxidation of the target surface (especially Ag) caused by oxygen-addition to the working gas was prevented. Additionally, cross-talk between the target materials was minimized by shutters between the target stations. The initial Argon-pressure during the AZO-deposition was set to 1.1 Pa as this was found in an earlier work to produce AZO films with a minimum resistivity [29]. In this study, the working pressure was also varied from 0.15 Pa to 1.6 Pa to investigate the influence of this parameter on the resulting structural, electrical, and optical properties of the AZO/Ag/AZO multilayer.
The optical spectra were recorded with scanning spectrophotometers (Perkin Elmer) in a spectral range from 300 nm – 2500 nm. Reflectance (R) and transmittance (T) were measured using an in-house developed VN-attachment allowing absolute reflectance measurements [30]. The sheet resistance was measured by linear four-point probing, and Hall measurements were carried out under van der Pauw geometry.
A Bruker D8 Discover was used for X-Ray reflectometry (XRR) and X-Ray diffraction (XRD) at a wavelength of λCuKα = 0.154 nm. SEM micrographs were taken with a Carl Zeiss Σigma scanning electron microscope and selected samples were analyzed by cross-sectional transmission electron microscopy (TEM) using a CM20-TEM (Philips) at Ulm University, Germany. The latter one was carried out on AZO/Ag/AZO stack deposited on Si substrate in the same deposition run as the glass substrates that were used for all other analysis methods.
The optical constants of thin AZO single films were determined by fitting the optical spectra with a Lorentzian multi-oscillator model by means of the LCalc software issued by Steffen Wilbrandt [30]. The thickness of the films was previously determined via XRR analysis and set constant during the optical fit. To enhance the accuracy, T and R measurements at various angles of incidence (6°, 45°, 60°) were taken into account.
3. Characterization results
In our previous work, the Ag and AZO film thicknesses for a low resistivity and high transmittance were optimized to 37 nm/10 nm/37 nm [28]. A sheet resistance RSh of ≈6.2 Ω/sq and an average transmittance of T400-800 = 79.9% (400 nm-800 nm) and maximum T550 = 87.4% (at 550 nm) were obtained by depositing the AZO film at an Argon working pressure of pAr = 1.1 Pa [28]. This pressure was obtained in an earlier work as the optimum working point for a minimum AZO resistivity [29]. With these values, a Figure-of-Merit (Haacke [31]) Φ = T10/RSh of Φ400-800 = 17.1 mΩ−1 and Φ550 = 42.0 mΩ−1 was obtained.
In order to reveal a correlation between the structural, electrical, and optical properties, this paper investigates the changes of the AZO base layer properties with the Argon working pressure and the resulting influence on the AZO/Ag/AZO electro-optical characteristics. For that purpose, the same tri-layer system was deposited at various AZO-working pressures ranging from 0.15 Pa to 1.6 Pa. The AZO-deposition rate did not change significantly but only by about 10% which was compensated by a slightly altering carrier speed during the deposition. The individual layer thicknesses in the AZO/Ag/AZO stack deposited at different AZO-working gas pressures were determined by XRR. The AZO thickness showed a variation between ≈36 nm and ≈38 nm without a clear trend concerning the AZO-working gas pressure. The Ag thickness was determined to 10.2 nm...10.8 nm. These thickness ranges include the reproducibility of the sputtering system as well as the error of thickness determination. The corresponding transmittance and reflectance spectra are shown in Fig. 1(a). With decreasing pAr, the transmittance in the visual and near infrared spectral range is increased. In the UV, the reflectance increases whereas in the near infrared spectral range the reflectance is slightly reduced with decreasing pAr. Figure 1(b) shows that the increase in T is mainly caused by reduced optical losses calculated via 100%-R-T. The structure at 860 nm is only a measurement artefact due to the detector change. The mean value T400-800 as well as the transmittance at 550 nm T550 is plotted in Fig. 2(a).
Fig. 1 Reflectance R and transmittance T spectra (a) as well as optical losses calculated via 100%-R-T (b).
Fig. 2 Sheet resistance and transmittance (a) and resulting Figure of Merit Φ (b) of 37 nm/10 nm/37 nm AZO/Ag/AZO films as a function of working pressure pAr during the AZO deposition. The line is only a guide for the eye.
At the same time, the sheet resistance of the AZO/Ag/AZO electrode changes. A reduction of pAr causes also a reduction of RSh from 7.2 Ω/sq at pAr = 1.6 Pa down to 4.7 Ω/sq at an Argon pressure of 0.15 Pa during the AZO deposition as it is illustrated in Fig. 2(a).
