The effect of Mg doping on the dielectric and tunable properties of Pb0.3Sr0.7TiO3 thin films prepared by sol–gel method

Xiaohua Sun Xiuneng Li Shuang Hou Caihua Huang Jun Zou Meiya Li Tianyou Peng Xing-zhong Zhao Mg doped Pb0.3Sr0.7TiO3 (PST) thin films were fabricated by the sol-gel method on a Pt/Ti/SiO2/Si substrate. The microstructure, surface morphology, dielectric and tunable properties of PST thin films were investigated as a function of Mg concentration. It is found that proper Mg doping dramatically improves the dielectric loss (0.0088 @ 1 MHz), furthermore, the crystallinity, dielectric constant, and tunability of films simultaneously decrease with the increase of Mg content. The 2 mol% Mg doped PST thin film shows the highest figure of merit (FOM) value of 36.8 for its the smallest dielectric loss and upper tunability. The dependence of Rayleigh coefficient on the doping concentration was examined, which indicated that the reduction of dielectric constant and tunability of films should be related to the MgTi-VO defect dipoles pinning the domain wall motion of residual polar clusters in PST. 1 Introduction Solid solutions of Pbx Sr1x TiO3 (PST) thin films have drawn great attention in recent years. Its Curie temperature can be adjusted linearly from 490 to 230 C with increasing Sr content and the process temperature of PST films is relatively low. It is considered as one of the most potential candidate materials for the future tunable microwave device components, such as phase shifters, filters, varactors, delay lines, etc. [14]. However, significant reductions in loss at high frequencies together with the improved dielectric tunability are needed for their realization in devices. In order to further improve the performance of PST films, many efforts have been tried in various ways. The effects of buffer layers [5], epitaxial or oriented growth [6, 7], compositionally graded films [8] and multilayered films structures [9] on the dielectric and ferroelectric properties of PST films were investigated. Besides, it has been identified that proper acceptor doping is another effective way to optimize the ferroelectric and dielectric properties of perovskite ferroelectric films. In fact, doping may be able to change the defect concentration even of defect types. Furthermore, the different defect concentration and defect types have an important effect on the electric properties of thin films. Miao et al. controlled defects in (Ba0.8Sr0.2)(Zr0.2Ti0.8)O3 films through Co acceptor doping to depress the leakage current and increase tunable properties of films [10]. In PST system, though some researches [11, 12] about acceptor doping have been carried out, most of them mainly focus on the effect of acceptor dopant on the dielectric loss, and the related physics mechanisms behind the element doping adjusting defect concentration and types and then changing polarization and dielectric properties still need to be explored. In this study, Mg doped PST thin films with dopant content from 06 mol% were prepared on Pt/Ti/SiO2/Si substrates by the solgel method. The microstructure and surface morphology of the Mg doped PST films were characterized by XRD and AFM. The Rayleigh law was used to characterize the effect of Mg as an acceptor dopant on the defects, polarization and dielectric properties of films. 2 Experimental procedure Mg doped Pb0.3Sr0.7TiO3 thin films were prepared according to the formula Pb0.3Sr0.7(Ti1x Mgx )O3, where x = 0, 2 %, 4 % and 6 %, with the solgel method. All the films with 10 % excess Pb were prepared using lead acetate tri-hydrate [Pb(CH3COO)23H2O], strontium acetate semi-hydrate [Sr(CH3COO)21/2H2O], magnesium acetate [Mg(CH3COO)24H2O] and titanium tetrabutoxide [Ti(OC4H9)4] as source materials. Glacial acetic acid, deionized water and 2-methoxyethanol were selected as solvents. Formamide, acetylacetone, and ethylene glycol were added to stabilize phase, adjust the viscosity and surface tension. The concentration of the precursor solution was adjusted to 0.5 mol/L. The precursor solution was coated on the Pt/TiO2/SiO2/Si substrates via a spin coating at a rate of 4000 rpm for 25 s and then to pyrolyzed at 400 C for 10 min and 480 C for 5 min. The spin-coating and heattreatment procedure was repeated several times to obtain desired thickness. Finally, all films were annealed at 700 C for 30 min for crystallization. The structural and dielectric properties of Mg doped PST thin films were characterized by various techniques. X-ray diffraction (XRD) was performed for phase identification using a Ultima IV X-ray diffractometer with Cu K radiation. The surface and roughness were observed by the SPM9500J3 atomic force microscope (AFM). The cross section and thickness of the films were examined with a JSM-7500F field emission scanning electron microscope (FE-SEM). Dielectric measurements were carried out using the metal insulatormetal (MIM) capacitor configuration. A gold top electrode with 0.3 mm diameter was deposited on the film by direct current magnetron sputtering system (JGP-560). The dielectric properties were measured using Agilent 4294A precision impedance analyzer. Ferroelectric hysteresis loops (P-E) of PST thin films were characterized by a ferroelectric tester (Precision Premier Workstation, Radiant Technology, USA). 3 Results and discussion Figure 1 shows the X-ray diffraction patterns of Pb0.3Sr0.7 (Ti1x Mgx )O3 thin films annealed at 700 C with different x. It can be noted that all crystal structures of Pb0.3Sr0.7(Ti1x Mgx )O3 thin films are cubic perovskite Fig. 1 X-ray diffraction patterns of the Pb0.3Sr0.7(Ti1x Mgx )O3 thin films annealed at 700 C with different x: (a) x = 0, (b) x = 2 %, (c) x = 4 %, (d) x = 6 % phase with no evidence of secondary phase formation and show no preferred orientation. Furthermore, the diffraction peaks shifted towards low angle gradually with the increase in x, which implies that the dopants have entered the unitcell maintaining the perovskite structure of solid solution. The change in the peak position should be ascribed to the substitution of Ti4+ by Mg2+ ions. The ionic radius of Mg2+ (0.720 ) in the 6-fold coordination is larger than that of Ti4+ (0.605 ) in the 6-fold coordination [13], which leads to expansion of the crystal cells. In addition, the intensity of Pb0.3Sr0.7(Ti1x Mgx )O3 thin films reduced slightly with increasing Mg content, which demonstrates that the crystallinity and/or grain size of Mg doped samples decreases. The shift of peak position and the reduction in crystallization and/or grain size by Mg dopant are in agreement with the results reported in other literature [1416]. Figure 2 shows the surface morphology of Pb0.3Sr0.7 (Ti1x Mgx )O3 thin films analyzed by AFM. As shown in Fig. 2, all the AFM images show granular microstructure. The surface root-mean-square (RMS) roughness values of Pb0.3Sr0.7(Ti1x Mgx )O3 thin films are 8.075.95 nm and the average grain sizes of films estimated using the linear intercept method are 5262 nm. It can be seen that the reduction of grain sizes for Pb0.3Sr (...truncated)