Investigation On The Growth Mechanism And Domain Switching Evolution Of Ferroelectric Thin Film

With the rapid development of modern microelectronics and optoelectronic technology, electronic devices move toward the miniaturization and integration. Perovskite ferroelectric thin films, such as BaTiO₃, PbTiO₃, Pb(Zr,Ti)O₃, (Pb, La)(Zr,Ti)O₃, have attracted much attention and become the research hotspot within new functional materials due to the excellent ferroelectric, dielectric, pyroelectric, electro-optic, acousto-optic, and piezoelectric properties. Meanwhile, studying the ferroelectric thin films growth mechanism, getting excellent preparation parameters, and exploring the physical properties of the ferroelectric thin films under the different environment have been the research topic in academic interests.Pulsed laser deposition (PLD) with remaining the target stoichiometry is a conventional technique, which is widely used for the growth of complex oxygen thin films. Although, a few studies about PLD growth of perovskite ferroelectric thin film have been reported, the growth mechanism of initial growth stage has rarely been investigated. Moreover, the effects of various experimental parameters on two and three dimensional growth of perovskite ferroelectric thin film still need to be further understood so that the three-dimensional growth mechanism can be figured out. Among the therotical ways to study the growth process, Kinetic Monte Carlo (KMC), which can provide atomic-scale point of views on the growth of thin films, was usually used to simulate the growth of semiconducting and metal thin films. However, the simulation works of perovskite ferroelectric thin films growth were not easy to be done because of the complexities of lattice structure and growth mechanism. Meanwhile, for ferroelectric thin films, the domain state is produced by its own uniformity and mechanical constraint. The structure, type, number and evolution of domain decide physical properties and the applied direction of ferroelectric thin films. Therefore, further research on the real time observation of domain structures is the foundation of understanding the properties of ferroelectric thin films. In this thesis, first of all two and three dimensional KMC models were constructed to simulate the submonolayer and multilayer growths of perovskite ferroelectric BaTiO₃ thin film via PLD respectively, for various experimental parameters. Secondly, the domain evalution of ferroelectric thin films is observed by using Piezoelectric Force Microscopy (PFM) under the different external fields. Finally, the phase transition, dielectric properties and piezoelectric properties of ferroelectric thin films are investigated theorically. The main contents are given as follows.(1) We proposed an energy-dependent kinetic Monte Carlo (KMC) approach to simulate BaTiO₃ thin film growth via PLD within the submonolayer regime, in which the coverageθis less than 1. Distinguishing with the traditional solid-on-solid (SOS) model, the adatom bonding is specially considered to describe the atom combine according to the perovskite structure, and the PLD growth of the perovskite thin film on the surface of square lattice substrate of homoepitaxial system which is considered as three stochastic incidents such as the deposition, diffusion, and bonding of adatoms. Varying the values of the laser repetition rate and pulse duration, the relative curves of the island density and island size vs coverage were obtained. The simulation results show that the island density increases while the island size decreases with the pulse frequency. When the pulse repetition rate is less than 1 KHz, there is no obvious variation for the curves of the island density and island size vs coverage. However, when the pulse repetition rate is larger than 1 KHz, the island density is no change forθ< 0.1, and with the pulse duration the island density increases while the island size decreases forθ< 0.1. In conclusion, it indicates that if pulse repetition rate is elevated to kilohertz or higher, pulse duration should be considered in growth model at the cases.(2) An energy-dependent KMC approach is proposed to simulate the multilayer growth of BaTiO₃ thin films via pulsed laser deposition, in which the four steps, such as the deposition of atoms, the diffusion of adatoms, the bonding of adatoms, and the surface migration of adatoms, are considered. Distinguishing with the traditional SOS model, the adatom bonding and the overhanging of atoms, according to the perovskite structure, are specially adopted to describe the ferroelectric thin film growth. The activation energy is considered from the interactions between the ions, which are calculated by BMH potential and Ehrilich Schwoebel (ES) is first considerd in calculations because of the adatoms hopping to subjacent atomic layers. From the simulation, the relationship between growth modes and the different PLD parameters is acquired. Moreover, the Reflection High-Energy Electron Diffraction (RHEED) is calculated in the simulation based on the three dimensional KMC model and the effects of different PLD parameters on the RHEED intensity are studied. The main conclusions are as follows. (a) The saturated coverage of each atomic layer is about 0.75 ML. With the increasing of incident kinetic energy, the growth mode of BaTiO₃ thin film can be transformed from the 3D growth to the layer-by-layer growth, and the surface roughness decreases. (b) With the increasing of laser repetition rates, the growth modes transformed from the layer-by-layer growth to the 3D growth, and the surface roughness increases. (c) For the mean deposition rates 0.1, 0.5 and 1.0 ML/s, the growth modes are respectively 2D-3D growth, 3D growth and 2D growth. The maximum value of surface roughness is for 0.5 ML/s, while the minimum value is for 1.0 ML/s. (d) For a higher incident kinetic energy, the RHEED intensity becomes stronger and the surface morphology becomes smoother. (e) With the increasing of laser repetition rates, the growth modes transformed from the layer-by-layer growth mode to the island growth mode, and the surface roughness increases. (f) With thehigher mean deposition rates, we conclude that the growth modes have the trendency to the layer-by-layer growth.