Both effects lead to an increase of the Figure-of-Merit to very high values of Φ400-800 = 33.6 mΩ−1 and Φ550 = 77.5 mΩ−1, respectively (Fig. 2(b)). To the best of our knowledge, this is the best performance of ZnO-based multilayer transparent electrodes prepared by industrially applicable DC magnetron sputtering published so far (see Table 1 for comparison of literature values).
Table 1. Electro-optical properties of In-free tri-layer transparent electrodes prepared by different deposition techniques: EBE (electron beam evaporation), DC MS: DC magnetron sputtering, RF MS: RF magnetron sputtering, IBS: ion beam sputtering. AZO: Al-doped ZnO, GZO: Ga-doped ZnO. FOM: Figure-of-Merit.
In the following, four different aspects are discussed that are influenced by the working pressure pAr during the AZO layer deposition. Besides the change of the electro-optical properties of AZO single layers itself, the textured Ag film growth on AZO, the interface roughness, and the Ag grain size have an influence on the resulting electro-optical properties of the AZO/Ag/AZO tri-layer system.
3.1 Electro-optical properties of AZO single layer
Decreasing pAr from 1.1 Pa to 0.15 Pa leads to an increase of the resistivity of the AZO layer as it can be seen in Fig. 3(a). This has a direct influence on the optical constants n and k (Fig. 3(b)) since they are connected to the electrical transport parameters via the Drude model.
Fig. 3 Specific Resistivity of AZO single layers with a thickness of ∼37 nm (a) and optical constants n and k determined from R and T spectra (b) as a function of pAr during the AZO deposition.
The AZO film with the lowest specific resistivity exhibits the highest extinction coefficient and lowest refractive index in the IR caused by the plasma resonance of the free carriers. This also influences the extinction in the Vis due to the tail of the Drude term. The reduced absorption in the AZO layer as well as the higher refractive index at low pAr are the reason for the enhanced transmittance and reduced reflectance of the AZO/Ag/AZO multilayer films.
At the same time, the increase of the AZO resistivity does not lead to an increase of the AZO/Ag/AZO sheet resistance, which highlights that the electrical properties of the tri-layer structure are mainly governed by the silver conductivity. As a matter of fact, RSh is even reduced when the Argon pressure decreases. The reasons for this behavior are discussed in the following sections.
3.2 Textured Ag film growth
Figure 4 shows the XRD spectra in symmetric θ-2θ scan geometry (Bragg-Brentano) of AZO/Ag/AZO systems prepared at different working pressures pAr during the AZO deposition. It can be seen that the AZO layer exhibits the typical (002) orientation with a peak at approximately 34.4° meaning that the c-axis is oriented perpendicular to the substrate surface. The Ag layer has also a preferred orientation with the (111) plane parallel to the substrate (2θ = 38.1°). With decreasing pAr both the AZO(002) peak intensity as well as the Ag(111) peak intensity increase implying an increased texture of both films. The more AZO crystallites are oriented in (002), the more Ag(111) crystallites exist. Thus, the underlying AZO(002) layer acts as a template for the textured Ag(111) film growth.
Fig. 4 XRD spectra of AZO/Ag/AZO multilayers at various gas pressures in symmetric θ-2θ scan geometry (Bragg-Brentano).
The rocking curve measurements in ω-2θ scan geometry with a constant detector angle of 2θ = 38.1° (Fig. 5) also reveal that the Ag texture increases with decreasing working pressure pAr during the AZO deposition.
Fig. 5 XRD spectra of AZO/Ag/AZO multilayers at various gas pressures in asymmetric ω-2θ scan geometry (rocking curve) at constant 2θ = 38.1°.
Comparing the crystal structures of the hexagonal ZnO wurtzit structure and the cubic fcc Ag lattice, it can be concluded that the AZO(002)/Ag(111) interface exhibits the same atomic arrangement of the AZO(002) and Ag(111) lattice planes as it was illustrated by Kato et al. [32]. Therefore, the AZO(002) surface can easily act as a template for the heteroepitaxial growth of the Ag(111) phase.
From Fig. 4 it can also be observed that the AZO(002) peak position shifts to lower angles 2θ when pAr is reduced. When the tri-layer system is treated as a simple parallel connection of resistors, the specific resistivity of the Ag layer can be calculated and consequently a clear correlation of the Ag-resistivity and the AZO(002) peak position becomes evident (Fig. 6). From the (002) peak position the AZO lattice plane distance d(002) perpendicular to the substrate surface can be calculated which is also included in Fig. 6.
