Low-Temperature Fabrication And Enhanced Pizoelectric Property Of (100)-oriented Bi_3.15 Nd_0.85 Ti 3 O₁₂ Films

As a typical bismuth-layered-perovskite material, Bi₄Ti₃O₁₂ (BTO) has been demonstrated to be lead-free promising candidates for the future ferro- and piezoelectric devices due to the excellent ferroelectric, piezoelectric, dielectric and pyroelectricity properties. It is well known that BTO has an anisotropic pseudo-orthorhombic structure, space group B2cb. The lattice parameters are a=0.541, b=0.544, and c=3.280 nm, the spontaneous polarizations along its a and c axes are about 50 and 4μC/cm2, respectively. In the past decade years, several groups have demonstrated successively that the direction of the major polarization vector of Lanthanides (La, Sm, Nd, and Pr)-doped Bi₄Ti₃O₁₂ (BLnT) is still close to the a-axis for doping contents less than 0.85 mol through fabricating (100)- and non-(100)-oriented BLnT films on different substrates. Therefore, fabrication of highly (100)-oriented BLnT films is the most effective way to improve the ferro- and piezoelectric properties.Lee et al. have reported that highly a-axis-oriented Bi_3.25La_0.75Ti₃O₁₂ (BLT0.75, a(100) =99%)can be formed on SrRuO3(110)/YSZ(100)/Si(100) substrates by pulsed laser deposition (PLD) through controlling the growth rate and oxygen pressure. The remanent polarization (2Pr) of the BLT0.75 films is 32μC/cm2. Several groups have prepared predominantly a-axis-oriented BLnT films using the chemical solution deposition methods (such as sol–gel and metal organic decomposition) combined with a layer-by-layer annealing process. However, the relative intensity of (100) diffraction peak in these films [I(200)/I(117)] is much lower than that reported by Lee et al.Aiming at these obstacles, We preparated Bi_3.15Nd_0.85Ti₃O₁₂ (BNT0.85) films using sol-gel method and investigated the effects of seeding layer and the excess of Bi on the structure, ferroelectric and piezoelectric of BNT0.85 films. The samples were studied by X-ray diffraction (XRD), atomic force microscope (AFM), scanning electron microscope (SEM), and a standard ferroelectric tester. The main contents and results are summarized as follows: 1. For the BLnT films annealed layer-by-layer, the first layer will not only sever as the seed for the next layer but also act as a barrier layer eliminating the negative influence of Pt electrodes on the nucleation and growth of (100)-oriented grains [note that there is no direct epitaxial relationship between (111) facet of Pt and (100) facet of BLnT]. However, the importance of the first deposited layer in the orientation control of chemical-solution-derived BLnT thin films has not attracted enough attention for the fact that the same heating treatment parameters have been used for all the layers during the annealing process in the previous works. And so far, no work has been reported on the preparation of the predominantly (100)-oriented BLnT single seeding layer.BNT0.85 thin films were fabricated on Si(100) substrates, the effects of the thicknesses of the first seeding layers and heating treatment parameters on the structure of BNT0.85 thin films are studied. It was found that the peak intensity and the ratio of I(100)/I(117) are very sensitive to the layer thickness, preheating temperature, annealing temperature and time. The predominantly (100)-oriented BNT0.85 film can be obtainted as the thickness of the first seeding is 70nm. Interestingly, 20-nm increase of layer thickness can lead to that the predominant peak changes from (100) in 70-nm-thick layer to (117) in 90-nm-thick one. The crystallinity of the films decrease with the increase of the preheating temperature, the crystallinity is favor for the whole films at the preheating temperature of 400°C. There exists a temperature window, in which the activation energies required for the nucleation and growth of (100)-oriented grains are lower than those for non a-axis-oriented grains as the films prepared by layer-by-layer. For a 70-nm-thick first layer annealed at 560°C for 180s, the relative intensity of (100) diffraction peak [I(200)/I(117) ] is as high as 1.5. Note that the layer thickness as well as the processing parameters for the preparation of (100)-preferred layer is independent on the substrates utilized, indicating that this method is transplantable.2. In order to evaluate the effects of the first layer on the ferro- and piezoelectric properties of the final films, a 500nm-thick BNT0.85 films were deposited on Pt(111)/TiO₂/SiO₂/Si substrate which the first seeding layer were annealed at 560°C for 180s and 570°C for 240s.(BNT0.85560°C-180s and BNT0.85570°C-240s) respectively. The results demonstrate that crystallinity of the first seeding layer is greatly influence the oriention of the final films. It also can decrease the thickness of the buffer layers which the competition of grain growth is more evident between (117)-and (200)-oriented grains. The ferro- and piezoelectric properties of the BNT0.85560°C -180s films are obviously improved. The aquare shapes of the P-E loops increases from 76% to 87%, the remanent piezoelectric coefficient of the BNT0.85560°C-180s films is 74 kV/cm, which is about 75% higher than that of the BNT0.85570°C-240s films.3. In the typical bismuth-layered-perovskite material, the volatile element Bi is very easily to evaporate from film surface and diffusion into the bottom electrode during annealing process. The deficiency of Bi on BLnT thin film strongly affects the structural, ferro- and piezoelectrical properties. In order to obtain stoichiometric BNT0.85 thin film, excess Bi is usually required. We prepared a serial of the BNT0.85 thin films with various excess Bi contents ranged from 0% to 25% and systematically studied structure, ferro- and piezoelectrical properties. All the films are predominantly (100)-oriented, the relative intensity of (100) diffraction peak [I(200)/I(117)] in these films is larger than 6, which may be due to the well crystallinity of the first seeding layer. When the value of excess Bi content is 15%, the remnant polarization (2Pr) value of the BNT0.85 is 34.25μC/cm2, the P-E loops of the films show well rectangularity of the electricity fields >180kV/cm. The different excesses of Bi greatly affect the piezoelectric coefficient of the BNT0.85 films. The BNT0.85 films with Bi excess of 15% and 20% have larger remanent piezoelectric coefficient.In summary, the relative intensity of (100) diffraction peak, ferro- and piezoelectrical properties of the BNT0.85 films are significantly improved by investigating the first seeding layer and the different excess Bi contents, which can pave the way for the development of future microelectronic devices.