(3) MOD method are used to prepare the Bi_3.15Eu_0.85Ti₃O₁₂(BET) and (1-x)Na_0.5Bi_0.5TiO_3-xK_0.5Bi_0.5TiO₃(NBT-KBT100x) ferroelectric thin films. The microstructure and ferroelectric properties of thin film are also characterized. The initial domain structures of ferroelectric thin films are obtained by PFM. Meanwhile, the domain evolution of ferroelectric thin films is observed by PFM under different external electric fields and forces. The main results are as follows. (a) We have observed 90°domain switching due to the external mechanical forces exerted by the SFM tip on the surface of BET thin film. With the increase of the mechanical force along the certain scan direction, some deformed grains are stretched up and the polarization is reversed to out-of-plane direction, corresponding to the phase transition from r phase to c phase. With the opposite scan direction, the grains are compressed and the polarization was reversed to in-plane direction, corresponding to the phase transition from c phase to aa phase. When the strongest force is applied, the weak piezoelectric signal is observed, and it is coincident with the previous experimental result. Meantime, the movement of 90°domain wall driven by the mechanical force is observed. (b) The surface morphology and domains of NBT-KBT100x with different components are investigated by PFM images. The results show that the NBT-KBT17 thin film presents the most single domain grains. Moreover, the LPFM amplitude and phase images of NBT-KBT17 thin film are also obtained, which indicate that the piezoelectric response is obvious to be energy harvesting device with d₃₁ mode. (c) The single grain of NBT-KBT17 thin film is selected to write the domain by opposite DC voltages. The written single grain was then keeping in environment for different duration to detect the retention of the thin film and the retention loss is low. At last, the different scan rates are applied on the NBT-KBT13 thin film. The results show that the polarization area of NBT-KBT13 thin film is much stronger at the lower scan rate 0.1 Hz. (d) The imprint of ferroelectric capacitor is investigated under the external mechanical force. The phase and amplitude-electric voltage hysteresis loops of NBT-KBT18 ferroelectric capacitor were obtained under the mechanical force. The observations indicate that the hysteresis loop and butterfly curve make a mobile, which means the imprint was produced by the external force.(4) Pb(Zr_0.52Ti_0.48)O₃ (PZT) ferroelectric thin film is prepared by PLD and the micro structure is characterized. The in-plane strain is evaluated by XRD.The single domain treatment on the selected single grain was performed by the negative DC bias in order to obtain the single-domain state, and the opposite color contrasts within the selected grain in piezoelectric phase images of PZT ferroelectric thin film were observed by PFM. Based on nonlinear thermodynamic theory, theα1c^- and r^- phases with the negative P 3 component are introduced to describe the electric-generated domain switching, and the external misfit strain-electric field phase diagram and the electric field-polarization components curve are simulated at the simplification of uniform stress/electric distribution for the single-domain state of a single grain. The main results are as follows. (a) PZT thin film is well crystallized with random orientation and no pyrochlore phase and the thicknesses of PZT thin film is estimated as 500 nm. (b) The piezoelectric phase images of the selected grain were observed by PFM under different DC voltages and the range of threshold electric field of 180°domain switching is 120-160 kV/cm. (c) According to the Bragg diffraction law, the out-plane strain S_{⊥^hikili} can be obtained by XRD. The mean out of plane strain can be evaluated by statistical average. Then, the in-plane strains measured by XRD are -0.001 for neglecting piezoelectric effect and -0.002 for considering piezoelectric couple. (d) The external stress and electric fields in PFM are reasonably regarded as the uniform distribution. Because of the opposite color contrasts within the selected grain in PFM, theα1c^- and r^- phases with negative P₃ component are introduced to describe the electric-generated domain switching. For the selected grain with the single-domain states, the misfit strain-external electric field phase diagram and the external electric field-polarization components curve are constructed based on nonlinear thermodynamic theory. In the misfit strain-external electric field phase diagram, the threshold of phase transition E₃ fromα1c^- phase to single-domain c⁺ phase is 139 kV/cm at the misfit strain -0.002, which is within the range of 120-160 kV/cm for electric-generated 180°domain switching. The in-plane strain -0.002 evaluated by XRD is reasonable considering the piezoelectric coupling and the simulation results are in agreement with the experimental observations.(5) The tradional nonlinear thermodynamic theory, which was used to simulate the phase transition of the ferroelectric thin film grown on the cubic substrate, is now amended to explore the effect of nonequally biaxal misfit strain, external stress and external temperature on phase states and physical properties of epitaxial Pb(Zr_1-xTi_x)O₃(PZT) thin films with different components grown on anisotropic substrates. The"misfit strain- misfit strain", the"misfit strain-external stress"and"misfit strain-temperature"phase diagrams are constructed aiming at single-domain PZT thin films grown on anisotropic substrates and the dielectric and piezoelectric responses under nonequally biaxal misfit strains are studied. The main contents are: (a) In misfit strain- misfit strain phase diagram, the nonequally biaxal misfit strains in the film plane may lead to the appearance of new phases:α1c andα1 phases. With the increasing of Ti contents, the areas of r andα1c phases decrease while the areas of c,α1 andα1α2 increase. An appropriate external stress could be influence the dielectric and piezoelectric responses of the film. (b) In misfit strain-external stress phase diagram for the known misfit strain e₁ = 0.005, the compressive stress may lead to the appearance of c phase while the tensile stress may lead to the appearance ofα1α2 phase. (c) In misfit strain-temperature phase diagram for the known misfit strain e₁ = 0.005, theα1c phase occurs at x≤0.7 and disappears gradually at x>0.7. The quadruple point is changed to the triple point under the nonequally biaxal misfit strains.