Fig. 6 Specific resistivity as a function of the AZO lattice plane distance d(002) and diffraction angle 2θ(002).
The reduction of the diffraction angle 2θ is equivalent to an increase of the lattice plane spacing d(002) while at the same time, the specific resistivity of Ag decreases. This can be attributed to the improved lattice match between the AZO template layer and the growing Ag film. Comparing the literature values from the powder diffraction database, the Ag-atomic distance inside the (111) plane a(111) = 2.89 Å is 11% smaller than the lattice constant a = 3.25 Å of the AZO layer within the (002) plane [33]. If the Argon pressure during the AZO deposition is now reduced, the mismatch between AZO and Ag is reduced since the increase in d(002) leads to a reduction of the AZO in-plane lattice constant a. This can be illustrated if the material is considered as an elastic rubber band which is constricted perpendicular to the stretch direction. With an improved match between the seed layer and the growing Ag film, the crystalline quality and texture of Ag is increased (e.g. less dislocations), hence the resistivity is lower. The same behavior was observed by Kato et al. who investigated the growth of Ag on ZnO films prepared by reactive DC magnetron sputtering [32].
3.3 Interface roughness
Because of the polycrystalline growth of AZO, the films’ surface can exhibit a non-negligible roughness which can affect both the Ag resistivity and the resulting transmittance of the whole AZO/Ag/AZO stack. For that purpose, the surface roughness of thin AZO single layers, as well as the interface roughness of AZO/Ag/AZO multilayers prepared at different pAr was determined via XRR. From both, it can be seen that the roughness at the first AZO/Ag interface increases with increasing pAr. At the same time we have seen at the beginning that the resistivity increases and the transmittance in the visual spectral range decreases with a raise of pAr. Figure 7 visualizes the correlation of the Argon pressure, the interface roughness at the first AZO/Ag interface, the resistivity and the transmittance of the AZO/Ag/AZO stack.
Fig. 7 Specific resistivity of Ag and transmittance of the AZO/Ag/AZO stack as a function of working pressure and roughness at the first AZO/Ag interface. Lines are only a guide for the eye.
SEM images of AZO single films prepared at different working gas pressures also reveal that the structure changes from a compact relatively smooth columnar structure to a more and more fine-grained rough surface (Fig. 8). This behavior is described by the modified Thornton model for ZnO:Al where the films grow compact, dense and smoother at low pressure [35].
From a cross-sectional TEM investigation of an AZO/Ag/AZO stack prepared at 1.1 Pa it can also be observed that the first interface exhibits a higher roughness caused by the columnar AZO film growth while the second interface appears smoother (Fig. 9).
Fig. 9 Cross-sectional TEM image of an AZO/Ag/AZO multilayer prepared at pAr = 1.1 Pa. The high resolution image visualizes the AZO(002) planes oriented parallel to the substrate surface. The roughness at the interface AZO(bottom)/Ag is higher than that of the Ag/AZO(top) interface approving the XRR-results (see Table 2).
In [34] a correlation between the surface roughness and the conductivity of the Ag films was described, hence the decrease of the sheet resistance with decreasing pAr observed in this paper can also be affected by the varying roughness of the underlying AZO layer (in addition to the improvement of the Ag texture). In the last section we will also show that the interface roughness can lead to a decrease of the transmittance of the multilayer system.
3.4 Ag grain size
Figure 10 shows exemplarily SEM images of thin AZO films (prepared at 0.15 Pa and 1.1 Pa) overcoated with 10 nm Ag. The smoother AZO under-layer prepared at 0.15 Pa also leads to the formation of larger Ag grains which can be also a reason for the lower resistivity at lower pressure since the scattering at grain boundaries is reduced.
In addition, the rocking curve measurements (Fig. 5) reveal an increase of the Ag grain size with decreasing pAr since the FWHM of the peaks is a criterion for the grain size in lateral direction. The broader the peak, the smaller are the grains within the polycrystalline Ag layer. All data are summarized in Table 2.
Table 2. Summary of electrical and structural properties of AZO/Ag/AZO films prepared at different sputter gas pressures pAr.
