# Low-Temperature Dielectric Response of Strontium Titanate Thin Films Manipulated by Zn Doping
## 3 Results and Discussion  
Figure 1a shows the SEM cross-sectional microstructure of SrTi\({}_{1-x}\)Zn\({}_{x}\)O\({}_{3-\delta}\) thin films with x = 0, 0.01 and 0.05 grown on platinized silicon substrates. The average film thickness is about 320 nm for both Zn-doped ST films, while that of undoped ST is about 350 nm. Moreover, the morphology of each film reveals several rounded and closely-packed grains across the film's thickness. EDS analysis of SrTi\({}_{1-x}\)Zn\({}_{x}\)O\({}_{3-\delta}\) thin films, presented in Figure 1b, clearly displays a Zn peak, whose intensity increases with the value of x. According to the spectra quantification, Zn concentrations in both Zn-doped ST thin films are close to targeted ones, while overall estimated elemental contents indicate the proximity of all the film compositions to nominal ones within error bars. Figure 1: SEM cross-section micrographs (**a**) and energy-dispersive spectra (**b**) for SrTi\({}_{1-x}\)Zn\({}_{x}\)O\({}_{3-\delta}\) thin films with x = 0, 0.01 and 0.05, deposited on Pt/TiO\({}_{2}\)/SiO\({}_{2}\)/Si substrates. The spectra quantification results are also presented in (**b**). XRD profiles of 1% and 5% Zn-doped and undoped ST films on Pt/TiO\({}_{2}\)/SiO\({}_{2}\)/Si substrates are shown in Figure 2a, revealing only peaks related to the cubic perovskite Pm-3m structure (PDF#35-0734) and those from the substrate layers, particularly Pt, whose XRD profile is also shown in Figure 2a. The cubic perovskite structure peak positions, however, slightly shift toward lower 26 values with increasing Zn content, as shown in Figure 2b for (200) and (211) peaks, best seen between the Pt peaks. Such a shift implies a lattice parameter increase, as displayed in the inset in Figure 2b. This finding, together with EDS confirmation of the presence of and increase in Zn peaks with nominal Zn concentration and an educated guess of similar strain in the films [29], proves the incorporation of Zn onto the Ti sites of the SrTiO\({}_{3}\) lattice, at least in its major part. The proof can be found by taking into account that the ionic size of Sr\({}^{2+}\) is larger than that of Zn\({}^{2+}\), whereas the ionic size of Zn\({}^{2+}\) is larger than that of Ti\({}^{4+}\)[27]. Therefore, it is reasonable to suppose the formation of a SrTi\({}_{1-x}\)Zn\({}_{x}\)O\({}_{3-\delta}\) solid solution, since, for Sr\({}_{1-x}\)Zn\({}_{x}\)TiO\({}_{3}\) solid solutions, the lattice parameter should decrease with Zn content. Moreover, the lattice parameter variation slope is similar to that reported for SrTi\({}_{1-x}\)Mg\({}_{x}\)O\({}_{3-\delta}\) ceramics [30]. Such similarity is also reasonable, since Mg\({}^{2+}\) possesses an ionic size close to that of Zn\({}^{2+}\)[27]. The \(\epsilon^{\prime}(T)\) dependence of the reference ST thin film shown in Figure 3a reveals an increase in cooling until a diffuse peak at about 60 K. This peak is also seen in the \(\epsilon^{\prime}(T)\)  
Figure 2: XRD profiles (**a**) and magnified view at reflections (200) and (211) (**b**) for SrTi\({}_{1-x}\)Zn\({}_{x}\)O\({}_{3-\delta}\) thin films with x = 0, 0.01 and 0.05, deposited on Pt/TiO\({}_{2}\)/SiO\({}_{2}\)/Si substrates with reflections related to perovskite structure of SrTiO\({}_{3}\) (card PDF#35-0734) marked by corresponding indexes. The X-ray diffraction profile for bare Pt/TiO\({}_{2}\)/SiO\({}_{2}\)/Si substrate is also presented in the bottom panel of (**a**). Inset in (**b**) shows the lattice parameter variation with Zn content. of SrTi\({}_{1-x}\)Zn\({}_{x}\)O\({}_{3-\delta}\) films with peak temperatures decreasing to about 46 K for x = 0.01 and 38 K for x = 0.05. The permittivity value also decreases with increasing Zn content. The permittivity decrease was also observed for SrTi\({}_{1-x}\)Mg\({}_{x}\)O\({}_{3-\delta}\) ceramics [30] and thin films [31] as well as for SrTi\({}_{1-x}\)Mn\({}_{x}\)O\({}_{3}\) ceramics [32], being explained by the substitution of smaller and highly polarizable Ti\({}^{4+}\) by larger and hence less polarizable dopant ions.