# Low-Temperature Dielectric Response of Strontium Titanate Thin Films Manipulated by Zn Doping
## 2 Materials and Methods  
For the deposition of SrTi\({}_{1-x}\)Zn\({}_{x}\)O\({}_{3-\delta}\) thin films with x = 0, 0.01 and 0.05, solutions with a concentration of about 0.2 M were prepared using strontium acetate C\({}_{4}\)H\({}_{6}\)O\({}_{4}\)Sr (98%, abcr GmbH, Karlsruhe, Germany), tetra-n-butyl orthothatinate C\({}_{16}\)H\({}_{36}\)O\({}_{4}\)Ti (98%, Merck KGaA, Darmstadt, Germany) and zinc acetate-2-hydrate C\({}_{4}\)H\({}_{6}\)O\({}_{4}\)Zn\(\cdot\)2H\({}_{2}\)O (99.5%, Riedel-de Haen, Seelze, Germany) as starting precursors. Acetic acid C\({}_{2}\)H\({}_{4}\)O\({}_{2}\) (99.8%, Merck KGaA, Darmstadt, Germany), 1,2-propanediol C\({}_{3}\)H\({}_{8}\)O\({}_{2}\) (99.5%, Riedel-de Haen, Seelze, Germany) and absolute ethanol C\({}_{2}\)H\({}_{6}\)O (99.8%, Merck KGaA, Darmstadt, Germany) were used as solvents. Strontium acetate was initially dissolved into heated acetic acid (\(T\sim\) 60 \({}^{\circ}\)C) followed by the addition of zinc acetate-2-hydrate under constant stirring to form a transparent solution. After cooling to room temperature, the former solution was diluted with 1,2-propanediol and then titanium isopropoxide was added. The resultant solution was continuously stirred in a closed flask for 12 h, at the end of which an ethanol was added as a final step. Using these transparent and homogeneous solutions, layers of Zn-doped SrTiO\({}_{3}\) were deposited on Pt/TiO\({}_{2}\)/SiO\({}_{2}\)/Si substrates (Inostek INC, Seoul, Korea) by spin-coating at 4000 rpm for 30 s, using a spin-coater KW-4A (Chemat Technology, Los Angeles, CA, USA). Before the deposition, the substrates were cleaned in boiling ethanol and dried on a hot plate. After the deposition of each wet layer on the substrate, they were heated on a hot plate at 350 \({}^{\circ}\)C for \(\sim\)1 min to ensure complete removal of the volatile species between the layers. After the complete deposition of the required number of layers (10 layers), they were annealed in air at 750 \({}^{\circ}\)C for 60 min with a heating/cooling rate of 5 \({}^{\circ}\)C/min.  
The thickness of the thin films was determined, and their cross-sectional morphology was observed using scanning electron microscopy (SEM, Hitachi S4100, Tokyo, Japan) under an acceleration voltage of 25 kV. Compositional analysis of the films was performed using an energy dispersive spectroscopy (EDS) system (QUANTAX 75/80, Bruker, Ettlingen, Germany) built in SEM (Hitachi TM4000 Plus, Tokyo, Japan) in the top-view geometry under an acceleration voltage of 10 kV. The thin-film crystal phase was analysed at room \(T\) using a Rigaku D/Max-B X-ray diffractometer (Rigaku, Tokyo, Japan). The X-ray diffraction (XRD) data were recorded in 0.02\({}^{\circ}\) step mode with a scanning rate of 1\({}^{\circ}\)/min from 20\({}^{\circ}\) to 80\({}^{\circ}\) using Cu K\(\alpha\) radiation. The lattice parameter was calculated by a least-squaresapproach Rietveld refinement fitting of the XRD data in the range of 45\({}^{\circ}\)-60\({}^{\circ}\). Dielectric spectroscopy and tunability measurements of Zn-doped ST films were performed using Au, sputtered through a mask onto the films, as top electrodes, and the substrate Pt layer as the bottom one. Complex dielectric permittivity, consisting of a real part \(\varepsilon^{\prime}\) and an imaginary part \(\varepsilon^{\prime\prime}\), as well as the dissipation factor tan\(\delta\) = \(\varepsilon^{\prime\prime}\)/\(\varepsilon^{\prime}\) were measured under an oscillation voltage of 50 mV (and a DC voltage up to 5 V for tunability measurements) at a frequency of 10 kHz, using a precision LCR meter (HP 4284A, Hewlett Packard, Palo Alto, CA, USA). A He closed-cycle cryogenic system (Displex APD-Cryostat HC-2, Allentown, PA, USA) equipped with silicon diode temperature sensors and a digital temperature controller (Scientific Instruments Model 9650, West Palm Beach, FL, USA) was used for \(T\) variation in the range of 10-300 K.