Altogether, the four different aspects discussed in the previous sections led to a change of the sheet resistance and transmittance of the AZO/Ag/AZO stack only by varying the AZO working gas pressure and keeping all other parameters - esp. thicknesses - constant. From the data collected here, it is not possible to identify a dominating effect since all changes (texture, grain size, and interface roughness) are related to each other and cannot be separated. Besides the AZO working gas pressure there are further parameters that influence the resistance and transmittance of the AZO/Ag/AZO stack. The specific resistivity of the Ag layer could also be reduced by an increasing Ag layer thickness as the electron scattering at interface and grain boundaries would become less dominant. However, this would also affect the transmittance of the tri-layer system (see [28]). A change of the AZO bottom thickness in our previous work revealed an influence of this parameter on the transmittance of the layer system but the sheet resistance did not change significantly [28].
4. Modeling approach
This section addresses the modeling of the optical spectra of AZO/Ag/AZO transparent electrodes. For that purpose, Fig. 11 shows simple forward calculations (OptiLayer Thin Film Software) of a tri-layer AZO/Ag/AZO system assuming smooth interfaces. The thicknesses were set equal to the values obtained via XRR. The AZO optical constants were determined for each working gas pressure as it was described in the previous section. For silver, the optical constants determined by Johnson and Christy were used [17,36]. As it was described in [37], these optical constants allowed a very good description of silver films prepared by sputtering technique.
Fig. 11 Comparison between measured optical spectra (AOI = 6°) of AZO/Ag/AZO systems prepared at different working gas pressures and simulation applying a simple tri-layer model with smooth interfaces. For AZO the optical constants shown in Fig. 3(b) are used whereas silver was described by the optical constants given in [17,36].
In general, the reflectance and transmittance behavior is described quite well using the above mentioned optical constants and thicknesses. However, it becomes obvious that the simulation always overestimates the transmittance in the visual and near infrared spectral range in comparison to the measurement. This discrepancy increases with increasing working pressure pAr. This might be attributed to the increasing interface roughness verified by XRR analysis and SEM/TEM images. In the following, an optical model is presented involving the surface roughness by the introduction of an effective interface layer consisting of a mixture of AZO and Ag. The very fine-grained structure allows us to neglect scattering losses within the considered visual and near-infrared spectral range since the lateral dimensions are considerably smaller than the wavelengths of the incoming light. Due to this roughness with only high spatial frequencies, the choice of an optical model which describes the mixing at the interface as a homogeneous film with a thickness deff and effective optical constants neff and keff is reasonable (Fig. 12). This metal-dielectric compound layer will exhibit optical losses because of the nature of the partner’s dielectric functions εAZO and εAg.
For the calculation of the effective dielectric function εeff of the interface mixing layer, a generalized Maxwell-Garnett approach is used [38], where AZO acts as a “host” material and silver is considered as “guest” with a filling factor pAg within the AZO matrix:
The geometry-dependent depolarization factor L describes the shape of inclusions in the host material (e.g. L = 1/3 for spherical shape, L = 0 for infinitely long needle with electric field parallel to symmetry axis, L = 1 for infinitely broad pancake with electric field parallel to symmetry axis [38]). In order to account for the statistical character of the surface geometry (including form and orientation of the Ag inclusions in AZO), the depolarization factor was described by a broad Gaussian distribution g(L) centered at L = 1/3 (spherical inclusions) with a broad standard deviation of 0.5 (Fig. 13). With this assumption, many different clusters – from infinitely long needle to broad pancake-like structures – are included into the model of the effective mixture of AZO and Ag.
Fig. 13 Gaussian distribution g(L) assumed for the depolarization factor L. The mean value is chosen to L = 1/3 and the standard deviation is 0.5.
In a second step, the silver filling factor was optimized. It was discovered, that only a very small filling factor pAg = 0.15 was necessary to obtain a satisfying accordance between the measured and simulated spectra. The resulting optical constants neff and keff of the effective interlayer (IL) were calculated from Eq. (1) via and are shown in Fig. 14. Higher values of pAg would cause characteristic and sharp extinction structures in the visual and near-infrared spectral range that were not observed in the AZO/Ag/AZO optical spectra.
Fig. 14 Optical constants neff and keff of the effective interlayer (IL) calculated by the generalized Maxwell-Garnett approach with the g(L)-distribution shown in Fig. 13 and a silver filling factor pAg of 15%.
