# Low-Temperature Dielectric Response of Strontium Titanate Thin Films Manipulated by Zn Doping
###### Abstract  
The voltage dependence of the dielectric permittivity \(\varepsilon^{\prime}\) and the low dielectric loss tan\(\delta\) of incipient ferroelectrics have drawn vast attention to the use of these materials for the development of tuning elements in electronics and telecommunications. Here, we study the DC electric field dependence of low-temperature \(\varepsilon^{\prime}\) in \(\sim\)320 nm thick sol-gel-derived SrTi\({}_{1-x}\)Zn\({}_{x}\)O\({}_{3-\delta}\) thin films with x = 0.01 and 0.05, deposited on Pt/TiO\({}_{2}\)/SiO\({}_{2}\)/Si substrates. Incorporation of Zn onto Ti sites is found to decrease \(\varepsilon^{\prime}\) compared to undoped SrTiO\({}_{3}\) films, while increasing the relative tunability \(n_{r}\) up to \(\sim\)32.9% under a DC electric field of 125 kV/cm at low temperatures. The hysteresis-free variation in \(\varepsilon^{\prime}\) with electric field and tan\(\delta\) values below 0.6% observed for SrTi\({}_{1-x}\)Zn\({}_{x}\)O\({}_{3-\delta}\) film with x = 0.01 make this compound more attractive for tunable device applications.  
perovskites; polar dielectrics; thin films; doping; dielectric tunability +
Footnote †: journal: materials  
The tan\(\delta\) of ST-based materials can be reduced by admixing with low-loss, non-tunable materials [16; 20] and by acceptor doping on Ti sites [21; 22; 23]. Nevertheless, reports of the dielectric response of Zn-doped ST are rather rare and ambiguous. In 2012, [PERSON] et al. reported that the low-\(T\) dielectric permittivity increases with a Zn content up to 0.9% in Sr\({}_{1-x}\)Zn\({}_{x}\)TiO\({}_{3}\) ceramics sintered at 1400 \({}^{\circ}\)C [24]. In 2020, [PERSON] et al. claimed that the room-\(T\) permittivity was similar to that of undoped SrTiO\({}_{3}\) at reduced losses for ceramics with nominal composition SrTi\({}_{0.995}\)Zn\({}_{0.015}\)O\({}_{3}\) sintered at 1500 \({}^{\circ}\)C [25]. However, the Zn effect in this case is doubtful as Zn is volatile at such high temperatures [26]. Accordingly, the increase in the lattice parameter when Zn was supposed by [PERSON] et al. to be substituted with Nb in their ceramics was observed instead of decrease [25]. The ionic size of Nb\({}^{5+}\) is smaller than that of Zn\({}^{2+}\) but larger than that of Ti\({}^{4+}\)[27]. Therefore, the presence of Zn in the final ceramics should lead to the lattice parameter decreasing with decreasing Zn content, whereas the observed increase can just be explained by Nb substitution for Ti with the absence of Zn in the lattice.  
More recently, the room-\(T\) dielectric permittivity was reported to decrease with increasing ZnTiO\({}_{3}\) content in sol-gel-derived SrTiO\({}_{3}\)/ZnTiO\({}_{3}\) heterostructures annealed at just 750 \({}^{\circ}\)C by [PERSON] et al. [28], thus avoiding the problem of high-\(T\) Zn volatility. However, no low-\(T\) dielectric characterization has been reported so far on Zn-doped ST films with Zn incorporation into Ti positions. Therefore, in this work, we performed a compositional, structural, as well as variable temperature and DC electric field dielectric characterisation of sol-gel-derived SrTi\({}_{1-x}\)Zn\({}_{x}\)O\({}_{3-\delta}\) thin films with x = 0.01 and 0.05, deposited on Pt/TiO\({}_{2}\)/SiO\({}_{2}\)/Si substrates, in comparison to similarly prepared films of undoped SrTiO\({}_{3}\).