Planar Capacitive Structures Based on Ferroelectric Barium Titanate–Stannate Films on Sapphire for Microwave Applications

Enhanced Tunability of BaTixSn1−xO3 Films on Dielectric Substrate

“…(ii) For films deposited in a medium containing 40% oxygen in a gas mixture, the positions of the reflexes correspond to BaSnO 3 with the lattice parameter 4.12 Å, without signs of the presence of a solid solution. The probable cause of the formation of the dielectric barium stannate may be the presence of impurity phases of titanium oxides on the substrate, the possible positions of XRD reflexes from which are marked in Figure 1 with red and blue dotted lines (PDF 21-1272 TiO 2 Anatase; PDF 8-117 TiO; PDF 29-1360 TiO 2 Brookite; PDF 34-133 BaTi 2 O 5 ), similar to [34,38]. (iii) The reflexes from the BTS phase in Figure 1 are significantly shifted towards large angles when the oxygen content in the gas medium decreases from 40 to 20%, that for polycrystalline samples means a change in the composition of the solid solution (the ratio of Ti and Sn) according to the Vegard's law (see the inset in Figure 1) [42].…”

“…Thus, the perovskite crystal structure of barium-strontium titanate without secondary phases is confidently formed in a purely oxygen medium on sapphire, silicon carbide, magnesium oxide, etc. On the other hand, it is noted in [34,38] that attempts to form FE film with substitution of Ti by Zr or Sn atoms on dielectric substrates in an oxygen-rich gas medium lead to the formation of a multiphase structure consisting of purely dielectric barium zirconates or barium stannates and secondary phases with a high content of titanium-simple oxides and barium polytitanates (BaTi 2 O 5 , BaTi 5 O 11 , etc.). This can be explained by the kinetics of Ti oxidation in a medium with a high oxygen content, when the rate of formation of secondary phases of titanium significantly exceeds the rate of formation of ferroelectric BaTiO 3 .…”

“…Depending on the ratio of the concentrations of Ti and Sn atoms, the value of the relative dielectric permittivity of BTS solid solution in the maximum can reach significant values (2-3) × 10 4 [15]; the introduction of chemically more stable Sn atoms into a solid solution instead of Ti can lead to a decrease in through conductivity and dielectric losses, to a significant increase in the electrical strength of the material, and, as a result, to the possibility of increasing the applied electric field and increasing the dielectric nonlinearity [16]. A number of papers have been published that investigate the structural and electrophysical properties of BTS thin films [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34], including works dedicated to microwave investigations [25,34,35]. The studied films were obtained by chemical solution deposition [22,23,36], ion-plasma sputtering [17][18][19]29,30,34,35], sol-gel technology [24][25][26][27][28]31,32] and laser ablation…”

“…A number of papers have been published that investigate the structural and electrophysical properties of BTS thin films [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34], including works dedicated to microwave investigations [25,34,35]. The studied films were obtained by chemical solution deposition [22,23,36], ion-plasma sputtering [17][18][19]29,30,34,35], sol-gel technology [24][25][26][27][28]31,32] and laser ablation [33]. In the vast majority of works, experimental data on dielectric losses in FE capacitive structures are given for the frequency range of 1 kHz-1 MHz.…”

“…In addition, it should be noted that the high dielectric nonlinearity of capacitive elements based on the studied films (tunability up to 72%) is demonstrated exclusively in plane-parallel metal-dielectric-metal (MDM) structures, i.e., in cases when the film is formed on a conducting electrode [22][23][24][28][29][30]33,35], while in works devoted to the growth of FE layers on dielectric materials data on tunability from 15 to 46% are given [25][26][27]31,32,34,36], and only on expensive monocrystal substrates. The design of MDM capacitors allows the use of small control voltages to change the capacitance, but exactly because of it, the use of such structures is possible only in small-signal devices.…”