For the final modeling of the optical spectra the following layer sequence was used: sub/AZO/IL/Ag/IL/AZO. To obtain more information, the optical spectra recorded at an angle of incidence of 45° under s- and p-polarization were also taken into account. Figure 15 shows exemplarily for the sample prepared at pAr = 1.6 Pa that the implementation the effective interlayer with only a small thickness of ∼2 nm leads to a significant improvement of the match between the measured and fitted spectra in the visual and near infrared spectral range. The very broad absorption within the effective interlayer leads to a reduction of the modeled transmittance. The reflectance in the infrared spectral range is not significantly affected.
Fig. 15 Comparison between measured and modeled optical spectra (pAr = 1.6 Pa) considering a simple tri-layer model with smooth interfaces as well as an extended model with interlayers describing the surface roughness: sub/AZO/IL/Ag/IL/AZO with the following thicknesses: 34.2 nm/2.5 nm/10.5 nm/1.1 nm/35.0 nm.
The filling factor pAg and the thickness of the effective interlayer could be interchanged to a certain extent without changing the fit results significantly since the applied model is overdetermined. However, the amount of silver within the intermixing zone would stay the same.
In addition, one can observe two characteristic dips at around 330 nm (T) and 2000 nm (R) in the spectrum recorded at 45° in p-polarization. This is caused by the so-called Berreman effect which is present in thin films near the zero-crossing of the dielectric function (here: AZO) [39]. This effect is based on the resonance of longitudinal phonons and plasmon-polaritons hence it only occurs on p-polarization. The deviation between the measured and simulated spectra (45° p-pol) in the IR might originate from a small uncertainty of the AZO optical constants which may be caused by only slight deviations of the conductivity of the AZO single layers.
Finally, the postulated Gaussian distribution, which was necessary to obtain a good agreement between measurement and optical model, will be discussed. The rather broad distribution indicates that high diversity of silver inclusions in the AZO host material has to be present. In particular, a high fraction of low L-values was necessary to obtain a good fitting result. Since a depolarization factor of L = 0 corresponds to an infinitely long Ag-needle, the applied broad Gaussian distribution g(L) also includes the percolation of the silver within the thin interlayer. At first glance, this seems to be contradictory to the low Ag-filling factor of only 15% used. However, a visualization of the real structure (Fig. 16) at the interface explains that an Ag-overcoating of the columnar AZO film can lead to an accumulation of silver in the valleys of the AZO surface. Therefore, also a percolation of silver is possible despite the low volume fraction within the effective interlayer.
In summary, the interlayer model led to an improvement of the match between measurement and model but did not result in a perfect overlap of the spectra. In general, every modeling approach is based on several simplifying assumptions that define a suitable balance between adherence to experimental results and predictive force of the model. The latter also includes aspects of manageability, such as the experimental accessibility of input parameters, the number of free parameters, stability and relative simplicity of the mathematical apparatus, and the like. In this connection, the remaining mismatch that is still present between modeled and measured spectra (Fig. 15) could have different causes:
First of all, the postulated Maxwell Garnett approach itself represents a strong idealization of the real system, by introducing a clear classification of the mixing partners into a host and a guest material. Furthermore, the Gaussian function used for the description of the interlayers morphology was only an empirically postulated distribution based on the observations made concerning the shape of the transmittance spectra and the real structure of the thin films. Another assumed distribution might lead to an even better match of the spectra, but increase the number of free parameters. A further improvement in fit quality might be achieved by freeing the optical constants of silver. However, the quantity of free parameters in the fit might on the one hand lead to a better fit result but on the other hand the physical meaning and the transferability on other systems might be questionable. With our approach we could demonstrate that the experimentally verified interface roughness can be described by a physical model taking into account the real structure of the layer stack, which resulted in a better match between measurement and model.
5. Heat treatment and thermal stability
Optimized AZO/Ag/AZO transparent electrodes (pAr = 0.15 Pa) were subsequently annealed in vacuum (1 mbar) at different temperatures to investigate the thermal stability. Figure 17 displays the sheet resistance, transmittance, and Figure-of-Merit as a function of temperature. It can be seen that RSh decreases further from 4.7 Ω/sq (as deposited) to ≈3.9 Ω/sq at an annealing temperature of 350°C to 400°C. Hall measurements (not shown here) revealed that this is caused by a slight increase of the carrier mobility while the carrier concentration did not change significantly. The increase in conductivity is most likely caused by an improvement of the structural quality of the silver film (e.g. healing of defects) and not by a considerable recrystallization of the silver since rocking curve scans did not reveal significant changes of the grain size of Ag. At the same time the transmittance in the visible spectral range increases which is mainly caused by a reduction of the AZO extinction coefficient. Both effects lead to a further improvement of the Figure-of-Merit up to a very high value of Φ400-800 = 45 mΩ−1 and Φ550 = 99 mΩ−1, respectively.
