*3.2. Optical and Electrical Studies*

The GAG-0 sample without any silver interlayer showed a high sheet resistance *R*<sup>S</sup> of 2500 Ω/sq and electrical resistivity <sup>ρ</sup> in the order of 10−<sup>2</sup> <sup>Ω</sup>·cm due to the low preparation temperature. The optical transmittance of GAG-0 is 88% in the visible range of wavelengths (400–700 nm), as shown in Figure 3a, which is consistent with early studies [23,26]. After covering this GZO film with the 10-nm thick Ag layer (GA sample), the resistivity decreases to 2.3 <sup>×</sup> 10−<sup>5</sup> <sup>Ω</sup>·cm. It is obvious that the conductivity of such a two-layer structure is mainly governed by the continuity and homogeneity of the Ag thin layer [39]. However, as can be seen from Figure 3a, the existence of the Ag layer on top of the 80-nm thick GZO layer substantially reduces the optical transmittance in the visible and near infrared (NIR) regions (the average visible transmittance *T*av is 41.5%) [28].

**Figure 3.** Optical transmittance spectra of the prepared multilayer structures: (**a**) with different Ag interlayer thicknesses; (**b**) with different thicknesses for the bottom and top GZO layers.

In Figure 3a, the optical transmittance spectra are presented of the GAG trilayered structures, consisting of two identical 40-nm GZO layers and Ag interlayers with different thicknesses. As shown in the figure, the top GZO layer antireflects the Ag layer in GAG structures to output higher transmittances than the GA bilayer structure by itself [40]. Moreover, the optical transmittances of the GAG multilayers were found to depend critically on the Ag interlayer thickness.

The average optical transmittance in the visible wavelength *T*av region of the GAG-1 sample is relatively low (75%) due to light scattering on various defects (pores inherent in ultrathin Ag films and resulting imperfections of the GZO–Ag interfaces). By increasing the Ag thickness to 10 nm, the average optical visible transmittance increases and there is a shift of the transmission peak due to the effects of surface plasmon resonance of the Ag interlayer with minimum voids [41]. In particular, GAG-3 shows high optical transmittance in the visible region, with maximum transmittances of approximately 89% at λ = 529 nm. Next to a wavelength of 550 nm, this sample has the highest optical transmittance, which is even higher than that of GAG-0, which has no silver interlayer.

Further increasing the Ag thickness above 10 nm results in a decrease of the transmittance because of increased light reflection from the continuous Ag interlayer. Additionally, all samples show an abrupt decrease in optical transmittance in the near infra-red region, which is correlated with the thickness of the metal interlayer and attributed to the reflection of long-wavelength light by the layered metal [33]. Thus, the best optical properties of the GAG structures are obtained when the Ag interlayer thickness is 10 nm. The obtained optimal thickness value of the Ag interlayer is similar to the one reported by other groups for ZnO/Ag/Zno multilayers deposited at low substrate temperatures [19,28,42–44].

Figure 3b depicts the transmittance spectra for the GAG structures with the optimum Ag interlayer thickness and different GZO thicknesses. With the increase of GZO thickness, the transmittance first shows an increase and then decreases. Simultaneously, the peak transmittance shifts towards the long wavelength regions. For clarity, the results of our optical measurements are summarized in the corresponding columns of Table 3. Based on these results, the GAG-3 sample with 40-nm thick GZO top and bottom layers and a 10-nm thick Ag interlayer was considered as the optimum choice in terms of optical properties.


**Table 3.** Optical and electrical parameters of GAG samples.

The results of our study on the dependence of electrical properties of the multilayer samples as sheet resistance resistivity (*R*S) and resistivity (ρ) on the thicknesses of the GZO and Ag layers are also presented in Table 3.

According to the presented results, the resistivity ρ can be decreased drastically by three orders of magnitude by inserting a thin Ag interlayer. From the fact that even at the Ag thickness of 6 nm, the specific resistance of the GAG-1 is significantly reduced, it can be assumed that in this case the Ag interlayer is already a continuous network of partially coalesced islands. The material of the top GZO layer partially fills the voids of Ag, therefore additionally shunting the gaps in the metal network. A further increase in the thickness of the Ag interlayer results in both an improvement in the crystalline perfection of the metal phase and a decrease in the size and number of voids in it. Thus, a monotonic decrease in resistance with increasing thickness of the Ag interlayer can be explained by an obvious increase in the total number of charge carriers (the effective carrier concentration) in GAG and also very likely by an increase of carrier mobility.

From the comparison of the electrical properties of the GAG-3, GAG-5, and GAG-6 samples (GZO thickness variation at a fixed Ag thickness of 10 nm), we can verify that the oxide layers in the oxide–Ag–oxide multilayer play only a minor role in the electrical properties of the conductive multilayer structures [45]. While the surface resistance decreases from 2.8 to 2.2 Ω/sq with increasing GZO thickness, there is an increase in resistivity from 2 <sup>×</sup> <sup>10</sup>−<sup>5</sup> to 2.45 <sup>×</sup> <sup>10</sup>−<sup>5</sup> <sup>Ω</sup>·cm.

Assuming that the total number of carriers in the metal layer (*N*Ag) is much greater than the number of carriers in the oxide layer (*N*GZO), the effective concentration of carriers (*n*) of the symmetric GAG structure is related to the thickness of the GZO layers by the following expression [46]:

$$n \sim N\_{\rm Ag} / (2 \times d\_{\rm GZD} + d\_{\rm Ag}) \tag{1}$$

where *d*Ag and *d*GZO are the thickness of the metal interlayer and top (bottom) oxide layer, respectively. From this relation, it can be clearly seen that the carrier concentration should be decreased as the GZO layer thickness increases. This is consistent with the above experiment results.

To evaluate the performance of transparent conductive films for various applications, the optical transmission and the electrical conduction of the films should not be considered separately. Simultaneous optimization of low resistivity and transparencies is needed. Usually, the objective evaluation can be carried out using Haacke's figure of merit (*FOM*) [47], defined as:

$$FOM = T^{10} / R\_{\odot} \tag{2}$$

where *T* is the transmittance at λ = 550 nm or the average visible transmittance. In the last column of Table 3, there are *FOM* values, which are calculated by using the value of the average visible transmittance *T*av for the all deposited samples. From samples of Ag of varying thickness, the maximum *FOM* of 5.15 <sup>×</sup> 10−<sup>2</sup> <sup>Ω</sup>−<sup>1</sup> corresponds to GAG-3, with Ag thickness measuring 10 nm and sheet resistance of 2.45 Ω/sq. This is despite the fact that GAG-4, with Ag thickness of 12 nm, showed a record low resistance (2.0 Ω/sq.). As can be seen, GAG-3 also demonstrates the maximum *FOM* value when comparing samples with the same Ag layer thickness. Additionally, for this sample the *FOM*' was also calculated by using the value *T*550nm of the transmittance at λ = 550 nm (this parameter is usually used to characterize transparent electrodes for LED and information display applications). The value of *FOM*' is equal 1.27 <sup>×</sup> <sup>10</sup>−<sup>1</sup> <sup>Ω</sup>−<sup>1</sup> due to the highest transmittance at 550 nm. Both values of the figure of merit of GAG-3 are superior to many of those reported in the literature [19,44–46]. This may be due to the better spreadability of the Ag layer on the GZO layer during DC magnetron deposition at low temperature, where the bottom GZO layer enhances the silver thin film crystallite size [34,48]. This prompts the formation of a uniform and continuous Ag layer at a much thinner thickness, thereby significantly improving its transparency and conductivity characteristics.
