Studies On The Preparation And Electric Properties Of (Sr, Ba) TiO3 Thin Films With Modifying Elements

The application of (Sr, Ba)TiO₃ thin films used in DRAM (Dynamic Random Acess Memory), phase shifters and tunable microwave devices has been one of the significance fields in the investigation and fabrications. The paper investigated the proportion technology, microstructure and properties of (Sr, Ba)TiO₃ thin films by sol-gal processing, when barium acetate (Ba(CH₃COO)₂), Sr(CH₃COO)₂ 1/2H₂O), and tetrabutoxide(Ti(OC₄H₉)₄) were used as sources of strontium, barium and titanium respectively.When the amounts of glacial acetate acid and ethanol used as solvents were 10 to 20ml, and acetylacetone and glycerin used as chelating and surface activation agent was 0.5 to 2ml respectively, the PH value of the sol was 3 to 4, its viscosity was from 3pc to 6pc, and the concentration 0.3 to 0.4mol/L. After a spin coating at 3500 r/min for 30s annealed at 750℃, the thickness of thin films with three layers for each layer was 450nm. The crystal grains distributed uniformly, and no cracking existed on the surface of thin films at 0.5 to l°C/min of hyperthermic or cooling treatment. The Pt electrode was stabilized by the structure of Pt/TiO₂/SiO₂/Si (100), and Pt separating from substrates was also avoided. Because of the transformation of structure from ketonic to enol form, acetylacetone (HAcAc) was easy to chelate with Ti(OC₄H₉)₄ to form network structure, in which the Ba²⁺, Sr²⁺ cations presented homogeneously. The formation of crystal phase and the growth of grains were improved, the forming temperature of perovskit was reduced to 624℃, and the average grain diameter was about 80nm. The best annealed temperature and thickness determined by the higher dielectric properties of thin films was 750°C and 450nm respectively.The Curie temperatures and maximum dielectric constants of BST thin films were not always the same, and they could be changed by varying the ratios of Sr/Ba in order to reach the best condition of dielectric constants. Maximum dielectricconstant er (er=78), minimum dielectric loss tand (tand=0.057) and the temperature Tm for maximum er (Curie temperature) (Tm=305K) were obtained, when the thin film with the ratio of Sr/Ba was 0.5, deposited on the substrate of silicon (100) with lower resistivity (3 to 60cm) at 1KHZ. While Sr/Ba=0.5, er=100.54, tand=0.060, and Tm=308K, just the base were different, the film which was prepared on the higher resistance silicon, (100) with (30000cm), possessed relatively some poor dielectric properties than those of the films prepared on the surface of low resistance silicon. But both the two thin films possess good dielectric frequency characteristics.Dopant Bi, Y, or Nb decreased tand (the contents of dopant Bi varied from 0.000 to 0.30mol, and Nb contents varied from 0.000 to 0.0412mol), and Tm moved to lower temperature direction at the same time, and evident characteristics of diffuse phase transition appeared. Only the suitable amount of dopant exists, the er can be increased obviously. For example, when at 1KHz, the amount of Y was 0.015mol, the er of Sro.5Bao.485Yo.oi5Ti03 thin film was 74.2 and its tan=d0.067, which was higher than that of the thin films with any other Y dopants. Tm and tand were decreased greatly by doped Nb, for example, the Tm of Sro.sBao.5Ti03 thin film, when with 0.0412mol of Nb was added, declined to 277Kfrom 305K, and its tand declined to 0.067 from 0.09. The different defect types and its concentration were formed, when Ba2+ or Ti4+ was substituted by modified elements, which affected the diffusion rate of ions, grain growth at the annealing treatment, and polarization of dipole, so better properties of the thin films were resulted.The ferroelectric domain in the SBT structure was neither 90° nor 180°, but its width was about 1 to 3 nm. With the increasement of dopant Bi, the hyteresis loop rectangle shape of the Bao.5-xBixTi03 also gets increased. When at the condition of lOOHz, 20V and the content of dopant Bi was 0.015mol, the remanent polarization (Pr), spontaneous polarization (Ps) and coercive field (Ec) were 0.22 /j, C/cm2, 0.32fiC/cm and 60KV/cm respectively.The microstructures of the SBT thin films were studied in the paper, such as the surface structure, cross section structure, the distribution of Yi4+, Ba2+, Sr2+ on the line, grain sizes, lattice fringes, crystal boundaries, etc. When Sr/Ba=0.5, the surface of the thin films was even and plain, no cracking, with grain distributed homogeneously, and all grains were about the sizes of 50 to llOnm. After Bi was added, some grains grew dramatically, which enlarged the grain size scope, such as in Sample B3, the grains were around 50¹10nm, with the average size was approximately 80nm. When Y3+ or Nb5+ was added, the grain sizes were reduced, for example, the average grain size of Sro.5Bao.493Yo.oo7Ti03 was about 70nm and that of Sr0.5Bao.5Ti0.9588Nbo.442Ti03 was about 60nm. The grains of the thin films deposited on the surface of high resistance silicon arranged compactly, and with little gas pockets. In all the experiments, whether some dopants existed, the eutectic boundary and non-eutectic boundary always exist. Grains grow in two different ways: one is growing in the same direction, and the other is growing randomly. All the thin films are with clear, even cross layers and even thickness in their structure. The average thickness of the film was about 450nm, which was derived from the distribution principle of Sr2+, Ba2+ and Ti4+ in the experiments.The structure of the SBT thin films is always tetragonal perovskites. With the contents of Ba"+ get increased gradually, the perovskites diffraction plain of Sri.xBaxTiO3 (110), characteristic diffraction plain of the tetragonal and their diffraction intensity are getting increased gradually too. After Bi3+, or Y3+, Nb5+ were substituted in the films for Ba2+ or Ti4+, still no other phases were resulted, the structure is also tetragonal perovstites, but ratio c/a changed. The influences of the changing value of c/a on the Curie temperature and dielectric constants vary in accordance with different samples. That is, while value c/a is getting higher, theequivalent maximum dielectric constants temperature (Tm) and er don't keep the same tendency of increasing or decreasing. When c/a=1.140 and c/a=1.152, the dielectric constants er and the dielectric losses tand in Sro.5Bao.493Yo.oo7Ti03 and Sro.5Bao.5Tio.9588Nbo.442TiC>3 reached their maximum and minimum value expectedly, that is, ermax in the former sample, Sro.5Bao.493Yo.oo7Ti03, is 74.2, which tandm;n is 0.067; ermax in the later is 58, which tandmin 0.073. The film, which was produced on the substrate of high resistance silicon, was more favorable for the development of tetragonal perovskites, so the film prepared here shows better dielectric properties.The influences of dopant Bi on the octahedron [TiOe] could be demonstrated by the variation of the two characteristic absorption peaks, 47.65cm"1 and 610cm"1 by FTIR analysis of Sro.5Bao.s.xBixTi03 powder, with the increasement of dopant Bi, the two characteristic absorption peaks of octahedron [TiC>6] drifted to the lower wavenumber, indicating that the longer the length of Ti-0 bond, the favorer of the formation of tetragonal perovskites.