Fig. 17 Sheet resistance, transmittance, and Figure-of-Merit of AZO/Ag/AZO transparent electrode prepared at pAr = 0.15 Pa as a function of annealing temperature.
At even higher temperatures the sheet resistance increases again and also the peak transmittance T550 decreases which causes a reduction of the Figure-of-Merit Φ550. Reasons for that might be agglomeration and diffusion of the silver into the surrounding oxide layer [8].
The AZO/Ag/AZO films exhibited a good stability in damp heat test (85°C, 85%r.H.) as shown in Fig. 18. Up to a test duration of 500 h the sheet resistance remained constant and also the transmittance did not decrease in contrary to the behavior of an AZO single layer which was similarly treated.
Fig. 18 Investigation of the damp heat stability of AZO/Ag/AZO films prepared at pAr = 0.15 Pa in comparison to an AZO single layer (d = 200 nm).
6. Conclusions
In summary, the influence of the argon working gas pressure during the AZO deposition pAr on the electro-optical properties of the AZO/Ag/AZO transparent electrode has been investigated. A reduction of pAr leads to an increase of the transmittance and a decrease of the sheet resistance of the tri-layer system. A low resistance of 4.7 Ω/sq and a high transmittance of T550 = 90.4% were achieved. The resulting Figure-of-Merit Φ550 = 77.5 mΩ−1 is the highest value reported so far for AZO/Ag/AZO systems prepared with DC magnetron sputtering.
With reduced pAr, the AZO refractive index is increased and extinction coefficient is decreased leading to an enhanced transmittance of the tri-layer system. By the use of XRR, XRD, SEM, and TEM analysis it was found out that the textured Ag growth in enhanced at lower pAr. Also the interface roughness between AZO and Ag is reduced and the Ag grain size is increased. Altogether, these effects lead to a reduction of the sheet resistance.
In addition, an optical modeling approach for the description of the reflectance and transmittance spectra of real-structure AZO/Ag/AZO multilayers was developed. An effective interfacial layer that describes the roughness at the AZO/Ag interface was introduced. An improved agreement between the measured and simulated optical spectra could be achieved using a generalized Maxwell-Garnett approach with a Gaussian distribution of the Depolarization factors to describe the interfacial layer.
External heat treatment of the layer system has shown that the performance can be improved by tempering the sample up to 350°C. The sheet resistance could be reduced to 3.9 Ω/sq resulting in a high Figure-of-Merit of Φ550 = 99 mΩ−1.
In conclusion, we have shown a detailed investigation of AZO/Ag/AZO electrodes prepared at room temperature. With that, the performance could be improved enabling the possibility of high performance transparent electrodes that can be prepared with an industrially applicable deposition method also suitable for the deposition on plastic substrates.
Acknowledgments
The authors would like to thank Sabrina Wolleb for the SEM investigation and Johannes Biskupek (TU Ulm) for the TEM analysis. The authors are grateful to Steffen Wilbrandt for supplying the LCalc software for this study.
