## 1 Introduction  
Electrically tunable devices such as varactors, phase shifters, and delay lines are key elements of modern microwave communication systems. Today, the main materials on the basis of which these devices are implemented are semiconductors, ferrites, and ferroelectrics (FE). All of them have their advantages and disadvantages determined by their physical nature; therefore, the search for the \"optimal for microwave applications\" material is still an urgent task. Following is a list of the main desirable characteristics of an \"ideal\" electrically tunable microwave element: high tunability, low losses, thermal stability of properties, high operating power, and fast response. It is easy to see that a device with such a set of characteristics is not implemented currently. Thus, semiconductor elements operate at low power and significantly lose Q-factor at frequencies above 10 GHz; ferrite elements are characterized by high-energy consumption and dimensions and low speed; the properties of FE devices depend on temperature [1]. It should be noted that FE materials exhibit properties that make them attractive for use in electrically tunable microwave devices, namely a strong dependence on the permittivity on the external electric field [2], relatively low dielectric losses at microwaves [3], the ability to operate at a high power level [4; 5; 6; 7], and fast responses [8]. The above-mentioned properties of FE materials are most effectively realized in film structures on dielectric substrates due to the possibility of forming oriented layers with low microwave losses, the possibility of reaching a high intensity in the control electric field in the film, and the perspective of integration into microwave microelectronic devices [4; 8]. Today, laboratory prototypes of electrically tunable capacitors, phase shifters, delay lines, and other microwave elements based on ferroelectric films have been implemented [9; 10; 11; 12; 13; 14]. The most studied FE material for microwave applications today is a solid solution of barium and strontium titanates, Ba\({}_{\infty}\)Sr\({}_{1\times}\)TiO\({}_{3}\) (BST), the nonlinear properties of which vary widely with changes in the component composition [15; 16]. Compositions with a barium content of 50-60%, for which the temperature of the phase transition from ferroelectric to paraelectric state is close to room temperature, exhibit pronounced nonlinear properties, with a low Q-factor and high temperature dependence of the properties [17]. The best combination of tunability and losses, determined by the commutation quality factor [18], is demonstrated by solid solutions with 30% Ba content [15]. A further decrease in the concentration of barium in the solid solution significantly improves the quality factor and reduces the nonlinearity of the material. Pure SrTiO\({}_{3}\) (STO) has the lowest microwave loss in this series of solid solutions [19]. Single-crystal strontium titanate is a centrosymmetric paraelectric material with a cubic perovskite structure (a = 3.905 A), which does not transform into a ferroelectric state when the temperature decreases down to 0 K. The dielectric permittivity \(\varepsilon\) of the STO crystal increases from 300 at room temperature to 24,000 at 4 K [20]. Strontium titanate is a bright representative of functional oxides. The electrical, optical, thermoelectric, and superconducting properties of SrTiO\({}_{3}\) can be used in various electronic devices [21; 22; 23; 24; 25]. Strontium titanate films are of interest for strain engineering [26; 27; 28], in the development of infrared radiation sensors [29], and for photocatalytic, biosensor [30], and energy-storage [31] applications. Based on strontium titanate films, the epitaxial heterostructures of a \"dielectric-semiconductor\" [32; 33], and multilayer structures for use in photodetectors, memory [34] and other devices [35; 36] can be realized. The electrical properties of strontium titanate, which make it a promising material for tunable microwave elements, are considered in [37].