*3.1. Surface Morphology and Structural Studies*

The surface morphology of the GAG samples was investigated by SEM. Figure 1 shows the typical SEM images of top view surfaces observed in this work. A single layer GAG-0 sample with a total thickness of oxide of 80 nm and a zero thickness Ag interlayer (Figure 1a) consists of well-defined continuous particles of nearly equal lateral size (~ 40 nm) uniformly covering the smooth substrate. After deposition of 10-nm thick Ag layer on the GZO surface, a well-marked change in morphology appears (Figure 1b). Forming a sufficiently continuous layer of silver makes the surface of the structure even smoother, although some nanovoids on the surface of the GA bilayered structure are still present due to the surface performance of the bottom GZO layer. The observed smoothing of the surface indicates that under the above growth conditions, the process of Ag growth should be described in the following scenario:


**Figure 1.** SEM images GAG-0 (**a**), GA (**b**), and GAG-3 samples (**c**).

In earlier reports [33,34], it was found that the spreading of Ag on the surface of ZnO was better than on SnO2 and In2O3 due to an ameliorated affinity between Ag and ZnO. Nevertheless, it was confirmed that the surface of ZnO was rougher than that of the latter.

As is shown in Figure 1c, the surface morphology of GAG-3 trilayered structure became less smooth but quite compact. The difference between the surfaces of GAG-1 and GAG-3 samples is due to both differences in the nucleation conditions of the top and bottom oxide layers, and the fact that the grain size usually increases with the thickness of the ZnO thin film [35,36].

It should be noted that the SEM studies of other samples did not reveal any noticeable differences in the surface morphology of the samples considering the thickness of the Ag interlayer at a fixed thickness of the GZO, as well as when changing the thickness of the GZO layers in the range of 30–50 nm. Thus, we can reach a conclusion that in our SEM experiment, the top GZO, which was sputtered at room temperature, showed surface features typical of nanocrystalline Ga-doped ZnO thin films, regardless of the Ag interlayer thickness and its morphology. At the same time, additional atomic-force microscopy (AFM) studies (Figure S2 in SM) showed that the surface roughness increases noticeably by introducing an Ag interlayer into GZO. The root mean square (RMS) roughness values of GZO-0 and GZO-1 samples are 0.897 and 1.226 nm, respectively, calculated from the AFM data. A slight decrease in roughness is observed with further increases of both Ag and GZO thicknesses in the trilayer structures.

Figure 2a shows the XRD plots of the GAG multilayered structures with Ag interlayers of different thicknesses. Only four broad peaks were present in the XRD spectra, two of which belong to the (002) ZnO and (004) ZnO reflections, and the other two to the (111) Ag and (222) Ag reflections. The presence of two (002) ZnO and (004) ZnO peaks corresponding to the nanocrystalline hexagonal ZnO wurtzite phase indicates that the GZO has a preferential orientation featuring the c-axis perpendicular to the substrate surface, regardless of the Ag interlayer thickness. The insertion of the Ag interlayer in the middle of the GAG structure does not affect the strongly preferred orientation of GZO toward (001). Additionally, for the GAG structures, the Ag interlayer has highly preferred orientation toward (111). It is often reported that the crystallized ZnO lattice promotes the silver growth along the (111) direction. This might be due to the fact that the (111) plane of a cubic structure has a similar symmetry to that of the (001) plane of ZnO [34]. The inset of Figure 2a shows the XRD spectral region in which the most intense (002) ZnO and (111) Ag peaks are located. The main features of both peaks are given in Table 2.

**Figure 2.** X-ray diffraction (XRD) plots of the prepared multilayer structures: (**a**) with different Ag interlayer thicknesses; (**b**) with different thicknesses for bottom and top GZO layers. The inset of Figure 2a shows the XRD spectral region with the most intense (002) ZnO and (111) Ag peaks.


**Table 2.** XRD data for GAG multilayer structures.

It can be seen that when the Ag interlayer is introduced into the GAG structure, thereby breaking the GZO layer into two equal parts by thickness, a decrease in intensity and some broadening of the (002) ZnO peak takes place. A further increase of the thickness of the Ag interlayer until 12 nm does not affect the crystallinity of the GZO phase, which is correlated with the SEM data results.

At the same time, the intensity of the (111) Ag peak increases and the integral breadth β decreases with a thickening of the Ag interlayer. The peak shifts from 38.21 to 38.25◦, with an increase in the Ag thickness from 6 to 8 nm, after which its position no longer changes.

Estimation of the averaged crystallite size (CS) from (002) ZnO and (111) Ag peak characteristics using the Scherrer equation (CS = 0.9λ/(βcosθ), where λ is the wavelength of CuKα x-rays, β is the peak integral breadth with no instrumental contribution, and θ is the peak Bragg angle) showed that the crystallite size of GZO decreases from 16 to 14 nm when the Ar interlayer is introduced, and the Ag crystallite size increases continuously with the increase of the Ag thickness from 7 to 10 nm.

The change in the ratio of the intensities I of the (111) Ag and (002) ZnO peaks (IAg/IZnO) is in agreement with the deposition regimes for these GAG structures.

Despite the low substrate temperature during the GZO and Ag sputtering process, the GAG multilayer structures consist of both nanocrystalline GZO and Ag layers (Figure S3 of SM). In addition, we can reach the conclusion that the crystallinity of the top GZO layer is independent of the Ag interlayer.

GZO thickness variation in the range of 30–50 nm at a fixed thickness of the Ag interlayer of 10 nm does not affect the preferential orientation of both GZO and Ag layers. Figure 2b shows the XRD plots of the GAG structures as a function of the top and bottom GZO thickness in the 2θ range of 31–41◦. XRD data for (002) ZnO and (111) Ag peaks of GAG-5 and GAG-6 are shown in Table 2 (in order to compare these with GAG-3).

Comparing these samples, the (002) ZnO peak shifts from 34.01 to 34.10◦ with an increase in the GZO thickness from 30 to 40 nm. A further increase of the GZO thickness does not change the peak position at 34.10. It can also be observed that as the thickness of GZO film increases, the intensity *I* is enhanced and the integral breadth β decreases for the (002) ZnO diffraction peak, indicating that the increased thickness of top and bottom layers improves the crystallinity of the GZO phase.

As for the (111) Ag peak, its features for this sample group were practically independent of the thickness of the oxide layers, which implies the invariability of the crystallinity of the Ag interlayer with the increase of the GZO thickness. By increasing the GZO thickness, the averaged crystallite size for GZO increases continuously from 12 to 15 nm, while the Ag crystallite size remains unchanged and remains in good agreement with the thickness of the Ag interlayer.

Thus, we can conclude that the crystallinity of the Ag interlayer is insensitive to changes in the thickness of the bottom GZO layer for the low substrate temperature sputtering process. In this case, the presence of this GZO layer itself is important as a seed layer for Ag. Changes in the nature of coalescence of Ag nuclei were observed in the presence of seed layers with a thickness of only a few nanometers [37,38].