References and links
1. U. Betz, M. Kharrazi Olsson, J. Marthy, M. F. Escola, and F. Atamny, “Thin films engineering of indium tin oxide: Large area flat panel displays application,” Surf. Coat. Tech. 200(20-21), 5751–5759 (2006). [CrossRef]
2. M. Katayama, “TFT-LCD technology,” Thin Solid Films 341(1-2), 140–147 (1999). [CrossRef]
3. M. Eritt, C. May, K. Leo, M. Toerker, and C. Radehaus, “OLED manufacturing for large area lighting applications,” Thin Solid Films 518(11), 3042–3045 (2010). [CrossRef]
4. M. Berginski, J. Hüpkes, M. Schulte, G. Schöpe, H. Stiebig, B. Rech, and M. Wuttig, “The effect of front ZnO:Al surface texture and optical transparency on efficient light trapping in silicon thin-film solar cells,” J. Appl. Phys. 101(7), 074903 (2007). [CrossRef]
5. B. Szyszka, W. Dewald, S. K. Gurram, A. Pflug, C. Schulz, M. Siemers, V. Sittinger, and S. Ulrich, “Recent developments in the field of transparent conductive oxide films for spectral selective coatings, electronics and photovoltaics,” Curr. Appl. Phys. 12, S2–S11 (2012). [CrossRef]
6. K. Ellmer, “Past achievements and future challenges in the development of optically transparent electrodes,” Nat. Photonics 6(12), 809–817 (2012). [CrossRef]
7. T. Minami, H. Sato, H. Imanoto, and S. Takata, “Substrate Temperature Dependence of Transparent Conducting Al-Doped ZnO Thin Films Prepared by Magnetron Sputtering,” Jpn. J. Appl. Phys. 31(Part 2, No. 3A), L257–L260 (1992). [CrossRef]
8. J. Szczyrbowski, A. Dietrich, and K. Hartig, “Bendable silver-based low emissivity coating on glass,” Sol. Energy Mater. 19(1-2), 43–53 (1989). [CrossRef]
9. M. Bender, W. Seelig, C. Daube, H. Frankenberger, B. Ocker, and J. Stollenwerk, “Dependence of film composition and thickness on optical and electrical properties of ITO-metal-ITO multilayers,” Thin Solid Films 326(1-2), 67–71 (1998). [CrossRef]
10. T. Dimopoulos, G. Radnoczi, B. Pécz, and H. Brückl, “Characterization of ZnO:Al/Au/ZnO:Al trilayers for high performance transparent conducting electrodes,” Thin Solid Films 519(4), 1470–1474 (2010). [CrossRef]
11. C. Guillén and J. Herrero, “ITO/metal/ITO multilayer structures based on Ag and Cu metal films for high-performance transparent electrodes,” Sol. Energy Mater. Sol. Cells 92(8), 938–941 (2008). [CrossRef]
12. A. Klöppel, W. Kriefseis, B. K. Meyer, A. Scharmann, C. Daube, J. Stollenwerk, and J. Trube, “Dependence of the electrical and optical behavior of ITO-silver-ITO multilayers on the silver properties,” Thin Solid Films 365(1), 139–146 (2000). [CrossRef]
13. M. Fahland, P. Karlsson, and C. Charton, “Low resistivity transparent electrodes for displays on polymer substrates,” Thin Solid Films 392(2), 334–337 (2001). [CrossRef]
14. S. Yu, W. Thang, L. Li, D. Yu, H. Dong, and Y. Jin, “Optimization of SnO2/Ag/SnO2 tri-layer films as transparent composite electrode with high figure of merit,” Thin Solid Films 552, 150–154 (2014). [CrossRef]
15. D. R. Sahu, S.-Y. Lin, and J.-L. Huang, “Investigation of conductive and transparent Al-doped ZnO/Ag/Al-doped ZnO multilayer coatings by electron beam evaporation,” Thin Solid Films 516(15), 4728–4732 (2008). [CrossRef]
16. W. M. Haynes, CRC Handbook of Chemistry and Physics (Taylor and Francis, 2012).
17. P. B. Johnson and R. W. Christy, “Optical Constants of the Noble Metals,” Phys. Rev. B 6(12), 4370–4379 (1972). [CrossRef]
18. S. Song, T. Yang, Y. Xin, L. Jiang, Y. Li, Z. Pang, M. Lv, and S. Han, “Effect of GZO thickness and annealing temperature on the structural, electrical and optical properties of GZO/Ag/GZO sandwich films,” Curr. Appl. Phys. 10(2), 452–456 (2010). [CrossRef]
19. I. Crupi, S. Boscarino, V. Strano, S. Mirabella, F. Simone, and A. Terrasi, “Optimization of ZnO:Al/Ag/ZnO:Al structures for ultra-thin high performance transparent electrodes,” Thin Solid Films 520(13), 4432–4435 (2012). [CrossRef]
20. H.-W. Wu, R.-Y. Yang, C.-M. Hsiung, and C.-H. Chu, “Influence of Ag thickness on aluminum-doped ZnO/Ag/aluminum-doped ZnO thin films,” Thin Solid Films 520(24), 7147–7152 (2012). [CrossRef]
21. J. H. Kim, Y.-J. Moon, S.-K. Kim, Y.-Z. Yoo, and T.-Y. Seong, “Al-doped ZnO/Ag/Al-doped ZnO multilayer films with a high figure of merit,” Ceram. Int. 41(10), 14805–14810 (2015). [CrossRef]
22. Y. S. Jung, Y. S. Park, K. H. Kim, and W.-J. Lee, “Properties of AZO/Ag/AZO Multilayer Thin Film Deposited on Polyethersulfone Substrate,” Trans. Electr. Electron. Mater. 14(1), 9–11 (2013). [CrossRef]
23. L. Zhou, X. Chen, F. Zhu, X. X. Sun, and Z. Sun, “Improving temperature-stable AZO-Ag-AZO multilayer transparent electrodes using thin Al layer modification,” J. Phys. D Appl. Phys. 45(50), 505103 (2012). [CrossRef]
24. H.-K. Park, J.-W. Kang, S.-I. Na, D.-Y. Kim, and H.-K. Kim, “Characteristics of indium-free GZO/Ag/GZO and AZO/Ag/AZO multilayer electrode grown by dual target DC sputtering at room temperature for low-cost organic photovoltaics,” Sol. Energy Mater. Sol. Cells 93(11), 1994–2002 (2009). [CrossRef]
25. M. Theuring, M. Vehse, K. von Maydell, and C. Agert, “AZO-Ag-AZO transparent electrode for amorphous silicon solar cells,” Thin Solid Films 558, 294–297 (2014). [CrossRef]
26. D. Zhang, H. Yabe, E. Akita, P. Wang, R. Murakami, and X. Song, “Effect of silver evolution on conductivity and transmittance of ZnO/Ag thin films,” J. Appl. Phys. 109(10), 104318 (2011). [CrossRef]
27. A. Sytchkova, M. L. Grilli, A. Rinaldi, S. Vedraine, P. Torchio, A. Piegari, and F. Flory, “Radio frequency sputtered Al:ZnO-Ag transparent conductor: A plasmonic nanostructure with enhanced optical and electrical properties,” J. Appl. Phys. 114(9), 094509 (2013). [CrossRef]
28. A. Bingel, M. Steglich, P. Naujok, R. Müller, U. Schulz, N. Kaiser, and A. Tünnermann, “Influence of the ZnO:Al dispersion on the performance of ZnO:Al/Ag/ZnO:Al transparent electrodes,” Thin Solid Films (submitted to).
29. A. Bingel, K. Füchsel, N. Kaiser, and A. Tünnermann, “ZnO:Al films prepared by inline DC magnetron sputtering,” Adv. Opt. Technol. 3(1), 103–111 (2014).
30. O. Stenzel, S. Wilbrandt, K. Friedrich, and N. Kaiser, “Realistische Modellierung der NIR/Vis/UV-optischen Konstanten dünner optischer Schichten im Rahmen des Oszillatormodells,” Vak. Forsch. Prax. 21(5), 15–23 (2009). [CrossRef]
31. G. Haacke, “New figure of merit for transparent conductors,” J. Appl. Phys. 47(9), 4086–4089 (1976). [CrossRef]
32. K. Kato, H. Omoto, and A. Takamatsu, “Optimum structure of metal oxide under-layer used in Ag-based multilayer,” Vacuum 83(3), 606–609 (2008). [CrossRef]
33. Powder Diffraction File, ICCD Database, Pattern 36–1451 (ZnO), 04–0783 (Ag), 1997.
34. E. Z. Luo, S. Heun, M. Kennedy, J. Wollschläger, and M. Henzler, “Surface Roughness and Conductivity of thin Ag films,” Phys. Rev. B Condens. Matter 49(7), 4858–4865 (1994). [CrossRef] [PubMed]
35. O. Kluth, G. Schöpe, J. Hüpkes, C. Agashe, J. Müller, and B. Rech, “Modified Thornton model for magnetron sputtered zinc oxide: film structure and etching behavior,” Thin Solid Films 442(1-2), 80–85 (2003). [CrossRef]
36. Optical constants from in-house database, in this case essentially a smoothed version of the data from [17].
37. P. J. Jobst, O. Stenzel, M. Schürmann, N. Modsching, S. Yulin, S. Wilbrandt, D. Gäbler, N. Kaiser, and A. Tünnermann, “Optical properties of unprotected and protected sputtered silver films: Surface morphology vs. UV/Vis reflectance,” Adv. Opt. Technol. 3(1), 91–102 (2014).
38. O. Stenzel and A. Macleod, “Metal-dielectric composite optical coatings: underlying physics, main models, characterization, design and application aspects,” Adv. Opt. Technol. 1(6), 463–481 (2012).
39. B. Harbecke, B. Heinz, and P. Grosse, “Optical Properties of Thin Films and the Berreman Effect,” Appl. Phys., A Mater. Sci. Process. 38(4), 263–267 (1985). [CrossRef]
